Task cancellation shields #3037
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Introduce a proposal for task cancellation shields to allow certain code to execute regardless of task cancellation status. Implementation WIP PR: swiftlang/swift#85637
Updated reference to SE-0493 to include a link.
FranzBusch
reviewed
Nov 21, 2025
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| public func withTaskCancellationShield<T, E>( | ||
| _ operation: () throws(E) -> T, | ||
| file: String = #fileID, line: Int = #line | ||
| ) throws(E) -> T | ||
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| public nonisolated(nonsending) func withTaskCancellationShield<T, E>( | ||
| _ operation: nonisolated(nonsending) () async throws(E) -> T, // FIXME: order of attrs | ||
| file: String = #fileID, line: Int = #line | ||
| ) async throws(E) -> T |
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Can we give the generic types a slightly more descriptive name so it shows up nicely in the docs. On the PR that introduced nonisolated(nonsending) to the withTaskCancellationHandler methods we used Return and Failure for those two generic types.
Suggested change
| public func withTaskCancellationShield<T, E>( | |
| _ operation: () throws(E) -> T, | |
| file: String = #fileID, line: Int = #line | |
| ) throws(E) -> T | |
| public nonisolated(nonsending) func withTaskCancellationShield<T, E>( | |
| _ operation: nonisolated(nonsending) () async throws(E) -> T, // FIXME: order of attrs | |
| file: String = #fileID, line: Int = #line | |
| ) async throws(E) -> T | |
| public func withTaskCancellationShield<Result, Failure>( | |
| _ operation: () throws(Failure) -> Result, | |
| file: String = #fileID, line: Int = #line | |
| ) throws(Failure) -> Result | |
| public nonisolated(nonsending) func withTaskCancellationShield<Result, Failure>( | |
| _ operation: nonisolated(nonsending) () async throws(Failure) -> Result, // FIXME: order of attrs | |
| file: String = #fileID, line: Int = #line | |
| ) async throws(Failure) -> Result |
| await withTaskCancellationShield { | ||
| await withDiscardingTaskGroup { group in | ||
| group.addTask { ... } | ||
| group.cancelAll() // cancels all tasks within the group, as expected |
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This is perfect and exactly how I expected it.
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| func cleanup() { | ||
| withTaskCancellationShield { | ||
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Member
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Is this missing a slowOperation call here?
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| ```swift | ||
| extension Task where Success == Never, Failure == Never { | ||
| public static func isCancelled(ignoringCancellationShield: Bool) -> Bool |
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+1 I think that's great for debugging purposes
jamieQ
reviewed
Nov 21, 2025
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| ## Motivation | ||
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| Task cancellation is _final_ and can not be ignored or undone. Once a task has been cancelled, it remains cancelled for the rest of its existance. |
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Suggested change
| Task cancellation is _final_ and can not be ignored or undone. Once a task has been cancelled, it remains cancelled for the rest of its existance. | |
| Task cancellation is _final_ and can not be ignored or undone. Once a task has been cancelled, it remains cancelled for the rest of its existence. |
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| Doing nothing is always an option, and we suggest developers have to keep using the unstructured task workaround. | ||
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| This doesn't seem viable though as the problem indeed is real, and the workaround is problematic scheduling wise, and may not even be usable in certain situations. |
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may not even be usable in certain situations
is the only example of this the synchronous context case cited above, or are there others?
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Introduce a proposal for task cancellation shields to allow certain code to execute regardless of task cancellation status.
Implementation WIP PR: swiftlang/swift#85637