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JavaScriptEventLoop: release the isSpinning latch before running jobs #809

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JavaScriptEventLoop: release the isSpinning latch before running jobs #809

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605f78e
JavaScriptEventLoop: release the isSpinning latch with defer
mansbernhardt Aug 25, 2026
8c6ab0b
Release the isSpinning latch before running jobs, not after
mansbernhardt Aug 29, 2026
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15 changes: 13 additions & 2 deletions Sources/JavaScriptEventLoop/JobQueue.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,19 @@ extension JavaScriptEventLoop {

func runAllJobs() {
assert(queueState.isSpinning)
// Release the latch BEFORE running any job, so `isSpinning` means "a
// drain microtask is pending" rather than "a drain is in progress".
//
// `runSynchronously` can unwind without running Swift cleanup — a
// runtime trap lowers to `unreachable`, and a JS exception crossing
// back into wasm unwinds the frames outright. Neither runs a `defer`.
// Clearing the latch only after the loop therefore leaves it stuck at
// `true` on those paths, and `insertJobQueue` never schedules another
// drain: the executor is dead for the lifetime of the process,
// silently. Clearing it up front costs one extra drain microtask per
// batch that enqueues, and an unwinding job leaves the queue merely
// undrained rather than unreachable — the next enqueue picks it up.
queueState.isSpinning = false

while let job = self.claimNextFromQueue() {
#if compiler(>=5.9)
Expand All @@ -50,8 +63,6 @@ extension JavaScriptEventLoop {
job._runSynchronously(on: self.asUnownedSerialExecutor())
#endif
}

queueState.isSpinning = false
}

func claimNextFromQueue() -> UnownedJob? {
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