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Fix race condition in AsyncCurrentValueSubject #40

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Fix race condition in AsyncCurrentValueSubject #40

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1f0729e
Merge pull request #33 from sideeffect-io/release/0.5.2
twittemb Jan 3, 2023
fc0dc63
Fix race condition in `AsyncCurrentValueSubject`
wfltaylor Jan 31, 2024
29ad29a
Fix race condition in `AsyncThrowingCurrentValueSubject`
wfltaylor Jan 31, 2024
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7 changes: 3 additions & 4 deletions Sources/AsyncSubjects/AsyncCurrentValueSubject.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -91,20 +91,19 @@ public final class AsyncCurrentValueSubject<Element>: AsyncSubject where Element
func handleNewConsumer() -> (iterator: AsyncBufferedChannel<Element>.Iterator, unregister: @Sendable () -> Void) {
let asyncBufferedChannel = AsyncBufferedChannel<Element>()

let (terminalState, current) = self.state.withCriticalRegion { state -> (Termination?, Element) in
(state.terminalState, state.current)
let terminalState = self.state.withCriticalRegion { state -> Termination? in
state.terminalState
}

if let terminalState = terminalState, terminalState.isFinished {
asyncBufferedChannel.finish()
return (asyncBufferedChannel.makeAsyncIterator(), {})
}

asyncBufferedChannel.send(current)

let consumerId = self.state.withCriticalRegion { state -> Int in
state.ids += 1
state.channels[state.ids] = asyncBufferedChannel
asyncBufferedChannel.send(state.current)
return state.ids
Comment on lines 103 to 107
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@chatgpt-codex-connector chatgpt-codex-connector bot Jan 20, 2026

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P1 Badge Avoid resuming consumers while holding subject lock

Calling asyncBufferedChannel.send(state.current) while inside state.withCriticalRegion can synchronously resume an awaiting consumer. If that consumer’s code touches the subject again (e.g., reads value or calls send), it will try to re-enter the same ManagedCriticalState that uses a non‑reentrant os_unfair_lock, leading to a deadlock. This only occurs when a consumer awaits the channel and then re-enters the subject on receipt, but that pattern is common in for‑await loops.

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@twittemb twittemb Jan 20, 2026 •
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@codex What do you suggest that solves the initial issue while avoiding re-entrency ?

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@codex What do you suggest that solves the initial issue while avoiding re-entrency ?

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Codex couldn't complete this request. Try again later.

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@twittemb twittemb Mar 31, 2026

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@codex What do you suggest that solves the initial issue while avoiding re-entrency ?

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Codex couldn't complete this request. Try again later.

}

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7 changes: 3 additions & 4 deletions Sources/AsyncSubjects/AsyncThrowingCurrentValueSubject.swift
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Expand Up @@ -97,8 +97,8 @@ public final class AsyncThrowingCurrentValueSubject<Element, Failure: Error>: As
) -> (iterator: AsyncThrowingBufferedChannel<Element, Error>.Iterator, unregister: @Sendable () -> Void) {
let asyncBufferedChannel = AsyncThrowingBufferedChannel<Element, Error>()

let (terminalState, current) = self.state.withCriticalRegion { state -> (Termination?, Element) in
(state.terminalState, state.current)
let terminalState = self.state.withCriticalRegion { state -> Termination? in
state.terminalState
}

if let terminalState = terminalState {
Expand All @@ -111,11 +111,10 @@ public final class AsyncThrowingCurrentValueSubject<Element, Failure: Error>: As
return (asyncBufferedChannel.makeAsyncIterator(), {})
}

asyncBufferedChannel.send(current)

let consumerId = self.state.withCriticalRegion { state -> Int in
state.ids += 1
state.channels[state.ids] = asyncBufferedChannel
asyncBufferedChannel.send(state.current)
return state.ids
Comment on lines 114 to 118
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P1 Badge Avoid resuming consumers while holding subject lock

Calling asyncBufferedChannel.send(state.current) while inside state.withCriticalRegion can synchronously resume an awaiting consumer. If that consumer’s code touches the subject again (e.g., reads value or calls send), it will try to re-enter the same ManagedCriticalState that uses a non‑reentrant os_unfair_lock, leading to a deadlock. This only occurs when a consumer awaits the channel and then re-enters the subject on receipt, but that pattern is common in for‑await loops.

Useful? React with 👍 / 👎.

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@twittemb twittemb Jan 20, 2026

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@codex What do you suggest that solves the initial issue while avoiding re-entrency ?

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@chatgpt-codex-connector chatgpt-codex-connector bot Jan 20, 2026

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To use Codex here, create an environment for this repo.

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@codex is there a real risk of deadlock here ?

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Codex Review: Didn't find any major issues. 👍

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

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  • Mark a draft as ready
  • Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

}

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