Module p2

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§Wasmtime’s WASIp2 Implementation

This module provides a Wasmtime host implementation of WASI 0.2 (aka WASIp2 aka Preview 2) and WASI 0.1 (aka WASIp1 aka Preview 1). WASI is implemented with the Rust crates tokio and cap-std primarily, meaning that operations are implemented in terms of their native platform equivalents by default.

§WASIp2 interfaces

This module contains implementations of the following interfaces:

All traits are implemented in terms of a WasiView trait which provides access to WasiCtx, which defines the configuration for WASI. The WasiView trait imples IoView, which provides access to a common ResourceTable, which owns all host-defined component model resources.

The wasmtime-wasi-io crate contains implementations of the following interfaces, and this module reuses those implementations:

These traits are implemented in terms of a IoView trait, which only provides access to a common ResourceTable. All aspects of wasmtime-wasi-io that are used by this module are re-exported. Unless you are implementing other host functionality that needs to interact with the WASI scheduler and don’t want to use other functionality provided by wasmtime-wasi, you don’t need to take a direct dependency on wasmtime-wasi-io.

§Generated Bindings

This module uses wasmtime::component::bindgen! to generate bindings for all WASI interfaces. Raw bindings are available in the bindings submodule of this module. Downstream users can either implement these traits themselves or you can use the built-in implementations in this module for WasiImpl<T: WasiView>.

§The `WasiView` trait

This module’s implementation of WASI is done in terms of an implementation of WasiView. This trait provides a “view” into WASI-related state that is contained within a Store<T>. WasiView implies the IoView trait, which provides access to common ResourceTable which owns all host-implemented component model resources.

For all of the generated bindings in this module (Host traits), implementations are provided looking like:

impl<T: WasiView> bindings::wasi::Host for WasiImpl<T> {
    // ...
}

The add_to_linker_sync and add_to_linker_async function then require that T: WasiView with Linker<T>.

To implement the WasiView and IoView trait you will first select a T to put in Store<T> (typically, by defining your own struct). Somewhere within T you’ll store:

ResourceTable - created through default constructors.
WasiCtx - created through WasiCtxBuilder.

You’ll then write implementations of the IoView and WasiView traits to access those items in your T. For example:

use wasmtime::component::ResourceTable;
use wasmtime_wasi::p2::{WasiCtx, IoView, WasiView};
struct MyCtx {
    table: ResourceTable,
    wasi: WasiCtx,
}
impl IoView for MyCtx {
    fn table(&mut self) -> &mut ResourceTable {
        &mut self.table
    }
}
impl WasiView for MyCtx {
    fn ctx(&mut self) -> &mut WasiCtx {
        &mut self.wasi
    }
}

§Async and Sync

As of WASI0.2, WASI functions are not blocking from WebAssembly’s point of view: a WebAssembly call into these functions returns when they are complete.

This module provides an implementation of those functions in the host, where for some functions, it is appropriate to implement them using async Rust and the Tokio executor, so that the host implementation can be nonblocking when Wasmtime’s Config::async_support is set. Synchronous wrappers are provided for all async implementations, which creates a private Tokio executor.

Users can choose between these modes of implementation using variants of the add_to_linker functions:

For non-async users (the default of Config), use add_to_linker_sync.
For async users, use add_to_linker_async.

Note that bindings are generated once for async and once for sync. Most interfaces do not change, however, so only interfaces with blocking functions have bindings generated twice. Bindings are organized as:

bindings - default location of all bindings, blocking functions are async
bindings::sync - blocking interfaces have synchronous versions here.

§Module-specific traits

This module’s default implementation of WASI bindings to native primitives for the platform that it is compiled for. For example opening a TCP socket uses the native platform to open a TCP socket (so long as WasiCtxBuilder allows it). There are a few important traits, however, that are specific to this module.

InputStream and OutputStream - these are the host traits behind the WASI input-stream and output-stream types in the wasi:io/streams interface. These enable embedders to build their own custom stream and insert them into a ResourceTable (as a boxed trait object, see DynInputStream and DynOutputStream) to be used from wasm.
Pollable - this trait enables building arbitrary logic to get hooked into a pollable resource from wasi:io/poll. A pollable resource is created through the subscribe function.
HostWallClock and HostMonotonicClock are used in conjunction with WasiCtxBuilder::wall_clock and WasiCtxBuilder::monotonic_clock if the defaults host’s clock should not be used.
StdinStream and StdoutStream are used to provide custom stdin/stdout streams if they’re not inherited (or null, which is the default).

These traits enable embedders to customize small portions of WASI interfaces provided while still providing all other interfaces.

§Examples

Usage of this module is done through a few steps to get everything hooked up:

First implement IoView and WasiView for your type which is the T in Store<T>.
Add WASI interfaces to a wasmtime::component::Linker<T>. This is either done through top-level functions like add_to_linker_sync or through individual add_to_linker functions in generated bindings throughout this module.
Create a WasiCtx for each Store<T> through WasiCtxBuilder. Each WASI context is “null” or “empty” by default, so items must be explicitly added to get accessed by wasm (such as env vars or program arguments).
Use the previous Linker<T> to instantiate a Component within a Store<T>.

For examples see each of WasiView, WasiCtx, WasiCtxBuilder, add_to_linker_sync, and bindings::Command.

Modules§

bindings: Auto-generated bindings for WASI interfaces.
pipe: Virtual pipes.

Structs§

AsyncStdinStream: An impl of StdinStream built on top of crate::p2::pipe::AsyncReadStream.
AsyncStdoutStream: A wrapper of crate::p2::pipe::AsyncWriteStream that implements StdoutStream. Note that the OutputStream impl for this is not correct when used for interleaved async IO.
DynPollable: The host representation of the wasi:io/poll.pollable resource.
FileInputStream
IoImpl: A small newtype wrapper which serves as the basis for implementations of Host WASI traits in this crate.
OutputFile: This implementation will yield output streams that block on writes, and output directly to a file. If truly async output is required, AsyncStdoutStream should be used instead.
Stderr: This implementation will yield output streams that block on writes, as they inherit the implementation directly from the rust std library. A different implementation of StdoutStream will be necessary if truly async output streams are required.
Stdin: Only public interface is the InputStream impl.
Stdout: This implementation will yield output streams that block on writes, as they inherit the implementation directly from the rust std library. A different implementation of StdoutStream will be necessary if truly async output streams are required.
WasiCtx: Per-Store state which holds state necessary to implement WASI from this crate.
WasiCtxBuilder: Builder-style structure used to create a WasiCtx.
WasiImpl: A small newtype wrapper which serves as the basis for implementations of Host WASI traits in this crate.

Enums§

IsATTY
StreamError

Traits§

InputStream: Host trait for implementing the wasi:io/streams.input-stream resource: A bytestream which can be read from.
IoView: A trait which provides access to the ResourceTable inside the embedder’s T of Store<T>.
OutputStream: Host trait for implementing the wasi:io/streams.output-stream resource: A bytestream which can be written to.
Pollable: The trait used to implement DynPollable to create a pollable resource in wasi:io/poll.
StdinStream: A trait used to represent the standard input to a guest program.
StdoutStream: Similar to StdinStream, except for output.
WasiView: A trait which provides access to the WasiCtx inside the embedder’s T of Store<T>.

Functions§

add_to_linker_async: Add all WASI interfaces from this crate into the linker provided.
add_to_linker_sync: Add all WASI interfaces from this crate into the linker provided.
add_to_linker_with_options_async: Similar to add_to_linker_async, but with the ability to enable unstable features.
add_to_linker_with_options_sync: Similar to add_to_linker_sync, but with the ability to enable unstable features.
stderr: Returns a stream that represents the host’s standard err.
stdin: Returns a stream that represents the host’s standard input.
stdout: Returns a stream that represents the host’s standard out.
subscribe: Creates a wasi:io/poll/pollable resource which is subscribed to the provided resource.

Type Aliases§

DynFuture
DynInputStream
DynOutputStream
FsError
FsResult
IoError: Representation of the error resource type in the wasi:io/error interface.
MakeFuture
SocketError
SocketResult
StreamResult

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