wasmtime/runtime/gc/enabled/rooting.rs
1//! Garbage collection rooting APIs.
2//!
3//! Rooting prevents GC objects from being collected while they are actively
4//! being used.
5//!
6//! ## Goals
7//!
8//! We have a few sometimes-conflicting goals with our GC rooting APIs:
9//!
10//! 1. Safety: It should never be possible to get a use-after-free bug because
11//! the user misused the rooting APIs, the collector "mistakenly" determined
12//! an object was unreachable and collected it, and then the user tried to
13//! access the object. This is our highest priority.
14//!
15//! 2. Moving GC: Our rooting APIs should moving collectors (such as
16//! generational and compacting collectors) where an object might get
17//! relocated after a collection and we need to update the GC root's pointer
18//! to the moved object. This means we either need cooperation and internal
19//! mutability from individual GC roots as well as the ability to enumerate
20//! all GC roots on the native Rust stack, or we need a level of indirection.
21//!
22//! 3. Performance: Our rooting APIs should generally be as low-overhead as
23//! possible. They definitely shouldn't require synchronization and locking
24//! to create, access, and drop GC roots.
25//!
26//! 4. Ergonomics: Our rooting APIs should be, if not a pleasure, then at least
27//! not a burden for users. Additionally, the API's types should be `Sync`
28//! and `Send` so that they work well with async Rust.
29//!
30//! For example, goals (3) and (4) are in conflict when we think about how to
31//! support (2). Ideally, for ergonomics, a root would automatically unroot
32//! itself when dropped. But in the general case that requires holding a
33//! reference to the store's root set, and that root set needs to be held
34//! simultaneously by all GC roots, and they each need to mutate the set to
35//! unroot themselves. That implies `Rc<RefCell<...>>` or `Arc<Mutex<...>>`! The
36//! former makes the store and GC root types not `Send` and not `Sync`. The
37//! latter imposes synchronization and locking overhead. So we instead make GC
38//! roots indirect and require passing in a store context explicitly to unroot
39//! in the general case. This trades worse ergonomics for better performance and
40//! support for moving GC.
41//!
42//! ## Two Flavors of Rooting API
43//!
44//! Okay, with that out of the way, this module provides two flavors of rooting
45//! API. One for the common, scoped lifetime case, and another for the rare case
46//! where we really need a GC root with an arbitrary, non-LIFO/non-scoped
47//! lifetime:
48//!
49//! 1. `RootScope` and `Rooted<T>`: These are used for temporarily rooting GC
50//! objects for the duration of a scope. The internal implementation takes
51//! advantage of the LIFO property inherent in scopes, making creating and
52//! dropping `Rooted<T>`s and `RootScope`s super fast and roughly equivalent
53//! to bump allocation.
54//!
55//! This type is vaguely similar to V8's [`HandleScope`].
56//!
57//! [`HandleScope`]: https://v8.github.io/api/head/classv8_1_1HandleScope.html
58//!
59//! Note that `Rooted<T>` can't be statically tied to its context scope via a
60//! lifetime parameter, unfortunately, as that would allow the creation of
61//! only one `Rooted<T>` at a time, since the `Rooted<T>` would take a borrow
62//! of the whole context.
63//!
64//! This supports the common use case for rooting and provides good
65//! ergonomics.
66//!
67//! 2. `ManuallyRooted<T>`: This is the fully general rooting API used for
68//! holding onto non-LIFO GC roots with arbitrary lifetimes. However, users
69//! must manually unroot them. Failure to manually unroot a
70//! `ManuallyRooted<T>` before it is dropped will result in the GC object
71//! (and everything it transitively references) leaking for the duration of
72//! the `Store`'s lifetime.
73//!
74//! This type is roughly similar to SpiderMonkey's [`PersistentRooted<T>`],
75//! although they avoid the manual-unrooting with internal mutation and
76//! shared references. (Our constraints mean we can't do those things, as
77//! mentioned explained above.)
78//!
79//! [`PersistentRooted<T>`]: http://devdoc.net/web/developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/JSAPI_reference/JS::PersistentRooted.html
80//!
81//! At the end of the day, both `Rooted<T>` and `ManuallyRooted<T>` are just
82//! tagged indices into the store's `RootSet`. This indirection allows working
83//! with Rust's borrowing discipline (we use `&mut Store` to represent mutable
84//! access to the GC heap) while still allowing rooted references to be moved
85//! around without tying up the whole store in borrows. Additionally, and
86//! crucially, this indirection allows us to update the *actual* GC pointers in
87//! the `RootSet` and support moving GCs (again, as mentioned above).
88//!
89//! ## Unrooted References
90//!
91//! We generally don't expose *unrooted* GC references in the Wasmtime API at
92//! this time -- and I expect it will be a very long time before we do, but in
93//! the limit we may want to let users define their own GC-managed types that
94//! participate in GC tracing and all that -- so we don't have to worry about
95//! failure to root an object causing use-after-free bugs or failing to update a
96//! GC root pointer after a moving GC as long as users stick to our safe rooting
97//! APIs. (The one exception is `ValRaw`, which does hold raw GC references. But
98//! with `ValRaw` all bets are off and safety is 100% up to the user.)
99//!
100//! We do, however, have to worry about these things internally. So first of
101//! all, try to avoid ever working with unrooted GC references if you
102//! can. However, if you really must, consider also using an `AutoAssertNoGc`
103//! across the block of code that is manipulating raw GC references.
104
105use crate::runtime::vm::{GcRootsList, GcStore, VMGcRef};
106use crate::{
107 AsContext, AsContextMut, GcRef, Result, RootedGcRef,
108 store::{AsStoreOpaque, AutoAssertNoGc, StoreId, StoreOpaque},
109};
110use crate::{ValRaw, prelude::*};
111use core::any;
112use core::marker;
113use core::mem::{self, MaybeUninit};
114use core::num::NonZeroU64;
115use core::{
116 fmt::{self, Debug},
117 hash::{Hash, Hasher},
118 ops::{Deref, DerefMut},
119};
120use wasmtime_slab::{Id as SlabId, Slab};
121
122mod sealed {
123 use super::*;
124
125 /// Sealed, `wasmtime`-internal trait for GC references.
126 ///
127 /// # Safety
128 ///
129 /// All types implementing this trait must:
130 ///
131 /// * Be a newtype of a `GcRootIndex`
132 ///
133 /// * Not implement `Copy` or `Clone`
134 ///
135 /// * Only have `&self` methods.
136 pub unsafe trait GcRefImpl: Sized {
137 /// Transmute a `&GcRootIndex` into an `&Self`.
138 fn transmute_ref(index: &GcRootIndex) -> &Self;
139 }
140
141 /// Sealed, `wasmtime`-internal trait for the common methods on rooted GC
142 /// references.
143 pub trait RootedGcRefImpl<T: GcRef> {
144 /// Get this rooted GC reference's raw `VMGcRef` out of the store's GC
145 /// root set.
146 ///
147 /// Returns `None` for objects that have since been unrooted (eg because
148 /// its associated `RootedScope` was dropped).
149 ///
150 /// Panics if this root is not associated with the given store.
151 fn get_gc_ref<'a>(&self, store: &'a StoreOpaque) -> Option<&'a VMGcRef>;
152
153 /// Same as `get_gc_ref` but returns an error instead of `None` for
154 /// objects that have been unrooted.
155 fn try_gc_ref<'a>(&self, store: &'a StoreOpaque) -> Result<&'a VMGcRef> {
156 self.get_gc_ref(store).ok_or_else(|| {
157 anyhow!("attempted to use a garbage-collected object that has been unrooted")
158 })
159 }
160
161 /// Get a clone of this rooted GC reference's raw `VMGcRef` out of the
162 /// store's GC root set.
163 ///
164 /// Returns `None` for objects that have since been unrooted (eg because
165 /// its associated `RootedScope` was dropped).
166 ///
167 /// Panics if this root is not associated with the given store.
168 fn clone_gc_ref(&self, store: &mut AutoAssertNoGc<'_>) -> Option<VMGcRef> {
169 let gc_ref = self.get_gc_ref(store)?.unchecked_copy();
170 Some(store.clone_gc_ref(&gc_ref))
171 }
172
173 /// Same as `clone_gc_ref` but returns an error instead of `None` for
174 /// objects that have been unrooted.
175 fn try_clone_gc_ref(&self, store: &mut AutoAssertNoGc<'_>) -> Result<VMGcRef> {
176 let gc_ref = self.try_gc_ref(store)?.unchecked_copy();
177 Ok(store.clone_gc_ref(&gc_ref))
178 }
179 }
180}
181pub(crate) use sealed::*;
182
183/// The index of a GC root inside a particular store's GC root set.
184///
185/// Can be either a LIFO- or manually-rooted object, depending on the
186/// `PackedIndex`.
187///
188/// Every `T` such that `T: GcRef` must be a newtype over this `GcRootIndex`.
189#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
190// Just `pub` to avoid `warn(private_interfaces)` in public APIs, which we can't
191// `allow(...)` on our MSRV yet.
192#[doc(hidden)]
193#[repr(C)] // NB: if this layout changes be sure to change the C API as well
194pub struct GcRootIndex {
195 store_id: StoreId,
196 generation: u32,
197 index: PackedIndex,
198}
199
200const _: () = {
201 // NB: these match the C API which should also be updated if this changes
202 assert!(mem::size_of::<GcRootIndex>() == 16);
203 assert!(mem::align_of::<GcRootIndex>() == mem::align_of::<u64>());
204};
205
206impl GcRootIndex {
207 #[inline]
208 pub(crate) fn comes_from_same_store(&self, store: &StoreOpaque) -> bool {
209 self.store_id == store.id()
210 }
211
212 /// Same as `RootedGcRefImpl::get_gc_ref` but not associated with any
213 /// particular `T: GcRef`.
214 ///
215 /// We must avoid triggering a GC while holding onto the resulting raw
216 /// `VMGcRef` to avoid use-after-free bugs and similar. The `'a` lifetime
217 /// threaded from the `store` to the result will normally prevent GCs
218 /// statically, at compile time, since performing a GC requires a mutable
219 /// borrow of the store. However, if you call `VMGcRef::unchecked_copy` on
220 /// the resulting GC reference, then all bets are off and this invariant is
221 /// up to you to manually uphold. Failure to uphold this invariant is memory
222 /// safe but will lead to general incorrectness such as panics and wrong
223 /// results.
224 ///
225 /// # Panics
226 ///
227 /// Panics if `self` is not associated with the given store.
228 pub(crate) fn get_gc_ref<'a>(&self, store: &'a StoreOpaque) -> Option<&'a VMGcRef> {
229 assert!(
230 self.comes_from_same_store(store),
231 "object used with wrong store"
232 );
233 if let Some(index) = self.index.as_lifo() {
234 let entry = store.gc_roots().lifo_roots.get(index)?;
235 if entry.generation == self.generation {
236 Some(&entry.gc_ref)
237 } else {
238 None
239 }
240 } else if let Some(id) = self.index.as_manual() {
241 let gc_ref = store.gc_roots().manually_rooted.get(id);
242 debug_assert!(gc_ref.is_some());
243 gc_ref
244 } else {
245 unreachable!()
246 }
247 }
248
249 /// Same as `get_gc_ref` but returns an error instead of `None` if
250 /// the GC reference has been unrooted.
251 ///
252 /// # Panics
253 ///
254 /// Panics if `self` is not associated with the given store.
255 pub(crate) fn try_gc_ref<'a>(&self, store: &'a StoreOpaque) -> Result<&'a VMGcRef> {
256 self.get_gc_ref(store).ok_or_else(|| {
257 anyhow!("attempted to use a garbage-collected object that has been unrooted")
258 })
259 }
260
261 /// Same as `RootedGcRefImpl::clone_gc_ref` but not associated with any
262 /// particular `T: GcRef`.
263 pub(crate) fn try_clone_gc_ref(&self, store: &mut AutoAssertNoGc<'_>) -> Result<VMGcRef> {
264 let gc_ref = self.try_gc_ref(store)?.unchecked_copy();
265 Ok(store.clone_gc_ref(&gc_ref))
266 }
267}
268
269/// This is a bit-packed version of
270///
271/// ```ignore
272/// enum {
273/// Lifo(usize),
274/// Manual(SlabId),
275/// }
276/// ```
277///
278/// where the high bit is the discriminant and the lower 31 bits are the
279/// payload.
280#[derive(Clone, Copy, PartialEq, Eq, Hash)]
281#[repr(transparent)]
282struct PackedIndex(u32);
283
284impl Debug for PackedIndex {
285 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
286 if let Some(index) = self.as_lifo() {
287 f.debug_tuple("PackedIndex::Lifo").field(&index).finish()
288 } else if let Some(id) = self.as_manual() {
289 f.debug_tuple("PackedIndex::Manual").field(&id).finish()
290 } else {
291 unreachable!()
292 }
293 }
294}
295
296impl PackedIndex {
297 const DISCRIMINANT_MASK: u32 = 0b1 << 31;
298 const LIFO_DISCRIMINANT: u32 = 0b0 << 31;
299 const MANUAL_DISCRIMINANT: u32 = 0b1 << 31;
300 const PAYLOAD_MASK: u32 = !Self::DISCRIMINANT_MASK;
301
302 fn new_lifo(index: usize) -> PackedIndex {
303 let index32 = u32::try_from(index).unwrap();
304 assert_eq!(index32 & Self::DISCRIMINANT_MASK, 0);
305 let packed = PackedIndex(Self::LIFO_DISCRIMINANT | index32);
306 debug_assert!(packed.is_lifo());
307 debug_assert_eq!(packed.as_lifo(), Some(index));
308 debug_assert!(!packed.is_manual());
309 debug_assert!(packed.as_manual().is_none());
310 packed
311 }
312
313 fn new_manual(id: SlabId) -> PackedIndex {
314 let raw = id.into_raw();
315 assert_eq!(raw & Self::DISCRIMINANT_MASK, 0);
316 let packed = PackedIndex(Self::MANUAL_DISCRIMINANT | raw);
317 debug_assert!(packed.is_manual());
318 debug_assert_eq!(packed.as_manual(), Some(id));
319 debug_assert!(!packed.is_lifo());
320 debug_assert!(packed.as_lifo().is_none());
321 packed
322 }
323
324 fn discriminant(&self) -> u32 {
325 self.0 & Self::DISCRIMINANT_MASK
326 }
327
328 fn is_lifo(&self) -> bool {
329 self.discriminant() == Self::LIFO_DISCRIMINANT
330 }
331
332 fn is_manual(&self) -> bool {
333 self.discriminant() == Self::MANUAL_DISCRIMINANT
334 }
335
336 fn payload(&self) -> u32 {
337 self.0 & Self::PAYLOAD_MASK
338 }
339
340 fn as_lifo(&self) -> Option<usize> {
341 if self.is_lifo() {
342 Some(usize::try_from(self.payload()).unwrap())
343 } else {
344 None
345 }
346 }
347
348 fn as_manual(&self) -> Option<SlabId> {
349 if self.is_manual() {
350 Some(SlabId::from_raw(self.payload()))
351 } else {
352 None
353 }
354 }
355}
356
357/// The set of all embedder-API GC roots in a single store/heap.
358#[derive(Debug, Default)]
359pub(crate) struct RootSet {
360 /// GC roots with arbitrary lifetime that are manually rooted and unrooted,
361 /// for use with `ManuallyRooted<T>`.
362 manually_rooted: Slab<VMGcRef>,
363
364 /// Strictly LIFO-ordered GC roots, for use with `RootScope` and
365 /// `Rooted<T>`.
366 lifo_roots: Vec<LifoRoot>,
367
368 /// Generation counter for entries to prevent ABA bugs with `RootScope` and
369 /// `Rooted<T>`.
370 lifo_generation: u32,
371}
372
373#[derive(Debug)]
374struct LifoRoot {
375 generation: u32,
376 gc_ref: VMGcRef,
377}
378
379impl RootSet {
380 pub(crate) fn trace_roots(&mut self, gc_roots_list: &mut GcRootsList) {
381 log::trace!("Begin trace user LIFO roots");
382 for root in &mut self.lifo_roots {
383 unsafe {
384 gc_roots_list.add_root((&mut root.gc_ref).into(), "user LIFO root");
385 }
386 }
387 log::trace!("End trace user LIFO roots");
388
389 log::trace!("Begin trace user manual roots");
390 for (_id, root) in self.manually_rooted.iter_mut() {
391 unsafe {
392 gc_roots_list.add_root(root.into(), "user manual root");
393 }
394 }
395 log::trace!("End trace user manual roots");
396 }
397
398 /// Enter a LIFO rooting scope.
399 ///
400 /// Returns an integer that should be passed unmodified to `exit_lifo_scope`
401 /// when the scope is finished.
402 ///
403 /// Calls to `{enter,exit}_lifo_scope` must happen in a strict LIFO order.
404 #[inline]
405 pub(crate) fn enter_lifo_scope(&self) -> usize {
406 self.lifo_roots.len()
407 }
408
409 /// Exit a LIFO rooting scope.
410 ///
411 /// The `scope` argument must be the result of the corresponding
412 /// `enter_lifo_scope` call.
413 ///
414 /// Calls to `{enter,exit}_lifo_scope` must happen in a strict LIFO order.
415 #[inline]
416 pub(crate) fn exit_lifo_scope(&mut self, gc_store: Option<&mut GcStore>, scope: usize) {
417 debug_assert!(self.lifo_roots.len() >= scope);
418
419 // If we actually have roots to unroot, call an out-of-line slow path.
420 if self.lifo_roots.len() > scope {
421 self.exit_lifo_scope_slow(gc_store, scope);
422 }
423 }
424
425 #[inline(never)]
426 #[cold]
427 fn exit_lifo_scope_slow(&mut self, mut gc_store: Option<&mut GcStore>, scope: usize) {
428 self.lifo_generation += 1;
429
430 // TODO: In the case where we have a tracing GC that doesn't need to
431 // drop barriers, this should really be:
432 //
433 // self.lifo_roots.truncate(scope);
434
435 let mut lifo_roots = mem::take(&mut self.lifo_roots);
436 for root in lifo_roots.drain(scope..) {
437 // Only drop the GC reference if we actually have a GC store. How
438 // can we have a GC reference but not a GC store? If we've only
439 // created `i31refs`, we never force a GC store's allocation. This
440 // is fine because `i31ref`s never need drop barriers.
441 if let Some(gc_store) = &mut gc_store {
442 gc_store.drop_gc_ref(root.gc_ref);
443 }
444 }
445 self.lifo_roots = lifo_roots;
446 }
447
448 pub(crate) fn with_lifo_scope<S, T>(store: &mut S, f: impl FnOnce(&mut S) -> T) -> T
449 where
450 S: ?Sized + DerefMut<Target = StoreOpaque>,
451 {
452 let scope = store.gc_roots().enter_lifo_scope();
453 let ret = f(store);
454 store.exit_gc_lifo_scope(scope);
455 ret
456 }
457
458 pub(crate) fn push_lifo_root(&mut self, store_id: StoreId, gc_ref: VMGcRef) -> GcRootIndex {
459 let generation = self.lifo_generation;
460 let index = self.lifo_roots.len();
461 let index = PackedIndex::new_lifo(index);
462 self.lifo_roots.push(LifoRoot { generation, gc_ref });
463 GcRootIndex {
464 store_id,
465 generation,
466 index,
467 }
468 }
469}
470
471/// A scoped, rooted reference to a garbage-collected `T`.
472///
473/// A `Rooted<T>` is a strong handle to a garbage-collected `T`, preventing its
474/// referent (and anything else transitively referenced) from being collected by
475/// the GC during the scope within which this `Rooted<T>` was created.
476///
477/// When the context exits this `Rooted<T>`'s scope, the underlying GC object is
478/// automatically unrooted and any further attempts to use access the underlying
479/// object will return errors or otherwise fail.
480///
481/// `Rooted<T>` dereferences to its underlying `T`, allowing you to call `T`'s
482/// methods.
483///
484/// # Example
485///
486/// ```
487/// # use wasmtime::*;
488/// # fn _foo() -> Result<()> {
489/// let mut store = Store::<()>::default();
490///
491/// // Allocating a GC object returns a `Rooted<T>`.
492/// let hello: Rooted<ExternRef> = ExternRef::new(&mut store, "hello")?;
493///
494/// // Because `Rooted<T>` derefs to `T`, we can call `T` methods on a
495/// // `Rooted<T>`. For example, we can call the `ExternRef::data` method when we
496/// // have a `Rooted<ExternRef>`.
497/// let data = hello
498/// .data(&store)?
499/// .ok_or_else(|| Error::msg("externref has no host data"))?
500/// .downcast_ref::<&str>()
501/// .ok_or_else(|| Error::msg("not a str"))?;
502/// assert_eq!(*data, "hello");
503///
504/// // A `Rooted<T>` roots its underlying GC object for the duration of the
505/// // scope of the store/caller/context that was passed to the method that created
506/// // it. If we only want to keep a GC reference rooted and alive temporarily, we
507/// // can introduce new scopes with `RootScope`.
508/// {
509/// let mut scope = RootScope::new(&mut store);
510///
511/// // This `Rooted<T>` is automatically unrooted after `scope` is dropped,
512/// // allowing the collector to reclaim its GC object in the next GC.
513/// let scoped_ref = ExternRef::new(&mut scope, "goodbye");
514/// }
515///
516/// let module = Module::new(store.engine(), r#"
517/// (module
518/// (global (export "global") (mut externref) (ref.null extern))
519/// (table (export "table") 10 externref)
520/// (func (export "func") (param externref) (result externref)
521/// local.get 0
522/// )
523/// )
524/// "#)?;
525/// let instance = Instance::new(&mut store, &module, &[])?;
526///
527/// // GC references returned from calls into Wasm also return (optional, if the
528/// // Wasm type is nullable) `Rooted<T>`s.
529/// let result: Option<Rooted<_>> = instance
530/// .get_typed_func::<Option<Rooted<ExternRef>>, Option<Rooted<ExternRef>>>(&mut store, "func")?
531/// .call(&mut store, Some(hello))?;
532///
533/// // Similarly, getting a GC reference from a Wasm instance's exported global
534/// // or table yields a `Rooted<T>`.
535///
536/// let global = instance
537/// .get_global(&mut store, "global")
538/// .ok_or_else(|| Error::msg("missing `global` export"))?;
539/// let global_val = global.get(&mut store);
540/// let global_ref: Option<&Rooted<_>> = global_val
541/// .externref()
542/// .ok_or_else(|| Error::msg("not an externref"))?;
543///
544/// let table = instance.get_table(&mut store, "table").unwrap();
545/// let table_elem = table
546/// .get(&mut store, 3)
547/// .ok_or_else(|| Error::msg("table out of bounds"))?;
548/// let table_elem_ref: Option<&Rooted<_>> = table_elem
549/// .as_extern()
550/// .ok_or_else(|| Error::msg("not an externref"))?;
551/// # Ok(())
552/// # }
553/// ```
554///
555/// # Differences Between `Rooted<T>` and `ManuallyRooted<T>`
556///
557/// While `Rooted<T>` is automatically unrooted when its scope is exited, this
558/// means that `Rooted<T>` is only valid for strictly last-in-first-out (LIFO,
559/// aka stack order) lifetimes. This is in contrast to
560/// [`ManuallyRooted<T>`][crate::ManuallyRooted], which supports rooting GC
561/// objects for arbitrary lifetimes, but requires manual unrooting.
562///
563/// | Type | Supported Lifetimes | Unrooting |
564/// |----------------------------------------------|-----------------------------|-----------|
565/// | [`Rooted<T>`][crate::Rooted] | Strictly LIFO / stack order | Automatic |
566/// | [`ManuallyRooted<T>`][crate::ManuallyRooted] | Arbitrary | Manual |
567///
568/// `Rooted<T>` should suffice for most use cases, and provides better
569/// ergonomics, but `ManuallyRooted<T>` exists as a fully-general escape hatch.
570///
571/// # Scopes
572///
573/// Wasmtime automatically creates two kinds of scopes:
574///
575/// 1. A [`Store`][crate::Store] is the outermost rooting scope. Creating a
576/// `Root<T>` directly inside a `Store` permanently roots the underlying
577/// object, similar to dropping a
578/// [`ManuallyRooted<T>`][crate::ManuallyRooted] without unrooting it.
579///
580/// 2. A [`Caller`][crate::Caller] provides a rooting scope for the duration of
581/// a call from Wasm into a host function. Any objects rooted in a `Caller`
582/// will be unrooted after the host function returns. Note that there can be
583/// nested `Caller` scopes in the case where Wasm calls a host function,
584/// creating the first `Caller` and its rooting scope , and then the host
585/// function calls a Wasm function which then calls another host function,
586/// creating a second `Caller` and a second rooting scope. This nesting can
587/// be arbitrarily deep.
588///
589/// Additionally, if you would like to define finer-grained rooting scopes,
590/// Wasmtime provides the [`RootScope`][crate::RootScope] type.
591///
592/// Scopes are always nested in a last-in-first-out (LIFO) order. An outer scope
593/// is never exited (and the `Rooted<T>`s defined within it are never
594/// automatically unrooted) while an inner scope is still active. All inner
595/// scopes are exited before their outer scopes.
596///
597/// The following diagram illustrates various rooting scopes over time, how they
598/// nest, and when their `Rooted<T>`s are automatically unrooted:
599///
600/// ```text
601/// ----- new Store
602/// |
603/// |
604/// | let a: Rooted<T> = ...;
605/// |
606/// |
607/// | ----- call into Wasm
608/// | |
609/// | |
610/// | | ----- Wasm calls host function F
611/// | | |
612/// | | |
613/// | | | let b: Rooted<T> = ...;
614/// | | |
615/// | | |
616/// | | | ----- F calls into Wasm
617/// | | | |
618/// | | | |
619/// | | | | ----- Wasm call host function G
620/// | | | | |
621/// | | | | |
622/// | | | | | let c: Rooted<T> = ...;
623/// | | | | |
624/// | | | | |
625/// | | | | ----- return to Wasm from host function G (unroots `c`)
626/// | | | |
627/// | | | |
628/// | | | ----- Wasm returns to F
629/// | | |
630/// | | |
631/// | | ----- return from host function F (unroots `b`)
632/// | |
633/// | |
634/// | ----- return from Wasm
635/// |
636/// |
637/// | ----- let scope1 = RootScope::new(...);
638/// | |
639/// | |
640/// | | let d: Rooted<T> = ...;
641/// | |
642/// | |
643/// | | ----- let scope2 = RootScope::new(...);
644/// | | |
645/// | | |
646/// | | | let e: Rooted<T> = ...;
647/// | | |
648/// | | |
649/// | | ----- drop `scope2` (unroots `e`)
650/// | |
651/// | |
652/// | ----- drop `scope1` (unroots `d`)
653/// |
654/// |
655/// ----- drop Store (unroots `a`)
656/// ```
657///
658/// A `Rooted<T>` can be used successfully as long as it is still rooted so, in
659/// the above diagram, `d` is valid inside `scope2` because `scope2` is wholly
660/// contained within the scope `d` was rooted within (`scope1`).
661///
662/// See also the documentation for [`RootScope`][crate::RootScope].
663#[repr(transparent)]
664pub struct Rooted<T: GcRef> {
665 inner: GcRootIndex,
666 _phantom: marker::PhantomData<T>,
667}
668
669impl<T: GcRef> Clone for Rooted<T> {
670 fn clone(&self) -> Self {
671 Rooted {
672 inner: self.inner,
673 _phantom: marker::PhantomData,
674 }
675 }
676}
677
678impl<T: GcRef> Copy for Rooted<T> {}
679
680impl<T: GcRef> Debug for Rooted<T> {
681 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
682 let name = format!("Rooted<{}>", any::type_name::<T>());
683 f.debug_struct(&name).field("inner", &self.inner).finish()
684 }
685}
686
687impl<T: GcRef> RootedGcRefImpl<T> for Rooted<T> {
688 fn get_gc_ref<'a>(&self, store: &'a StoreOpaque) -> Option<&'a VMGcRef> {
689 assert!(
690 self.comes_from_same_store(store),
691 "object used with wrong store"
692 );
693 let index = self.inner.index.as_lifo().unwrap();
694 let entry = store.gc_roots().lifo_roots.get(index)?;
695 if entry.generation == self.inner.generation {
696 Some(&entry.gc_ref)
697 } else {
698 None
699 }
700 }
701}
702
703impl<T: GcRef> Deref for Rooted<T> {
704 type Target = T;
705
706 fn deref(&self) -> &Self::Target {
707 T::transmute_ref(&self.inner)
708 }
709}
710
711impl<T: GcRef> Rooted<T> {
712 /// Push the given `VMGcRef` onto our LIFO root set.
713 ///
714 /// `gc_ref` should belong to `store`'s heap; failure to uphold this is
715 /// memory safe but will result in general failures down the line such as
716 /// panics or incorrect results.
717 ///
718 /// `gc_ref` should be a GC reference pointing to an instance of the GC type
719 /// that `T` represents. Failure to uphold this invariant is memory safe but
720 /// will result in general incorrectness such as panics and wrong results.
721 pub(crate) fn new(store: &mut AutoAssertNoGc<'_>, gc_ref: VMGcRef) -> Rooted<T> {
722 let id = store.id();
723 let roots = store.gc_roots_mut();
724 let inner = roots.push_lifo_root(id, gc_ref);
725 Rooted {
726 inner,
727 _phantom: marker::PhantomData,
728 }
729 }
730
731 /// Create a new `Rooted<T>` from a `GcRootIndex`.
732 ///
733 /// Note that `Rooted::from_gc_root_index(my_rooted.index)` is not
734 /// necessarily an identity function, as it allows changing the `T` type
735 /// parameter.
736 ///
737 /// The given index should be a LIFO index of a GC reference pointing to an
738 /// instance of the GC type that `T` represents. Failure to uphold this
739 /// invariant is memory safe but will result in general incorrectness such
740 /// as panics and wrong results.
741 pub(crate) fn from_gc_root_index(inner: GcRootIndex) -> Rooted<T> {
742 debug_assert!(inner.index.is_lifo());
743 Rooted {
744 inner,
745 _phantom: marker::PhantomData,
746 }
747 }
748
749 #[inline]
750 pub(crate) fn comes_from_same_store(&self, store: &StoreOpaque) -> bool {
751 debug_assert!(self.inner.index.is_lifo());
752 self.inner.comes_from_same_store(store)
753 }
754
755 /// Create a [`ManuallyRooted<T>`][crate::ManuallyRooted] holding onto the
756 /// same GC object as `self`.
757 ///
758 /// Returns `None` if `self` is used outside of its scope and has therefore
759 /// been unrooted.
760 ///
761 /// This does not unroot `self`, and `self` remains valid until its
762 /// associated scope is exited.
763 ///
764 /// # Panics
765 ///
766 /// Panics if this object is not associate with the given store.
767 ///
768 /// # Example
769 ///
770 /// ```
771 /// # use wasmtime::*;
772 /// # fn _foo() -> Result<()> {
773 /// let mut store = Store::<()>::default();
774 ///
775 /// let y: ManuallyRooted<_> = {
776 /// // Create a nested rooting scope.
777 /// let mut scope = RootScope::new(&mut store);
778 ///
779 /// // `x` is only rooted within this nested scope.
780 /// let x: Rooted<_> = ExternRef::new(&mut scope, "hello!")?;
781 ///
782 /// // Extend `x`'s rooting past its scope's lifetime by converting it
783 /// // to a `ManuallyRooted`.
784 /// x.to_manually_rooted(&mut scope)?
785 /// };
786 ///
787 /// // Now we can still access the reference outside the scope it was
788 /// // originally defined within.
789 /// let data = y.data(&store)?.expect("should have host data");
790 /// let data = data.downcast_ref::<&str>().expect("host data should be str");
791 /// assert_eq!(*data, "hello!");
792 ///
793 /// // But we have to manually unroot `y`.
794 /// y.unroot(&mut store);
795 /// # Ok(())
796 /// # }
797 /// ```
798 pub fn to_manually_rooted(&self, mut store: impl AsContextMut) -> Result<ManuallyRooted<T>> {
799 self._to_manually_rooted(store.as_context_mut().0)
800 }
801
802 pub(crate) fn _to_manually_rooted(&self, store: &mut StoreOpaque) -> Result<ManuallyRooted<T>> {
803 let mut store = AutoAssertNoGc::new(store);
804 let gc_ref = self.try_clone_gc_ref(&mut store)?;
805 Ok(ManuallyRooted::new(&mut store, gc_ref))
806 }
807
808 /// Are these two `Rooted<T>`s the same GC root?
809 ///
810 /// Note that this function can return `false` even when `a` and `b` are
811 /// rooting the same underlying GC object, but the object was rooted
812 /// multiple times (for example in different scopes). Use
813 /// [`Rooted::ref_eq`][crate::Rooted::ref_eq] to test whether these are
814 /// references to the same underlying GC object or not.
815 ///
816 /// # Example
817 ///
818 /// ```
819 /// # use wasmtime::*;
820 /// # fn foo() -> Result<()> {
821 /// let mut store = Store::<()>::default();
822 ///
823 /// let a = ExternRef::new(&mut store, "hello")?;
824 /// let b = a;
825 ///
826 /// // `a` and `b` are the same GC root.
827 /// assert!(Rooted::rooted_eq(a, b));
828 ///
829 /// {
830 /// let mut scope = RootScope::new(&mut store);
831 ///
832 /// // `c` is a different GC root, in a different scope, even though it
833 /// // is rooting the same object.
834 /// let c = a.to_manually_rooted(&mut scope)?.into_rooted(&mut scope);
835 /// assert!(!Rooted::rooted_eq(a, c));
836 /// }
837 ///
838 /// let x = ExternRef::new(&mut store, "goodbye")?;
839 ///
840 /// // `a` and `x` are different GC roots, rooting different objects.
841 /// assert!(!Rooted::rooted_eq(a, x));
842 /// # Ok(())
843 /// # }
844 /// ```
845 pub fn rooted_eq(a: Self, b: Self) -> bool {
846 a.inner == b.inner
847 }
848
849 /// Are these two GC roots referencing the same underlying GC object?
850 ///
851 /// This function will return `true` even when `a` and `b` are different GC
852 /// roots (for example because they were rooted in different scopes) if they
853 /// are rooting the same underlying GC object. To only test whether they are
854 /// the same GC root, and not whether they are rooting the same GC object,
855 /// use [`Rooted::rooted_eq`][crate::Rooted::rooted_eq].
856 ///
857 /// Returns an error if either `a` or `b` has been unrooted, for example
858 /// because the scope it was rooted within has been exited.
859 ///
860 /// Because this method takes any `impl RootedGcRef<T>` arguments, it can be
861 /// used to compare, for example, a `Rooted<T>` and a `ManuallyRooted<T>`.
862 ///
863 /// # Panics
864 ///
865 /// Panics if either `a` or `b` is not associated with the given `store`.
866 ///
867 /// # Example
868 ///
869 /// ```
870 /// # use wasmtime::*;
871 /// # fn foo() -> Result<()> {
872 /// let mut store = Store::<()>::default();
873 ///
874 /// let a = ExternRef::new(&mut store, "hello")?;
875 /// let b = a;
876 ///
877 /// // `a` and `b` are rooting the same object.
878 /// assert!(Rooted::ref_eq(&store, &a, &b)?);
879 ///
880 /// {
881 /// let mut scope = RootScope::new(&mut store);
882 ///
883 /// // `c` is a different GC root, in a different scope, but still
884 /// // rooting the same object.
885 /// let c = a.to_manually_rooted(&mut scope)?.into_rooted(&mut scope);
886 /// assert!(!Rooted::ref_eq(&scope, &a, &c)?);
887 /// }
888 ///
889 /// let x = ExternRef::new(&mut store, "goodbye")?;
890 ///
891 /// // `a` and `x` are rooting different objects.
892 /// assert!(!Rooted::ref_eq(&store, &a, &x)?);
893 ///
894 /// // You can also compare `Rooted<T>`s and `ManuallyRooted<T>`s with this
895 /// // function.
896 /// let d = a.to_manually_rooted(&mut store)?;
897 /// assert!(Rooted::ref_eq(&store, &a, &d)?);
898 ///
899 /// d.unroot(&mut store);
900 /// # Ok(())
901 /// # }
902 /// ```
903 pub fn ref_eq(
904 store: impl AsContext,
905 a: &impl RootedGcRef<T>,
906 b: &impl RootedGcRef<T>,
907 ) -> Result<bool> {
908 let store = store.as_context().0;
909 Self::_ref_eq(store, a, b)
910 }
911
912 pub(crate) fn _ref_eq(
913 store: &StoreOpaque,
914 a: &impl RootedGcRef<T>,
915 b: &impl RootedGcRef<T>,
916 ) -> Result<bool> {
917 let a = a.try_gc_ref(store)?;
918 let b = b.try_gc_ref(store)?;
919 Ok(a == b)
920 }
921
922 /// Hash this root.
923 ///
924 /// Note that, similar to `Rooted::rooted_eq`, this only operates on the
925 /// root and *not* the underlying GC reference. That means that two
926 /// different rootings of the same object will hash to different values
927 /// (modulo hash collisions). If this is undesirable, use the
928 /// [`ref_hash`][crate::Rooted::ref_hash] method instead.
929 pub fn rooted_hash<H>(&self, state: &mut H)
930 where
931 H: Hasher,
932 {
933 self.inner.hash(state);
934 }
935
936 /// Hash the underlying rooted object reference.
937 ///
938 /// Note that, similar to `Rooted::ref_eq`, and operates on the underlying
939 /// rooted GC object reference, not the root. That means that two
940 /// *different* rootings of the same object will hash to the *same*
941 /// value. If this is undesirable, use the
942 /// [`rooted_hash`][crate::Rooted::rooted_hash] method instead.
943 pub fn ref_hash<H>(&self, store: impl AsContext, state: &mut H) -> Result<()>
944 where
945 H: Hasher,
946 {
947 let gc_ref = self.try_gc_ref(store.as_context().0)?;
948 gc_ref.hash(state);
949 Ok(())
950 }
951
952 /// Cast `self` to a `Rooted<U>`.
953 ///
954 /// It is the caller's responsibility to ensure that `self` is actually a
955 /// `U`. Failure to uphold this invariant will be memory safe but will
956 /// result in general incorrectness such as panics and wrong results.
957 pub(crate) fn unchecked_cast<U: GcRef>(self) -> Rooted<U> {
958 Rooted::from_gc_root_index(self.inner)
959 }
960
961 /// Common implementation of the `WasmTy::store` trait method for all
962 /// `Rooted<T>`s.
963 pub(super) fn wasm_ty_store(
964 self,
965 store: &mut AutoAssertNoGc<'_>,
966 ptr: &mut MaybeUninit<ValRaw>,
967 val_raw: impl Fn(u32) -> ValRaw,
968 ) -> Result<()> {
969 let gc_ref = self.inner.try_clone_gc_ref(store)?;
970
971 let raw = match store.optional_gc_store_mut() {
972 Some(s) => s.expose_gc_ref_to_wasm(gc_ref),
973 None => {
974 // NB: do not force the allocation of a GC heap just because the
975 // program is using `i31ref`s.
976 debug_assert!(gc_ref.is_i31());
977 gc_ref.as_raw_non_zero_u32()
978 }
979 };
980
981 ptr.write(val_raw(raw.get()));
982 Ok(())
983 }
984
985 /// Common implementation of the `WasmTy::load` trait method for all
986 /// `Rooted<T>`s.
987 pub(super) fn wasm_ty_load(
988 store: &mut AutoAssertNoGc<'_>,
989 raw_gc_ref: u32,
990 from_cloned_gc_ref: impl Fn(&mut AutoAssertNoGc<'_>, VMGcRef) -> Self,
991 ) -> Self {
992 debug_assert_ne!(raw_gc_ref, 0);
993 let gc_ref = VMGcRef::from_raw_u32(raw_gc_ref).expect("non-null");
994
995 let gc_ref = match store.optional_gc_store_mut() {
996 Some(s) => s.clone_gc_ref(&gc_ref),
997 None => {
998 // NB: do not force the allocation of a GC heap just because the
999 // program is using `i31ref`s.
1000 debug_assert!(gc_ref.is_i31());
1001 gc_ref.unchecked_copy()
1002 }
1003 };
1004
1005 from_cloned_gc_ref(store, gc_ref)
1006 }
1007
1008 /// Common implementation of the `WasmTy::store` trait method for all
1009 /// `Option<Rooted<T>>`s.
1010 pub(super) fn wasm_ty_option_store(
1011 me: Option<Self>,
1012 store: &mut AutoAssertNoGc<'_>,
1013 ptr: &mut MaybeUninit<ValRaw>,
1014 val_raw: impl Fn(u32) -> ValRaw,
1015 ) -> Result<()> {
1016 match me {
1017 Some(me) => me.wasm_ty_store(store, ptr, val_raw),
1018 None => {
1019 ptr.write(val_raw(0));
1020 Ok(())
1021 }
1022 }
1023 }
1024
1025 /// Common implementation of the `WasmTy::load` trait method for all
1026 /// `Option<Rooted<T>>`s.
1027 pub(super) fn wasm_ty_option_load(
1028 store: &mut AutoAssertNoGc<'_>,
1029 raw_gc_ref: u32,
1030 from_cloned_gc_ref: impl Fn(&mut AutoAssertNoGc<'_>, VMGcRef) -> Self,
1031 ) -> Option<Self> {
1032 let gc_ref = VMGcRef::from_raw_u32(raw_gc_ref)?;
1033 let gc_ref = store.clone_gc_ref(&gc_ref);
1034 Some(from_cloned_gc_ref(store, gc_ref))
1035 }
1036}
1037
1038/// Nested rooting scopes.
1039///
1040/// `RootScope` allows the creation or nested rooting scopes for use with
1041/// [`Rooted<T>`][crate::Rooted]. This allows for fine-grained control over how
1042/// long a set of [`Rooted<T>`][crate::Rooted]s are strongly held alive, giving
1043/// gives you the tools necessary to avoid holding onto GC objects longer than
1044/// necessary. `Rooted<T>`s created within a `RootScope` are automatically
1045/// unrooted when the `RootScope` is dropped. For more details on
1046/// [`Rooted<T>`][crate::Rooted] lifetimes and their interaction with rooting
1047/// scopes, see [`Rooted<T>`][crate::Rooted]'s documentation.
1048///
1049/// A `RootScope<C>` wraps a `C: AsContextMut` (that is, anything that
1050/// represents exclusive access to a [`Store`][crate::Store]) and in turn
1051/// implements [`AsContext`][crate::AsContext] and
1052/// [`AsContextMut`][crate::AsContextMut] in terms of its underlying
1053/// `C`. Therefore, `RootScope<C>` can be used anywhere you would use the
1054/// underlying `C`, for example in the [`Global::get`][crate::Global::get]
1055/// method. Any `Rooted<T>`s created by a method that a `RootScope<C>` was
1056/// passed as context to are tied to the `RootScope<C>`'s scope and
1057/// automatically unrooted when the scope is dropped.
1058///
1059/// # Example
1060///
1061/// ```
1062/// # use wasmtime::*;
1063/// # fn _foo() -> Result<()> {
1064/// let mut store = Store::<()>::default();
1065///
1066/// let a: Rooted<_>;
1067/// let b: Rooted<_>;
1068/// let c: Rooted<_>;
1069///
1070/// // Root `a` in the store's scope. It will be rooted for the duration of the
1071/// // store's lifetime.
1072/// a = ExternRef::new(&mut store, 42)?;
1073///
1074/// // `a` is rooted, so we can access its data successfully.
1075/// assert!(a.data(&store).is_ok());
1076///
1077/// {
1078/// let mut scope1 = RootScope::new(&mut store);
1079///
1080/// // Root `b` in `scope1`.
1081/// b = ExternRef::new(&mut scope1, 36)?;
1082///
1083/// // Both `a` and `b` are rooted.
1084/// assert!(a.data(&scope1).is_ok());
1085/// assert!(b.data(&scope1).is_ok());
1086///
1087/// {
1088/// let mut scope2 = RootScope::new(&mut scope1);
1089///
1090/// // Root `c` in `scope2`.
1091/// c = ExternRef::new(&mut scope2, 36)?;
1092///
1093/// // All of `a`, `b`, and `c` are rooted.
1094/// assert!(a.data(&scope2).is_ok());
1095/// assert!(b.data(&scope2).is_ok());
1096/// assert!(c.data(&scope2).is_ok());
1097///
1098/// // Drop `scope2`.
1099/// }
1100///
1101/// // Now `a` and `b` are still rooted, but `c` was unrooted when we dropped
1102/// // `scope2`.
1103/// assert!(a.data(&scope1).is_ok());
1104/// assert!(b.data(&scope1).is_ok());
1105/// assert!(c.data(&scope1).is_err());
1106///
1107/// // Drop `scope1`.
1108/// }
1109///
1110/// // And now only `a` is still rooted. Both `b` and `c` were unrooted when we
1111/// // dropped their respective rooting scopes.
1112/// assert!(a.data(&store).is_ok());
1113/// assert!(b.data(&store).is_err());
1114/// assert!(c.data(&store).is_err());
1115/// # Ok(())
1116/// # }
1117/// ```
1118pub struct RootScope<C>
1119where
1120 C: AsContextMut,
1121{
1122 store: C,
1123 scope: usize,
1124}
1125
1126impl<C> Drop for RootScope<C>
1127where
1128 C: AsContextMut,
1129{
1130 fn drop(&mut self) {
1131 self.store.as_context_mut().0.exit_gc_lifo_scope(self.scope);
1132 }
1133}
1134
1135impl<C> RootScope<C>
1136where
1137 C: AsContextMut,
1138{
1139 // NB: we MUST NOT expose a method like
1140 //
1141 // pub fn store(&mut self) -> &mut Store { ... }
1142 //
1143 // because callers could do treacherous things like
1144 //
1145 // let scope1 = RootScope::new(&mut store1);
1146 // let scope2 = RootScope::new(&mut store2);
1147 // std::mem::swap(scope1.store(), scope2.store());
1148 //
1149 // and then we would start truncate the store's GC root set's LIFO roots to
1150 // the wrong lengths.
1151 //
1152 // Instead, we just implement `AsContext[Mut]` for `RootScope`.
1153
1154 /// Construct a new scope for rooting GC objects.
1155 ///
1156 /// # Example
1157 ///
1158 /// ```
1159 /// # use wasmtime::*;
1160 /// let mut store = Store::<()>::default();
1161 ///
1162 /// {
1163 /// let mut scope = RootScope::new(&mut store);
1164 ///
1165 /// // Temporarily root GC objects in this nested rooting scope...
1166 /// }
1167 /// ```
1168 pub fn new(store: C) -> Self {
1169 let scope = store.as_context().0.gc_roots().enter_lifo_scope();
1170 RootScope { store, scope }
1171 }
1172
1173 fn gc_roots(&mut self) -> &mut RootSet {
1174 self.store.as_context_mut().0.gc_roots_mut()
1175 }
1176
1177 fn lifo_roots(&mut self) -> &mut Vec<LifoRoot> {
1178 &mut self.gc_roots().lifo_roots
1179 }
1180
1181 /// Reserve enough capacity for `additional` GC roots in this scope.
1182 ///
1183 /// # Example
1184 ///
1185 /// ```
1186 /// # use wasmtime::*;
1187 /// let mut store = Store::<()>::default();
1188 ///
1189 /// {
1190 /// let mut scope = RootScope::new(&mut store);
1191 ///
1192 /// // Ensure we have enough storage pre-allocated to root five GC
1193 /// // references inside this scope without any underlying reallocation.
1194 /// scope.reserve(5);
1195 ///
1196 /// // ...
1197 /// }
1198 /// ```
1199 pub fn reserve(&mut self, additional: usize) {
1200 self.lifo_roots().reserve(additional);
1201 }
1202}
1203
1204impl<T> AsContext for RootScope<T>
1205where
1206 T: AsContextMut,
1207{
1208 type Data = T::Data;
1209
1210 fn as_context(&self) -> crate::StoreContext<'_, Self::Data> {
1211 self.store.as_context()
1212 }
1213}
1214
1215impl<T> AsContextMut for RootScope<T>
1216where
1217 T: AsContextMut,
1218{
1219 fn as_context_mut(&mut self) -> crate::StoreContextMut<'_, Self::Data> {
1220 self.store.as_context_mut()
1221 }
1222}
1223
1224/// Internal version of `RootScope` that only wraps a `&mut StoreOpaque` rather
1225/// than a whole `impl AsContextMut<Data = T>`.
1226pub(crate) struct OpaqueRootScope<S>
1227where
1228 S: AsStoreOpaque,
1229{
1230 store: S,
1231 scope: usize,
1232}
1233
1234impl<S> Drop for OpaqueRootScope<S>
1235where
1236 S: AsStoreOpaque,
1237{
1238 fn drop(&mut self) {
1239 self.store.as_store_opaque().exit_gc_lifo_scope(self.scope);
1240 }
1241}
1242
1243impl<S> Deref for OpaqueRootScope<S>
1244where
1245 S: AsStoreOpaque,
1246{
1247 type Target = S;
1248
1249 fn deref(&self) -> &Self::Target {
1250 &self.store
1251 }
1252}
1253
1254// XXX: Don't use this `DerefMut` implementation to `mem::{swap,replace}` or
1255// etc... the underlying `StoreOpaque` in a `OpaqueRootScope`! That will result
1256// in truncating the store's GC root set's LIFO roots to the wrong length.
1257//
1258// We don't implement `DerefMut` for `RootScope` for exactly this reason, but
1259// allow it for `OpaqueRootScope` because it is only Wasmtime-internal and not
1260// publicly exported.
1261impl<S> DerefMut for OpaqueRootScope<S>
1262where
1263 S: AsStoreOpaque,
1264{
1265 fn deref_mut(&mut self) -> &mut Self::Target {
1266 &mut self.store
1267 }
1268}
1269
1270impl<S> OpaqueRootScope<S>
1271where
1272 S: AsStoreOpaque,
1273{
1274 pub(crate) fn new(mut store: S) -> Self {
1275 let scope = store.as_store_opaque().gc_roots().enter_lifo_scope();
1276 OpaqueRootScope { store, scope }
1277 }
1278}
1279
1280/// A rooted reference to a garbage-collected `T` with arbitrary lifetime.
1281///
1282/// A `ManuallyRooted<T>` is a strong handle to a garbage-collected `T`,
1283/// preventing its referent (and anything else transitively referenced) from
1284/// being collected by the GC until [`unroot`][crate::ManuallyRooted::unroot] is
1285/// explicitly called.
1286///
1287/// The primary way to create a `ManuallyRooted<T>` is to promote a temporary
1288/// `Rooted<T>` into a `ManuallyRooted<T>` via its
1289/// [`to_manually_rooted`][crate::Rooted::to_manually_rooted] method.
1290///
1291/// `ManuallyRooted<T>` dereferences to its underlying `T`, allowing you to call
1292/// `T`'s methods.
1293///
1294/// # Example
1295///
1296/// ```
1297/// # use wasmtime::*;
1298/// # fn _foo() -> Result<()> {
1299/// let mut store = Store::<Option<ManuallyRooted<ExternRef>>>::default();
1300///
1301/// // Create our `ManuallyRooted` in a nested scope to avoid rooting it for
1302/// // the duration of the store's lifetime.
1303/// let x = {
1304/// let mut scope = RootScope::new(&mut store);
1305/// let x = ExternRef::new(&mut scope, 1234)?;
1306/// x.to_manually_rooted(&mut scope)?
1307/// };
1308///
1309/// // Place `x` into our store.
1310/// *store.data_mut() = Some(x);
1311///
1312/// // Do a bunch stuff that may or may not access, replace, or take `x`...
1313///
1314/// // At any time, in any arbitrary scope, we can remove `x` from the store
1315/// // and unroot it:
1316/// if let Some(x) = store.data_mut().take() {
1317/// x.unroot(&mut store);
1318/// }
1319/// # Ok(())
1320/// # }
1321/// ```
1322///
1323/// # Differences Between `ManuallyRooted<T>` and `Rooted<T>`
1324///
1325/// While `ManuallyRooted<T>` can have arbitrary lifetimes, it requires manual
1326/// unrooting. This is in contrast to [`Rooted<T>`][crate::Rooted] which is
1327/// restricted to strictly last-in-first-out (LIFO, aka stack order) lifetimes,
1328/// but comes with automatic unrooting.
1329///
1330/// | Type | Supported Lifetimes | Unrooting |
1331/// |----------------------------------------------|-----------------------------|-----------|
1332/// | [`Rooted<T>`][crate::Rooted] | Strictly LIFO / stack order | Automatic |
1333/// | [`ManuallyRooted<T>`][crate::ManuallyRooted] | Arbitrary | Manual |
1334///
1335/// `Rooted<T>` should suffice for most use cases, and provides better
1336/// ergonomics, but `ManuallyRooted<T>` exists as a fully-general escape hatch.
1337///
1338/// # Manual Unrooting
1339///
1340/// Failure to explicitly call [`unroot`][crate::ManuallyRooted::unroot] (or
1341/// another method that consumes `self` and unroots the reference, such as
1342/// [`into_rooted`][crate::ManuallyRooted::into_rooted]) will leak the
1343/// underlying GC object, preventing it from being garbage collected until its
1344/// owning [`Store`][crate::Store] is dropped. That means all of the following
1345/// will result in permanently rooting the underlying GC object:
1346///
1347/// * Implicitly dropping a `ManuallyRooted<T>`:
1348///
1349/// ```no_run
1350/// # use wasmtime::*;
1351/// # let get_manually_rooted = || -> ManuallyRooted<ExternRef> { todo!() };
1352/// {
1353/// let perma_root: ManuallyRooted<_> = get_manually_rooted();
1354///
1355/// // `perma_root` is implicitly dropped at the end of its scope,
1356/// // permanently rooting/leaking its referent.
1357/// }
1358/// ```
1359///
1360/// * Explicitly dropping a `ManuallyRooted<T>`: `drop(my_manually_rooted)`.
1361///
1362/// * Forgetting a `ManuallyRooted<T>`: `std::mem::forget(my_manually_rooted)`.
1363///
1364/// * Inserting a `ManuallyRooted<T>` into a `std::sync::Arc` or `std::rc::Rc`
1365/// cycle.
1366///
1367/// * Etc...
1368///
1369/// Wasmtime does *not* assert that a `ManuallyRooted<T>` is unrooted on `Drop`,
1370/// or otherwise raise a panic, log a warning, or etc... on failure to manually
1371/// unroot. Sometimes leaking is intentional and desirable, particularly when
1372/// dealing with short-lived [`Store`][crate::Store]s where unrooting would just
1373/// be busy work since the whole store is about to be dropped.
1374#[repr(transparent)] // NB: the C API relies on this
1375pub struct ManuallyRooted<T>
1376where
1377 T: GcRef,
1378{
1379 inner: GcRootIndex,
1380 _phantom: marker::PhantomData<T>,
1381}
1382
1383const _: () = {
1384 use crate::{AnyRef, ExternRef};
1385
1386 // NB: these match the C API which should also be updated if this changes
1387 assert!(mem::size_of::<ManuallyRooted<AnyRef>>() == 16);
1388 assert!(mem::align_of::<ManuallyRooted<AnyRef>>() == mem::align_of::<u64>());
1389 assert!(mem::size_of::<ManuallyRooted<ExternRef>>() == 16);
1390 assert!(mem::align_of::<ManuallyRooted<ExternRef>>() == mem::align_of::<u64>());
1391};
1392
1393impl<T: GcRef> Debug for ManuallyRooted<T> {
1394 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1395 let name = format!("ManuallyRooted<{}>", any::type_name::<T>());
1396 f.debug_struct(&name).field("inner", &self.inner).finish()
1397 }
1398}
1399
1400impl<T: GcRef> Deref for ManuallyRooted<T> {
1401 type Target = T;
1402
1403 fn deref(&self) -> &Self::Target {
1404 T::transmute_ref(&self.inner)
1405 }
1406}
1407
1408impl<T> ManuallyRooted<T>
1409where
1410 T: GcRef,
1411{
1412 /// Construct a new manually-rooted GC root.
1413 ///
1414 /// `gc_ref` should belong to `store`'s heap; failure to uphold this is
1415 /// memory safe but will result in general failures down the line such as
1416 /// panics or incorrect results.
1417 ///
1418 /// `gc_ref` should be a GC reference pointing to an instance of the GC type
1419 /// that `T` represents. Failure to uphold this invariant is memory safe but
1420 /// will result in general incorrectness such as panics and wrong results.
1421 pub(crate) fn new(store: &mut AutoAssertNoGc<'_>, gc_ref: VMGcRef) -> Self {
1422 let id = store.gc_roots_mut().manually_rooted.alloc(gc_ref);
1423 ManuallyRooted {
1424 inner: GcRootIndex {
1425 store_id: store.id(),
1426 generation: 0,
1427 index: PackedIndex::new_manual(id),
1428 },
1429 _phantom: marker::PhantomData,
1430 }
1431 }
1432
1433 #[inline]
1434 pub(crate) fn comes_from_same_store(&self, store: &StoreOpaque) -> bool {
1435 debug_assert!(self.inner.index.is_manual());
1436 self.inner.comes_from_same_store(store)
1437 }
1438
1439 /// Clone this `ManuallyRooted`.
1440 ///
1441 /// Does not consume or unroot `self`: both `self` and the new
1442 /// `ManuallyRooted` return value will need to be manually unrooted.
1443 ///
1444 /// # Panics
1445 ///
1446 /// Panics if `self` is not associated with the given `store`.
1447 ///
1448 /// # Example
1449 ///
1450 /// ```
1451 /// # use wasmtime::*;
1452 /// # fn _foo() -> Result<()> {
1453 /// let mut store = Store::<Vec<ManuallyRooted<ExternRef>>>::default();
1454 ///
1455 /// // Create our `ManuallyRooted` in a nested scope to avoid rooting it for
1456 /// // the duration of the store's lifetime.
1457 /// let x = {
1458 /// let mut scope = RootScope::new(&mut store);
1459 /// let x = ExternRef::new(&mut scope, 1234)?;
1460 /// x.to_manually_rooted(&mut scope)?
1461 /// };
1462 ///
1463 /// // Push five clones of `x` into our store.
1464 /// for _ in 0..5 {
1465 /// let x_clone = x.clone(&mut store);
1466 /// store.data_mut().push(x_clone);
1467 /// }
1468 /// # Ok(())
1469 /// # }
1470 /// ```
1471 pub fn clone(&self, mut store: impl AsContextMut) -> Self {
1472 self._clone(store.as_context_mut().0)
1473 }
1474
1475 pub(crate) fn _clone(&self, store: &mut StoreOpaque) -> Self {
1476 let mut store = AutoAssertNoGc::new(store);
1477 let gc_ref = self
1478 .clone_gc_ref(&mut store)
1479 .expect("ManuallyRooted always has a gc ref");
1480 Self::new(&mut store, gc_ref)
1481 }
1482
1483 /// Unroot this GC object.
1484 ///
1485 /// Failure to call this method will result in the GC object, and anything
1486 /// it transitively references, being kept alive (aka "leaking") for the
1487 /// entirety of the store's lifetime.
1488 ///
1489 /// See the type-level docs for example usage.
1490 pub fn unroot(self, mut store: impl AsContextMut) {
1491 self._unroot(store.as_context_mut().0)
1492 }
1493
1494 pub(crate) fn _unroot(self, store: &mut StoreOpaque) {
1495 assert!(
1496 self.comes_from_same_store(store),
1497 "object used with wrong store"
1498 );
1499
1500 let mut store = AutoAssertNoGc::new(store);
1501 let id = self.inner.index.as_manual().unwrap();
1502 let roots = store.gc_roots_mut();
1503 let gc_ref = roots.manually_rooted.dealloc(id);
1504 store.unwrap_gc_store_mut().drop_gc_ref(gc_ref);
1505 }
1506
1507 /// Clone this `ManuallyRooted<T>` into a `Rooted<T>`.
1508 ///
1509 /// This operation does not consume or unroot this `ManuallyRooted<T>`.
1510 ///
1511 /// The underlying GC object is re-rooted in the given context's scope. The
1512 /// resulting `Rooted<T>` is only valid during the given context's
1513 /// scope. See the [`Rooted<T>`][crate::Rooted] documentation for more
1514 /// details on rooting scopes.
1515 ///
1516 /// This operation does not consume or unroot this `ManuallyRooted<T>`.
1517 ///
1518 /// # Panics
1519 ///
1520 /// Panics if this object is not associated with the given context's store.
1521 ///
1522 /// # Example
1523 ///
1524 /// ```
1525 /// # use wasmtime::*;
1526 /// # fn _foo() -> Result<()> {
1527 /// let mut store = Store::<()>::default();
1528 ///
1529 /// let root1: Rooted<_>;
1530 ///
1531 /// let manual = {
1532 /// let mut scope = RootScope::new(&mut store);
1533 /// root1 = ExternRef::new(&mut scope, 1234)?;
1534 /// root1.to_manually_rooted(&mut scope)?
1535 /// };
1536 ///
1537 /// // `root1` is no longer accessible because it was unrooted when `scope`
1538 /// // was dropped.
1539 /// assert!(root1.data(&store).is_err());
1540 ///
1541 /// // But we can re-root `manual` into this scope.
1542 /// let root2 = manual.to_rooted(&mut store);
1543 /// assert!(root2.data(&store).is_ok());
1544 ///
1545 /// // And we also still have access to `manual` and we still have to
1546 /// // manually unroot it.
1547 /// assert!(manual.data(&store).is_ok());
1548 /// manual.unroot(&mut store);
1549 /// # Ok(())
1550 /// # }
1551 /// ```
1552 pub fn to_rooted(&self, mut context: impl AsContextMut) -> Rooted<T> {
1553 self._to_rooted(context.as_context_mut().0)
1554 }
1555
1556 pub(crate) fn _to_rooted(&self, store: &mut StoreOpaque) -> Rooted<T> {
1557 assert!(
1558 self.comes_from_same_store(store),
1559 "object used with wrong store"
1560 );
1561 let mut store = AutoAssertNoGc::new(store);
1562 let gc_ref = self.clone_gc_ref(&mut store).unwrap();
1563 Rooted::new(&mut store, gc_ref)
1564 }
1565
1566 /// Convert this `ManuallyRooted<T>` into a `Rooted<T>`.
1567 ///
1568 /// The underlying GC object is re-rooted in the given context's scope. The
1569 /// resulting `Rooted<T>` is only valid during the given context's
1570 /// scope. See the [`Rooted<T>`][crate::Rooted] documentation for more
1571 /// details on rooting scopes.
1572 ///
1573 /// This operation consumes and unroots this `ManuallyRooted<T>`.
1574 ///
1575 /// # Panics
1576 ///
1577 /// Panics if this object is not associate with the given context's store.
1578 ///
1579 /// # Example
1580 ///
1581 /// ```
1582 /// # use wasmtime::*;
1583 /// # fn _foo() -> Result<()> {
1584 /// let mut store = Store::<()>::default();
1585 ///
1586 /// let root1: Rooted<_>;
1587 ///
1588 /// let manual = {
1589 /// let mut scope = RootScope::new(&mut store);
1590 /// root1 = ExternRef::new(&mut scope, 1234)?;
1591 /// root1.to_manually_rooted(&mut scope)?
1592 /// };
1593 ///
1594 /// // `root1` is no longer accessible because it was unrooted when `scope`
1595 /// // was dropped.
1596 /// assert!(root1.data(&store).is_err());
1597 ///
1598 /// // But we can re-root `manual` into this scope.
1599 /// let root2 = manual.into_rooted(&mut store);
1600 /// assert!(root2.data(&store).is_ok());
1601 ///
1602 /// // `manual` was consumed by the `into_rooted` call, and we no longer
1603 /// // have access to it, nor need to manually unroot it.
1604 /// # Ok(())
1605 /// # }
1606 /// ```
1607 pub fn into_rooted(self, mut context: impl AsContextMut) -> Rooted<T> {
1608 self._into_rooted(context.as_context_mut().0)
1609 }
1610
1611 pub(crate) fn _into_rooted(self, store: &mut StoreOpaque) -> Rooted<T> {
1612 assert!(
1613 self.comes_from_same_store(store),
1614 "object used with wrong store"
1615 );
1616 let rooted = self._to_rooted(store);
1617 self._unroot(store);
1618 rooted
1619 }
1620
1621 /// Are these two GC roots referencing the same underlying GC object?
1622 ///
1623 /// This function will return `true` even when `a` and `b` are different GC
1624 /// roots (for example because they were rooted in different scopes) if they
1625 /// are rooting the same underlying GC object.
1626 ///
1627 /// Because this method takes any `impl RootedGcRef<T>` arguments, it can be
1628 /// used to compare, for example, a `Rooted<T>` and a `ManuallyRooted<T>`.
1629 ///
1630 /// # Panics
1631 ///
1632 /// Panics if either `a` or `b` is not associated with the given `store`.
1633 ///
1634 /// # Example
1635 ///
1636 /// ```
1637 /// # use wasmtime::*;
1638 /// # fn foo() -> Result<()> {
1639 /// let mut store = Store::<()>::default();
1640 ///
1641 /// let a;
1642 /// let b;
1643 /// let x;
1644 ///
1645 /// {
1646 /// let mut scope = RootScope::new(&mut store);
1647 ///
1648 /// a = ExternRef::new(&mut scope, "hello")?.to_manually_rooted(&mut scope)?;
1649 /// b = a.clone(&mut scope);
1650 ///
1651 /// // `a` and `b` are rooting the same object.
1652 /// assert!(ManuallyRooted::ref_eq(&scope, &a, &b)?);
1653 ///
1654 /// // `c` is a different GC root, is in a different scope, and is a
1655 /// // `Rooted<T>` instead of a `ManuallyRooted<T>`, but is still rooting
1656 /// // the same object.
1657 /// let c = a.to_rooted(&mut scope);
1658 /// assert!(ManuallyRooted::ref_eq(&scope, &a, &c)?);
1659 ///
1660 /// x = ExternRef::new(&mut scope, "goodbye")?.to_manually_rooted(&mut scope)?;
1661 ///
1662 /// // `a` and `x` are rooting different objects.
1663 /// assert!(!ManuallyRooted::ref_eq(&scope, &a, &x)?);
1664 /// }
1665 ///
1666 /// // Unroot our manually-rooted GC references.
1667 /// a.unroot(&mut store);
1668 /// b.unroot(&mut store);
1669 /// x.unroot(&mut store);
1670 /// # Ok(())
1671 /// # }
1672 /// ```
1673 pub fn ref_eq(
1674 store: impl AsContext,
1675 a: &impl RootedGcRef<T>,
1676 b: &impl RootedGcRef<T>,
1677 ) -> Result<bool> {
1678 Rooted::ref_eq(store, a, b)
1679 }
1680
1681 /// Hash this root.
1682 ///
1683 /// Note that, similar to `Rooted::rooted_eq`, this only operates on the
1684 /// root and *not* the underlying GC reference. That means that two
1685 /// different rootings of the same object will hash to different values
1686 /// (modulo hash collisions). If this is undesirable, use the
1687 /// [`ref_hash`][crate::ManuallyRooted::ref_hash] method instead.
1688 pub fn rooted_hash<H>(&self, state: &mut H)
1689 where
1690 H: Hasher,
1691 {
1692 self.inner.hash(state);
1693 }
1694
1695 /// Hash the underlying rooted object reference.
1696 ///
1697 /// Note that, similar to `Rooted::ref_eq`, and operates on the underlying
1698 /// rooted GC object reference, not the root. That means that two
1699 /// *different* rootings of the same object will hash to the *same*
1700 /// value. If this is undesirable, use the
1701 /// [`rooted_hash`][crate::Rooted::rooted_hash] method instead.
1702 pub fn ref_hash<H>(&self, store: impl AsContext, state: &mut H)
1703 where
1704 H: Hasher,
1705 {
1706 let gc_ref = self
1707 .get_gc_ref(store.as_context().0)
1708 .expect("ManuallyRooted's get_gc_ref is infallible");
1709 gc_ref.hash(state);
1710 }
1711
1712 #[doc(hidden)]
1713 pub fn into_parts_for_c_api(self) -> (NonZeroU64, u32, u32) {
1714 (
1715 self.inner.store_id.as_raw(),
1716 self.inner.generation,
1717 self.inner.index.0,
1718 )
1719 }
1720
1721 #[doc(hidden)]
1722 pub unsafe fn from_raw_parts_for_c_api(a: NonZeroU64, b: u32, c: u32) -> ManuallyRooted<T> {
1723 ManuallyRooted {
1724 inner: GcRootIndex {
1725 store_id: StoreId::from_raw(a),
1726 generation: b,
1727 index: PackedIndex(c),
1728 },
1729 _phantom: marker::PhantomData,
1730 }
1731 }
1732
1733 /// Cast `self` to a `ManuallyRooted<U>`.
1734 ///
1735 /// It is the caller's responsibility to ensure that `self` is actually a
1736 /// `U`. Failure to uphold this invariant will be memory safe but will
1737 /// result in general incorrectness such as panics and wrong results.
1738 pub(crate) fn unchecked_cast<U: GcRef>(self) -> ManuallyRooted<U> {
1739 let u = ManuallyRooted {
1740 inner: self.inner,
1741 _phantom: core::marker::PhantomData,
1742 };
1743 core::mem::forget(self);
1744 u
1745 }
1746
1747 /// Common implementation of the `WasmTy::store` trait method for all
1748 /// `ManuallyRooted<T>`s.
1749 pub(super) fn wasm_ty_store(
1750 self,
1751 store: &mut AutoAssertNoGc<'_>,
1752 ptr: &mut MaybeUninit<ValRaw>,
1753 val_raw: impl Fn(u32) -> ValRaw,
1754 ) -> Result<()> {
1755 let gc_ref = self.try_clone_gc_ref(store)?;
1756 let raw = store.require_gc_store_mut()?.expose_gc_ref_to_wasm(gc_ref);
1757 ptr.write(val_raw(raw.get()));
1758 Ok(())
1759 }
1760
1761 /// Common implementation of the `WasmTy::load` trait method for all
1762 /// `ManuallyRooted<T>`s.
1763 pub(super) fn wasm_ty_load(
1764 store: &mut AutoAssertNoGc<'_>,
1765 raw_gc_ref: u32,
1766 from_cloned_gc_ref: impl Fn(&mut AutoAssertNoGc<'_>, VMGcRef) -> Rooted<T>,
1767 ) -> Self {
1768 debug_assert_ne!(raw_gc_ref, 0);
1769 let gc_ref = VMGcRef::from_raw_u32(raw_gc_ref).expect("non-null");
1770 let gc_ref = store.clone_gc_ref(&gc_ref);
1771 RootSet::with_lifo_scope(store, |store| {
1772 let rooted = from_cloned_gc_ref(store, gc_ref);
1773 rooted
1774 ._to_manually_rooted(store)
1775 .expect("rooted is in scope")
1776 })
1777 }
1778
1779 /// Common implementation of the `WasmTy::store` trait method for all
1780 /// `Option<ManuallyRooted<T>>`s.
1781 pub(super) fn wasm_ty_option_store(
1782 me: Option<Self>,
1783 store: &mut AutoAssertNoGc<'_>,
1784 ptr: &mut MaybeUninit<ValRaw>,
1785 val_raw: impl Fn(u32) -> ValRaw,
1786 ) -> Result<()> {
1787 match me {
1788 Some(me) => me.wasm_ty_store(store, ptr, val_raw),
1789 None => {
1790 ptr.write(val_raw(0));
1791 Ok(())
1792 }
1793 }
1794 }
1795
1796 /// Common implementation of the `WasmTy::load` trait method for all
1797 /// `Option<ManuallyRooted<T>>`s.
1798 pub(super) fn wasm_ty_option_load(
1799 store: &mut AutoAssertNoGc<'_>,
1800 raw_gc_ref: u32,
1801 from_cloned_gc_ref: impl Fn(&mut AutoAssertNoGc<'_>, VMGcRef) -> Rooted<T>,
1802 ) -> Option<Self> {
1803 let gc_ref = VMGcRef::from_raw_u32(raw_gc_ref)?;
1804 let gc_ref = store.clone_gc_ref(&gc_ref);
1805 RootSet::with_lifo_scope(store, |store| {
1806 let rooted = from_cloned_gc_ref(store, gc_ref);
1807 Some(
1808 rooted
1809 ._to_manually_rooted(store)
1810 .expect("rooted is in scope"),
1811 )
1812 })
1813 }
1814}
1815
1816impl<T: GcRef> RootedGcRefImpl<T> for ManuallyRooted<T> {
1817 fn get_gc_ref<'a>(&self, store: &'a StoreOpaque) -> Option<&'a VMGcRef> {
1818 assert!(
1819 self.comes_from_same_store(store),
1820 "object used with wrong store"
1821 );
1822
1823 let id = self.inner.index.as_manual().unwrap();
1824 store.gc_roots().manually_rooted.get(id)
1825 }
1826}
1827
1828#[cfg(test)]
1829mod tests {
1830 use crate::ExternRef;
1831
1832 use super::*;
1833
1834 #[test]
1835 fn sizes() {
1836 // Try to keep tabs on the size of these things. Don't want them growing
1837 // unintentionally.
1838 assert_eq!(std::mem::size_of::<Rooted<ExternRef>>(), 16);
1839 assert_eq!(std::mem::size_of::<ManuallyRooted<ExternRef>>(), 16);
1840 }
1841}