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Discarding new disposable on a disposed SerialDisposable #688

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Discarding new disposable on a disposed SerialDisposable #688

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8 changes: 4 additions & 4 deletions Sources/Disposable.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -350,10 +350,10 @@ public final class SerialDisposable: Disposable {
}

set(disposable) {
_inner.swap(disposable)?.dispose()

if let disposable = disposable, isDisposed {
disposable.dispose()
if isDisposed {
disposable?.dispose()
} else {
_inner.swap(disposable)?.dispose()
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We need to check isDisposed again after swapping, since self could be disposed of in parallel during the process, racing against https://github.com/ReactiveCocoa/ReactiveSwift/pull/688/files#diff-7056bb5d45375e26f2eeb3adeabac16bR373.

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@DevAndArtist DevAndArtist Dec 4, 2018 •
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Can you provide a change suggestion and elaborate more why we need to do it, because I'm not sure I follow. I always thought that such operations should not be done in a multi-threading fashion, so it will be the users responsibility to serialize correctly.

If this PR gets approved, I'll add more documentations because this slightly changes the behavior of the type after it's disposed.

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Change suggestions do not seem to be available in this repo.

Essentially, we need to do this after the swapping in the else path:

if isDisposed {
    _inner.swap(nil)?.dispose()
}

Because dispose() can be called in parallel to inner = new on different threads. Depending on the order observed by the system memory, it could yield a boundary case where isDisposed is true but the inner disposable is left untouched. The conflicting path may

I always thought that such operations should not be done in a multi-threading fashion, so it will be the users responsibility to serialize correctly.

Disposables have always meant to be thread safe, despite us not explicitly having written this as part of the contract.

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Change suggestions do not seem to be available in this repo

Weird. I don't see a setting for it.

}
}
}
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