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feat(integration,event_loop): add wasm / wasm-gc scheduler-only run_async_main shim #389

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feat(integration,event_loop): add wasm / wasm-gc scheduler-only run_async_main shim #389

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6c5519e
feat(integration,event_loop): add wasm / wasm-gc scheduler-only run_a…
mizchi May 23, 2026
4c1b564
fix(event_loop): wasm reschedule must gate on has_immediately_ready_t…
mizchi May 23, 2026
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24 changes: 24 additions & 0 deletions src/integration.mbt
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Original file line number Diff line number Diff line change
Expand Up @@ -41,3 +41,27 @@ pub fn run_async_main(main : async () -> Unit) -> Unit {
let _ = @coroutine.spawn(main)
@event_loop.reschedule()
}

///|
/// Scheduler-only entry for wasm / wasm-gc. The async library does not
/// provide IO on these targets (`sleep`, `Timer`, sockets, fs, etc. all
/// `abort` via the existing `unimplemented.mbt` stubs), but pure
/// coroutine code (`pause`, `with_task_group`, `aqueue`, `semaphore`,
/// `cond_var`) does not depend on the event loop and works fine — this
/// shim just drains the ready queue.
#coverage.skip
#cfg(target="wasm")
#doc(hidden)
pub fn run_async_main(main : async () -> Unit) -> Unit {
let _ = @coroutine.spawn(main)
@event_loop.reschedule()
}

///|
#coverage.skip
#cfg(target="wasm-gc")
#doc(hidden)
pub fn run_async_main(main : async () -> Unit) -> Unit {
let _ = @coroutine.spawn(main)
@event_loop.reschedule()
}
9 changes: 9 additions & 0 deletions src/internal/event_loop/unimplemented.mbt
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Expand Up @@ -38,3 +38,12 @@ pub fn Timer::cancel(timer : Timer) -> Unit {
ignore(timer)
abort("`Timer::cancel()` is unimplemented on WASM backend")
}

///|
/// WASM stub for the scheduler-only `reschedule` entry point. Mirrors
/// the JS shim — no IO, just drains ready coroutines until none remain.
pub fn reschedule() -> Unit {
while !@coroutine.no_more_work() {
@coroutine.reschedule()

@chatgpt-codex-connector chatgpt-codex-connector Bot May 23, 2026

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P1 Badge Avoid spinning when no runnable coroutine exists

reschedule() loops until @coroutine.no_more_work() is true, but no_more_work() also depends on the internal blocking counter. If a coroutine calls @coroutine.suspend() and no coroutine is ready (for example, waiting for an external callback or a deadlocked wait), this condition stays false while the ready queue is empty, so this loop becomes a tight CPU spin that never yields back to the host. On wasm/wasm-gc this can deadlock embedding scenarios the shim is meant to support, because the host event loop cannot run while this function is busy-looping.

Useful? React with 👍 / 👎.

@mizchi mizchi May 23, 2026

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Good catch, fixed in 4c1b564. Switched the guard from !@coroutine.no_more_work() to @coroutine.has_immediately_ready_task() (= !scheduler.run_later.is_empty()). Now the loop drains only immediately-ready work; if all ready coroutines are processed and only blocked ones remain (e.g., suspended on an external host callback), reschedule() returns and yields back to the host. The host can re-enter via run_async_main when new work becomes available, mirroring the spirit of the JS shim (which delegates continuation to set_timeout(0, reschedule)).

}
}

@Guest0x0 Guest0x0 May 29, 2026

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inline this loop into call sites

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