Merge pull request swiftlang#80896 from al45tair/eng/PR-149346132

[Concurrency] Make initial executor construction fully thread safe.

Author: al45tair

stdlib/public/Concurrency/Executor.swift

@@ -546,6 +546,14 @@ public func _createExecutors<F: ExecutorFactory>(factory: F.Type) {
   Task._defaultExecutor = factory.defaultExecutor
 }
 
+@available(SwiftStdlib 6.2, *)
+@_silgen_name("swift_createDefaultExecutors")
+func _createDefaultExecutors() {
+  if Task._defaultExecutor == nil {
+    _createExecutors(factory: DefaultExecutorFactory.self)
+  }
+}
+
 #if !$Embedded && !SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY
 extension MainActor {
   @available(SwiftStdlib 6.2, *)
@@ -558,9 +566,8 @@ extension MainActor {
   /// executor is a fatal error.
   @available(SwiftStdlib 6.2, *)
   public static var executor: any MainExecutor {
-    if _executor == nil {
-      _executor = DefaultExecutorFactory.mainExecutor
-    }
+    // It would be good if there was a Swift way to do this
+    _createDefaultExecutorsOnce()
     return _executor!
   }
 }
@@ -577,9 +584,8 @@ extension Task where Success == Never, Failure == Never {
   /// executor is a fatal error.
   @available(SwiftStdlib 6.2, *)
   public static var defaultExecutor: any TaskExecutor {
-    if _defaultExecutor == nil {
-      _defaultExecutor = DefaultExecutorFactory.defaultExecutor
-    }
+    // It would be good if there was a Swift way to do this
+    _createDefaultExecutorsOnce()
     return _defaultExecutor!
   }
 }

stdlib/public/Concurrency/ExecutorBridge.cpp

@@ -14,6 +14,8 @@
 #include <dispatch/dispatch.h>
 #endif
 
+#include "swift/Threading/Once.h"
+
 #include "Error.h"
 #include "ExecutorBridge.h"
 
@@ -27,6 +29,13 @@ void _swift_exit(int result) {
   exit(result);
 }
 
+extern "C" SWIFT_CC(swift)
+void swift_createDefaultExecutorsOnce() {
+  static swift::once_t createExecutorsOnce;
+
+  swift::once(createExecutorsOnce, swift_createDefaultExecutors);
+}
+
 #if SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY
 extern "C" SWIFT_CC(swift)
 SerialExecutorRef swift_getMainExecutor() {

stdlib/public/Concurrency/ExecutorBridge.h

@@ -33,10 +33,16 @@ void swift_destroyDispatchEventC(void *event);
 extern "C" SWIFT_CC(swift)
 void swift_signalDispatchEvent(void *event);
 #endif // !SWIFT_CONCURRENCY_EMBEDDED
- 
+
 extern "C" SWIFT_CC(swift) __attribute__((noreturn))
 void swift_dispatchMain();
 
+extern "C" SWIFT_CC(swift)
+void swift_createDefaultExecutors();
+
+extern "C" SWIFT_CC(swift)
+void swift_createDefaultExecutorsOnce();
+
 #pragma clang diagnostic pop
 
 } // namespace swift

stdlib/public/Concurrency/ExecutorBridge.swift

@@ -119,3 +119,6 @@ internal func _dispatchEnqueueWithDeadline(_ global: CBool,
 @available(SwiftStdlib 6.2, *)
 @_silgen_name("swift_dispatchAssertMainQueue")
 internal func _dispatchAssertMainQueue()
+
+@_silgen_name("swift_createDefaultExecutorsOnce")
+func _createDefaultExecutorsOnce()