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Make MPSCAsyncChannel #isolation methods use nonisolated(nonsending)`. #384

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Make MPSCAsyncChannel #isolation methods use nonisolated(nonsending)`. #384

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4 changes: 2 additions & 2 deletions ...MultiProducerSingleConsumerChannel/MultiProducerSingleConsumerAsyncChannel+Internal.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -391,7 +391,7 @@ extension MultiProducerSingleConsumerAsyncChannel {
}

@inlinable
func next(isolation: isolated (any Actor)? = #isolation) async throws -> Element? {
nonisolated(nonsending) func next() async throws -> Element? {
let action = self._stateMachine.withLock {
$0.next()
}
Expand Down Expand Up @@ -431,7 +431,7 @@ extension MultiProducerSingleConsumerAsyncChannel {
}

@inlinable
func suspendNext(isolation: isolated (any Actor)? = #isolation) async throws -> Element? {
nonisolated(nonsending) func suspendNext() async throws -> Element? {
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@jamieQ jamieQ Dec 15, 2025

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in my recent testing, it seems nonisolated(nonsending) does not grant the ability to actually procure a reference to the isolated parameter in a way that allows isolating closures in the same manner that capturing an explicit isolated parameter does. i suppose in this case it doesn't matter since the parameter isn't being captured by the body closures, but that does make me wonder if those closures are actually not being isolated in the intended manner currently.

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@FranzBusch FranzBusch Dec 18, 2025

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Could you briefly expand what closures you are worried are not isolated in particular?

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@jamieQ jamieQ Dec 18, 2025 •
edited
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so, the concern would be that even though the method inherits isolation by default (in the original formulation), the isolated parameter is unused within the body closures. this is true in the prior version with an isolated parameter, and seems to also be true even after converting to implicit caller isolation inheritance. this means the actual 'work' invoked by the with* runtime methods isn't isolated, so IIUC it's going to require an executor hop if the caller is actor-isolated. this can be seen in this sort of example: https://swift.godbolt.org/z/GP4dPhfKv.

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@FranzBusch FranzBusch Dec 18, 2025 •
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Ah so you are talking about the withTaskCancellationHandler and withCheckedContinuation handler. Both of them have open PRs to address these issues swiftlang/swift#85191 & swiftlang/swift#85518

Edit: I see that you left a comment on the latter PR. The hop should be eliminated and I think @gottesmm is looking into that.

try await withTaskCancellationHandler {
try await withUnsafeThrowingContinuation { (continuation: UnsafeContinuation<Element?, Error>) in
let action = self._stateMachine.withLock {
Expand Down
8 changes: 2 additions & 6 deletions ...gorithms/MultiProducerSingleConsumerChannel/MultiProducerSingleConsumerAsyncChannel.swift
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Expand Up @@ -201,9 +201,7 @@ public struct MultiProducerSingleConsumerAsyncChannel<Element, Failure: Error>:
/// - Parameter isolation: The callers isolation.
/// - Returns: The next buffered element.
@inlinable
public mutating func next(
isolation: isolated (any Actor)? = #isolation
) async throws(Failure) -> Element? {
nonisolated(nonsending) public mutating func next() async throws(Failure) -> Element? {
do {
return try await self.storage.next()
} catch {
Expand Down Expand Up @@ -710,9 +708,7 @@ extension MultiProducerSingleConsumerAsyncChannel.ChannelAsyncSequence where Ele
}

@inlinable
mutating func next(
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@FranzBusch FranzBusch Dec 9, 2025

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I don't think we can change this one since this is a protocol requirement of AsyncSequence unless nonisolated(nonsending) methods are capable of fulfilling isolated protocol requirement

isolation actor: isolated (any Actor)? = #isolation
) async throws(Failure) -> Element? {
nonisolated(nonsending) mutating func next() async throws(Failure) -> Element? {
do {
return try await self._backing.storage.next(isolation: actor)
} catch {
Expand Down
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