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Tolerate spurious wakeups on the background worker thread. #350

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Merged
iCharlesHu merged 1 commit into from
Jul 9, 2026
Merged

Tolerate spurious wakeups on the background worker thread. #350

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14 changes: 11 additions & 3 deletions Sources/Subprocess/Thread.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -91,11 +91,13 @@ private struct BackgroundWorkItem {
// exposed so we can use it with `pthread_cond_wait`.
private final class WorkQueue: Sendable {
private nonisolated(unsafe) var queue: [BackgroundWorkItem]
private nonisolated(unsafe) var isShuttingDown: Bool
internal nonisolated(unsafe) let mutex: UnsafeMutablePointer<MutexType>
internal nonisolated(unsafe) let waitCondition: UnsafeMutablePointer<ConditionType>

init() {
self.queue = []
self.isShuttingDown = false
self.mutex = UnsafeMutablePointer<MutexType>.allocate(capacity: 1)
self.waitCondition = UnsafeMutablePointer<ConditionType>.allocate(capacity: 1)
#if canImport(WinSDK)
Expand Down Expand Up @@ -128,7 +130,11 @@ private final class WorkQueue: Sendable {
pthread_mutex_unlock(mutex)
}
#endif
if condition(&queue) {
// Use a `while` loop (not `if`) so that spurious wakeups and signal
// interruptions of `pthread_cond_wait` do not fall through the wait.
// `condition` is responsible for also returning `false` when shutting
// down, so the worker can wake and exit rather than sleeping forever.
while condition(&queue) {
#if canImport(WinSDK)
SleepConditionVariableCS(self.waitCondition, mutex, INFINITE)
#else
Expand All @@ -141,8 +147,9 @@ private final class WorkQueue: Sendable {
// Only called in worker thread. Sleeps the thread if there's no more item
func dequeue() -> BackgroundWorkItem? {
return self.withUnsafeUnderlyingLock { queue in
// Sleep the worker thread if there's no more work
queue.isEmpty
// Sleep the worker thread while there's no work and we're not
// shutting down. Returning `false` here ends the wait.
queue.isEmpty && !self.isShuttingDown
} body: { mutex, queue in
guard !queue.isEmpty else {
return nil
Expand All @@ -166,6 +173,7 @@ private final class WorkQueue: Sendable {
func shutdown() {
self.withLock { queue in
queue.removeAll()
self.isShuttingDown = true
#if canImport(WinSDK)
WakeConditionVariable(self.waitCondition)
#else
Expand Down