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Tweak SemaphoreDisposer to null out the held semaphore #57831

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Nov 22, 2021
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Tweak SemaphoreDisposer to null out the held semaphore #57831

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20 changes: 18 additions & 2 deletions src/Compilers/Core/Portable/InternalUtilities/SemaphoreSlimExtensions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -26,7 +26,7 @@ public static async ValueTask<SemaphoreDisposer> DisposableWaitAsync(this Semaph
[NonCopyable]
internal struct SemaphoreDisposer : IDisposable
{
private readonly SemaphoreSlim _semaphore;
private SemaphoreSlim? _semaphore;

public SemaphoreDisposer(SemaphoreSlim semaphore)
{
Expand All @@ -35,7 +35,23 @@ public SemaphoreDisposer(SemaphoreSlim semaphore)

public void Dispose()
{
_semaphore.Release();
// Officially, Dispose() being called more than once is allowable, but in this case
// if that were to ever happen that means something is very, very wrong. Since it's an internal
// type, better to be strict.

// Nulling this out also means it's a bit easier to diagnose some async deadlocks; if you have an
// async deadlock where a SemaphoreSlim is held but you're unsure why, as long all the users of the
// SemaphoreSlim used the Disposable helpers, you can search memory and find the instance that
// is pointing to the SemaphoreSlim that hasn't nulled out this field yet; in that case you know
// that's holding the lock and can figure out who is holding that SemaphoreDisposer.
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we're in a struct. so this does only work as long as the NonCopyable part above really is maintained. i can allow this :)

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@CyrusNajmabadi Indeed, the [NonCopyable] being violated does mean we could end up with "surprises", but at this point, this is the least of your concerns IMHO. 😄

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@jaredpar jaredpar Nov 18, 2021 •
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so this does only work as long as the NonCopyable part above really is maintained

Instances of this type are copied twice in this very file so [NonCopyable] is more like [NonCopyableForSomeVersionsOfCopy]

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@sharwell sharwell Nov 18, 2021 •
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Instances of this type are copied twice in this very file

There are no copies of this type in this file (moves are not copies). A copy is a case where the new location and the old location can still be accessed with the value; the analyzer ideally would ignore cases where the old location is accessible but never accessed, but it doesn't pay attention to that right now.

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There are two returns which are copies. You label them moves but that doesn't change that copies occur.

var semaphoreToDispose = Interlocked.Exchange(ref _semaphore, null);

if (semaphoreToDispose is null)
{
throw new ObjectDisposedException($"Somehow a {nameof(SemaphoreDisposer)} is being disposed twice.");
}

semaphoreToDispose.Release();
}
}
}
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