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[Concurrency] Make initial executor construction fully thread safe. #80896

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Merged
merged 1 commit into from
Apr 22, 2025
Merged

[Concurrency] Make initial executor construction fully thread safe. #80896

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18 changes: 12 additions & 6 deletions stdlib/public/Concurrency/Executor.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -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, *)
Expand All @@ -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!
}
}
Expand All @@ -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
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@beccadax beccadax Apr 17, 2025 •
edited
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Wouldn't something like this get thread-safe lazy initialization? I believe we do that for static lets.

public static let defaultExector: any TaskExecutor = DefaultExecutorFactory.defaultExecutor

edit: come to think of it this might need to be moved into a backing global, but the point stands.

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It's a bit more complicated than it looks :-)

Yes, initialising a static variable does happen in a thread-safe manner, but we don't actually want to initialise these to the default executor (since that means constructing the default executor always, even when we don't actually want it, and the act of constructing it might have side effects).

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I think we could leverage static variable initialisation, but I think to do so we'd need to add an extra variable and make the static initialisation be a closure call. I'm not sure that's really an improvement overall. What would be an improvement is the ability to do a once explicitly and directly in Swift, since that's what we really want here.

_createDefaultExecutorsOnce()
return _defaultExecutor!
}
}
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9 changes: 9 additions & 0 deletions stdlib/public/Concurrency/ExecutorBridge.cpp
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Expand Up @@ -14,6 +14,8 @@
#include <dispatch/dispatch.h>
#endif

#include "swift/Threading/Once.h"

#include "Error.h"
#include "ExecutorBridge.h"

Expand All @@ -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() {
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8 changes: 7 additions & 1 deletion stdlib/public/Concurrency/ExecutorBridge.h
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Expand Up @@ -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
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3 changes: 3 additions & 0 deletions stdlib/public/Concurrency/ExecutorBridge.swift
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Expand Up @@ -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()