Stream: git-wasmtime

Topic: wasmtime / PR #11454 Relax the `Send` bound on fiber cre...

Wasmtime GitHub notifications bot (Aug 18 2025 at 20:19):

alexcrichton opened PR #11454 from alexcrichton:remove-send-bound-from-async-stuff to bytecodealliance:main:

This commit removes the need for Send for all usages of fiber-related
bits. The goal is that the future returned from async functions is
Send if all the inputs are Send, but there's no actual need to
require that in the function signatures themselves. This property was
identified in upcoming work to make more internals of Wasmtime async
where async functions are going to be used to implement the
synchronous path through Wasmtime where Send isn't a bound today, nor
ideally do we want to have it there.

The way this commit works is:

• First make_fiber now has a Send bound which makes it sound. Before
  https://github.com/bytecodealliance/wasmtime/pull/11444 it took &mut dyn VMStore which is never Send and the
  refactoring there missed adding this bound. This change required a few
  Send bounds in concurrent.rs where it's expected to have no
  impact as everything there is basically already Send.

• Next make_fiber_unchecked is added which is the same as make_fiber
  except without a Send bound. The on_fiber function is then updated
  to use this *_unchecked variant. This means that fibers can now be
  used with non-Send stores. Crucially though the structure of
  internals in play means that future produced is still only Send if
  T: Send meaning that there is no loss in safety.

• Next some Send bounds were dropped throughout async functions in
  Wasmtime as a proof-of-concept to require these changes to on_fiber.
  This means that these entrypoints into Wasmtime no longer require
  Send, but still naturally require Send if the result future is
  sent to various threads.

• Finally a new doctest is added to Instance::new_async with a
  compile_fail example to ensure that this fails. This is
  unfortunately a very brittle test because all we can assert is that
  compilation failed, not why compilation failed. That means that this
  is likely to regress over time, but it's the best we can do at this
  time.

Note that the goal here is NOT to remove Send bounds throughout
Wasmtime. Many of them are still required, for example all the ones
related to Linker. The realization is that we can remove some of the
"edge" bounds on entrypoints where the resulting future can be
conditionally Send depending on T meaning we don't actually have to
write down any bounds ourselves.

This refactoring will enable future refactorings to have sync-and-async
functions use the same internals and neither entrypoint needs to have a
Send bound.

Wasmtime GitHub notifications bot (Aug 18 2025 at 20:19):

alexcrichton requested pchickey for a review on PR #11454.

Wasmtime GitHub notifications bot (Aug 18 2025 at 20:19):

alexcrichton requested wasmtime-core-reviewers for a review on PR #11454.

Wasmtime GitHub notifications bot (Aug 18 2025 at 20:20):

alexcrichton commented on PR #11454:

Procedurally this is built on https://github.com/bytecodealliance/wasmtime/pull/11444 and https://github.com/bytecodealliance/wasmtime/pull/11453. The goal is to work towards https://github.com/bytecodealliance/wasmtime/pull/11430

Wasmtime GitHub notifications bot (Aug 18 2025 at 20:20):

alexcrichton updated PR #11454.

Wasmtime GitHub notifications bot (Aug 18 2025 at 21:19):

fitzgen requested fitzgen for a review on PR #11454.

Wasmtime GitHub notifications bot (Aug 18 2025 at 21:19):

fitzgen commented on PR #11454:

I can review this once it is rebased, now that the things it was based on top of are approved (and presumably will land shortly)

Wasmtime GitHub notifications bot (Aug 18 2025 at 22:46):

alexcrichton updated PR #11454.

Wasmtime GitHub notifications bot (Aug 18 2025 at 22:47):

alexcrichton commented on PR #11454:

I've rebased on the merge commit of https://github.com/bytecodealliance/wasmtime/pull/11444 since I need to head out soon, but if/when that PR merges github won't actually remove the commit from this PR, I think I'll have to open/close it or something like that (it's weird)

Wasmtime GitHub notifications bot (Aug 18 2025 at 22:52):

fitzgen submitted PR review.

Wasmtime GitHub notifications bot (Aug 18 2025 at 22:52):

fitzgen created PR review comment:

Maybe add a comment here that Rc is not Send, just to make it a little easier to follow the example. Alternatively, below say "Store<Rc<()>> is not Send because Rc<()> is not Send" or something.

Wasmtime GitHub notifications bot (Aug 18 2025 at 22:52):

fitzgen created PR review comment:

?

Wasmtime GitHub notifications bot (Aug 18 2025 at 23:21):

alexcrichton submitted PR review.

Wasmtime GitHub notifications bot (Aug 18 2025 at 23:21):

alexcrichton created PR review comment:

what

(Forgot to delete after testing)

Wasmtime GitHub notifications bot (Aug 19 2025 at 02:19):

github-actions[bot] commented on PR #11454:

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Wasmtime GitHub notifications bot (Aug 19 2025 at 05:08):

alexcrichton updated PR #11454.

Wasmtime GitHub notifications bot (Aug 19 2025 at 05:09):

alexcrichton updated PR #11454.

Wasmtime GitHub notifications bot (Aug 19 2025 at 05:09):

alexcrichton has enabled auto merge for PR #11454.

Wasmtime GitHub notifications bot (Aug 19 2025 at 05:43):

alexcrichton merged PR #11454.

Last updated: Dec 06 2025 at 06:05 UTC