wasmtime/runtime/component/concurrent/

futures_and_streams.rs

use super::table::{TableDebug, TableId};
use super::{Event, GlobalErrorContextRefCount, Waitable, WaitableCommon};
use crate::component::concurrent::{ConcurrentState, CurrentThread, WorkItem, tls};
use crate::component::func::{self, LiftContext, LowerContext};
use crate::component::matching::InstanceType;
use crate::component::resources::{HostResourceIndex, HostResourceTables};
use crate::component::types;
use crate::component::values::ErrorContextAny;
use crate::component::{
    AsAccessor, ComponentInstanceId, ComponentType, FutureAny, Instance, Lift, Lower,
    RuntimeInstance, StreamAny, Val, WasmList,
};
use crate::prelude::*;
use crate::store::{StoreOpaque, StoreToken};
use crate::try_mutex::{TryMutex, TryMutexGuard};
use crate::vm::component::{ComponentInstance, HandleCounter, HandleTable, TransmitLocalState};
use crate::vm::{AlwaysMut, VMStore};
use crate::{AsContext, AsContextMut, StoreContextMut, ValRaw};
use crate::{
    Error, Result, Trap, bail, bail_bug, ensure,
    error::{Context as _, format_err},
};
use alloc::sync::Arc;
use buffers::{Extender, SliceBuffer, UntypedWriteBuffer};
use core::any::{Any, TypeId};
use core::fmt;
use core::future;
use core::iter;
use core::marker::PhantomData;
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::ops::{Deref, DerefMut};
use core::pin::Pin;
use core::task::{Context, Poll, Waker, ready};
use futures::channel::oneshot;
use futures::{FutureExt as _, stream};
use wasmtime_environ::component::{
    CanonicalAbiInfo, ComponentTypes, InterfaceType, OptionsIndex, RuntimeComponentInstanceIndex,
    TypeComponentGlobalErrorContextTableIndex, TypeComponentLocalErrorContextTableIndex,
    TypeFutureTableIndex, TypeStreamTableIndex,
};

pub use buffers::{ReadBuffer, VecBuffer, WriteBuffer};

mod buffers;

/// Enum for distinguishing between a stream or future in functions that handle
/// both.
#[derive(Copy, Clone, Debug)]
pub enum TransmitKind {
    Stream,
    Future,
}

/// Represents `{stream,future}.{read,write}` results.
#[derive(Copy, Clone, Debug, PartialEq)]
pub enum ReturnCode {
    Blocked,
    Completed(ItemCount),
    Dropped(ItemCount),
    Cancelled(ItemCount),
}

impl ReturnCode {
    /// Pack `self` into a single 32-bit integer that may be returned to the
    /// guest.
    ///
    /// This corresponds to `pack_copy_result` in the Component Model spec.
    pub fn encode(&self) -> u32 {
        const BLOCKED: u32 = 0xffff_ffff;
        const COMPLETED: u32 = 0x0;
        const DROPPED: u32 = 0x1;
        const CANCELLED: u32 = 0x2;
        match self {
            ReturnCode::Blocked => BLOCKED,
            ReturnCode::Completed(n) => (n.as_u32() << 4) | COMPLETED,
            ReturnCode::Dropped(n) => (n.as_u32() << 4) | DROPPED,
            ReturnCode::Cancelled(n) => (n.as_u32() << 4) | CANCELLED,
        }
    }

    /// Returns `Self::Completed` with the specified count (or zero if
    /// `matches!(kind, TransmitKind::Future)`)
    fn completed(kind: TransmitKind, count: ItemCount) -> Self {
        Self::Completed(if let TransmitKind::Future = kind {
            ItemCount::ZERO
        } else {
            count
        })
    }
}

/// Representation of how many items are being operated on in a stream read or
/// write.
///
/// The component model requires that stream operations are limited to `1<<28`
/// items in one go. This type is a newtype wrapper around `u32` with the
/// invariant that the internal value is limited by this amount.
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord)]
#[repr(transparent)]
pub struct ItemCount {
    raw: u32,
}

impl ItemCount {
    const MAX: u32 = 1 << 28;
    const ZERO: ItemCount = ItemCount { raw: 0 };

    /// Creates a new `ItemCount` with the specified count, or a trap if it's
    /// too large.
    fn new(count: u32) -> Result<Self, Trap> {
        if count < Self::MAX {
            Ok(Self { raw: count })
        } else {
            Err(Trap::StreamOpTooBig)
        }
    }

    /// Same as `Self::new` but takes a `usize`.
    fn new_usize(count: usize) -> Result<Self, Trap> {
        let count = u32::try_from(count).map_err(|_| Trap::StreamOpTooBig)?;
        Self::new(count)
    }

    fn as_u32(&self) -> u32 {
        self.raw
    }

    fn as_usize(&self) -> usize {
        usize::try_from(self.raw).unwrap()
    }

    /// Increments `self` by `amt`, returning a trap if the amount would exceed
    /// the maximum item count.
    fn inc(&mut self, amt: usize) -> Result<(), Trap> {
        let amt = u32::try_from(amt).map_err(|_| Trap::StreamOpTooBig)?;
        let new_raw = self.raw.checked_add(amt).ok_or(Trap::StreamOpTooBig)?;
        if new_raw < Self::MAX {
            self.raw = new_raw;
            Ok(())
        } else {
            Err(Trap::StreamOpTooBig)
        }
    }

    /// Helper to add two `ItemCount`s together, fallibly.
    ///
    /// It's considered a bug if this overflows, so this is only suitable in
    /// situations where overflow and/or exceeding the total item count is known
    /// that it may be possible.
    fn add(&self, other: ItemCount) -> Result<ItemCount> {
        match self.raw.checked_add(other.raw) {
            Some(raw) => Ok(ItemCount::new(raw)?),
            None => bail_bug!("overflow in `ItemCount::add`"),
        }
    }

    /// Same as `add`, but for subtraction.
    ///
    /// Like with `add` this is only suitable for situations where the result is
    /// known to not underflow.
    fn sub(&self, other: ItemCount) -> Result<ItemCount> {
        match self.raw.checked_sub(other.raw) {
            Some(raw) => Ok(ItemCount { raw }),
            None => bail_bug!("underflow in `ItemCount::sub`"),
        }
    }
}

impl fmt::Display for ItemCount {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.raw.fmt(f)
    }
}

impl fmt::Debug for ItemCount {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        self.raw.fmt(f)
    }
}

impl PartialEq<u32> for ItemCount {
    fn eq(&self, other: &u32) -> bool {
        self.raw == *other
    }
}

impl PartialOrd<u32> for ItemCount {
    fn partial_cmp(&self, other: &u32) -> Option<core::cmp::Ordering> {
        self.raw.partial_cmp(other)
    }
}

/// Represents a stream or future type index.
///
/// This is useful as a parameter type for functions which operate on either a
/// future or a stream.
#[derive(Copy, Clone, Debug)]
pub enum TransmitIndex {
    Stream(TypeStreamTableIndex),
    Future(TypeFutureTableIndex),
}

impl TransmitIndex {
    pub fn kind(&self) -> TransmitKind {
        match self {
            TransmitIndex::Stream(_) => TransmitKind::Stream,
            TransmitIndex::Future(_) => TransmitKind::Future,
        }
    }

    /// Retrieve the payload type of the specified stream or future, or `None`
    /// if it has no payload type.
    fn payload<'a>(&self, types: &'a ComponentTypes) -> Option<&'a InterfaceType> {
        match self {
            TransmitIndex::Stream(i) => {
                let ty = types[*i].ty;
                types[ty].payload.as_ref()
            }
            TransmitIndex::Future(i) => {
                let ty = types[*i].ty;
                types[ty].payload.as_ref()
            }
        }
    }
}

/// Retrieve the host rep and state for the specified guest-visible waitable
/// handle.
fn get_mut_by_index_from(
    handle_table: &mut HandleTable,
    ty: TransmitIndex,
    index: u32,
) -> Result<(u32, &mut TransmitLocalState)> {
    match ty {
        TransmitIndex::Stream(ty) => handle_table.stream_rep(ty, index),
        TransmitIndex::Future(ty) => handle_table.future_rep(ty, index),
    }
}

fn lower<T: func::Lower + Send + 'static, B: WriteBuffer<T>, U: 'static>(
    mut store: StoreContextMut<U>,
    instance: Instance,
    options: OptionsIndex,
    ty: TransmitIndex,
    address: usize,
    count: usize,
    buffer: &mut B,
) -> Result<()> {
    let count = buffer.remaining().len().min(count);

    let lower = &mut if T::MAY_REQUIRE_REALLOC {
        LowerContext::new(store.as_context_mut(), options, instance)
    } else {
        LowerContext::new_without_realloc(store.as_context_mut(), options, instance)
    };

    // Note that validation only happens with operations that have a payload.
    if let Some(ty) = ty.payload(lower.types).copied() {
        if address % usize::try_from(T::ALIGN32)? != 0 {
            bail!("read pointer not aligned");
        }
        lower
            .as_slice_mut()
            .get_mut(address..)
            .and_then(|b| b.get_mut(..T::SIZE32 * count))
            .ok_or_else(|| crate::format_err!("read pointer out of bounds of memory"))?;

        T::linear_store_list_to_memory(lower, ty, address, &buffer.remaining()[..count])?;
    }

    buffer.skip(count);

    Ok(())
}

fn lift<T: func::Lift + Send + 'static, B: ReadBuffer<T>>(
    lift: &mut LiftContext<'_>,
    ty: Option<InterfaceType>,
    buffer: &mut B,
    address: usize,
    count: usize,
) -> Result<()> {
    let count = count.min(buffer.remaining_capacity());
    if T::IS_RUST_UNIT_TYPE {
        // SAFETY: `T::IS_RUST_UNIT_TYPE` is only true for `()`, a
        // zero-sized type, so `MaybeUninit::uninit().assume_init()`
        // is a valid way to populate the zero-sized buffer.
        buffer.extend(
            iter::repeat_with(|| unsafe { MaybeUninit::uninit().assume_init() }).take(count),
        )
    } else {
        let ty = match ty {
            Some(ty) => ty,
            None => bail_bug!("type required for non-unit lift"),
        };
        if address % usize::try_from(T::ALIGN32)? != 0 {
            bail!("write pointer not aligned");
        }
        lift.memory()
            .get(address..)
            .and_then(|b| b.get(..T::SIZE32 * count))
            .ok_or_else(|| crate::format_err!("write pointer out of bounds of memory"))?;

        let list = &WasmList::new(address, count, lift, ty)?;
        T::linear_lift_into_from_memory(lift, list, &mut Extender(buffer))?
    }
    Ok(())
}

/// Represents the state associated with an error context
#[derive(Debug, PartialEq, Eq, PartialOrd)]
pub(super) struct ErrorContextState {
    /// Debug message associated with the error context
    pub(crate) debug_msg: String,
}

/// Represents the size and alignment for a "flat" Component Model type,
/// i.e. one containing no pointers or handles.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(super) struct FlatAbi {
    pub(super) size: u32,
    pub(super) align: u32,
}

struct HostBuffer<'a> {
    dst: &'a mut Vec<u8>,
    marked_written: &'a mut usize,
}

impl HostBuffer<'_> {
    fn reborrow(&mut self) -> HostBuffer<'_> {
        HostBuffer {
            dst: &mut *self.dst,
            marked_written: &mut *self.marked_written,
        }
    }
}

/// Represents the buffer for a host- or guest-initiated stream read.
pub struct Destination<'a, T, B> {
    id: TableId<TransmitState>,
    buffer: &'a mut B,
    host_buffer: Option<HostBuffer<'a>>,
    _phantom: PhantomData<fn() -> T>,
}

impl<'a, T, B> Destination<'a, T, B> {
    /// Reborrow `self` so it can be used again later.
    pub fn reborrow(&mut self) -> Destination<'_, T, B> {
        Destination {
            id: self.id,
            buffer: &mut *self.buffer,
            host_buffer: self.host_buffer.as_mut().map(|b| b.reborrow()),
            _phantom: PhantomData,
        }
    }

    /// Take the buffer out of `self`, leaving a default-initialized one in its
    /// place.
    ///
    /// This can be useful for reusing the previously-stored buffer's capacity
    /// instead of allocating a fresh one.
    pub fn take_buffer(&mut self) -> B
    where
        B: Default,
    {
        mem::take(self.buffer)
    }

    /// Store the specified buffer in `self`.
    ///
    /// Any items contained in the buffer will be delivered to the reader after
    /// the `StreamProducer::poll_produce` call to which this `Destination` was
    /// passed returns (unless overwritten by another call to `set_buffer`).
    ///
    /// If items are stored via this buffer _and_ written via a
    /// `DirectDestination` view of `self`, then the items in the buffer will be
    /// delivered after the ones written using `DirectDestination`.
    pub fn set_buffer(&mut self, buffer: B) {
        *self.buffer = buffer;
    }

    /// Return the remaining number of items the current read has capacity to
    /// accept, if known.
    ///
    /// This will return `Some(_)` if the reader is a guest; it will return
    /// `None` if the reader is the host.
    ///
    /// Note that this can return `Some(0)`. This means that the guest is
    /// attempting to perform a zero-length read which typically means that it's
    /// trying to wait for this stream to be ready-to-read but is not actually
    /// ready to receive the items yet. The host in this case is allowed to
    /// either block waiting for readiness or immediately complete the
    /// operation. The guest is expected to handle both cases. Some more
    /// discussion about this case can be found in the discussion of ["Stream
    /// Readiness" in the component-model repo][docs].
    ///
    /// [docs]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Concurrency.md#stream-readiness
    pub fn remaining(&self, mut store: impl AsContextMut) -> Result<Option<usize>> {
        self.remaining_(store.as_context_mut().0)
    }

    fn remaining_(&self, store: &mut StoreOpaque) -> Result<Option<usize>> {
        let transmit = store.concurrent_state_mut()?.get_mut(self.id)?;

        if let &ReadState::GuestReady { count, .. } = &transmit.read {
            let &WriteState::HostReady { guest_offset, .. } = &transmit.write else {
                bail_bug!("expected WriteState::HostReady")
            };

            Ok(Some(count.as_usize() - guest_offset.as_usize()))
        } else {
            Ok(None)
        }
    }
}

impl<'a, B> Destination<'a, u8, B> {
    /// Return a `DirectDestination` view of `self`.
    ///
    /// If the reader is a guest, this will provide direct access to the guest's
    /// read buffer.  If the reader is a host, this will provide access to a
    /// buffer which will be delivered to the host before any items stored using
    /// `Destination::set_buffer`.
    ///
    /// `capacity` will only be used if the reader is a host, in which case it
    /// will update the length of the buffer, possibly zero-initializing the new
    /// elements if the new length is larger than the old length.
    pub fn as_direct<D>(
        mut self,
        store: StoreContextMut<'a, D>,
        capacity: usize,
    ) -> DirectDestination<'a, D> {
        if let Some(buffer) = &mut self.host_buffer {
            *buffer.marked_written = 0;
            buffer.dst.resize(capacity, 0);
        }

        DirectDestination {
            id: self.id,
            host_buffer: self.host_buffer,
            store,
        }
    }
}

/// Represents a read from a `stream<u8>`, providing direct access to the
/// writer's buffer.
pub struct DirectDestination<'a, D: 'static> {
    id: TableId<TransmitState>,
    host_buffer: Option<HostBuffer<'a>>,
    store: StoreContextMut<'a, D>,
}

#[cfg(feature = "std")]
impl<D: 'static> std::io::Write for DirectDestination<'_, D> {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        let rem = self.remaining().map_err(std::io::Error::other)?;
        let n = rem.len().min(buf.len());
        rem[..n].copy_from_slice(&buf[..n]);
        self.mark_written(n);
        Ok(n)
    }

    fn flush(&mut self) -> std::io::Result<()> {
        Ok(())
    }
}

impl<D: 'static> DirectDestination<'_, D> {
    /// Provide direct access to the writer's buffer.
    pub fn remaining(&mut self) -> Result<&mut [u8]> {
        if let Some(buffer) = self.host_buffer.as_mut() {
            return Ok(buffer.dst);
        }
        let transmit = self
            .store
            .as_context_mut()
            .0
            .concurrent_state_mut()?
            .get_mut(self.id)?;

        let &ReadState::GuestReady {
            address,
            count,
            options,
            instance,
            ..
        } = &transmit.read
        else {
            bail_bug!("expected ReadState::GuestReady")
        };

        let &WriteState::HostReady { guest_offset, .. } = &transmit.write else {
            bail_bug!("expected WriteState::HostReady")
        };

        let memory = instance
            .options_memory_mut(self.store.0, options)
            .get_mut(
                if count == 0 {
                    0
                } else {
                    address + guest_offset.as_usize()
                }..,
            )
            .and_then(|b| b.get_mut(..(count.as_usize() - guest_offset.as_usize())));
        match memory {
            Some(memory) => Ok(memory),
            None => bail_bug!("guest buffer unexpectedly out of bounds"),
        }
    }

    /// Mark the specified number of bytes as written to the writer's buffer.
    ///
    /// # Panics
    ///
    /// This will panic if the count is larger than the size of the
    /// buffer returned by `Self::remaining`.
    pub fn mark_written(&mut self, count: usize) {
        // Note that this unwrap should only trigger for bugs in Wasmtime, and
        // this is modeled here to centralize the `.unwrap()` for this method in
        // one location.
        self.mark_written_(count).unwrap()
    }

    fn mark_written_(&mut self, count: usize) -> Result<()> {
        if let Some(buffer) = self.host_buffer.as_mut() {
            // Note that this `.unwrap` is a documented panic condition of
            // `mark_written`.
            *buffer.marked_written = buffer.marked_written.checked_add(count).unwrap();
        } else {
            let transmit = self
                .store
                .as_context_mut()
                .0
                .concurrent_state_mut()?
                .get_mut(self.id)?;

            let ReadState::GuestReady {
                count: read_count, ..
            } = &transmit.read
            else {
                bail_bug!("expected ReadState::GuestReady")
            };

            let WriteState::HostReady { guest_offset, .. } = &mut transmit.write else {
                bail_bug!("expected WriteState::HostReady");
            };

            if guest_offset.as_usize() + count > read_count.as_usize() {
                // Note that this `panic` is a documented panic condition of
                // `mark_written`.
                panic!(
                    "write count ({count}) must be less than or equal to read count ({read_count})"
                )
            } else {
                guest_offset.inc(count)?;
            }
        }
        Ok(())
    }
}

/// Represents the state of a `Stream{Producer,Consumer}`.
#[derive(Copy, Clone, Debug)]
pub enum StreamResult {
    /// The operation completed normally, and the producer or consumer may be
    /// able to produce or consume more items, respectively.
    Completed,
    /// The operation was interrupted (i.e. it wrapped up early after receiving
    /// a `finish` parameter value of true in a call to `poll_produce` or
    /// `poll_consume`), and the producer or consumer may be able to produce or
    /// consume more items, respectively.
    Cancelled,
    /// The operation completed normally, but the producer or consumer will
    /// _not_ able to produce or consume more items, respectively.
    Dropped,
}

/// Represents the host-owned write end of a stream.
pub trait StreamProducer<D>: Send + 'static {
    /// The payload type of this stream.
    type Item;

    /// The `WriteBuffer` type to use when delivering items.
    type Buffer: WriteBuffer<Self::Item> + Default;

    /// Handle a host- or guest-initiated read by delivering zero or more items
    /// to the specified destination.
    ///
    /// This will be called whenever the reader starts a read.
    ///
    /// # Arguments
    ///
    /// * `self` - a `Pin`'d version of self to perform Rust-level
    ///   future-related operations on.
    /// * `cx` - a Rust-related [`Context`] which is passed to other
    ///   future-related operations or used to acquire a waker.
    /// * `store` - the Wasmtime store that this operation is happening within.
    ///   Used, for example, to consult the state `D` associated with the store.
    /// * `destination` - the location that items are to be written to.
    /// * `finish` - a flag indicating whether the host should strive to
    ///   immediately complete/cancel any pending operation. See below for more
    ///   details.
    ///
    /// # Behavior
    ///
    /// If the implementation is able to produce one or more items immediately,
    /// it should write them to `destination` and return either
    /// `Poll::Ready(Ok(StreamResult::Completed))` if it expects to produce more
    /// items, or `Poll::Ready(Ok(StreamResult::Dropped))` if it cannot produce
    /// any more items.
    ///
    /// If the implementation is unable to produce any items immediately, but
    /// expects to do so later, and `finish` is _false_, it should store the
    /// waker from `cx` for later and return `Poll::Pending` without writing
    /// anything to `destination`.  Later, it should alert the waker when either
    /// the items arrive, the stream has ended, or an error occurs.
    ///
    /// If more items are written to `destination` than the reader has immediate
    /// capacity to accept, they will be retained in memory by the caller and
    /// used to satisfy future reads, in which case `poll_produce` will only be
    /// called again once all those items have been delivered.
    ///
    /// # Zero-length reads
    ///
    /// This function may be called with a zero-length capacity buffer
    /// (i.e. `Destination::remaining` returns `Some(0)`). This indicates that
    /// the guest wants to wait to see if an item is ready without actually
    /// reading the item. For example think of a UNIX `poll` function run on a
    /// TCP stream, seeing if it's readable without actually reading it.
    ///
    /// In this situation the host is allowed to either return immediately or
    /// wait for readiness. Note that waiting for readiness is not always
    /// possible. For example it's impossible to test if a Rust-native `Future`
    /// is ready without actually reading the item. Stream-specific
    /// optimizations, such as testing if a TCP stream is readable, may be
    /// possible however.
    ///
    /// For a zero-length read, the host is allowed to:
    ///
    /// - Return `Poll::Ready(Ok(StreamResult::Completed))` without writing
    ///   anything if it expects to be able to produce items immediately (i.e.
    ///   without first returning `Poll::Pending`) the next time `poll_produce`
    ///   is called with non-zero capacity. This is the best-case scenario of
    ///   fulfilling the guest's desire -- items aren't read/buffered but the
    ///   host is saying it's ready when the guest is.
    ///
    /// - Return `Poll::Ready(Ok(StreamResult::Completed))` without actually
    ///   testing for readiness. The guest doesn't know this yet, but the guest
    ///   will realize that zero-length reads won't work on this stream when a
    ///   subsequent nonzero read attempt is made which returns `Poll::Pending`
    ///   here.
    ///
    /// - Return `Poll::Pending` if the host has performed necessary async work
    ///   to wait for this stream to be readable without actually reading
    ///   anything. This is also a best-case scenario where the host is letting
    ///   the guest know that nothing is ready yet. Later the zero-length read
    ///   will complete and then the guest will attempt a nonzero-length read to
    ///   actually read some bytes.
    ///
    /// - Return `Poll::Ready(Ok(StreamResult::Completed))` after calling
    ///   `Destination::set_buffer` with one more more items. Note, however,
    ///   that this creates the hazard that the items will never be received by
    ///   the guest if it decides not to do another non-zero-length read before
    ///   closing the stream.  Moreover, if `Self::Item` is e.g. a
    ///   `Resource<_>`, they may end up leaking in that scenario. It is not
    ///   recommended to do this and it's better to return
    ///   `StreamResult::Completed` without buffering anything instead.
    ///
    /// For more discussion on zero-length reads see the [documentation in the
    /// component-model repo itself][docs].
    ///
    /// [docs]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Concurrency.md#stream-readiness
    ///
    /// # Return
    ///
    /// This function can return a number of possible cases from this function:
    ///
    /// * `Poll::Pending` - this operation cannot complete at this time. The
    ///   Rust-level `Future::poll` contract applies here where a waker should
    ///   be stored from the `cx` argument and be arranged to receive a
    ///   notification when this implementation can make progress. For example
    ///   if you call `Future::poll` on a sub-future, that's enough. If items
    ///   were written to `destination` then a trap in the guest will be raised.
    ///
    ///   Note that implementations should strive to avoid this return value
    ///   when `finish` is `true`. In such a situation the guest is attempting
    ///   to, for example, cancel a previous operation. By returning
    ///   `Poll::Pending` the guest will be blocked during the cancellation
    ///   request. If `finish` is `true` then `StreamResult::Cancelled` is
    ///   favored to indicate that no items were read. If a short read happened,
    ///   however, it's ok to return `StreamResult::Completed` indicating some
    ///   items were read.
    ///
    /// * `Poll::Ok(StreamResult::Completed)` - items, if applicable, were
    ///   written to the `destination`. Only zero-length reads (see above) may
    ///   complete without writing anything; if nothing was written for a
    ///   nonzero-length read, or for any read by the host, then a trap in the
    ///   guest will be raised.
    ///
    /// * `Poll::Ok(StreamResult::Cancelled)` - used when `finish` is `true` and
    ///   the implementation was able to successfully cancel any async work that
    ///   a previous read kicked off, if any. The host should not buffer values
    ///   received after returning `Cancelled` because the guest will not be
    ///   aware of these values and the guest could close the stream after
    ///   cancelling a read. Hosts should only return `Cancelled` when there are
    ///   no more async operations in flight for a previous read.
    ///
    ///   If items were written to `destination` then a trap in the guest will
    ///   be raised. If `finish` is `false` then this return value will raise a
    ///   trap in the guest.
    ///
    /// * `Poll::Ok(StreamResult::Dropped)` - end-of-stream marker, indicating
    ///   that this producer should not be polled again. Note that items may
    ///   still be written to `destination`.
    ///
    /// # Errors
    ///
    /// The implementation may alternatively choose to return `Err(_)` to
    /// indicate an unrecoverable error. This will cause the guest (if any) to
    /// trap and render the component instance (if any) unusable. The
    /// implementation should report errors that _are_ recoverable by other
    /// means (e.g. by writing to a `future`) and return
    /// `Poll::Ready(Ok(StreamResult::Dropped))`.
    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        store: StoreContextMut<'a, D>,
        destination: Destination<'a, Self::Item, Self::Buffer>,
        finish: bool,
    ) -> Poll<Result<StreamResult>>;

    /// Attempt to convert the specified object into a `Box<dyn Any>` which may
    /// be downcast to the specified type.
    ///
    /// The implementation must ensure that, if it returns `Ok(_)`, a downcast
    /// to the specified type is guaranteed to succeed.
    fn try_into(me: Pin<Box<Self>>, _ty: TypeId) -> Result<Box<dyn Any>, Pin<Box<Self>>> {
        Err(me)
    }
}

impl<T, D> StreamProducer<D> for iter::Empty<T>
where
    T: Send + Sync + 'static,
{
    type Item = T;
    type Buffer = Option<Self::Item>;

    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        _: &mut Context<'_>,
        _: StoreContextMut<'a, D>,
        _: Destination<'a, Self::Item, Self::Buffer>,
        _: bool,
    ) -> Poll<Result<StreamResult>> {
        Poll::Ready(Ok(StreamResult::Dropped))
    }
}

impl<T, D> StreamProducer<D> for stream::Empty<T>
where
    T: Send + Sync + 'static,
{
    type Item = T;
    type Buffer = Option<Self::Item>;

    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        _: &mut Context<'_>,
        _: StoreContextMut<'a, D>,
        _: Destination<'a, Self::Item, Self::Buffer>,
        _: bool,
    ) -> Poll<Result<StreamResult>> {
        Poll::Ready(Ok(StreamResult::Dropped))
    }
}

impl<T, D> StreamProducer<D> for Vec<T>
where
    T: Unpin + Send + Sync + 'static,
{
    type Item = T;
    type Buffer = VecBuffer<T>;

    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        _: &mut Context<'_>,
        _: StoreContextMut<'a, D>,
        mut dst: Destination<'a, Self::Item, Self::Buffer>,
        _: bool,
    ) -> Poll<Result<StreamResult>> {
        dst.set_buffer(mem::take(self.get_mut()).into());
        Poll::Ready(Ok(StreamResult::Dropped))
    }
}

impl<T, D> StreamProducer<D> for Box<[T]>
where
    T: Unpin + Send + Sync + 'static,
{
    type Item = T;
    type Buffer = VecBuffer<T>;

    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        _: &mut Context<'_>,
        _: StoreContextMut<'a, D>,
        mut dst: Destination<'a, Self::Item, Self::Buffer>,
        _: bool,
    ) -> Poll<Result<StreamResult>> {
        dst.set_buffer(mem::take(self.get_mut()).into_vec().into());
        Poll::Ready(Ok(StreamResult::Dropped))
    }
}

#[cfg(feature = "component-model-bytes")]
impl<D> StreamProducer<D> for bytes::Bytes {
    type Item = u8;
    type Buffer = Self;

    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        _: &mut Context<'_>,
        _store: StoreContextMut<'a, D>,
        mut dst: Destination<'a, Self::Item, Self::Buffer>,
        _: bool,
    ) -> Poll<Result<StreamResult>> {
        dst.set_buffer(mem::take(self.get_mut()));
        Poll::Ready(Ok(StreamResult::Dropped))
    }
}

#[cfg(feature = "component-model-bytes")]
impl<D> StreamProducer<D> for bytes::BytesMut {
    type Item = u8;
    type Buffer = Self;

    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        _: &mut Context<'_>,
        _store: StoreContextMut<'a, D>,
        mut dst: Destination<'a, Self::Item, Self::Buffer>,
        _: bool,
    ) -> Poll<Result<StreamResult>> {
        dst.set_buffer(mem::take(self.get_mut()));
        Poll::Ready(Ok(StreamResult::Dropped))
    }
}

/// Represents the buffer for a host- or guest-initiated stream write.
pub struct Source<'a, T> {
    id: TableId<TransmitState>,
    host_buffer: Option<&'a mut dyn WriteBuffer<T>>,
}

impl<'a, T> Source<'a, T> {
    /// Reborrow `self` so it can be used again later.
    pub fn reborrow(&mut self) -> Source<'_, T> {
        Source {
            id: self.id,
            host_buffer: self.host_buffer.as_deref_mut(),
        }
    }

    /// Accept zero or more items from the writer.
    pub fn read<B, S: AsContextMut>(&mut self, mut store: S, buffer: &mut B) -> Result<()>
    where
        T: func::Lift + 'static,
        B: ReadBuffer<T>,
    {
        if let Some(input) = &mut self.host_buffer {
            let count = input.remaining().len().min(buffer.remaining_capacity());
            buffer.move_from(*input, count);
        } else {
            let store = store.as_context_mut();
            let transmit = store.0.concurrent_state_mut()?.get_mut(self.id)?;

            let &ReadState::HostReady { guest_offset, .. } = &transmit.read else {
                bail_bug!("expected ReadState::HostReady");
            };

            let &WriteState::GuestReady {
                ty,
                address,
                count,
                options,
                instance,
                ..
            } = &transmit.write
            else {
                bail_bug!("expected WriteState::GuestReady");
            };

            // Zero-length writes are exempt from bounds/alignment checks, so
            // there's nothing to lift.
            if guest_offset >= count {
                return Ok(());
            }

            let cx = &mut LiftContext::new(store.0.store_opaque_mut(), options, instance)?;
            let ty = ty.payload(cx.types);
            let old_remaining = buffer.remaining_capacity();
            lift::<T, B>(
                cx,
                ty.copied(),
                buffer,
                address + (T::SIZE32 * guest_offset.as_usize()),
                count.as_usize() - guest_offset.as_usize(),
            )?;

            let transmit = store.0.concurrent_state_mut()?.get_mut(self.id)?;

            let ReadState::HostReady { guest_offset, .. } = &mut transmit.read else {
                bail_bug!("expected ReadState::HostReady");
            };

            guest_offset.inc(old_remaining - buffer.remaining_capacity())?;
        }

        Ok(())
    }

    /// Return the number of items remaining to be read from the current write
    /// operation.
    pub fn remaining(&self, mut store: impl AsContextMut) -> Result<usize>
    where
        T: 'static,
    {
        self.remaining_(store.as_context_mut().0)
    }

    fn remaining_(&self, store: &mut StoreOpaque) -> Result<usize>
    where
        T: 'static,
    {
        let transmit = store.concurrent_state_mut()?.get_mut(self.id)?;

        if let &WriteState::GuestReady { count, .. } = &transmit.write {
            let &ReadState::HostReady { guest_offset, .. } = &transmit.read else {
                bail_bug!("expected ReadState::HostReady")
            };

            Ok(count.as_usize() - guest_offset.as_usize())
        } else if let Some(host_buffer) = &self.host_buffer {
            Ok(host_buffer.remaining().len())
        } else {
            bail_bug!("expected either WriteState::GuestReady or host buffer")
        }
    }
}

impl<'a> Source<'a, u8> {
    /// Return a `DirectSource` view of `self`.
    pub fn as_direct<D>(self, store: StoreContextMut<'a, D>) -> DirectSource<'a, D> {
        DirectSource {
            id: self.id,
            host_buffer: self.host_buffer,
            store,
        }
    }
}

/// Represents a write to a `stream<u8>`, providing direct access to the
/// writer's buffer.
pub struct DirectSource<'a, D: 'static> {
    id: TableId<TransmitState>,
    host_buffer: Option<&'a mut dyn WriteBuffer<u8>>,
    store: StoreContextMut<'a, D>,
}

#[cfg(feature = "std")]
impl<D: 'static> std::io::Read for DirectSource<'_, D> {
    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
        let rem = self.remaining().map_err(std::io::Error::other)?;
        let n = rem.len().min(buf.len());
        buf[..n].copy_from_slice(&rem[..n]);
        self.mark_read(n);
        Ok(n)
    }
}

impl<D: 'static> DirectSource<'_, D> {
    /// Provide direct access to the writer's buffer.
    pub fn remaining(&mut self) -> Result<&[u8]> {
        if let Some(buffer) = self.host_buffer.as_deref_mut() {
            return Ok(buffer.remaining());
        }
        let transmit = self
            .store
            .as_context_mut()
            .0
            .concurrent_state_mut()?
            .get_mut(self.id)?;

        let &WriteState::GuestReady {
            address,
            count,
            options,
            instance,
            ..
        } = &transmit.write
        else {
            bail_bug!("expected WriteState::GuestReady")
        };

        let &ReadState::HostReady { guest_offset, .. } = &transmit.read else {
            bail_bug!("expected ReadState::HostReady")
        };

        let memory = instance
            .options_memory(self.store.0, options)
            .get(
                if count == 0 {
                    0
                } else {
                    address + guest_offset.as_usize()
                }..,
            )
            .and_then(|b| b.get(..(count.as_usize() - guest_offset.as_usize())));
        match memory {
            Some(memory) => Ok(memory),
            None => bail_bug!("guest buffer unexpectedly out of bounds"),
        }
    }

    /// Mark the specified number of bytes as read from the writer's buffer.
    ///
    /// # Panics
    ///
    /// This will panic if the count is larger than the size of the buffer
    /// returned by `Self::remaining`.
    pub fn mark_read(&mut self, count: usize) {
        // Note that this unwrap should only trigger for bugs in Wasmtime, and
        // this is modeled here to centralize the `.unwrap()` for this method in
        // one location.
        self.mark_read_(count).unwrap()
    }

    fn mark_read_(&mut self, count: usize) -> Result<()> {
        if let Some(buffer) = self.host_buffer.as_deref_mut() {
            buffer.skip(count);
            return Ok(());
        }

        let transmit = self
            .store
            .as_context_mut()
            .0
            .concurrent_state_mut()?
            .get_mut(self.id)?;

        let WriteState::GuestReady {
            count: write_count, ..
        } = &transmit.write
        else {
            bail_bug!("expected WriteState::GuestReady");
        };

        let ReadState::HostReady { guest_offset, .. } = &mut transmit.read else {
            bail_bug!("expected ReadState::HostReady");
        };

        if guest_offset.as_usize() + count > write_count.as_usize() {
            // Note that this is a documented panic condition of `mark_read`.
            panic!("read count ({count}) must be less than or equal to write count ({write_count})")
        } else {
            guest_offset.inc(count)?;
        }
        Ok(())
    }
}

/// Represents the host-owned read end of a stream.
pub trait StreamConsumer<D>: Send + 'static {
    /// The payload type of this stream.
    type Item;

    /// Handle a host- or guest-initiated write by accepting zero or more items
    /// from the specified source.
    ///
    /// This will be called whenever the writer starts a write.
    ///
    /// If the implementation is able to consume one or more items immediately,
    /// it should take them from `source` and return either
    /// `Poll::Ready(Ok(StreamResult::Completed))` if it expects to be able to consume
    /// more items, or `Poll::Ready(Ok(StreamResult::Dropped))` if it cannot
    /// accept any more items.  Alternatively, it may return `Poll::Pending` to
    /// indicate that the caller should delay sending a `COMPLETED` event to the
    /// writer until a later call to this function returns `Poll::Ready(_)`.
    /// For more about that, see the `Backpressure` section below.
    ///
    /// If the implementation cannot consume any items immediately and `finish`
    /// is _false_, it should store the waker from `cx` for later and return
    /// `Poll::Pending` without writing anything to `destination`.  Later, it
    /// should alert the waker when either (1) the items arrive, (2) the stream
    /// has ended, or (3) an error occurs.
    ///
    /// If the implementation cannot consume any items immediately and `finish`
    /// is _true_, it should, if possible, return
    /// `Poll::Ready(Ok(StreamResult::Cancelled))` immediately without taking
    /// anything from `source`.  However, that might not be possible if an
    /// earlier call to `poll_consume` kicked off an asynchronous operation
    /// which needs to be completed (and possibly interrupted) gracefully, in
    /// which case the implementation may return `Poll::Pending` and later alert
    /// the waker as described above.  In other words, when `finish` is true,
    /// the implementation should prioritize returning a result to the reader
    /// (even if no items can be consumed) rather than wait indefinitely for at
    /// capacity to free up.
    ///
    /// In all of the above cases, the implementation may alternatively choose
    /// to return `Err(_)` to indicate an unrecoverable error.  This will cause
    /// the guest (if any) to trap and render the component instance (if any)
    /// unusable.  The implementation should report errors that _are_
    /// recoverable by other means (e.g. by writing to a `future`) and return
    /// `Poll::Ready(Ok(StreamResult::Dropped))`.
    ///
    /// Note that the implementation should only return
    /// `Poll::Ready(Ok(StreamResult::Cancelled))` without having taken any
    /// items from `source` if called with `finish` set to true.  If it does so
    /// when `finish` is false, the caller will trap.  Additionally, it should
    /// only return `Poll::Ready(Ok(StreamResult::Completed))` after taking at
    /// least one item from `source` if there is an item available; otherwise,
    /// the caller will trap.  Items taken during earlier calls for the same
    /// write which returned `Poll::Pending` count towards this, so it's fine
    /// to return `Completed` without taking more items once those have been
    /// forwarded (see "Backpressure" below).  If `poll_consume` is called with
    /// no items in
    /// `source`, it should only return `Poll::Ready(_)` once it is able to
    /// accept at least one item during the next call to `poll_consume`.
    ///
    /// Note that any items which the implementation of this trait takes from
    /// `source` become the responsibility of that implementation.  For that
    /// reason, an implementation which forwards items to an upstream sink
    /// should reserve capacity in that sink before taking items out of
    /// `source`, if possible.  Alternatively, it might buffer items which can't
    /// be forwarded immediately and send them once capacity is freed up.
    ///
    /// ## Backpressure
    ///
    /// As mentioned above, an implementation might choose to return
    /// `Poll::Pending` after taking items from `source`, which tells the caller
    /// to delay sending a `COMPLETED` event to the writer.  This can be used as
    /// a form of backpressure when the items are forwarded to an upstream sink
    /// asynchronously.  Note, however, that it's not possible to "put back"
    /// items into `source` once they've been taken out, so if the upstream sink
    /// is unable to accept all the items, that cannot be communicated to the
    /// writer at this level of abstraction.  Just as with application-specific,
    /// recoverable errors, information about which items could be forwarded and
    /// which could not must be communicated out-of-band, e.g. by writing to an
    /// application-specific `future`.
    ///
    /// Similarly, if the writer cancels the write after items have been taken
    /// from `source` but before the items have all been forwarded to an
    /// upstream sink, `poll_consume` will be called with `finish` set to true,
    /// and the implementation may either:
    ///
    /// - Interrupt the forwarding process gracefully.  This may be preferable
    /// if there is an out-of-band channel for communicating to the writer how
    /// many items were forwarded before being interrupted.
    ///
    /// - Allow the forwarding to complete without interrupting it.  This is
    /// usually preferable if there's no out-of-band channel for reporting back
    /// to the writer how many items were forwarded.
    fn poll_consume(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        store: StoreContextMut<D>,
        source: Source<'_, Self::Item>,
        finish: bool,
    ) -> Poll<Result<StreamResult>>;
}

/// Represents a host-owned write end of a future.
pub trait FutureProducer<D>: Send + 'static {
    /// The payload type of this future.
    type Item;

    /// Handle a host- or guest-initiated read by producing a value.
    ///
    /// This is equivalent to `StreamProducer::poll_produce`, but with a
    /// simplified interface for futures.
    ///
    /// If `finish` is true, the implementation may return
    /// `Poll::Ready(Ok(None))` to indicate the operation was canceled before it
    /// could produce a value.  Otherwise, it must either return
    /// `Poll::Ready(Ok(Some(_)))`, `Poll::Ready(Err(_))`, or `Poll::Pending`.
    fn poll_produce(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        store: StoreContextMut<D>,
        finish: bool,
    ) -> Poll<Result<Option<Self::Item>>>;
}

impl<T, E, D, Fut> FutureProducer<D> for Fut
where
    E: Into<Error>,
    Fut: Future<Output = Result<T, E>> + ?Sized + Send + 'static,
{
    type Item = T;

    fn poll_produce<'a>(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        _: StoreContextMut<'a, D>,
        finish: bool,
    ) -> Poll<Result<Option<T>>> {
        match self.poll(cx) {
            Poll::Ready(Ok(v)) => Poll::Ready(Ok(Some(v))),
            Poll::Ready(Err(err)) => Poll::Ready(Err(err.into())),
            Poll::Pending if finish => Poll::Ready(Ok(None)),
            Poll::Pending => Poll::Pending,
        }
    }
}

/// Represents a host-owned read end of a future.
pub trait FutureConsumer<D>: Send + 'static {
    /// The payload type of this future.
    type Item;

    /// Handle a host- or guest-initiated write by consuming a value.
    ///
    /// This is equivalent to `StreamProducer::poll_produce`, but with a
    /// simplified interface for futures.
    ///
    /// If `finish` is true, the implementation may return `Poll::Ready(Ok(()))`
    /// without taking the item from `source`, which indicates the operation was
    /// canceled before it could consume the value.  Otherwise, it must either
    /// take the item from `source` and return `Poll::Ready(Ok(()))`, or else
    /// return `Poll::Ready(Err(_))` or `Poll::Pending` (with or without taking
    /// the item).
    fn poll_consume(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        store: StoreContextMut<D>,
        source: Source<'_, Self::Item>,
        finish: bool,
    ) -> Poll<Result<()>>;
}

/// Represents the readable end of a Component Model `future`.
///
/// Note that `FutureReader` instances must be disposed of using either `pipe`
/// or `close`; otherwise the in-store representation will leak and the writer
/// end will hang indefinitely.  Consider using [`GuardedFutureReader`] to
/// ensure that disposal happens automatically.
pub struct FutureReader<T> {
    idx: HostResourceIndex,
    _phantom: PhantomData<T>,
}

impl<T> FutureReader<T> {
    /// Create a new future with the specified producer.
    ///
    /// # Errors
    ///
    /// Returns an error if the resource table for this store is full or if
    /// [`Config::concurrency_support`] is not enabled.
    ///
    /// [`Config::concurrency_support`]: crate::Config::concurrency_support
    pub fn new<S: AsContextMut>(
        mut store: S,
        producer: impl FutureProducer<S::Data, Item = T>,
    ) -> Result<Self>
    where
        T: func::Lower + func::Lift + Send + Sync + 'static,
    {
        ensure!(
            store.as_context().0.concurrency_support(),
            "concurrency support is not enabled"
        );

        struct Producer<P>(P);

        impl<D, T: func::Lower + 'static, P: FutureProducer<D, Item = T>> StreamProducer<D>
            for Producer<P>
        {
            type Item = P::Item;
            type Buffer = Option<P::Item>;

            fn poll_produce<'a>(
                self: Pin<&mut Self>,
                cx: &mut Context<'_>,
                store: StoreContextMut<D>,
                mut destination: Destination<'a, Self::Item, Self::Buffer>,
                finish: bool,
            ) -> Poll<Result<StreamResult>> {
                // SAFETY: This is a standard pin-projection, and we never move
                // out of `self`.
                let producer = unsafe { self.map_unchecked_mut(|v| &mut v.0) };

                Poll::Ready(Ok(
                    if let Some(value) = ready!(producer.poll_produce(cx, store, finish))? {
                        destination.set_buffer(Some(value));

                        // Here we return `StreamResult::Completed` even though
                        // we've produced the last item we'll ever produce.
                        // That's because the ABI expects
                        // `ReturnCode::Completed(1)` rather than
                        // `ReturnCode::Dropped(1)`.  In any case, we won't be
                        // called again since the future will have resolved.
                        StreamResult::Completed
                    } else {
                        StreamResult::Cancelled
                    },
                ))
            }
        }

        Ok(Self::new_(
            store
                .as_context_mut()
                .new_transmit(TransmitKind::Future, Producer(producer))?,
        ))
    }

    pub(super) fn new_(idx: HostResourceIndex) -> Self {
        Self {
            idx,
            _phantom: PhantomData,
        }
    }

    pub(super) fn idx(&self) -> HostResourceIndex {
        self.idx
    }

    /// Set the consumer that accepts the result of this future.
    ///
    /// # Errors
    ///
    /// Returns an error if this future has already been closed or otherwise
    /// transferred (e.g. lowered into a guest), or if it does not belong to
    /// `store`.
    pub fn pipe<S: AsContextMut>(
        self,
        mut store: S,
        consumer: impl FutureConsumer<S::Data, Item = T> + Unpin,
    ) -> Result<()>
    where
        T: func::Lift + 'static,
    {
        struct Consumer<C>(C);

        impl<D: 'static, T: func::Lift + 'static, C: FutureConsumer<D, Item = T>> StreamConsumer<D>
            for Consumer<C>
        {
            type Item = T;

            fn poll_consume(
                self: Pin<&mut Self>,
                cx: &mut Context<'_>,
                mut store: StoreContextMut<D>,
                mut source: Source<Self::Item>,
                finish: bool,
            ) -> Poll<Result<StreamResult>> {
                // SAFETY: This is a standard pin-projection, and we never move
                // out of `self`.
                let consumer = unsafe { self.map_unchecked_mut(|v| &mut v.0) };

                ready!(consumer.poll_consume(
                    cx,
                    store.as_context_mut(),
                    source.reborrow(),
                    finish
                ))?;

                Poll::Ready(Ok(if source.remaining(store)? == 0 {
                    // Here we return `StreamResult::Completed` even though
                    // we've consumed the last item we'll ever consume.  That's
                    // because the ABI expects `ReturnCode::Completed(1)` rather
                    // than `ReturnCode::Dropped(1)`.  In any case, we won't be
                    // called again since the future will have resolved.
                    StreamResult::Completed
                } else {
                    StreamResult::Cancelled
                }))
            }
        }

        store
            .as_context_mut()
            .set_consumer(self.idx, TransmitKind::Future, Consumer(consumer))
    }

    /// Transfer ownership of the read end of a future from a guest to the host.
    fn lift_from_index(cx: &mut LiftContext<'_>, ty: InterfaceType, index: u32) -> Result<Self> {
        Ok(Self::new_(lift_index_to_future(cx, ty, index)?))
    }

    /// Close this `FutureReader`.
    ///
    /// This will close this half of the future which will signal to a pending
    /// write, if any, that the reader side is dropped. If the writer half has
    /// not yet written a value then when it attempts to write a value it will
    /// see that this end is closed.
    ///
    /// # Errors
    ///
    /// Returns an error if this future has already been closed or otherwise
    /// transferred (e.g. lowered into a guest), or if it does not belong to
    /// `store`.
    pub fn close(&mut self, mut store: impl AsContextMut) -> Result<()> {
        future_close(store.as_context_mut().0, self.idx)
    }

    /// Convenience method around [`Self::close`].
    pub fn close_with(&mut self, accessor: impl AsAccessor) -> Result<()> {
        accessor.as_accessor().with(|access| self.close(access))
    }

    /// Returns a [`GuardedFutureReader`] which will auto-close this future on
    /// drop and clean it up from the store.
    ///
    /// Note that the `accessor` provided must own this future and is
    /// additionally transferred to the `GuardedFutureReader` return value.
    pub fn guard<A>(self, accessor: A) -> GuardedFutureReader<T, A>
    where
        A: AsAccessor,
    {
        GuardedFutureReader::new(accessor, self)
    }

    /// Attempts to convert this [`FutureReader<T>`] to a [`FutureAny`].
    ///
    /// # Errors
    ///
    /// This function will return an error if `self` does not belong to
    /// `store`.
    pub fn try_into_future_any(self, store: impl AsContextMut) -> Result<FutureAny>
    where
        T: ComponentType + 'static,
    {
        FutureAny::try_from_future_reader(store, self)
    }

    /// Attempts to convert a [`FutureAny`] into a [`FutureReader<T>`].
    ///
    /// # Errors
    ///
    /// This function will fail if `T` doesn't match the type of the value that
    /// `future` is sending.
    pub fn try_from_future_any(future: FutureAny) -> Result<Self>
    where
        T: ComponentType + 'static,
    {
        future.try_into_future_reader()
    }
}

impl<T> fmt::Debug for FutureReader<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("FutureReader")
            .field("idx", &self.idx)
            .finish()
    }
}

pub(super) fn future_close(store: &mut StoreOpaque, idx: HostResourceIndex) -> Result<()> {
    let id = store.host_reader_take(idx)?;
    store.host_drop_reader(id, TransmitKind::Future)
}

/// Transfer ownership of the read end of a future from a guest to the host.
pub(super) fn lift_index_to_future(
    cx: &mut LiftContext<'_>,
    ty: InterfaceType,
    index: u32,
) -> Result<HostResourceIndex> {
    match ty {
        InterfaceType::Future(src) => {
            let id = {
                let (state, instance, counter) = cx.concurrent_state_and_instance_mut();
                lift_index_to_transmit(instance, counter, state, TransmitIndex::Future(src), index)?
            };
            cx.host_resource_lower_own(id.rep(), None, None)
        }
        _ => func::bad_type_info(),
    }
}

/// Checks that the host-owned read end `idx` may be lowered into a guest as a
/// future or stream with the payload type `payload`.
///
/// Note that this is technically redundant for `StreamReader<T>` where its
/// static typecheck already verified this, but this is otherwise required in
/// the face of the `{Stream,Future}Any` abstraction to handle various use cases
/// of types dynamically being labeled or possibly switched.
fn typecheck_lowered_transmit<U>(
    idx: HostResourceIndex,
    cx: &mut LowerContext<'_, U>,
    payload: Option<&InterfaceType>,
) -> Result<()> {
    let origin = cx.store.0.transmit_origin(idx)?;
    let dst = cx.instance_type();
    let equivalent = match origin {
        TransmitOrigin::Host { typecheck } => {
            return typecheck(payload, &dst).context("future or stream payload types differ");
        }
        TransmitOrigin::GuestFuture(id, ty) => {
            let src = InstanceType::new(cx.store.0.component_instance(id));
            types::FutureType::from(src.types[ty].ty, &src).equivalent_payload_guest(&dst, payload)
        }
        TransmitOrigin::GuestStream(id, ty) => {
            let src = InstanceType::new(cx.store.0.component_instance(id));
            types::StreamType::from(src.types[ty].ty, &src).equivalent_payload_guest(&dst, payload)
        }
    };
    if !equivalent {
        bail!("future or stream payload types differ");
    }
    Ok(())
}

/// Transfer ownership of the read end of a future from the host to a guest.
pub(super) fn lower_future_to_index<U>(
    idx: HostResourceIndex,
    cx: &mut LowerContext<'_, U>,
    ty: InterfaceType,
) -> Result<u32> {
    match ty {
        InterfaceType::Future(dst) => {
            let types = cx.types;
            typecheck_lowered_transmit(idx, cx, types[types[dst].ty].payload.as_ref())?;
            let id = cx.store.0.host_reader_take(idx)?;
            cx.instance_handle()
                .lower_transmit_to_index(cx.store.0, TransmitIndex::Future(dst), id)
        }
        _ => func::bad_type_info(),
    }
}

// SAFETY: This relies on the `ComponentType` implementation for `u32` being
// safe and correct since we lift and lower future handles as `u32`s.
unsafe impl<T: ComponentType> ComponentType for FutureReader<T> {
    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::SCALAR4;

    type Lower = <u32 as func::ComponentType>::Lower;

    fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::Future(ty) => {
                let ty = types.types[*ty].ty;
                types::typecheck_payload::<T>(types.types[ty].payload.as_ref(), types)
            }
            other => bail!("expected `future`, found `{}`", func::desc(other)),
        }
    }
}

// SAFETY: See the comment on the `ComponentType` `impl` for this type.
unsafe impl<T: ComponentType> func::Lower for FutureReader<T> {
    fn linear_lower_to_flat<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()> {
        lower_future_to_index(self.idx, cx, ty)?.linear_lower_to_flat(cx, InterfaceType::U32, dst)
    }

    fn linear_lower_to_memory<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        lower_future_to_index(self.idx, cx, ty)?.linear_lower_to_memory(
            cx,
            InterfaceType::U32,
            offset,
        )
    }
}

// SAFETY: See the comment on the `ComponentType` `impl` for this type.
unsafe impl<T: ComponentType> func::Lift for FutureReader<T> {
    fn linear_lift_from_flat(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        src: &Self::Lower,
    ) -> Result<Self> {
        let index = u32::linear_lift_from_flat(cx, InterfaceType::U32, src)?;
        Self::lift_from_index(cx, ty, index)
    }

    fn linear_lift_from_memory(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        bytes: &[u8],
    ) -> Result<Self> {
        let index = u32::linear_lift_from_memory(cx, InterfaceType::U32, bytes)?;
        Self::lift_from_index(cx, ty, index)
    }
}

/// A [`FutureReader`] paired with an [`Accessor`].
///
/// This is an RAII wrapper around [`FutureReader`] that ensures it is closed
/// when dropped. This can be created through [`GuardedFutureReader::new`] or
/// [`FutureReader::guard`].
///
/// [`Accessor`]: crate::component::Accessor
pub struct GuardedFutureReader<T, A>
where
    A: AsAccessor,
{
    // This field is `None` to implement the conversion from this guard back to
    // `FutureReader`. When `None` is seen in the destructor it will cause the
    // destructor to do nothing.
    reader: Option<FutureReader<T>>,
    accessor: A,
}

impl<T, A> GuardedFutureReader<T, A>
where
    A: AsAccessor,
{
    /// Create a new `GuardedFutureReader` with the specified `accessor` and `reader`.
    ///
    /// # Panics
    ///
    /// Panics if [`Config::concurrency_support`] is not enabled.
    ///
    /// [`Config::concurrency_support`]: crate::Config::concurrency_support
    pub fn new(accessor: A, reader: FutureReader<T>) -> Self {
        assert!(
            accessor
                .as_accessor()
                .with(|a| a.as_context().0.concurrency_support())
        );
        Self {
            reader: Some(reader),
            accessor,
        }
    }

    /// Extracts the underlying [`FutureReader`] from this guard, returning it
    /// back.
    pub fn into_future(self) -> FutureReader<T> {
        self.into()
    }
}

impl<T, A> From<GuardedFutureReader<T, A>> for FutureReader<T>
where
    A: AsAccessor,
{
    fn from(mut guard: GuardedFutureReader<T, A>) -> Self {
        guard.reader.take().unwrap()
    }
}

impl<T, A> Drop for GuardedFutureReader<T, A>
where
    A: AsAccessor,
{
    fn drop(&mut self) {
        if let Some(reader) = &mut self.reader {
            // Currently this can only fail if the future is closed twice or
            // the handle was cloned/closed elsewhere, all of which the embedder
            // is responsible for when using this guard. Hint during
            // development that's a bad idea, but otherwise ignore the error in
            // release builds.
            let result = reader.close_with(&self.accessor);
            debug_assert!(result.is_ok());
        }
    }
}

/// Represents the readable end of a Component Model `stream`.
///
/// Note that `StreamReader` instances must be disposed of using `close`;
/// otherwise the in-store representation will leak and the writer end will hang
/// indefinitely.  Consider using [`GuardedStreamReader`] to ensure that
/// disposal happens automatically.
pub struct StreamReader<T> {
    idx: HostResourceIndex,
    _phantom: PhantomData<T>,
}

impl<T> StreamReader<T> {
    /// Create a new stream with the specified producer.
    ///
    /// # Errors
    ///
    /// Returns an error if the resource table for this store is full or if
    /// [`Config::concurrency_support`] is not enabled.
    ///
    /// [`Config::concurrency_support`]: crate::Config::concurrency_support
    pub fn new<S: AsContextMut>(
        mut store: S,
        producer: impl StreamProducer<S::Data, Item = T>,
    ) -> Result<Self>
    where
        T: func::Lower + func::Lift + Send + Sync + 'static,
    {
        ensure!(
            store.as_context().0.concurrency_support(),
            "concurrency support is not enabled",
        );
        Ok(Self::new_(
            store
                .as_context_mut()
                .new_transmit(TransmitKind::Stream, producer)?,
        ))
    }

    pub(super) fn new_(idx: HostResourceIndex) -> Self {
        Self {
            idx,
            _phantom: PhantomData,
        }
    }

    pub(super) fn idx(&self) -> HostResourceIndex {
        self.idx
    }

    /// Attempt to consume this object by converting it into the specified type.
    ///
    /// This can be useful for "short-circuiting" host-to-host streams,
    /// bypassing the guest entirely.  For example, if a guest task returns a
    /// host-created stream and then exits, this function may be used to
    /// retrieve the write end, after which the guest instance and store may be
    /// disposed of if no longer needed.
    ///
    /// This will return `Ok(_)` if and only if the following conditions are
    /// met:
    ///
    /// - The stream was created by the host (i.e. not by the guest).
    ///
    /// - The `StreamProducer::try_into` function returns `Ok(_)` when given the
    /// producer provided to `StreamReader::new` when the stream was created,
    /// along with `TypeId::of::<V>()`.
    ///
    /// # Panics
    ///
    /// Panics if this stream has already been closed, or if this stream doesn't
    /// belong to the specified `store`.
    pub fn try_into<V: 'static>(mut self, mut store: impl AsContextMut) -> Result<V, Self> {
        let store = store.as_context_mut();
        let id = store.0.host_reader_rep(self.idx).unwrap();
        let state = store.0.concurrent_state_mut_already_forced_current_thread();
        let id = state.get_mut(id).unwrap().state;
        if let WriteState::HostReady { try_into, .. } = &state.get_mut(id).unwrap().write {
            match try_into(TypeId::of::<V>()) {
                Some(result) => {
                    self.close(store).unwrap();
                    Ok(*result.downcast::<V>().unwrap())
                }
                None => Err(self),
            }
        } else {
            Err(self)
        }
    }

    /// Set the consumer that accepts the items delivered to this stream.
    ///
    /// # Errors
    ///
    /// Returns an error if this stream has already been closed or otherwise
    /// transferred (e.g. lowered into a guest), or if it does not belong to
    /// `store`.
    pub fn pipe<S: AsContextMut>(
        self,
        mut store: S,
        consumer: impl StreamConsumer<S::Data, Item = T>,
    ) -> Result<()>
    where
        T: 'static,
    {
        store
            .as_context_mut()
            .set_consumer(self.idx, TransmitKind::Stream, consumer)
    }

    /// Transfer ownership of the read end of a stream from a guest to the host.
    fn lift_from_index(cx: &mut LiftContext<'_>, ty: InterfaceType, index: u32) -> Result<Self> {
        Ok(Self::new_(lift_index_to_stream(cx, ty, index)?))
    }

    /// Close this `StreamReader`.
    ///
    /// This will signal that this portion of the stream is closed causing all
    /// future writes to return immediately with "DROPPED".
    ///
    /// # Errors
    ///
    /// Returns an error if this stream has already been closed or otherwise
    /// transferred (e.g. lowered into a guest), or if it does not belong to
    /// `store`.
    pub fn close(&mut self, mut store: impl AsContextMut) -> Result<()> {
        stream_close(store.as_context_mut().0, self.idx)
    }

    /// Convenience method around [`Self::close`].
    pub fn close_with(&mut self, accessor: impl AsAccessor) -> Result<()> {
        accessor.as_accessor().with(|access| self.close(access))
    }

    /// Returns a [`GuardedStreamReader`] which will auto-close this stream on
    /// drop and clean it up from the store.
    ///
    /// Note that the `accessor` provided must own this future and is
    /// additionally transferred to the `GuardedStreamReader` return value.
    pub fn guard<A>(self, accessor: A) -> GuardedStreamReader<T, A>
    where
        A: AsAccessor,
    {
        GuardedStreamReader::new(accessor, self)
    }

    /// Attempts to convert this [`StreamReader<T>`] to a [`StreamAny`].
    ///
    /// # Errors
    ///
    /// This function will return an error if `self` does not belong to
    /// `store`.
    pub fn try_into_stream_any(self, store: impl AsContextMut) -> Result<StreamAny>
    where
        T: ComponentType + 'static,
    {
        StreamAny::try_from_stream_reader(store, self)
    }

    /// Attempts to convert a [`StreamAny`] into a [`StreamReader<T>`].
    ///
    /// # Errors
    ///
    /// This function will fail if `T` doesn't match the type of the value that
    /// `stream` is sending.
    pub fn try_from_stream_any(stream: StreamAny) -> Result<Self>
    where
        T: ComponentType + 'static,
    {
        stream.try_into_stream_reader()
    }
}

impl<T> fmt::Debug for StreamReader<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("StreamReader")
            .field("idx", &self.idx)
            .finish()
    }
}

pub(super) fn stream_close(store: &mut StoreOpaque, idx: HostResourceIndex) -> Result<()> {
    let id = store.host_reader_take(idx)?;
    store.host_drop_reader(id, TransmitKind::Stream)
}

/// Transfer ownership of the read end of a stream from a guest to the host.
pub(super) fn lift_index_to_stream(
    cx: &mut LiftContext<'_>,
    ty: InterfaceType,
    index: u32,
) -> Result<HostResourceIndex> {
    match ty {
        InterfaceType::Stream(src) => {
            let id = {
                let (state, instance, counter) = cx.concurrent_state_and_instance_mut();
                lift_index_to_transmit(instance, counter, state, TransmitIndex::Stream(src), index)?
            };
            cx.host_resource_lower_own(id.rep(), None, None)
        }
        _ => func::bad_type_info(),
    }
}

/// Transfer ownership of the read end of a stream from the host to a guest.
pub(super) fn lower_stream_to_index<U>(
    idx: HostResourceIndex,
    cx: &mut LowerContext<'_, U>,
    ty: InterfaceType,
) -> Result<u32> {
    match ty {
        InterfaceType::Stream(dst) => {
            let types = cx.types;
            typecheck_lowered_transmit(idx, cx, types[types[dst].ty].payload.as_ref())?;
            let id = cx.store.0.host_reader_take(idx)?;
            cx.instance_handle()
                .lower_transmit_to_index(cx.store.0, TransmitIndex::Stream(dst), id)
        }
        _ => func::bad_type_info(),
    }
}

// SAFETY: This relies on the `ComponentType` implementation for `u32` being
// safe and correct since we lift and lower stream handles as `u32`s.
unsafe impl<T: ComponentType> ComponentType for StreamReader<T> {
    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::SCALAR4;

    type Lower = <u32 as func::ComponentType>::Lower;

    fn typecheck(ty: &InterfaceType, types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::Stream(ty) => {
                let ty = types.types[*ty].ty;
                types::typecheck_payload::<T>(types.types[ty].payload.as_ref(), types)
            }
            other => bail!("expected `stream`, found `{}`", func::desc(other)),
        }
    }
}

// SAFETY: See the comment on the `ComponentType` `impl` for this type.
unsafe impl<T: ComponentType> func::Lower for StreamReader<T> {
    fn linear_lower_to_flat<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()> {
        lower_stream_to_index(self.idx, cx, ty)?.linear_lower_to_flat(cx, InterfaceType::U32, dst)
    }

    fn linear_lower_to_memory<U>(
        &self,
        cx: &mut LowerContext<'_, U>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        lower_stream_to_index(self.idx, cx, ty)?.linear_lower_to_memory(
            cx,
            InterfaceType::U32,
            offset,
        )
    }
}

// SAFETY: See the comment on the `ComponentType` `impl` for this type.
unsafe impl<T: ComponentType> func::Lift for StreamReader<T> {
    fn linear_lift_from_flat(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        src: &Self::Lower,
    ) -> Result<Self> {
        let index = u32::linear_lift_from_flat(cx, InterfaceType::U32, src)?;
        Self::lift_from_index(cx, ty, index)
    }

    fn linear_lift_from_memory(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        bytes: &[u8],
    ) -> Result<Self> {
        let index = u32::linear_lift_from_memory(cx, InterfaceType::U32, bytes)?;
        Self::lift_from_index(cx, ty, index)
    }
}

/// A [`StreamReader`] paired with an [`Accessor`].
///
/// This is an RAII wrapper around [`StreamReader`] that ensures it is closed
/// when dropped. This can be created through [`GuardedStreamReader::new`] or
/// [`StreamReader::guard`].
///
/// [`Accessor`]: crate::component::Accessor
pub struct GuardedStreamReader<T, A>
where
    A: AsAccessor,
{
    // This field is `None` to implement the conversion from this guard back to
    // `StreamReader`. When `None` is seen in the destructor it will cause the
    // destructor to do nothing.
    reader: Option<StreamReader<T>>,
    accessor: A,
}

impl<T, A> GuardedStreamReader<T, A>
where
    A: AsAccessor,
{
    /// Create a new `GuardedStreamReader` with the specified `accessor` and
    /// `reader`.
    ///
    /// # Panics
    ///
    /// Panics if [`Config::concurrency_support`] is not enabled.
    ///
    /// [`Config::concurrency_support`]: crate::Config::concurrency_support
    pub fn new(accessor: A, reader: StreamReader<T>) -> Self {
        assert!(
            accessor
                .as_accessor()
                .with(|a| a.as_context().0.concurrency_support())
        );
        Self {
            reader: Some(reader),
            accessor,
        }
    }

    /// Extracts the underlying [`StreamReader`] from this guard, returning it
    /// back.
    pub fn into_stream(self) -> StreamReader<T> {
        self.into()
    }
}

impl<T, A> From<GuardedStreamReader<T, A>> for StreamReader<T>
where
    A: AsAccessor,
{
    fn from(mut guard: GuardedStreamReader<T, A>) -> Self {
        guard.reader.take().unwrap()
    }
}

impl<T, A> Drop for GuardedStreamReader<T, A>
where
    A: AsAccessor,
{
    fn drop(&mut self) {
        if let Some(reader) = &mut self.reader {
            // See comments in `Drop for GuardedFutureReader` for this assert.
            let result = reader.close_with(&self.accessor);
            debug_assert!(result.is_ok());
        }
    }
}

/// Represents a Component Model `error-context`.
pub struct ErrorContext {
    rep: u32,
}

impl ErrorContext {
    pub(crate) fn new(rep: u32) -> Self {
        Self { rep }
    }

    /// Convert this `ErrorContext` into a [`Val`].
    pub fn into_val(self) -> Val {
        Val::ErrorContext(ErrorContextAny(self.rep))
    }

    /// Attempt to convert the specified [`Val`] to a `ErrorContext`.
    pub fn from_val(_: impl AsContextMut, value: &Val) -> Result<Self> {
        let Val::ErrorContext(ErrorContextAny(rep)) = value else {
            bail!("expected `error-context`; got `{}`", value.desc());
        };
        Ok(Self::new(*rep))
    }

    fn lift_from_index(cx: &mut LiftContext<'_>, ty: InterfaceType, index: u32) -> Result<Self> {
        match ty {
            InterfaceType::ErrorContext(src) => {
                let rep = cx
                    .instance_mut()
                    .table_for_error_context(src)
                    .error_context_rep(index)?;

                Ok(Self { rep })
            }
            _ => func::bad_type_info(),
        }
    }
}

pub(crate) fn lower_error_context_to_index<U>(
    rep: u32,
    cx: &mut LowerContext<'_, U>,
    ty: InterfaceType,
) -> Result<u32> {
    match ty {
        InterfaceType::ErrorContext(dst) => {
            let (instance, counter) = cx
                .instance_handle()
                .id()
                .get_mut_and_handle_counter(cx.store.0);
            instance
                .table_for_error_context(dst)
                .error_context_insert(counter, rep)
        }
        _ => func::bad_type_info(),
    }
}
// SAFETY: This relies on the `ComponentType` implementation for `u32` being
// safe and correct since we lift and lower future handles as `u32`s.
unsafe impl func::ComponentType for ErrorContext {
    const ABI: CanonicalAbiInfo = CanonicalAbiInfo::SCALAR4;

    type Lower = <u32 as func::ComponentType>::Lower;

    fn typecheck(ty: &InterfaceType, _types: &InstanceType<'_>) -> Result<()> {
        match ty {
            InterfaceType::ErrorContext(_) => Ok(()),
            other => bail!("expected `error`, found `{}`", func::desc(other)),
        }
    }
}

// SAFETY: See the comment on the `ComponentType` `impl` for this type.
unsafe impl func::Lower for ErrorContext {
    fn linear_lower_to_flat<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        dst: &mut MaybeUninit<Self::Lower>,
    ) -> Result<()> {
        lower_error_context_to_index(self.rep, cx, ty)?.linear_lower_to_flat(
            cx,
            InterfaceType::U32,
            dst,
        )
    }

    fn linear_lower_to_memory<T>(
        &self,
        cx: &mut LowerContext<'_, T>,
        ty: InterfaceType,
        offset: usize,
    ) -> Result<()> {
        lower_error_context_to_index(self.rep, cx, ty)?.linear_lower_to_memory(
            cx,
            InterfaceType::U32,
            offset,
        )
    }
}

// SAFETY: See the comment on the `ComponentType` `impl` for this type.
unsafe impl func::Lift for ErrorContext {
    fn linear_lift_from_flat(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        src: &Self::Lower,
    ) -> Result<Self> {
        let index = u32::linear_lift_from_flat(cx, InterfaceType::U32, src)?;
        Self::lift_from_index(cx, ty, index)
    }

    fn linear_lift_from_memory(
        cx: &mut LiftContext<'_>,
        ty: InterfaceType,
        bytes: &[u8],
    ) -> Result<Self> {
        let index = u32::linear_lift_from_memory(cx, InterfaceType::U32, bytes)?;
        Self::lift_from_index(cx, ty, index)
    }
}

/// Represents the read or write end of a stream or future.
pub(super) struct TransmitHandle {
    pub(super) common: WaitableCommon,
    /// See `TransmitState`
    state: TableId<TransmitState>,
}

impl TransmitHandle {
    fn new(state: TableId<TransmitState>) -> Self {
        Self {
            common: WaitableCommon::default(),
            state,
        }
    }
}

impl TableDebug for TransmitHandle {
    fn type_name() -> &'static str {
        "TransmitHandle"
    }
}

/// Represents the state of a stream or future.
struct TransmitState {
    /// The write end of the stream or future.
    write_handle: TableId<TransmitHandle>,
    /// The read end of the stream or future.
    read_handle: TableId<TransmitHandle>,
    /// See `WriteState`
    write: WriteState,
    /// See `ReadState`
    read: ReadState,
    /// Whether further values may be transmitted via this stream or future.
    done: bool,
    /// The original creator of this stream or future, whose payload type is
    /// checked against a guest's whenever the host lowers the read end into
    /// that guest, and which `{Future,Stream}Any` use as their initial type.
    pub(super) origin: TransmitOrigin,
}

#[derive(Copy, Clone)]
pub(super) enum TransmitOrigin {
    Host {
        /// Type-checks the producer's items against the guest-supplied payload.
        typecheck: fn(Option<&InterfaceType>, &InstanceType<'_>) -> Result<()>,
    },
    GuestFuture(ComponentInstanceId, TypeFutureTableIndex),
    GuestStream(ComponentInstanceId, TypeStreamTableIndex),
}

impl TransmitState {
    fn new(origin: TransmitOrigin) -> Self {
        Self {
            write_handle: TableId::new(u32::MAX),
            read_handle: TableId::new(u32::MAX),
            read: ReadState::Open,
            write: WriteState::Open,
            done: false,
            origin,
        }
    }
}

impl TableDebug for TransmitState {
    fn type_name() -> &'static str {
        "TransmitState"
    }
}

impl TransmitOrigin {
    fn guest(id: ComponentInstanceId, index: TransmitIndex) -> Self {
        match index {
            TransmitIndex::Future(ty) => TransmitOrigin::GuestFuture(id, ty),
            TransmitIndex::Stream(ty) => TransmitOrigin::GuestStream(id, ty),
        }
    }
}

type PollStream = Box<
    dyn Fn() -> Pin<Box<dyn Future<Output = Result<StreamResult>> + Send + 'static>> + Send + Sync,
>;

type TryInto = Box<dyn Fn(TypeId) -> Option<Box<dyn Any>> + Send + Sync>;

/// Represents the state of the write end of a stream or future.
enum WriteState {
    /// The write end is open, but no write is pending.
    Open,
    /// The write end is owned by a guest task and a write is pending.
    GuestReady {
        instance: Instance,
        caller: RuntimeComponentInstanceIndex,
        ty: TransmitIndex,
        flat_abi: Option<FlatAbi>,
        options: OptionsIndex,
        address: usize,
        count: ItemCount,
        handle: u32,
    },
    /// The write end is owned by the host, which is ready to produce items.
    HostReady {
        produce: PollStream,
        try_into: TryInto,
        guest_offset: ItemCount,
        cancel: bool,
        cancel_waker: Option<Waker>,
    },
    /// The write end has been dropped.
    Dropped,
}

impl fmt::Debug for WriteState {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Self::Open => f.debug_tuple("Open").finish(),
            Self::GuestReady { .. } => f.debug_tuple("GuestReady").finish(),
            Self::HostReady { .. } => f.debug_tuple("HostReady").finish(),
            Self::Dropped => f.debug_tuple("Dropped").finish(),
        }
    }
}

/// Represents the state of the read end of a stream or future.
enum ReadState {
    /// The read end is open, but no read is pending.
    Open,
    /// The read end is owned by a guest task and a read is pending.
    GuestReady {
        ty: TransmitIndex,
        caller_instance: RuntimeComponentInstanceIndex,
        flat_abi: Option<FlatAbi>,
        instance: Instance,
        options: OptionsIndex,
        address: usize,
        count: ItemCount,
        handle: u32,
    },
    /// The read end is owned by a host task, and it is ready to consume items.
    HostReady {
        consume: PollStream,
        guest_offset: ItemCount,
        cancel: bool,
        cancel_waker: Option<Waker>,
    },
    /// Both the read and write ends are owned by the host.
    HostToHost {
        accept: Box<
            dyn for<'a> Fn(
                    &'a mut UntypedWriteBuffer<'a>,
                )
                    -> Pin<Box<dyn Future<Output = Result<StreamResult>> + Send + 'a>>
                + Send
                + Sync,
        >,
        buffer: Vec<u8>,
        limit: usize,
    },
    /// The read end has been dropped.
    Dropped,
}

impl fmt::Debug for ReadState {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self {
            Self::Open => f.debug_tuple("Open").finish(),
            Self::GuestReady { .. } => f.debug_tuple("GuestReady").finish(),
            Self::HostReady { .. } => f.debug_tuple("HostReady").finish(),
            Self::HostToHost { .. } => f.debug_tuple("HostToHost").finish(),
            Self::Dropped => f.debug_tuple("Dropped").finish(),
        }
    }
}

fn return_code(kind: TransmitKind, state: StreamResult, count: ItemCount) -> Result<ReturnCode> {
    Ok(match state {
        StreamResult::Dropped => ReturnCode::Dropped(count),
        StreamResult::Completed => ReturnCode::completed(kind, count),
        StreamResult::Cancelled => ReturnCode::Cancelled(count),
    })
}

fn settle_host_read(
    transmit: &mut TransmitState,
    kind: TransmitKind,
    state: StreamResult,
) -> Result<ReturnCode> {
    let ReadState::HostReady {
        consume,
        guest_offset,
        ..
    } = mem::replace(&mut transmit.read, ReadState::Open)
    else {
        bail_bug!("expected ReadState::HostReady")
    };
    let code = return_code(kind, state, guest_offset)?;
    transmit.read = match state {
        StreamResult::Dropped => ReadState::Dropped,
        StreamResult::Completed | StreamResult::Cancelled => ReadState::HostReady {
            consume,
            guest_offset: ItemCount::ZERO,
            cancel: false,
            cancel_waker: None,
        },
    };
    Ok(code)
}

fn settle_host_write(
    transmit: &mut TransmitState,
    kind: TransmitKind,
    state: StreamResult,
) -> Result<ReturnCode> {
    let WriteState::HostReady {
        produce,
        try_into,
        guest_offset,
        ..
    } = mem::replace(&mut transmit.write, WriteState::Open)
    else {
        bail_bug!("expected WriteState::HostReady")
    };
    let code = return_code(kind, state, guest_offset)?;
    transmit.write = match state {
        StreamResult::Dropped => WriteState::Dropped,
        StreamResult::Completed | StreamResult::Cancelled => WriteState::HostReady {
            produce,
            try_into,
            guest_offset: ItemCount::ZERO,
            cancel: false,
            cancel_waker: None,
        },
    };
    Ok(code)
}

impl StoreOpaque {
    fn pipe_from_guest(
        &mut self,
        kind: TransmitKind,
        id: TableId<TransmitState>,
        future: Pin<Box<dyn Future<Output = Result<StreamResult>> + Send + 'static>>,
    ) {
        let future = async move {
            let stream_state = future.await?;
            tls::get(|store| {
                let state = store.concurrent_state_mut()?;
                let transmit = state.get_mut(id)?;
                let code = settle_host_read(transmit, kind, stream_state)?;
                let WriteState::GuestReady { ty, handle, .. } =
                    mem::replace(&mut transmit.write, WriteState::Open)
                else {
                    bail_bug!("expected WriteState::GuestReady")
                };
                state.send_write_result(ty, id, handle, code)?;
                Ok(())
            })
        };

        self.concurrent_state_mut_already_forced_current_thread()
            .push_future(future.boxed());
    }

    fn pipe_to_guest(
        &mut self,
        kind: TransmitKind,
        id: TableId<TransmitState>,
        future: Pin<Box<dyn Future<Output = Result<StreamResult>> + Send + 'static>>,
    ) {
        let future = async move {
            let stream_state = future.await?;
            tls::get(|store| {
                let state = store.concurrent_state_mut()?;
                let transmit = state.get_mut(id)?;
                let code = settle_host_write(transmit, kind, stream_state)?;
                let ReadState::GuestReady { ty, handle, .. } =
                    mem::replace(&mut transmit.read, ReadState::Open)
                else {
                    bail_bug!("expected ReadState::GuestReady")
                };
                state.send_read_result(ty, id, handle, code)?;
                Ok(())
            })
        };

        self.concurrent_state_mut_already_forced_current_thread()
            .push_future(future.boxed());
    }

    /// Drop the read end of a stream or future read from the host.
    fn host_drop_reader(&mut self, id: TableId<TransmitHandle>, kind: TransmitKind) -> Result<()> {
        let state = self.concurrent_state_mut()?;
        Waitable::Transmit(id).join(state, None)?;
        let transmit_id = state.get_mut(id)?.state;
        let transmit = state
            .get_mut(transmit_id)
            .with_context(|| format!("error closing reader {transmit_id:?}"))?;
        log::trace!(
            "host_drop_reader state {transmit_id:?}; read state {:?} write state {:?}",
            transmit.read,
            transmit.write
        );

        transmit.read = ReadState::Dropped;

        // If the write end is already dropped, it should stay dropped,
        // otherwise, it should be opened.
        let new_state = if let WriteState::Dropped = &transmit.write {
            WriteState::Dropped
        } else {
            WriteState::Open
        };

        let write_handle = transmit.write_handle;
        let done = transmit.done;

        match mem::replace(&mut transmit.write, new_state) {
            // If a guest is waiting to write, notify it that the read end has
            // been dropped.
            WriteState::GuestReady { ty, handle, .. } => {
                state.update_event(
                    write_handle.rep(),
                    match ty {
                        TransmitIndex::Future(ty) => Event::FutureWrite {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: Some((ty, handle)),
                        },
                        TransmitIndex::Stream(ty) => Event::StreamWrite {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: Some((ty, handle)),
                        },
                    },
                )?;
            }

            // Otherwise notify the idle write end that the read end has been
            // dropped, unless it's already done.
            WriteState::Open if done => {}
            WriteState::Open => {
                state.update_event(
                    write_handle.rep(),
                    match kind {
                        TransmitKind::Future => Event::FutureWrite {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: None,
                        },
                        TransmitKind::Stream => Event::StreamWrite {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: None,
                        },
                    },
                )?;
            }

            // If the writer has already been dropped, then this cleans out the
            // state that the reader is using. If the write is host-owned then
            // by cleaning this out we run the host's `Drop` implementation
            // which notifies it of this drop.
            WriteState::Dropped | WriteState::HostReady { .. } => {
                log::trace!("host_drop_reader delete {transmit_id:?}");
                state.delete_transmit(transmit_id)?;
            }
        }
        Ok(())
    }

    /// Drop the write end of a stream or future read from the host.
    fn host_drop_writer(
        &mut self,
        id: TableId<TransmitHandle>,
        on_drop_open: Option<fn() -> Result<()>>,
    ) -> Result<()> {
        let state = self.concurrent_state_mut()?;
        Waitable::Transmit(id).join(state, None)?;
        let transmit_id = state.get_mut(id)?.state;
        let transmit = state
            .get_mut(transmit_id)
            .with_context(|| format!("error closing writer {transmit_id:?}"))?;
        log::trace!(
            "host_drop_writer state {transmit_id:?}; read state {:?} writer state {:?}",
            transmit.read,
            transmit.write
        );

        // Existing queued transmits must be updated with information for the impending writer closure
        match &mut transmit.write {
            WriteState::GuestReady { .. } => {
                bail_bug!("can't call `host_drop_writer` on a guest-owned writer");
            }
            WriteState::HostReady { .. } => {}
            v @ WriteState::Open => {
                if let (Some(on_drop_open), false) = (on_drop_open, transmit.done) {
                    on_drop_open()?;
                } else {
                    *v = WriteState::Dropped;
                }
            }
            WriteState::Dropped => bail_bug!("write state is already dropped"),
        }

        let transmit = self.concurrent_state_mut()?.get_mut(transmit_id)?;

        // If the existing read state is dropped, then there's nothing to read
        // and we can keep it that way.
        //
        // If the read state was any other state, then we must set the new state to open
        // to indicate that there *is* data to be read
        let new_state = if let ReadState::Dropped = &transmit.read {
            ReadState::Dropped
        } else {
            ReadState::Open
        };

        let read_handle = transmit.read_handle;
        let done = transmit.done;

        // Swap in the new read state
        match mem::replace(&mut transmit.read, new_state) {
            // If the guest was ready to read, then we cannot drop the reader (or writer);
            // we must deliver the event, and update the state associated with the handle to
            // represent that a read must be performed
            ReadState::GuestReady { ty, handle, .. } => {
                // Ensure the final read of the guest is queued, with appropriate closure indicator
                self.concurrent_state_mut()?.update_event(
                    read_handle.rep(),
                    match ty {
                        TransmitIndex::Future(ty) => Event::FutureRead {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: Some((ty, handle)),
                        },
                        TransmitIndex::Stream(ty) => Event::StreamRead {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: Some((ty, handle)),
                        },
                    },
                )?;
            }

            // Notify the read-end of the drop, unless we're already done.
            ReadState::Open if done => {}
            ReadState::Open => {
                self.concurrent_state_mut()?.update_event(
                    read_handle.rep(),
                    match on_drop_open {
                        Some(_) => Event::FutureRead {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: None,
                        },
                        None => Event::StreamRead {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            pending: None,
                        },
                    },
                )?;
            }

            // If the read state was already dropped, then we can remove the
            // transmit state completely (both writer and reader have been
            // dropped). If the read state is host-owned then it's additionally
            // deleted here as a notification that the read end has gone away.
            // Running the host's `Drop` implementation is what notifies it of
            // this event.
            ReadState::Dropped | ReadState::HostReady { .. } | ReadState::HostToHost { .. } => {
                log::trace!("host_drop_writer delete {transmit_id:?}");
                self.concurrent_state_mut()?.delete_transmit(transmit_id)?;
            }
        }
        Ok(())
    }

    pub(super) fn transmit_origin(&mut self, idx: HostResourceIndex) -> Result<TransmitOrigin> {
        let id = self.host_reader_rep(idx)?;
        let state = self.concurrent_state_mut()?;
        let state_id = state.get_mut(id)?.state;
        Ok(state.get_mut(state_id)?.origin)
    }

    /// Insert `id` within the store, returning a `HostResourceIndex` suitable
    /// to re-acquire it via the `host_reader_take` and `host_reader_rep`
    /// methods below.
    fn host_reader_insert(&mut self, id: TableId<TransmitHandle>) -> Result<HostResourceIndex> {
        HostResourceTables::new_host(self)?.host_resource_lower_own(id.rep(), None, None)
    }

    /// Removes `idx` from the store, returning the internal `id` it was
    /// created with.
    fn host_reader_take(&mut self, idx: HostResourceIndex) -> Result<TableId<TransmitHandle>> {
        let rep = HostResourceTables::new_host(self)?.host_resource_lift_own(idx)?;
        Ok(TableId::new(rep))
    }

    /// Looks up `idx` within the store, returning the internal `id` it was
    /// created with.
    fn host_reader_rep(&mut self, idx: HostResourceIndex) -> Result<TableId<TransmitHandle>> {
        let rep = HostResourceTables::new_host(self)?.host_resource_rep(idx)?;
        Ok(TableId::new(rep))
    }
}

impl<T> StoreContextMut<'_, T> {
    fn new_transmit<P: StreamProducer<T>>(
        mut self,
        kind: TransmitKind,
        producer: P,
    ) -> Result<HostResourceIndex>
    where
        P::Item: func::Lower,
    {
        let token = StoreToken::new(self.as_context_mut());
        let state = self.0.concurrent_state_mut()?;
        let (_, read) = state.new_transmit(TransmitOrigin::Host {
            typecheck: types::typecheck_payload::<P::Item>,
        })?;
        let producer = Arc::new(LockedState::new((Box::pin(producer), P::Buffer::default())));
        let id = state.get_mut(read)?.state;
        let mut dropped = false;
        let produce = Box::new({
            let producer = producer.clone();
            move || {
                let producer = producer.clone();
                async move {
                    let mut state = producer.take()?;
                    let (mine, buffer) = &mut *state;

                    let (result, cancelled) = if buffer.remaining().is_empty() {
                        future::poll_fn(|cx| {
                            tls::get(|store| {
                                let transmit = store.concurrent_state_mut()?.get_mut(id)?;

                                let &WriteState::HostReady { cancel, .. } = &transmit.write else {
                                    bail_bug!("expected WriteState::HostReady")
                                };

                                let mut host_written = 0;
                                let mut host_buffer =
                                    if let ReadState::HostToHost { buffer, .. } = &mut transmit.read {
                                        Some(mem::take(buffer))
                                    } else {
                                        None
                                    };

                                let poll = mine.as_mut().poll_produce(
                                    cx,
                                    token.as_context_mut(store),
                                    Destination {
                                        id,
                                        buffer,
                                        host_buffer: host_buffer.as_mut().map(|b| {
                                            HostBuffer {
                                                dst: b,
                                                marked_written: &mut host_written,
                                            }
                                        }),
                                        _phantom: PhantomData,
                                    },
                                    cancel,
                                );

                                let transmit = store.concurrent_state_mut()?.get_mut(id)?;

                                let host_offset = if let (
                                    Some(host_buffer),
                                    ReadState::HostToHost { buffer, limit, .. },
                                ) = (host_buffer, &mut transmit.read)
                                {
                                    *limit = host_written;
                                    *buffer = host_buffer;
                                    *limit
                                } else {
                                    0
                                };

                                {
                                    let WriteState::HostReady {
                                        guest_offset,
                                        cancel,
                                        cancel_waker,
                                        ..
                                    } = &mut transmit.write
                                    else {
                                        bail_bug!("expected WriteState::HostReady")
                                    };

                                    if poll.is_pending() {
                                        if !buffer.remaining().is_empty()
                                            || *guest_offset > 0
                                            || host_offset > 0
                                        {
                                            bail!(
                                                "StreamProducer::poll_produce returned Poll::Pending \
                                                 after producing at least one item"
                                            )
                                        }
                                        *cancel_waker = Some(cx.waker().clone());
                                    } else {
                                        *cancel_waker = None;
                                        *cancel = false;
                                    }
                                }

                                Ok(poll.map(|v| v.map(|result| (result, cancel))))
                            })?
                        })
                            .await?
                    } else {
                        (StreamResult::Completed, false)
                    };

                    let (guest_offset, host_offset, count) = tls::get(|store| {
                        let transmit = store.concurrent_state_mut()?.get_mut(id)?;
                        let (count, host_offset) = match &transmit.read {
                            &ReadState::GuestReady { count, .. } => (count.as_u32(), 0),
                            &ReadState::HostToHost { limit, .. } => (1, limit),
                            _ => bail_bug!("invalid read state"),
                        };
                        let guest_offset = match &transmit.write {
                            &WriteState::HostReady { guest_offset, .. } => guest_offset,
                            _ => bail_bug!("invalid write state"),
                        };
                        Ok((guest_offset, host_offset, count))
                    })?;

                    let produced =
                        !buffer.remaining().is_empty() || guest_offset > 0 || host_offset > 0;

                    match result {
                        StreamResult::Completed => {
                            // Only zero-length reads may complete without
                            // producing anything. Note that host-to-host reads
                            // always have a nonzero `count`.
                            if count > 0 && !produced {
                                bail!(
                                    "StreamProducer::poll_produce returned StreamResult::Completed \
                                     without producing any items"
                                );
                            }
                        }
                        StreamResult::Cancelled => {
                            if !cancelled {
                                bail!(
                                    "StreamProducer::poll_produce returned StreamResult::Cancelled \
                                     without being given a `finish` parameter value of true"
                                );
                            }
                            if produced {
                                bail!(
                                    "StreamProducer::poll_produce returned StreamResult::Cancelled \
                                     after producing at least one item"
                                );
                            }
                        }
                        StreamResult::Dropped => {
                            dropped = true;
                        }
                    }

                    let write_buffer = !buffer.remaining().is_empty() || host_offset > 0;

                    drop(state);

                    if write_buffer {
                        write(token, id, producer.clone(), kind).await?;
                    }

                    Ok(if dropped {
                        if producer.with(|p| p.1.remaining().is_empty())?  {
                            StreamResult::Dropped
                        } else {
                            StreamResult::Completed
                        }
                    } else {
                        result
                    })
                }
                .boxed()
            }
        });
        let try_into = Box::new(move |ty| {
            let (mine, buffer) = producer.try_lock().ok()?.take()?;
            match P::try_into(mine, ty) {
                Ok(value) => Some(value),
                Err(mine) => {
                    *producer.try_lock().ok()? = Some((mine, buffer));
                    None
                }
            }
        });
        state.get_mut(id)?.write = WriteState::HostReady {
            produce,
            try_into,
            guest_offset: ItemCount::ZERO,
            cancel: false,
            cancel_waker: None,
        };
        self.0.host_reader_insert(read)
    }

    fn set_consumer<C: StreamConsumer<T>>(
        mut self,
        idx: HostResourceIndex,
        kind: TransmitKind,
        consumer: C,
    ) -> Result<()> {
        let token = StoreToken::new(self.as_context_mut());
        let id = self.0.host_reader_take(idx)?;
        let state = self.0.concurrent_state_mut()?;
        let id = state.get_mut(id)?.state;
        let transmit = state.get_mut(id)?;
        let consumer = Arc::new(LockedState::new(Box::pin(consumer)));
        let consume_with_buffer = {
            let consumer = consumer.clone();
            async move |mut host_buffer: Option<&mut dyn WriteBuffer<C::Item>>| {
                let mut mine = consumer.take()?;

                let host_buffer_remaining_before =
                    host_buffer.as_deref_mut().map(|v| v.remaining().len());

                let (result, cancelled) = future::poll_fn(|cx| {
                    tls::get(|store| {
                        let cancel = match &store.concurrent_state_mut()?.get_mut(id)?.read {
                            &ReadState::HostReady { cancel, .. } => cancel,
                            ReadState::Open => false,
                            _ => bail_bug!("unexpected read state"),
                        };

                        let poll = mine.as_mut().poll_consume(
                            cx,
                            token.as_context_mut(store),
                            Source {
                                id,
                                host_buffer: host_buffer.as_deref_mut(),
                            },
                            cancel,
                        );

                        if let ReadState::HostReady {
                            cancel_waker,
                            cancel,
                            ..
                        } = &mut store.concurrent_state_mut()?.get_mut(id)?.read
                        {
                            if poll.is_pending() {
                                *cancel_waker = Some(cx.waker().clone());
                            } else {
                                *cancel_waker = None;
                                *cancel = false;
                            }
                        }

                        Ok(poll.map(|v| v.map(|result| (result, cancel))))
                    })?
                })
                .await?;

                let (guest_offset, count) = tls::get(|store| {
                    let transmit = store.concurrent_state_mut()?.get_mut(id)?;
                    Ok((
                        match &transmit.read {
                            &ReadState::HostReady { guest_offset, .. } => guest_offset,
                            ReadState::Open => ItemCount::ZERO,
                            _ => bail_bug!("invalid read state"),
                        },
                        match &transmit.write {
                            WriteState::GuestReady { count, .. } => count.as_usize(),
                            WriteState::HostReady { .. } => match host_buffer_remaining_before {
                                Some(n) => n,
                                None => bail_bug!("host_buffer_remaining_before should be set"),
                            },
                            _ => bail_bug!("invalid write state"),
                        },
                    ))
                })?;

                match result {
                    StreamResult::Completed => {
                        // Progress for a host writer is tracked by its buffer
                        // shrinking, whereas for a guest writer it's tracked
                        // in `guest_offset`.
                        let consumed = match (host_buffer_remaining_before, host_buffer) {
                            (Some(before), Some(after)) => before != after.remaining().len(),
                            _ => guest_offset > 0,
                        };
                        if count > 0 && !consumed {
                            bail!(
                                "StreamConsumer::poll_consume returned StreamResult::Completed \
                                 without consuming any items"
                            );
                        }

                        if let TransmitKind::Future = kind {
                            tls::get(|store| {
                                store.concurrent_state_mut()?.get_mut(id)?.done = true;
                                crate::error::Ok(())
                            })?;
                        }
                    }
                    StreamResult::Cancelled => {
                        if !cancelled {
                            bail!(
                                "StreamConsumer::poll_consume returned StreamResult::Cancelled \
                                 without being given a `finish` parameter value of true"
                            );
                        }
                    }
                    StreamResult::Dropped => {}
                }

                Ok(result)
            }
        };
        let consume = {
            let consume = consume_with_buffer.clone();
            Box::new(move || {
                let consume = consume.clone();
                async move { consume(None).await }.boxed()
            })
        };

        match &transmit.write {
            WriteState::Open => {
                transmit.read = ReadState::HostReady {
                    consume,
                    guest_offset: ItemCount::ZERO,
                    cancel: false,
                    cancel_waker: None,
                };
            }
            &WriteState::GuestReady { .. } => {
                // If the guest writer has a pending event, then don't start
                // another read event here even if there's some partial data
                // available. Wait for that to be delivered and wait for the
                // writer to do something else before a read is triggered.
                let write_handle = transmit.write_handle;
                let write_has_event = Waitable::Transmit(write_handle)
                    .common(state)?
                    .event
                    .is_some();
                let future = if write_has_event {
                    None
                } else {
                    Some(consume())
                };
                state.get_mut(id)?.read = ReadState::HostReady {
                    consume,
                    guest_offset: ItemCount::ZERO,
                    cancel: false,
                    cancel_waker: None,
                };
                if let Some(future) = future {
                    self.0.pipe_from_guest(kind, id, future);
                }
            }
            WriteState::HostReady { .. } => {
                let WriteState::HostReady { produce, .. } = mem::replace(
                    &mut transmit.write,
                    WriteState::HostReady {
                        produce: Box::new(|| {
                            Box::pin(async { bail_bug!("unexpected invocation of `produce`") })
                        }),
                        try_into: Box::new(|_| None),
                        guest_offset: ItemCount::ZERO,
                        cancel: false,
                        cancel_waker: None,
                    },
                ) else {
                    bail_bug!("expected WriteState::HostReady")
                };

                transmit.read = ReadState::HostToHost {
                    accept: Box::new(move |input| {
                        let consume = consume_with_buffer.clone();
                        async move { consume(Some(input.get_mut::<C::Item>()?)).await }.boxed()
                    }),
                    buffer: Vec::new(),
                    limit: 0,
                };

                let future = async move {
                    loop {
                        if tls::get(|store| {
                            crate::error::Ok(matches!(
                                store.concurrent_state_mut()?.get_mut(id)?.read,
                                ReadState::Dropped
                            ))
                        })? {
                            break Ok(());
                        }

                        match produce().await? {
                            StreamResult::Completed | StreamResult::Cancelled => {}
                            StreamResult::Dropped => break Ok(()),
                        }

                        if let TransmitKind::Future = kind {
                            break Ok(());
                        }
                    }
                }
                .map(move |result| {
                    tls::get(|store| store.concurrent_state_mut()?.delete_transmit(id))?;
                    result
                });

                state.push_future(Box::pin(future));
            }
            WriteState::Dropped => {
                let reader = transmit.read_handle;
                self.0.host_drop_reader(reader, kind)?;
            }
        }
        Ok(())
    }
}

async fn write<D: 'static, P: Send + 'static, T: func::Lower + 'static, B: WriteBuffer<T>>(
    token: StoreToken<D>,
    id: TableId<TransmitState>,
    pair: Arc<LockedState<(P, B)>>,
    kind: TransmitKind,
) -> Result<()> {
    let (read, guest_offset) = tls::get(|store| {
        let transmit = store.concurrent_state_mut()?.get_mut(id)?;

        let guest_offset = if let &WriteState::HostReady { guest_offset, .. } = &transmit.write {
            Some(guest_offset)
        } else {
            None
        };

        crate::error::Ok((
            mem::replace(&mut transmit.read, ReadState::Open),
            guest_offset,
        ))
    })?;

    match read {
        ReadState::GuestReady {
            ty,
            flat_abi,
            options,
            address,
            count,
            handle,
            instance,
            caller_instance,
        } => {
            let guest_offset = match guest_offset {
                Some(i) => i,
                None => bail_bug!("guest_offset should be present if ready"),
            };

            if let TransmitKind::Future = kind {
                tls::get(|store| {
                    store.concurrent_state_mut()?.get_mut(id)?.done = true;
                    crate::error::Ok(())
                })?;
            }

            let old_remaining = pair.with(|p| p.1.remaining().len())?;
            let accept = {
                let pair = pair.clone();
                move |mut store: StoreContextMut<D>| {
                    let mut state = pair.take()?;
                    lower::<T, B, D>(
                        store.as_context_mut(),
                        instance,
                        options,
                        ty,
                        address + (T::SIZE32 * guest_offset.as_usize()),
                        count.as_usize() - guest_offset.as_usize(),
                        &mut state.1,
                    )?;
                    crate::error::Ok(())
                }
            };

            if guest_offset < count {
                if T::MAY_REQUIRE_REALLOC {
                    // For payloads which may require a realloc call, use a
                    // oneshot::channel and background task.  This is
                    // necessary because calling the guest while there are
                    // host embedder frames on the stack is unsound.
                    let (tx, rx) = oneshot::channel();
                    tls::get(move |store| {
                        store
                            .concurrent_state_mut()?
                            .push_high_priority(WorkItem::WorkerFunction(AlwaysMut::new(
                                Box::new(move |store| {
                                    _ = tx.send(accept(token.as_context_mut(store))?);
                                    Ok(())
                                }),
                            )));
                        crate::error::Ok(())
                    })?;
                    match rx.await {
                        Ok(r) => r,
                        // The worker function was dropped without sending a
                        // result, which can happen when lowering trapped. This
                        // isn't a bug but this specific handling here might be
                        // the last hurray of this task in the event queue, so
                        // don't kill anything other than our caller. The
                        // original trap should make its way back to the best
                        // location naturally.
                        Err(oneshot::Canceled) => {
                            bail!("stream write cancelled before it could complete")
                        }
                    }
                } else {
                    // Optimize flat payloads (i.e. those which do not
                    // require calling the guest's realloc function) by
                    // lowering directly instead of using a oneshot::channel
                    // and background task.
                    tls::get(|store| accept(token.as_context_mut(store)))?
                }
            }

            tls::get(|store| {
                let count = old_remaining - pair.with(|p| p.1.remaining().len())?;

                let transmit = store.concurrent_state_mut()?.get_mut(id)?;

                let WriteState::HostReady { guest_offset, .. } = &mut transmit.write else {
                    bail_bug!("expected WriteState::HostReady")
                };

                guest_offset.inc(count)?;

                transmit.read = ReadState::GuestReady {
                    ty,
                    flat_abi,
                    options,
                    address,
                    count: ItemCount::new_usize(count)?,
                    handle,
                    instance,
                    caller_instance,
                };

                crate::error::Ok(())
            })?;

            Ok(())
        }

        ReadState::HostToHost {
            accept,
            mut buffer,
            limit,
        } => {
            let mut state = StreamResult::Completed;
            let mut position = 0;

            while !matches!(state, StreamResult::Dropped) && position < limit {
                let mut slice_buffer = SliceBuffer::new(buffer, position, limit);
                state = accept(&mut UntypedWriteBuffer::new(&mut slice_buffer)).await?;
                (buffer, position, _) = slice_buffer.into_parts();
            }

            {
                let mut pair = pair.take()?;
                let (_, buffer) = &mut *pair;

                while !(matches!(state, StreamResult::Dropped) || buffer.remaining().is_empty()) {
                    state = accept(&mut UntypedWriteBuffer::new(buffer)).await?;
                }
            }

            tls::get(|store| {
                store.concurrent_state_mut()?.get_mut(id)?.read = match state {
                    StreamResult::Dropped => ReadState::Dropped,
                    StreamResult::Completed | StreamResult::Cancelled => ReadState::HostToHost {
                        accept,
                        buffer,
                        limit: 0,
                    },
                };

                crate::error::Ok(())
            })?;
            Ok(())
        }

        _ => bail_bug!("unexpected read state"),
    }
}

/// Polls a `StreamProducer` or `StreamConsumer` future once, inline, from
/// within a guest's `{stream,future}.{read,write}` intrinsic.
///
/// The producer or consumer is embedder code which doesn't belong to the
/// reading or writing guest's task, so it's polled as though from the
/// top-level host, just as it is when it's later polled from the event loop.
fn poll_inline(
    store: &mut dyn VMStore,
    future: &mut Pin<Box<dyn Future<Output = Result<StreamResult>> + Send + 'static>>,
) -> Result<Poll<Result<StreamResult>>> {
    let old_thread = store.set_thread(CurrentThread::None)?;
    let poll = tls::set(store, || {
        future
            .as_mut()
            .poll(&mut Context::from_waker(&Waker::noop()))
    });
    store.set_thread(old_thread)?;
    Ok(poll)
}

impl Instance {
    /// Handle a guest-initiated write by delivering the item(s) to the
    /// `StreamConsumer` for the specified stream or future.
    fn consume(
        self,
        store: &mut dyn VMStore,
        kind: TransmitKind,
        transmit_id: TableId<TransmitState>,
        consume: PollStream,
        guest_offset: ItemCount,
        cancel: bool,
    ) -> Result<ReturnCode> {
        let mut future = consume();
        store.concurrent_state_mut()?.get_mut(transmit_id)?.read = ReadState::HostReady {
            consume,
            guest_offset,
            cancel,
            cancel_waker: None,
        };
        let poll = poll_inline(store, &mut future)?;

        Ok(match poll {
            Poll::Ready(state) => {
                let transmit = store.concurrent_state_mut()?.get_mut(transmit_id)?;
                let code = settle_host_read(transmit, kind, state?)?;
                transmit.write = WriteState::Open;
                code
            }
            Poll::Pending => {
                store.pipe_from_guest(kind, transmit_id, future);
                ReturnCode::Blocked
            }
        })
    }

    /// Handle a guest-initiated read by polling the `StreamProducer` for the
    /// specified stream or future for items.
    fn produce(
        self,
        store: &mut dyn VMStore,
        kind: TransmitKind,
        transmit_id: TableId<TransmitState>,
        produce: PollStream,
        try_into: TryInto,
        guest_offset: ItemCount,
        cancel: bool,
    ) -> Result<ReturnCode> {
        let mut future = produce();
        store.concurrent_state_mut()?.get_mut(transmit_id)?.write = WriteState::HostReady {
            produce,
            try_into,
            guest_offset,
            cancel,
            cancel_waker: None,
        };
        let poll = poll_inline(store, &mut future)?;

        Ok(match poll {
            Poll::Ready(state) => {
                let transmit = store.concurrent_state_mut()?.get_mut(transmit_id)?;
                let code = settle_host_write(transmit, kind, state?)?;
                transmit.read = ReadState::Open;
                code
            }
            Poll::Pending => {
                store.pipe_to_guest(kind, transmit_id, future);
                ReturnCode::Blocked
            }
        })
    }

    /// Drop the writable end of the specified stream or future from the guest.
    pub(super) fn guest_drop_writable(
        self,
        store: &mut StoreOpaque,
        ty: TransmitIndex,
        writer: u32,
    ) -> Result<()> {
        let (instance, counter) = self.id().get_mut_and_handle_counter(store);
        let table = instance.table_for_transmit(ty);
        let (transmit_rep, is_done) = match ty {
            TransmitIndex::Future(ty) => table.future_remove_writable(counter, ty, writer)?,
            TransmitIndex::Stream(ty) => {
                (table.stream_remove_writable(counter, ty, writer)?, false)
            }
        };

        let id = TableId::<TransmitHandle>::new(transmit_rep);
        log::trace!("guest_drop_writable: drop writer {id:?}");
        match ty {
            TransmitIndex::Stream(_) => store.host_drop_writer(id, None),
            TransmitIndex::Future(_) => store.host_drop_writer(
                id,
                if is_done {
                    None
                } else {
                    Some(|| {
                        Err(format_err!(
                            "cannot drop future write end without first writing a value"
                        ))
                    })
                },
            ),
        }
    }

    /// Copy `count` items from `read_address` to `write_address` for the
    /// specified stream or future.
    fn copy<T: 'static>(
        store: StoreContextMut<T>,
        flat_abi: Option<FlatAbi>,
        write_runtime_instance: RuntimeInstance,
        write_ty: TransmitIndex,
        write_options: OptionsIndex,
        write_address: usize,
        read_runtime_instance: RuntimeInstance,
        read_ty: TransmitIndex,
        read_options: OptionsIndex,
        read_address: usize,
        count: ItemCount,
        rep: u32,
    ) -> Result<()> {
        let write_instance = Instance::from_runtime_instance(store.0, write_runtime_instance);
        let read_instance = Instance::from_runtime_instance(store.0, read_runtime_instance);
        let (write_component, store) = write_instance.component_and_store_mut(store.0);
        let (read_component, mut store) = read_instance.component_and_store_mut(store);
        let write_types = write_component.types();
        let read_types = read_component.types();
        let count = count.as_usize();

        // Validate `write_ty` w.r.t. `write_address` to ensure it's properly
        // aligned and in-bounds.
        //
        // FIXME(#4311): this and the rest of this function use the 32-bit
        // layout of payloads, which needs memory64 handling.
        let write_payload_ty = write_ty.payload(write_types);
        let write_abi = match write_payload_ty {
            Some(ty) => write_types.canonical_abi(ty),
            None => &CanonicalAbiInfo::ZERO,
        };
        let write_length_in_bytes = match flat_abi {
            Some(abi) => usize::try_from(abi.size)? * count,
            None => usize::try_from(write_abi.size32)? * count,
        };
        if write_length_in_bytes > 0 {
            if write_address % usize::try_from(write_abi.align32)? != 0 {
                bail!("write pointer not aligned");
            }
            write_instance
                .options_memory(store, write_options)
                .get(write_address..)
                .and_then(|b| b.get(..write_length_in_bytes))
                .ok_or_else(|| crate::format_err!("write pointer out of bounds"))?;
        }

        let read_payload_ty = read_ty.payload(read_types);
        let read_abi = match read_payload_ty {
            Some(ty) => read_types.canonical_abi(ty),
            None => &CanonicalAbiInfo::ZERO,
        };
        let read_length_in_bytes = match flat_abi {
            Some(abi) => usize::try_from(abi.size)? * count,
            None => usize::try_from(read_abi.size32)? * count,
        };
        if read_length_in_bytes > 0 {
            if read_address % usize::try_from(read_abi.align32)? != 0 {
                bail!("read pointer not aligned");
            }
            read_instance
                .options_memory(store, read_options)
                .get(read_address..)
                .and_then(|b| b.get(..read_length_in_bytes))
                .ok_or_else(|| crate::format_err!("read pointer out of bounds"))?;
        }

        if write_runtime_instance == read_runtime_instance
            && !allow_intra_component_read_write(write_payload_ty)
        {
            bail!(
                "cannot read from and write to intra-component future/stream with non-numeric payload"
            )
        }

        match (write_ty, read_ty) {
            (TransmitIndex::Future(_), TransmitIndex::Future(_)) => {
                if count != 1 {
                    bail_bug!("futures can only send 1 item");
                }

                let val = write_payload_ty
                    .map(|ty| {
                        let lift = &mut LiftContext::new(store, write_options, write_instance)?;
                        let bytes = &lift.memory()[write_address..][..write_length_in_bytes];
                        Val::load(lift, *ty, bytes)
                    })
                    .transpose()?;

                if let Some(val) = val {
                    let lower =
                        &mut LowerContext::new(store.as_context_mut(), read_options, read_instance);
                    let ptr = func::validate_inbounds_dynamic(
                        read_abi,
                        lower.as_slice_mut(),
                        &ValRaw::u32(read_address.try_into()?),
                    )?;
                    let ty = match read_payload_ty {
                        Some(ty) => ty,
                        None => bail_bug!("expected read payload type to be present"),
                    };
                    val.store(lower, *ty, ptr)?;
                }
            }
            (TransmitIndex::Stream(_), TransmitIndex::Stream(_)) => {
                if write_length_in_bytes == 0 {
                    return Ok(());
                }
                let write_payload_ty = match write_payload_ty {
                    Some(ty) => ty,
                    None => bail_bug!("expected write payload type to be present"),
                };
                let read_payload_ty = match read_payload_ty {
                    Some(ty) => ty,
                    None => bail_bug!("expected read payload type to be present"),
                };
                if flat_abi.is_some() {
                    // Fast path memcpy for "flat" (i.e. no pointers or handles) payloads:
                    let store_opaque = store.store_opaque_mut();

                    assert_eq!(read_length_in_bytes, write_length_in_bytes);

                    if read_instance
                        .options_memory(store_opaque, read_options)
                        .as_ptr()
                        == write_instance
                            .options_memory(store_opaque, write_options)
                            .as_ptr()
                    {
                        let memory = read_instance.options_memory_mut(store_opaque, read_options);
                        memory.copy_within(
                            write_address..write_address + write_length_in_bytes,
                            read_address,
                        );
                    } else {
                        let src = write_instance.options_memory(store_opaque, write_options)
                            [write_address..][..write_length_in_bytes]
                            .as_ptr();
                        let dst = read_instance.options_memory_mut(store_opaque, read_options)
                            [read_address..][..read_length_in_bytes]
                            .as_mut_ptr();

                        // SAFETY: Both `src` and `dst` have been validated
                        // above to be valid pointers as they're derived from
                        // slices that have the desired length with the desired
                        // read/write permission. The `unsafe` bit here is that
                        // the memories are disjoint (different base pointers)
                        // and there's no easy way to borrow both
                        // simultaneously from the store. Different memories
                        // are guaranteed to be disjoint, however, so the
                        // `unsafe` here should be ok.
                        unsafe {
                            src.copy_to_nonoverlapping(dst, write_length_in_bytes);
                        }
                    }
                } else {
                    let store_opaque = store.store_opaque_mut();
                    let lift = &mut LiftContext::new(store_opaque, write_options, write_instance)?;
                    let bytes = &lift.memory()[write_address..][..write_length_in_bytes];
                    lift.consume_fuel_array(count, size_of::<Val>())?;

                    let values = (0..count)
                        .map(|index| {
                            let size = usize::try_from(write_abi.size32)?;
                            Val::load(lift, *write_payload_ty, &bytes[(index * size)..][..size])
                        })
                        .collect::<Result<Vec<_>>>()?;

                    let id = TableId::<TransmitHandle>::new(rep);
                    log::trace!("copy values {values:?} for {id:?}");

                    let lower =
                        &mut LowerContext::new(store.as_context_mut(), read_options, read_instance);
                    let mut ptr = read_address;
                    for value in values {
                        value.store(lower, *read_payload_ty, ptr)?;
                        ptr += usize::try_from(read_abi.size32)?;
                    }
                }
            }
            _ => bail_bug!("mismatched transmit types in copy"),
        }

        Ok(())
    }

    fn check_bounds(
        self,
        store: &StoreOpaque,
        options: OptionsIndex,
        ty: TransmitIndex,
        address: usize,
        count: usize,
    ) -> Result<()> {
        let types = self.id().get(store).component().types();
        let payload = match ty {
            TransmitIndex::Future(ty) => types[types[ty].ty].payload,
            TransmitIndex::Stream(ty) => types[types[ty].ty].payload,
        };
        let Some(payload) = payload else {
            return Ok(());
        };
        // As-written the spec currently exempts zero-length reads/writes from
        // bounds/alignment checks.
        if count == 0 {
            return Ok(());
        }
        let abi = types.canonical_abi(&payload);
        // FIXME(#4311): needs memory64 handling
        let size = usize::try_from(abi.size32)?;
        let align = usize::try_from(abi.align32)?;
        if address % align != 0 {
            bail!("buffer pointer not aligned");
        }
        self.options_memory(store, options)
            .get(address..)
            .and_then(|b| b.get(..size.checked_mul(count)?))
            .map(drop)
            .ok_or_else(|| crate::format_err!("buffer pointer out of bounds of memory"))
    }

    /// Write to the specified stream or future from the guest.
    pub(super) fn guest_write<T: 'static>(
        self,
        mut store: StoreContextMut<T>,
        caller: RuntimeComponentInstanceIndex,
        ty: TransmitIndex,
        options: OptionsIndex,
        flat_abi: Option<FlatAbi>,
        handle: u32,
        address: u32,
        count: u32,
    ) -> Result<ReturnCode> {
        let count = ItemCount::new(count)?;

        let address = usize::try_from(address)?;
        self.check_bounds(store.0, options, ty, address, count.as_usize())?;
        let async_ = self.options(store.0, options).async_;
        let (rep, state) = self.id().get_mut(store.0).get_mut_by_index(ty, handle)?;
        let TransmitLocalState::Write { done } = *state else {
            bail!(Trap::ConcurrentFutureStreamOp);
        };

        *state = TransmitLocalState::Busy;
        let transmit_handle = TableId::<TransmitHandle>::new(rep);
        let concurrent_state = store.0.concurrent_state_mut()?;
        let transmit_id = concurrent_state.get_mut(transmit_handle)?.state;
        let transmit = concurrent_state.get_mut(transmit_id)?;
        log::trace!(
            "guest_write {count} to {transmit_handle:?} (handle {handle}; state {transmit_id:?}); {:?}",
            transmit.read
        );

        if done || transmit.done {
            bail!(match ty {
                TransmitIndex::Future(_) => Trap::WriteToDroppedFuture,
                TransmitIndex::Stream(_) => Trap::WriteToDroppedStream,
            });
        }

        if !async_ {
            Waitable::Transmit(transmit_handle).trap_if_in_waitable_set(concurrent_state)?;
        }
        let transmit = concurrent_state.get_mut(transmit_id)?;

        let new_state = if let ReadState::Dropped = &transmit.read {
            ReadState::Dropped
        } else {
            ReadState::Open
        };

        let set_guest_ready = |me: &mut ConcurrentState| {
            let transmit = me.get_mut(transmit_id)?;
            if !matches!(&transmit.write, WriteState::Open) {
                bail_bug!("expected `WriteState::Open`; got `{:?}`", transmit.write);
            }
            transmit.write = WriteState::GuestReady {
                instance: self,
                caller,
                ty,
                flat_abi,
                options,
                address,
                count,
                handle,
            };
            Ok::<_, crate::Error>(())
        };

        let mut result = match mem::replace(&mut transmit.read, new_state) {
            ReadState::GuestReady {
                ty: read_ty,
                flat_abi: read_flat_abi,
                options: read_options,
                address: read_address,
                count: read_count,
                handle: read_handle,
                instance: read_instance,
                caller_instance: read_caller_instance,
            } => {
                if flat_abi != read_flat_abi {
                    bail_bug!("expected flat ABI calculations to be the same");
                }

                if let TransmitIndex::Future(_) = ty {
                    transmit.done = true;
                }

                // Note that zero-length reads and writes are handling specially
                // by the spec to allow each end to signal readiness to the
                // other.  Quoting the spec:
                //
                // ```
                // The meaning of a read or write when the length is 0 is that
                // the caller is querying the "readiness" of the other
                // side. When a 0-length read/write rendezvous with a
                // non-0-length read/write, only the 0-length read/write
                // completes; the non-0-length read/write is kept pending (and
                // ready for a subsequent rendezvous).
                //
                // In the corner case where a 0-length read and write
                // rendezvous, only the writer is notified of readiness. To
                // avoid livelock, the Canonical ABI requires that a writer must
                // (eventually) follow a completed 0-length write with a
                // non-0-length write that is allowed to block (allowing the
                // reader end to run and rendezvous with its own non-0-length
                // read).
                // ```

                let write_count = count;
                let write_complete = count == 0 || read_count > 0;
                let read_complete = count > 0;
                let read_buffer_remaining = count < read_count;

                let read_handle_rep = transmit.read_handle.rep();

                let count = count.min(read_count);

                Instance::copy(
                    store.as_context_mut(),
                    flat_abi,
                    self.runtime_instance(caller),
                    ty,
                    options,
                    address,
                    read_instance.runtime_instance(read_caller_instance),
                    read_ty,
                    read_options,
                    read_address,
                    count,
                    rep,
                )?;

                let instance = read_instance.id().get(store.0);
                let types = instance.component().types();
                // FIXME(#4311): needs memory64 handling
                let item_size = match read_ty.payload(types) {
                    Some(ty) => usize::try_from(types.canonical_abi(ty).size32)?,
                    None => 0,
                };
                let concurrent_state = store.0.concurrent_state_mut()?;
                if read_complete {
                    let total = if let Some(Event::StreamRead {
                        code: ReturnCode::Completed(old_total),
                        ..
                    }) = concurrent_state.take_event(read_handle_rep)?
                    {
                        count.add(old_total)?
                    } else {
                        count
                    };

                    let code = ReturnCode::completed(ty.kind(), total);

                    concurrent_state.send_read_result(read_ty, transmit_id, read_handle, code)?;
                }

                // If the reader still has buffer remaining, or if this was a
                // zero-length rendezvous, then restore the state of the reader
                // back to what it was when we found it. Note that for the
                // zero-length rendezvous case this specifically won't execute
                // the `read_complete` logic above, which is intentional, as the
                // reader remains blocked.
                if read_buffer_remaining || (write_count == 0 && read_count == 0) {
                    let transmit = concurrent_state.get_mut(transmit_id)?;
                    transmit.read = ReadState::GuestReady {
                        ty: read_ty,
                        flat_abi: read_flat_abi,
                        options: read_options,
                        address: read_address + (count.as_usize() * item_size),
                        count: read_count.sub(count)?,
                        handle: read_handle,
                        instance: read_instance,
                        caller_instance: read_caller_instance,
                    };
                }

                if write_complete {
                    ReturnCode::completed(ty.kind(), count)
                } else {
                    set_guest_ready(concurrent_state)?;
                    ReturnCode::Blocked
                }
            }

            ReadState::HostReady {
                consume,
                guest_offset,
                cancel,
                cancel_waker,
            } => {
                if cancel_waker.is_some() {
                    bail_bug!("expected cancel_waker to be none");
                }
                if cancel {
                    bail_bug!("expected cancel to be false");
                }
                if guest_offset != 0 {
                    bail_bug!("expected guest_offset to be 0");
                }

                set_guest_ready(concurrent_state)?;
                self.consume(
                    store.0,
                    ty.kind(),
                    transmit_id,
                    consume,
                    ItemCount::ZERO,
                    false,
                )?
            }

            ReadState::HostToHost { .. } => bail_bug!("unexpected HostToHost"),

            ReadState::Open => {
                set_guest_ready(concurrent_state)?;
                ReturnCode::Blocked
            }

            ReadState::Dropped => {
                if let TransmitIndex::Future(_) = ty {
                    transmit.done = true;
                }

                match Waitable::Transmit(transmit_handle).take_event(concurrent_state)? {
                    Some(
                        Event::StreamWrite {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            ..
                        }
                        | Event::FutureWrite {
                            code: ReturnCode::Dropped(ItemCount::ZERO),
                            ..
                        },
                    ) => {}
                    event => bail_bug!("expected pending dropped event for writer; got {event:?}"),
                }

                ReturnCode::Dropped(ItemCount::ZERO)
            }
        };

        if result == ReturnCode::Blocked && !async_ {
            result = self.wait_for_write(store.0, caller, transmit_handle)?;
        }

        if result != ReturnCode::Blocked {
            *self.id().get_mut(store.0).get_mut_by_index(ty, handle)?.1 =
                TransmitLocalState::Write {
                    done: matches!(result, ReturnCode::Dropped(_)),
                };
        }

        log::trace!(
            "guest_write result for {transmit_handle:?} (handle {handle}; state {transmit_id:?}): {result:?}",
        );

        Ok(result)
    }

    /// Read from the specified stream or future from the guest.
    pub(super) fn guest_read<T: 'static>(
        self,
        mut store: StoreContextMut<T>,
        caller_instance: RuntimeComponentInstanceIndex,
        ty: TransmitIndex,
        options: OptionsIndex,
        flat_abi: Option<FlatAbi>,
        handle: u32,
        address: u32,
        count: u32,
    ) -> Result<ReturnCode> {
        let count = ItemCount::new(count)?;

        let address = usize::try_from(address)?;
        self.check_bounds(store.0, options, ty, address, count.as_usize())?;
        let async_ = self.options(store.0, options).async_;
        let (rep, state) = self.id().get_mut(store.0).get_mut_by_index(ty, handle)?;
        let TransmitLocalState::Read { done } = *state else {
            bail!(Trap::ConcurrentFutureStreamOp);
        };

        *state = TransmitLocalState::Busy;
        let transmit_handle = TableId::<TransmitHandle>::new(rep);
        let concurrent_state = store.0.concurrent_state_mut()?;
        let transmit_id = concurrent_state.get_mut(transmit_handle)?.state;
        let transmit = concurrent_state.get_mut(transmit_id)?;
        log::trace!(
            "guest_read {count} from {transmit_handle:?} (handle {handle}; state {transmit_id:?}); {:?}",
            transmit.write
        );

        if done || transmit.done {
            match ty {
                TransmitIndex::Future(_) => {
                    bail!("cannot read from future after previous read succeeded")
                }
                TransmitIndex::Stream(_) => bail!(Trap::ReadFromDroppedStream),
            }
        }

        if !async_ {
            Waitable::Transmit(transmit_handle).trap_if_in_waitable_set(concurrent_state)?;
        }
        let transmit = concurrent_state.get_mut(transmit_id)?;

        let new_state = if let WriteState::Dropped = &transmit.write {
            WriteState::Dropped
        } else {
            WriteState::Open
        };

        let set_guest_ready = |me: &mut ConcurrentState| {
            let transmit = me.get_mut(transmit_id)?;
            if !matches!(&transmit.read, ReadState::Open) {
                bail_bug!("expected `ReadState::Open`; got `{:?}`", transmit.read);
            }
            transmit.read = ReadState::GuestReady {
                ty,
                flat_abi,
                options,
                address,
                count,
                handle,
                instance: self,
                caller_instance,
            };
            Ok::<_, crate::Error>(())
        };

        let mut result = match mem::replace(&mut transmit.write, new_state) {
            WriteState::GuestReady {
                instance: write_instance,
                ty: write_ty,
                flat_abi: write_flat_abi,
                options: write_options,
                address: write_address,
                count: write_count,
                handle: write_handle,
                caller: write_caller,
            } => {
                if flat_abi != write_flat_abi {
                    bail_bug!("expected flat ABI calculations to be the same");
                }

                if let TransmitIndex::Future(_) = ty {
                    transmit.done = true;
                }

                let write_handle_rep = transmit.write_handle.rep();

                // See the comment in `guest_write` for the
                // `ReadState::GuestReady` case concerning zero-length reads and
                // writes.

                let write_complete = write_count == 0 || count > 0;
                let read_complete = write_count > 0;
                let write_buffer_remaining = count < write_count;

                let count = count.min(write_count);

                Instance::copy(
                    store.as_context_mut(),
                    flat_abi,
                    write_instance.runtime_instance(write_caller),
                    write_ty,
                    write_options,
                    write_address,
                    self.runtime_instance(caller_instance),
                    ty,
                    options,
                    address,
                    count,
                    rep,
                )?;

                let instance = write_instance.id().get(store.0);
                let types = instance.component().types();
                // FIXME(#4311): needs memory64 handling
                let item_size = match write_ty.payload(types) {
                    Some(ty) => usize::try_from(types.canonical_abi(ty).size32)?,
                    None => 0,
                };
                let concurrent_state = store.0.concurrent_state_mut()?;

                if write_complete {
                    let total = if let Some(Event::StreamWrite {
                        code: ReturnCode::Completed(old_total),
                        ..
                    }) = concurrent_state.take_event(write_handle_rep)?
                    {
                        count.add(old_total)?
                    } else {
                        count
                    };

                    let code = ReturnCode::completed(ty.kind(), total);

                    concurrent_state.send_write_result(
                        write_ty,
                        transmit_id,
                        write_handle,
                        code,
                    )?;
                }

                if write_buffer_remaining {
                    let transmit = concurrent_state.get_mut(transmit_id)?;
                    transmit.write = WriteState::GuestReady {
                        instance: write_instance,
                        caller: write_caller,
                        ty: write_ty,
                        flat_abi: write_flat_abi,
                        options: write_options,
                        address: write_address + (count.as_usize() * item_size),
                        count: write_count.sub(count)?,
                        handle: write_handle,
                    };
                }

                if read_complete {
                    ReturnCode::completed(ty.kind(), count)
                } else {
                    set_guest_ready(concurrent_state)?;
                    ReturnCode::Blocked
                }
            }

            WriteState::HostReady {
                produce,
                try_into,
                guest_offset,
                cancel,
                cancel_waker,
            } => {
                if cancel_waker.is_some() {
                    bail_bug!("expected cancel_waker to be none");
                }
                if cancel {
                    bail_bug!("expected cancel to be false");
                }
                if guest_offset != 0 {
                    bail_bug!("expected guest_offset to be 0");
                }

                set_guest_ready(concurrent_state)?;

                let code = self.produce(
                    store.0,
                    ty.kind(),
                    transmit_id,
                    produce,
                    try_into,
                    ItemCount::ZERO,
                    false,
                )?;

                if let (TransmitIndex::Future(_), ReturnCode::Completed(_)) = (ty, code) {
                    store.0.concurrent_state_mut()?.get_mut(transmit_id)?.done = true;
                }

                code
            }

            WriteState::Open => {
                set_guest_ready(concurrent_state)?;
                ReturnCode::Blocked
            }

            WriteState::Dropped => {
                if let TransmitIndex::Future(_) = ty {
                    bail_bug!(
                        "should not be possible to read from a future whose write end was dropped"
                    );
                }

                match Waitable::Transmit(transmit_handle).take_event(concurrent_state)? {
                    Some(Event::StreamRead {
                        code: ReturnCode::Dropped(ItemCount::ZERO),
                        ..
                    }) => {}
                    event => bail_bug!("expected pending dropped event for reader; got {event:?}"),
                }

                ReturnCode::Dropped(ItemCount::ZERO)
            }
        };

        if result == ReturnCode::Blocked && !async_ {
            result = self.wait_for_read(store.0, caller_instance, transmit_handle)?;
        }

        if result != ReturnCode::Blocked {
            *self.id().get_mut(store.0).get_mut_by_index(ty, handle)?.1 =
                TransmitLocalState::Read {
                    done: matches!(
                        (result, ty),
                        (ReturnCode::Dropped(_), TransmitIndex::Stream(_))
                    ),
                };
        }

        log::trace!(
            "guest_read result for {transmit_handle:?} (handle {handle}; state {transmit_id:?}): {result:?}",
        );

        Ok(result)
    }

    fn wait_for_write(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        handle: TableId<TransmitHandle>,
    ) -> Result<ReturnCode> {
        let waitable = Waitable::Transmit(handle);
        store.wait_for_event(self.runtime_instance(caller), waitable)?;
        let event = waitable.take_event(store.concurrent_state_mut()?)?;
        if let Some(event @ (Event::StreamWrite { code, .. } | Event::FutureWrite { code, .. })) =
            event
        {
            waitable.on_delivery(store, self, event)?;
            Ok(code)
        } else {
            bail_bug!("expected either a stream or future write event")
        }
    }

    /// Cancel a pending stream or future write.
    fn cancel_write(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        transmit_id: TableId<TransmitState>,
        async_: bool,
    ) -> Result<ReturnCode> {
        let state = store.concurrent_state_mut()?;
        let transmit = state.get_mut(transmit_id)?;
        log::trace!(
            "host_cancel_write state {transmit_id:?}; write state {:?} read state {:?}",
            transmit.read,
            transmit.write
        );
        let waitable = Waitable::Transmit(transmit.write_handle);

        waitable.trap_if_has_sync_waiter(state)?;
        if !async_ {
            waitable.trap_if_in_waitable_set(state)?;
        }

        let code = if let Some(event) = waitable.take_event(state)? {
            let (Event::FutureWrite { code, .. } | Event::StreamWrite { code, .. }) = event else {
                bail_bug!("expected either a stream or future write event")
            };
            waitable.on_delivery(store, self, event)?;
            match (code, event) {
                (ReturnCode::Completed(count), Event::StreamWrite { .. }) => {
                    ReturnCode::Cancelled(count)
                }
                (ReturnCode::Dropped(_) | ReturnCode::Completed(_), _) => code,
                _ => bail_bug!("unexpected code/event combo"),
            }
        } else if let ReadState::HostReady {
            cancel,
            cancel_waker,
            ..
        } = &mut state.get_mut(transmit_id)?.read
        {
            *cancel = true;
            if let Some(waker) = cancel_waker.take() {
                waker.wake();
            }

            if async_ {
                ReturnCode::Blocked
            } else {
                let handle = store
                    .concurrent_state_mut()?
                    .get_mut(transmit_id)?
                    .write_handle;
                self.wait_for_write(store, caller, handle)?
            }
        } else {
            ReturnCode::Cancelled(ItemCount::ZERO)
        };

        if !matches!(code, ReturnCode::Blocked) {
            let transmit = store.concurrent_state_mut()?.get_mut(transmit_id)?;

            match &transmit.write {
                WriteState::GuestReady { .. } => {
                    transmit.write = WriteState::Open;
                }
                WriteState::HostReady { .. } => bail_bug!("support host write cancellation"),
                WriteState::Open | WriteState::Dropped => {}
            }
        }

        log::trace!("cancelled write {transmit_id:?}: {code:?}");

        Ok(code)
    }

    fn wait_for_read(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        handle: TableId<TransmitHandle>,
    ) -> Result<ReturnCode> {
        let waitable = Waitable::Transmit(handle);
        store.wait_for_event(self.runtime_instance(caller), waitable)?;
        let event = waitable.take_event(store.concurrent_state_mut()?)?;
        if let Some(event @ (Event::StreamRead { code, .. } | Event::FutureRead { code, .. })) =
            event
        {
            waitable.on_delivery(store, self, event)?;
            Ok(code)
        } else {
            bail_bug!("expected either a stream or future read event")
        }
    }

    /// Cancel a pending stream or future read.
    fn cancel_read(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        transmit_id: TableId<TransmitState>,
        async_: bool,
    ) -> Result<ReturnCode> {
        let state = store.concurrent_state_mut()?;
        let transmit = state.get_mut(transmit_id)?;
        log::trace!(
            "host_cancel_read state {transmit_id:?}; read state {:?} write state {:?}",
            transmit.read,
            transmit.write
        );

        let waitable = Waitable::Transmit(transmit.read_handle);

        waitable.trap_if_has_sync_waiter(state)?;
        if !async_ {
            waitable.trap_if_in_waitable_set(state)?;
        }

        let code = if let Some(event) = waitable.take_event(state)? {
            let (Event::FutureRead { code, .. } | Event::StreamRead { code, .. }) = event else {
                bail_bug!("expected either a stream or future read event")
            };
            waitable.on_delivery(store, self, event)?;
            match (code, event) {
                (ReturnCode::Completed(count), Event::StreamRead { .. }) => {
                    ReturnCode::Cancelled(count)
                }
                (ReturnCode::Dropped(_) | ReturnCode::Completed(_), _) => code,
                _ => bail_bug!("unexpected code/event combo"),
            }
        } else if let WriteState::HostReady {
            cancel,
            cancel_waker,
            ..
        } = &mut state.get_mut(transmit_id)?.write
        {
            *cancel = true;
            if let Some(waker) = cancel_waker.take() {
                waker.wake();
            }

            if async_ {
                ReturnCode::Blocked
            } else {
                let handle = store
                    .concurrent_state_mut()?
                    .get_mut(transmit_id)?
                    .read_handle;
                self.wait_for_read(store, caller, handle)?
            }
        } else {
            ReturnCode::Cancelled(ItemCount::ZERO)
        };

        if !matches!(code, ReturnCode::Blocked) {
            let transmit = store.concurrent_state_mut()?.get_mut(transmit_id)?;

            match &transmit.read {
                ReadState::GuestReady { .. } => {
                    transmit.read = ReadState::Open;
                }
                ReadState::HostReady { .. } | ReadState::HostToHost { .. } => {
                    bail_bug!("support host read cancellation")
                }
                ReadState::Open | ReadState::Dropped => {}
            }
        }

        log::trace!("cancelled read {transmit_id:?}: {code:?}");

        Ok(code)
    }

    /// Cancel a pending write for the specified stream or future from the guest.
    fn guest_cancel_write(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        ty: TransmitIndex,
        async_: bool,
        writer: u32,
    ) -> Result<ReturnCode> {
        let (rep, state) =
            get_mut_by_index_from(self.id().get_mut(store).table_for_transmit(ty), ty, writer)?;
        let id = TableId::<TransmitHandle>::new(rep);
        log::trace!("guest cancel write {id:?} (handle {writer})");
        match state {
            TransmitLocalState::Write { .. } => {
                bail!("stream or future write cancelled when no write is pending")
            }
            TransmitLocalState::Read { .. } => {
                bail!("passed read end to `{{stream|future}}.cancel-write`")
            }
            TransmitLocalState::Cancelling => {
                bail!("stream or future write cancelled when a cancellation is already pending")
            }
            TransmitLocalState::Busy => {}
        }
        let transmit_id = store.concurrent_state_mut()?.get_mut(id)?.state;
        let code = self.cancel_write(store, caller, transmit_id, async_)?;
        let state =
            get_mut_by_index_from(self.id().get_mut(store).table_for_transmit(ty), ty, writer)?.1;
        if let TransmitLocalState::Busy = state {
            *state = if matches!(code, ReturnCode::Blocked) {
                TransmitLocalState::Cancelling
            } else {
                TransmitLocalState::Write { done: false }
            };
        }
        Ok(code)
    }

    /// Cancel a pending read for the specified stream or future from the guest.
    fn guest_cancel_read(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        ty: TransmitIndex,
        async_: bool,
        reader: u32,
    ) -> Result<ReturnCode> {
        let (rep, state) =
            get_mut_by_index_from(self.id().get_mut(store).table_for_transmit(ty), ty, reader)?;
        let id = TableId::<TransmitHandle>::new(rep);
        log::trace!("guest cancel read {id:?} (handle {reader})");
        match state {
            TransmitLocalState::Read { .. } => {
                bail!("stream or future read cancelled when no read is pending")
            }
            TransmitLocalState::Write { .. } => {
                bail!("passed write end to `{{stream|future}}.cancel-read`")
            }
            TransmitLocalState::Cancelling => {
                bail!("stream or future read cancelled when a cancellation is already pending")
            }
            TransmitLocalState::Busy => {}
        }
        let transmit_id = store.concurrent_state_mut()?.get_mut(id)?.state;
        let code = self.cancel_read(store, caller, transmit_id, async_)?;
        let state =
            get_mut_by_index_from(self.id().get_mut(store).table_for_transmit(ty), ty, reader)?.1;
        if let TransmitLocalState::Busy = state {
            *state = if matches!(code, ReturnCode::Blocked) {
                TransmitLocalState::Cancelling
            } else {
                TransmitLocalState::Read { done: false }
            };
        }
        Ok(code)
    }

    /// Drop the readable end of the specified stream or future from the guest.
    fn guest_drop_readable(
        self,
        store: &mut StoreOpaque,
        ty: TransmitIndex,
        reader: u32,
    ) -> Result<()> {
        let (instance, counter) = self.id().get_mut_and_handle_counter(store);
        let table = instance.table_for_transmit(ty);
        let (rep, _is_done) = match ty {
            TransmitIndex::Stream(ty) => table.stream_remove_readable(counter, ty, reader)?,
            TransmitIndex::Future(ty) => table.future_remove_readable(counter, ty, reader)?,
        };
        let kind = match ty {
            TransmitIndex::Stream(_) => TransmitKind::Stream,
            TransmitIndex::Future(_) => TransmitKind::Future,
        };
        let id = TableId::<TransmitHandle>::new(rep);
        log::trace!("guest_drop_readable: drop reader {id:?}");
        store.host_drop_reader(id, kind)
    }

    /// Create a new error context for the given component.
    pub(crate) fn error_context_new(
        self,
        store: &mut StoreOpaque,
        ty: TypeComponentLocalErrorContextTableIndex,
        options: OptionsIndex,
        debug_msg_address: u32,
        debug_msg_len: u32,
    ) -> Result<u32> {
        let lift_ctx = &mut LiftContext::new(store, options, self)?;
        let debug_msg = String::linear_lift_from_flat(
            lift_ctx,
            InterfaceType::String,
            &[ValRaw::u32(debug_msg_address), ValRaw::u32(debug_msg_len)],
        )?;

        // Create a new ErrorContext that is tracked along with other concurrent state
        let err_ctx = ErrorContextState { debug_msg };
        let state = store.concurrent_state_mut()?;
        let table_id = state.push(err_ctx)?;
        let global_ref_count_idx =
            TypeComponentGlobalErrorContextTableIndex::from_u32(table_id.rep());

        // Add to the global error context ref counts
        let _ = state
            .global_error_context_ref_counts
            .insert(global_ref_count_idx, GlobalErrorContextRefCount(1));

        // Error context are tracked both locally (to a single component instance) and globally
        // the counts for both must stay in sync.
        //
        // Here we reflect the newly created global concurrent error context state into the
        // component instance's locally tracked count, along with the appropriate key into the global
        // ref tracking data structures to enable later lookup
        let (instance, counter) = self.id().get_mut_and_handle_counter(store);
        let local_idx = instance
            .table_for_error_context(ty)
            .error_context_insert(counter, table_id.rep())?;

        Ok(local_idx)
    }

    /// Retrieve the debug message from the specified error context.
    pub(super) fn error_context_debug_message<T>(
        self,
        store: StoreContextMut<T>,
        ty: TypeComponentLocalErrorContextTableIndex,
        options: OptionsIndex,
        err_ctx_handle: u32,
        debug_msg_address: u32,
    ) -> Result<()> {
        // Retrieve the error context and internal debug message
        let handle_table_id_rep = self
            .id()
            .get_mut(store.0)
            .table_for_error_context(ty)
            .error_context_rep(err_ctx_handle)?;

        let state = store.0.concurrent_state_mut()?;
        // Get the state associated with the error context
        let ErrorContextState { debug_msg } =
            state.get_mut(TableId::<ErrorContextState>::new(handle_table_id_rep))?;
        let debug_msg = debug_msg.clone();

        let lower_cx = &mut LowerContext::new(store, options, self);
        let debug_msg_address = usize::try_from(debug_msg_address)?;
        if debug_msg_address % usize::try_from(<str as ComponentType>::ALIGN32)? != 0 {
            bail!("invalid debug message pointer: not aligned");
        }
        // Lower the string into the component's memory.
        //
        // Note that the "8" here is the size of a WIT `string` in linear
        // memory, the ptr+length. This'll need to be updated when `memory64`
        // comes along. (FIXME(#4311))
        let offset = lower_cx
            .as_slice_mut()
            .get(debug_msg_address..)
            .and_then(|b| b.get(..8))
            .map(|_| debug_msg_address)
            .ok_or_else(|| crate::format_err!("invalid debug message pointer: out of bounds"))?;
        debug_msg
            .as_str()
            .linear_lower_to_memory(lower_cx, InterfaceType::String, offset)?;

        Ok(())
    }

    /// Implements the `future.cancel-read` intrinsic.
    pub(crate) fn future_cancel_read(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        ty: TypeFutureTableIndex,
        async_: bool,
        reader: u32,
    ) -> Result<u32> {
        self.guest_cancel_read(store, caller, TransmitIndex::Future(ty), async_, reader)
            .map(|v| v.encode())
    }

    /// Implements the `future.cancel-write` intrinsic.
    pub(crate) fn future_cancel_write(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        ty: TypeFutureTableIndex,
        async_: bool,
        writer: u32,
    ) -> Result<u32> {
        self.guest_cancel_write(store, caller, TransmitIndex::Future(ty), async_, writer)
            .map(|v| v.encode())
    }

    /// Implements the `stream.cancel-read` intrinsic.
    pub(crate) fn stream_cancel_read(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        ty: TypeStreamTableIndex,
        async_: bool,
        reader: u32,
    ) -> Result<u32> {
        self.guest_cancel_read(store, caller, TransmitIndex::Stream(ty), async_, reader)
            .map(|v| v.encode())
    }

    /// Implements the `stream.cancel-write` intrinsic.
    pub(crate) fn stream_cancel_write(
        self,
        store: &mut StoreOpaque,
        caller: RuntimeComponentInstanceIndex,
        ty: TypeStreamTableIndex,
        async_: bool,
        writer: u32,
    ) -> Result<u32> {
        self.guest_cancel_write(store, caller, TransmitIndex::Stream(ty), async_, writer)
            .map(|v| v.encode())
    }

    /// Implements the `future.drop-readable` intrinsic.
    pub(crate) fn future_drop_readable(
        self,
        store: &mut StoreOpaque,
        ty: TypeFutureTableIndex,
        reader: u32,
    ) -> Result<()> {
        self.guest_drop_readable(store, TransmitIndex::Future(ty), reader)
    }

    /// Implements the `stream.drop-readable` intrinsic.
    pub(crate) fn stream_drop_readable(
        self,
        store: &mut StoreOpaque,
        ty: TypeStreamTableIndex,
        reader: u32,
    ) -> Result<()> {
        self.guest_drop_readable(store, TransmitIndex::Stream(ty), reader)
    }

    /// Allocate a new future or stream and grant ownership of both the read and
    /// write ends to the (sub-)component instance to which the specified
    /// `TransmitIndex` belongs.
    fn guest_new(self, store: &mut StoreOpaque, ty: TransmitIndex) -> Result<ResourcePair> {
        let (write, read) = store
            .concurrent_state_mut()?
            .new_transmit(TransmitOrigin::guest(self.id().instance(), ty))?;

        let (instance, counter) = self.id().get_mut_and_handle_counter(store);
        let table = instance.table_for_transmit(ty);
        let (read_handle, write_handle) = match ty {
            TransmitIndex::Future(ty) => (
                table.future_insert_read(counter, ty, read.rep())?,
                table.future_insert_write(counter, ty, write.rep())?,
            ),
            TransmitIndex::Stream(ty) => (
                table.stream_insert_read(counter, ty, read.rep())?,
                table.stream_insert_write(counter, ty, write.rep())?,
            ),
        };

        let state = store.concurrent_state_mut()?;
        state.get_mut(read)?.common.handle = Some(read_handle);
        state.get_mut(write)?.common.handle = Some(write_handle);

        Ok(ResourcePair {
            write: write_handle,
            read: read_handle,
        })
    }

    /// Drop the specified error context.
    pub(crate) fn error_context_drop(
        self,
        store: &mut StoreOpaque,
        ty: TypeComponentLocalErrorContextTableIndex,
        error_context: u32,
    ) -> Result<()> {
        let (instance, counter) = self.id().get_mut_and_handle_counter(store);

        let local_handle_table = instance.table_for_error_context(ty);

        let rep = local_handle_table.error_context_drop(counter, error_context)?;

        let global_ref_count_idx = TypeComponentGlobalErrorContextTableIndex::from_u32(rep);

        let state = store.concurrent_state_mut()?;
        let Some(GlobalErrorContextRefCount(global_ref_count)) = state
            .global_error_context_ref_counts
            .get_mut(&global_ref_count_idx)
        else {
            bail_bug!("retrieve concurrent state for error context during drop")
        };

        // Reduce the component-global ref count, removing tracking if necessary
        if *global_ref_count < 1 {
            bail_bug!("ref count unexpectedly zero");
        }
        *global_ref_count -= 1;
        if *global_ref_count == 0 {
            state
                .global_error_context_ref_counts
                .remove(&global_ref_count_idx);

            state
                .delete(TableId::<ErrorContextState>::new(rep))
                .context("deleting component-global error context data")?;
        }

        Ok(())
    }

    /// Transfer ownership of the specified stream or future read end from one
    /// guest to another.
    fn guest_transfer(
        self,
        store: &mut StoreOpaque,
        src_idx: u32,
        src: TransmitIndex,
        dst: TransmitIndex,
    ) -> Result<u32> {
        let id = self.lift_index_to_transmit(store, src, src_idx)?;
        self.lower_transmit_to_index(store, dst, id)
    }

    fn lift_index_to_transmit(
        self,
        store: &mut StoreOpaque,
        ty: TransmitIndex,
        src_idx: u32,
    ) -> Result<TableId<TransmitHandle>> {
        let (state, _, counter, _, instance) = store.lift_context_parts(self);
        lift_index_to_transmit(instance, counter, state.concurrent_state_mut(), ty, src_idx)
    }

    fn lower_transmit_to_index(
        self,
        store: &mut StoreOpaque,
        ty: TransmitIndex,
        id: TableId<TransmitHandle>,
    ) -> Result<u32> {
        let (state, _, counter, _, instance) = store.lift_context_parts(self);
        lower_transmit_to_index(instance, counter, state.concurrent_state_mut(), ty, id)
    }

    /// Implements the `future.new` intrinsic.
    pub(crate) fn future_new(
        self,
        store: &mut StoreOpaque,
        ty: TypeFutureTableIndex,
    ) -> Result<ResourcePair> {
        self.guest_new(store, TransmitIndex::Future(ty))
    }

    /// Implements the `stream.new` intrinsic.
    pub(crate) fn stream_new(
        self,
        store: &mut StoreOpaque,
        ty: TypeStreamTableIndex,
    ) -> Result<ResourcePair> {
        self.guest_new(store, TransmitIndex::Stream(ty))
    }

    /// Transfer ownership of the specified future read end from one guest to
    /// another.
    pub(crate) fn future_transfer(
        self,
        store: &mut StoreOpaque,
        src_idx: u32,
        src: TypeFutureTableIndex,
        dst: TypeFutureTableIndex,
    ) -> Result<u32> {
        self.guest_transfer(
            store,
            src_idx,
            TransmitIndex::Future(src),
            TransmitIndex::Future(dst),
        )
    }

    /// Transfer ownership of the specified stream read end from one guest to
    /// another.
    pub(crate) fn stream_transfer(
        self,
        store: &mut StoreOpaque,
        src_idx: u32,
        src: TypeStreamTableIndex,
        dst: TypeStreamTableIndex,
    ) -> Result<u32> {
        self.guest_transfer(
            store,
            src_idx,
            TransmitIndex::Stream(src),
            TransmitIndex::Stream(dst),
        )
    }

    /// Copy the specified error context from one component to another.
    pub(crate) fn error_context_transfer(
        self,
        store: &mut StoreOpaque,
        src_idx: u32,
        src: TypeComponentLocalErrorContextTableIndex,
        dst: TypeComponentLocalErrorContextTableIndex,
    ) -> Result<u32> {
        let (mut instance, counter) = self.id().get_mut_and_handle_counter(store);
        let rep = instance
            .as_mut()
            .table_for_error_context(src)
            .error_context_rep(src_idx)?;
        let dst_idx = instance
            .table_for_error_context(dst)
            .error_context_insert(counter, rep)?;

        // Update the global (cross-subcomponent) count for error contexts
        // as the new component has essentially created a new reference that will
        // be dropped/handled independently
        let global_ref_count = store
            .concurrent_state_mut()?
            .global_error_context_ref_counts
            .get_mut(&TypeComponentGlobalErrorContextTableIndex::from_u32(rep))
            .context("global ref count present for existing (sub)component error context")?;

        global_ref_count.0 = global_ref_count
            .0
            .checked_add(1)
            .ok_or_else(|| format_err!(Trap::ReferenceCountOverflow))?;

        Ok(dst_idx)
    }
}

/// Performs the opertion of lifting a future or stream from and instance into
/// the `TransmitHandle` for it.
///
/// The `src_idx` is the guest-specified index within `instance` and `ty` is the
/// expected type of future/stream.
fn lift_index_to_transmit(
    instance: Pin<&mut ComponentInstance>,
    counter: &mut HandleCounter,
    concurrent_state: &mut ConcurrentState,
    ty: TransmitIndex,
    src_idx: u32,
) -> Result<TableId<TransmitHandle>> {
    let handle_table = instance.table_for_transmit(ty);
    let (rep, is_done) = match ty {
        TransmitIndex::Future(idx) => handle_table.future_remove_readable(counter, idx, src_idx)?,
        TransmitIndex::Stream(idx) => handle_table.stream_remove_readable(counter, idx, src_idx)?,
    };
    let desc = match ty {
        TransmitIndex::Future(_) => "future",
        TransmitIndex::Stream(_) => "stream",
    };
    if is_done {
        bail!("cannot lift {desc} after being notified that the writable end dropped");
    }
    let id = TableId::<TransmitHandle>::new(rep);
    let future = concurrent_state.get_mut(id)?;
    if future.common.set.is_some() {
        bail!("cannot lift {desc} while it's in a waitable set");
    }
    future.common.handle = None;

    let state = future.state;
    if concurrent_state.get_mut(state)?.done {
        match ty {
            TransmitIndex::Future(_) => bail!("cannot lift {desc} after previous read succeeded"),
            TransmitIndex::Stream(_) => bail!(Trap::LiftDroppedStream),
        };
    }

    Ok(id)
}

/// Performs the opertion of lowering a future or stream `TransmitHandle` into
/// an instance.
fn lower_transmit_to_index(
    instance: Pin<&mut ComponentInstance>,
    counter: &mut HandleCounter,
    concurrent_state: &mut ConcurrentState,
    ty: TransmitIndex,
    id: TableId<TransmitHandle>,
) -> Result<u32> {
    let state = concurrent_state.get_mut(id)?.state;
    debug_assert_eq!(concurrent_state.get_mut(state)?.read_handle, id);
    let handle_table = instance.table_for_transmit(ty);
    let handle = match ty {
        TransmitIndex::Future(idx) => handle_table.future_insert_read(counter, idx, id.rep()),
        TransmitIndex::Stream(idx) => handle_table.stream_insert_read(counter, idx, id.rep()),
    }?;
    concurrent_state.get_mut(id)?.common.handle = Some(handle);
    Ok(handle)
}

impl ComponentInstance {
    fn table_for_transmit(self: Pin<&mut Self>, ty: TransmitIndex) -> &mut HandleTable {
        let (states, types) = self.instance_states();
        let runtime_instance = match ty {
            TransmitIndex::Stream(ty) => types[ty].instance,
            TransmitIndex::Future(ty) => types[ty].instance,
        };
        states[runtime_instance].handle_table()
    }

    fn table_for_error_context(
        self: Pin<&mut Self>,
        ty: TypeComponentLocalErrorContextTableIndex,
    ) -> &mut HandleTable {
        let (states, types) = self.instance_states();
        let runtime_instance = types[ty].instance;
        states[runtime_instance].handle_table()
    }

    fn get_mut_by_index(
        self: Pin<&mut Self>,
        ty: TransmitIndex,
        index: u32,
    ) -> Result<(u32, &mut TransmitLocalState)> {
        get_mut_by_index_from(self.table_for_transmit(ty), ty, index)
    }
}

impl ConcurrentState {
    fn send_write_result(
        &mut self,
        ty: TransmitIndex,
        id: TableId<TransmitState>,
        handle: u32,
        code: ReturnCode,
    ) -> Result<()> {
        let write_handle = self.get_mut(id)?.write_handle.rep();
        self.set_event(
            write_handle,
            match ty {
                TransmitIndex::Future(ty) => Event::FutureWrite {
                    code,
                    pending: Some((ty, handle)),
                },
                TransmitIndex::Stream(ty) => Event::StreamWrite {
                    code,
                    pending: Some((ty, handle)),
                },
            },
        )
    }

    fn send_read_result(
        &mut self,
        ty: TransmitIndex,
        id: TableId<TransmitState>,
        handle: u32,
        code: ReturnCode,
    ) -> Result<()> {
        let read_handle = self.get_mut(id)?.read_handle.rep();
        self.set_event(
            read_handle,
            match ty {
                TransmitIndex::Future(ty) => Event::FutureRead {
                    code,
                    pending: Some((ty, handle)),
                },
                TransmitIndex::Stream(ty) => Event::StreamRead {
                    code,
                    pending: Some((ty, handle)),
                },
            },
        )
    }

    fn take_event(&mut self, waitable: u32) -> Result<Option<Event>> {
        Waitable::Transmit(TableId::<TransmitHandle>::new(waitable)).take_event(self)
    }

    fn set_event(&mut self, waitable: u32, event: Event) -> Result<()> {
        Waitable::Transmit(TableId::<TransmitHandle>::new(waitable)).set_event(self, Some(event))
    }

    /// Set or update the event for the specified waitable.
    ///
    /// If there is already an event set for this waitable, we assert that it is
    /// of the same variant as the new one and reuse the `ReturnCode` count and
    /// the `pending` field if applicable.
    // TODO: This is a bit awkward due to how
    // `Event::{Stream,Future}{Write,Read}` and
    // `ReturnCode::{Completed,Dropped,Cancelled}` are currently represented.
    // Consider updating those representations in a way that allows this
    // function to be simplified.
    fn update_event(&mut self, waitable: u32, event: Event) -> Result<()> {
        let waitable = Waitable::Transmit(TableId::<TransmitHandle>::new(waitable));

        fn update_code(old: ReturnCode, new: ReturnCode) -> Result<ReturnCode> {
            let (ReturnCode::Completed(count)
            | ReturnCode::Dropped(count)
            | ReturnCode::Cancelled(count)) = old
            else {
                bail_bug!("unexpected old return code")
            };

            Ok(match new {
                ReturnCode::Dropped(ItemCount::ZERO) => ReturnCode::Dropped(count),
                ReturnCode::Cancelled(ItemCount::ZERO) => ReturnCode::Cancelled(count),
                _ => bail_bug!("unexpected new return code"),
            })
        }

        let event = match (waitable.take_event(self)?, event) {
            (None, _) => event,
            (Some(old @ Event::FutureWrite { .. }), Event::FutureWrite { .. }) => old,
            (Some(old @ Event::FutureRead { .. }), Event::FutureRead { .. }) => old,
            (
                Some(Event::StreamWrite {
                    code: old_code,
                    pending: old_pending,
                }),
                Event::StreamWrite { code, pending },
            ) => Event::StreamWrite {
                code: update_code(old_code, code)?,
                pending: old_pending.or(pending),
            },
            (
                Some(Event::StreamRead {
                    code: old_code,
                    pending: old_pending,
                }),
                Event::StreamRead { code, pending },
            ) => Event::StreamRead {
                code: update_code(old_code, code)?,
                pending: old_pending.or(pending),
            },
            _ => bail_bug!("unexpected event combination"),
        };

        waitable.set_event(self, Some(event))
    }

    /// Allocate a new future or stream, including the `TransmitState` and the
    /// `TransmitHandle`s corresponding to the read and write ends.
    fn new_transmit(
        &mut self,
        origin: TransmitOrigin,
    ) -> Result<(TableId<TransmitHandle>, TableId<TransmitHandle>)> {
        // All three entries must fit before inserting any of them so a full
        // table does not leave an inaccessible state or handle behind.
        self.table().ensure_capacity(3)?;
        let state_id = self.push(TransmitState::new(origin))?;

        let write = self.push(TransmitHandle::new(state_id))?;
        let read = self.push(TransmitHandle::new(state_id))?;

        let state = self.get_mut(state_id)?;
        state.write_handle = write;
        state.read_handle = read;

        log::trace!("new transmit: state {state_id:?}; write {write:?}; read {read:?}",);

        Ok((write, read))
    }

    /// Delete the specified future or stream, including the read and write ends.
    fn delete_transmit(&mut self, state_id: TableId<TransmitState>) -> Result<()> {
        let state = self.delete(state_id)?;
        self.delete(state.write_handle)?;
        self.delete(state.read_handle)?;

        log::trace!(
            "delete transmit: state {state_id:?}; write {:?}; read {:?}",
            state.write_handle,
            state.read_handle,
        );

        Ok(())
    }
}

pub(crate) struct ResourcePair {
    pub(crate) write: u32,
    pub(crate) read: u32,
}

impl Waitable {
    /// Handle the imminent delivery of the specified event, e.g. by updating
    /// the state of the stream or future.
    pub(super) fn on_delivery(
        &self,
        store: &mut StoreOpaque,
        instance: Instance,
        event: Event,
    ) -> Result<()> {
        if let Event::FutureRead {
            code: ReturnCode::Dropped(_),
            ..
        }
        | Event::FutureWrite {
            code: ReturnCode::Dropped(_),
            ..
        }
        | Event::StreamRead {
            code: ReturnCode::Dropped(_),
            ..
        }
        | Event::StreamWrite {
            code: ReturnCode::Dropped(_),
            ..
        } = event
        {
            let Waitable::Transmit(transmit_handle) = self else {
                bail_bug!("unexpected `{event:?}` for `{self:?}`");
            };
            let state = store.concurrent_state_mut()?;
            let transmit_id = state.get_mut(*transmit_handle)?.state;
            state.get_mut(transmit_id)?.done = true;
        }

        let instance = instance.id().get_mut(store);
        let (rep, state, code) = match event {
            Event::FutureRead {
                pending: Some((ty, handle)),
                code,
            }
            | Event::FutureWrite {
                pending: Some((ty, handle)),
                code,
            } => {
                let runtime_instance = instance.component().types()[ty].instance;
                let (rep, state) = instance.instance_states().0[runtime_instance]
                    .handle_table()
                    .future_rep(ty, handle)?;
                (rep, state, code)
            }
            Event::StreamRead {
                pending: Some((ty, handle)),
                code,
            }
            | Event::StreamWrite {
                pending: Some((ty, handle)),
                code,
            } => {
                let runtime_instance = instance.component().types()[ty].instance;
                let (rep, state) = instance.instance_states().0[runtime_instance]
                    .handle_table()
                    .stream_rep(ty, handle)?;
                (rep, state, code)
            }
            _ => return Ok(()),
        };
        if rep != self.rep() {
            bail_bug!("unexpected rep mismatch");
        }
        if !matches!(
            *state,
            TransmitLocalState::Busy | TransmitLocalState::Cancelling
        ) {
            bail_bug!("expected state to be busy or cancelling");
        }
        let done = matches!(code, ReturnCode::Dropped(_));
        *state = match event {
            Event::FutureRead { .. } | Event::StreamRead { .. } => {
                TransmitLocalState::Read { done }
            }
            Event::FutureWrite { .. } | Event::StreamWrite { .. } => {
                TransmitLocalState::Write { done }
            }
            _ => bail_bug!("unexpected event for stream"),
        };

        let transmit_handle = TableId::<TransmitHandle>::new(rep);
        let state = store.concurrent_state_mut()?;
        let transmit_id = state.get_mut(transmit_handle)?.state;
        let transmit = state.get_mut(transmit_id)?;

        match event {
            Event::StreamRead { .. } => {
                transmit.read = ReadState::Open;
            }
            Event::StreamWrite { .. } => transmit.write = WriteState::Open,
            _ => {}
        }
        Ok(())
    }
}

/// Determine whether an intra-component read/write is allowed for the specified
/// `stream` or `future` payload type according to the component model
/// specification.
fn allow_intra_component_read_write(ty: Option<&InterfaceType>) -> bool {
    matches!(
        ty,
        None | Some(
            InterfaceType::S8
                | InterfaceType::U8
                | InterfaceType::S16
                | InterfaceType::U16
                | InterfaceType::S32
                | InterfaceType::U32
                | InterfaceType::S64
                | InterfaceType::U64
                | InterfaceType::Float32
                | InterfaceType::Float64
        )
    )
}

/// Helper structure to manage moving a `T` in/out of an interior `Mutex` which
/// contains an
/// `Option<T>`
struct LockedState<T> {
    inner: TryMutex<Option<T>>,
}

impl<T> LockedState<T> {
    /// Creates a new initial state with `value` stored.
    fn new(value: T) -> Self {
        Self {
            inner: TryMutex::new(Some(value)),
        }
    }

    /// Attempts to lock the inner mutex and return its guard.
    ///
    /// # Errors
    ///
    /// Fails if this lock is either poisoned or if it's currently locked.
    /// As-used in this file there should never actually be contention on this
    /// lock nor recursive access so failing to acquire the lock is a fatal
    /// error that gets propagated upwards.
    fn try_lock(&self) -> Result<TryMutexGuard<'_, Option<T>>> {
        match self.inner.try_lock() {
            Some(lock) => Ok(lock),
            None => bail_bug!("should not have contention on state lock"),
        }
    }

    /// Takes the inner `T` out of this state, returning it as a guard which
    /// will put it back when finished.
    ///
    /// # Errors
    ///
    /// Returns an error if the state `T` isn't present.
    fn take(&self) -> Result<LockedStateGuard<'_, T>> {
        let result = self.try_lock()?.take();
        match result {
            Some(result) => Ok(LockedStateGuard {
                value: ManuallyDrop::new(result),
                state: self,
            }),
            None => bail_bug!("lock value unexpectedly missing"),
        }
    }

    /// Performs the operation `f` on the inner state `&mut T`.
    ///
    /// This will acquire the internal lock and invoke `f`, so `f` should not
    /// expect to be able to recursively acquire this lock.
    ///
    /// # Errors
    ///
    /// Returns an error if the state `T` isn't present.
    fn with<R>(&self, f: impl FnOnce(&mut T) -> R) -> Result<R> {
        let mut inner = self.try_lock()?;
        match &mut *inner {
            Some(state) => Ok(f(state)),
            None => bail_bug!("lock value unexpectedly missing"),
        }
    }
}

/// Helper structure returned from [`LockedState::take`] which will put the
/// state specified by `value` back into the original lock once this is dropped.
struct LockedStateGuard<'a, T> {
    value: ManuallyDrop<T>,
    state: &'a LockedState<T>,
}

impl<T> Deref for LockedStateGuard<'_, T> {
    type Target = T;

    fn deref(&self) -> &T {
        &self.value
    }
}

impl<T> DerefMut for LockedStateGuard<'_, T> {
    fn deref_mut(&mut self) -> &mut T {
        &mut self.value
    }
}

impl<T> Drop for LockedStateGuard<'_, T> {
    fn drop(&mut self) {
        // SAFETY: `ManuallyDrop::take` requires that after invoked the
        // original value is not read. This is the `Drop` for this type which
        // means we have exclusive ownership and it is not read further in the
        // destructor, satisfying this requirement.
        let value = unsafe { ManuallyDrop::take(&mut self.value) };

        // If this fails due to contention that's a bug, but we're not in a
        // position to panic due to this being a destructor nor return an error,
        // so defer the bug to showing up later.
        if let Ok(mut lock) = self.state.try_lock() {
            *lock = Some(value);
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{Engine, Store};
    use core::future::pending;
    use core::pin::pin;
    use std::sync::LazyLock;

    static ENGINE: LazyLock<Engine> = LazyLock::new(Engine::default);

    fn poll_future_producer<T>(rx: Pin<&mut T>, finish: bool) -> Poll<Result<Option<T::Item>>>
    where
        T: FutureProducer<()>,
    {
        rx.poll_produce(
            &mut Context::from_waker(Waker::noop()),
            Store::new(&ENGINE, ()).as_context_mut(),
            finish,
        )
    }

    #[test]
    fn future_producer() {
        let mut fut = pin!(async { crate::error::Ok(()) });
        assert!(matches!(
            poll_future_producer(fut.as_mut(), false),
            Poll::Ready(Ok(Some(()))),
        ));

        let mut fut = pin!(async { crate::error::Ok(()) });
        assert!(matches!(
            poll_future_producer(fut.as_mut(), true),
            Poll::Ready(Ok(Some(()))),
        ));

        let mut fut = pin!(pending::<Result<()>>());
        assert!(matches!(
            poll_future_producer(fut.as_mut(), false),
            Poll::Pending,
        ));
        assert!(matches!(
            poll_future_producer(fut.as_mut(), true),
            Poll::Ready(Ok(None)),
        ));

        let (tx, rx) = oneshot::channel();
        let mut rx = pin!(rx);
        assert!(matches!(
            poll_future_producer(rx.as_mut(), false),
            Poll::Pending,
        ));
        assert!(matches!(
            poll_future_producer(rx.as_mut(), true),
            Poll::Ready(Ok(None)),
        ));
        tx.send(()).unwrap();
        assert!(matches!(
            poll_future_producer(rx.as_mut(), true),
            Poll::Ready(Ok(Some(()))),
        ));

        let (tx, rx) = oneshot::channel();
        let mut rx = pin!(rx);
        tx.send(()).unwrap();
        assert!(matches!(
            poll_future_producer(rx.as_mut(), false),
            Poll::Ready(Ok(Some(()))),
        ));

        let (tx, rx) = oneshot::channel::<()>();
        let mut rx = pin!(rx);
        drop(tx);
        assert!(matches!(
            poll_future_producer(rx.as_mut(), false),
            Poll::Ready(Err(..)),
        ));

        let (tx, rx) = oneshot::channel::<()>();
        let mut rx = pin!(rx);
        drop(tx);
        assert!(matches!(
            poll_future_producer(rx.as_mut(), true),
            Poll::Ready(Err(..)),
        ));
    }
}