Update WorkQueue.queue to be a reference type instead of an array.

We pass WorkQueue.queue 'by reference' through inout parameters and across thread suspension points. We noticed a stale value issue in release builds when passing Array with inout, likely due to the copy-on-write mechanism. To avoid stale values after a thread resumes from suspension, switch to a dedicated reference type Queue.

Author: iCharlesHu

.github/workflows/pull_request.yml

@@ -28,14 +28,14 @@ jobs:
           # Test dependencies
           yum install -y procps
         fi
-      linux_build_command: 'swift-format lint -s -r --configuration ./.swift-format . && swift test && swift test --disable-default-traits'
+      linux_build_command: 'swift-format lint -s -r --configuration ./.swift-format . && swift test && swift test -c release && swift test --disable-default-traits'
       windows_swift_versions: '["6.1", "nightly-main"]'
       windows_build_command: |
         Invoke-Program swift test
         Invoke-Program swift test --disable-default-traits
       enable_macos_checks: true
       macos_xcode_versions: '["16.3"]'
-      macos_build_command: 'xcrun swift-format lint -s -r --configuration ./.swift-format . && xcrun swift test && xcrun swift test --disable-default-traits'
+      macos_build_command: 'xcrun swift-format lint -s -r --configuration ./.swift-format . && xcrun swift test && xcrun swift test -c release && xcrun swift test --disable-default-traits'
       enable_linux_static_sdk_build: true
       linux_static_sdk_versions: '["6.1", "nightly-6.2"]'
       linux_static_sdk_build_command: |

Sources/Subprocess/Thread.swift

@@ -77,12 +77,33 @@ private struct BackgroundWorkItem {
 // We can't use Mutex directly here because we need the underlying `pthread_mutex_t` to be
 // exposed so we can use it with `pthread_cond_wait`.
 private final class WorkQueue: Sendable {
-    private nonisolated(unsafe) var queue: [BackgroundWorkItem]
+    // Queue needs to be a reference type because we pass it inout
+    final class Queue: Sendable {
+        internal nonisolated(unsafe) var queue: [BackgroundWorkItem]
+
+        var isEmpty: Bool { self.queue.isEmpty }
+
+        func append(_ item: BackgroundWorkItem) {
+            self.queue.append(item)
+        }
+
+        func removeFirst() -> BackgroundWorkItem {
+            return self.queue.removeFirst()
+        }
+
+        func removeAll() { self.queue.removeAll() }
+
+        init() {
+            self.queue = []
+        }
+    }
+
+    private nonisolated(unsafe) var queue: Queue
     internal nonisolated(unsafe) let mutex: UnsafeMutablePointer<MutexType>
     internal nonisolated(unsafe) let waitCondition: UnsafeMutablePointer<ConditionType>
 
     init() {
-        self.queue = []
+        self.queue = Queue()
         self.mutex = UnsafeMutablePointer<MutexType>.allocate(capacity: 1)
         self.waitCondition = UnsafeMutablePointer<ConditionType>.allocate(capacity: 1)
         #if canImport(WinSDK)
@@ -94,14 +115,14 @@ private final class WorkQueue: Sendable {
         #endif
     }
 
-    func withLock<R>(_ body: (inout [BackgroundWorkItem]) throws -> R) rethrows -> R {
+    func withLock<R>(_ body: (inout Queue) throws -> R) rethrows -> R {
         try withUnsafeUnderlyingLock { _, queue in
             try body(&queue)
         }
     }
 
     private func withUnsafeUnderlyingLock<R>(
-        _ body: (UnsafeMutablePointer<MutexType>, inout [BackgroundWorkItem]) throws -> R
+        _ body: (UnsafeMutablePointer<MutexType>, inout Queue) throws -> R
     ) rethrows -> R {
         #if canImport(WinSDK)
         EnterCriticalSection(self.mutex)