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Tweak TimeProviderTaskExtensions handling of CreateTimer #85200

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Apr 22, 2023
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Tweak TimeProviderTaskExtensions handling of CreateTimer #85200

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Tweak TimeProviderTaskExtensions handling of CreateTimer 
Avoid the separate Timer.Change call by tweaking how cleanup happens.
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@stephentoub
stephentoub committed Apr 22, 2023
commit caf23e31544173303eacd49cc4f93423e251e70e
36 changes: 17 additions & 19 deletions ...aries/Microsoft.Bcl.TimeProvider/src/System/Threading/Tasks/TimeProviderTaskExtensions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -64,32 +64,34 @@ public static Task Delay(this TimeProvider timeProvider, TimeSpan delay, Cancell

DelayState state = new();

// To prevent a race condition where the timer may fire before being assigned to s.Timer,
// we initialize it with an InfiniteTimeSpan and then set it to the state variable, followed by calling Time.Change.
state.Timer = timeProvider.CreateTimer(delayState =>
{
DelayState s = (DelayState)delayState;
DelayState s = (DelayState)delayState!;
s.TrySetResult(true);
s.Registration.Dispose();
s.Timer.Dispose();
}, state, Timeout.InfiniteTimeSpan, Timeout.InfiniteTimeSpan);

state.Timer.Change(delay, Timeout.InfiniteTimeSpan);
s?.Timer.Dispose();
}, state, delay, Timeout.InfiniteTimeSpan);

state.Registration = cancellationToken.Register(delayState =>
{
DelayState s = (DelayState)delayState;
DelayState s = (DelayState)delayState!;
s.TrySetCanceled(cancellationToken);
s.Timer.Dispose();
s.Registration.Dispose();
s?.Timer.Dispose();
}, state);

// To prevent a race condition where the timer fires after we have attached the cancellation callback
// but before the registration is stored in state.Registration, we perform a subsequent check to ensure
// that the registration is not left dangling.
// There are race conditions where the timer fires after we have attached the cancellation callback but before the
// registration is stored in state.Registration, or where cancellation is requested prior to the registration being
// stored into state.Registration, or where the timer could fire after it's been createdbut before it's been stored
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nit, 'createdbut'

// in state.Timer. In such cases, the cancellation registration and/or the Timer might be stored into state after the
// callbacks and thus left undisposed. So, we do a subsequent check here. If the task isn't completed by this point,
// then the callbacks won't have called TrySetResult (the callbacks invoke TrySetResult before disposing of the fields),
// in which case it will see both the timer and registration set and be able to Dispose them. If the task is completed
// by this point, then this is guaranteed to see s.Timer as non-null because it was deterministically set above.
if (state.Task.IsCompleted)
{
state.Registration.Dispose();
state.Timer.Dispose();
}

return state.Task;
Expand Down Expand Up @@ -149,8 +151,6 @@ public static Task WaitAsync(this Task task, TimeSpan timeout, TimeProvider time

var state = new WaitAsyncState();

// To prevent a race condition where the timer may fire before being assigned to s.Timer,
// we initialize it with an InfiniteTimeSpan and then set it to the state variable, followed by calling Time.Change.
state.Timer = timeProvider.CreateTimer(static s =>
{
var state = (WaitAsyncState)s!;
Expand All @@ -160,8 +160,7 @@ public static Task WaitAsync(this Task task, TimeSpan timeout, TimeProvider time
state.Registration.Dispose();
state.Timer!.Dispose();
state.ContinuationCancellation.Cancel();
}, state, Timeout.InfiniteTimeSpan, Timeout.InfiniteTimeSpan);
state.Timer.Change(timeout, Timeout.InfiniteTimeSpan);
}, state, timeout, Timeout.InfiniteTimeSpan);

_ = task.ContinueWith(static (t, s) =>
{
Expand All @@ -185,12 +184,11 @@ public static Task WaitAsync(this Task task, TimeSpan timeout, TimeProvider time
state.ContinuationCancellation.Cancel();
}, state);

// To prevent a race condition where the timer fires after we have attached the cancellation callback
// but before the registration is stored in state.Registration, we perform a subsequent check to ensure
// that the registration is not left dangling.
// See explanation in Delay for this final check
if (state.Task.IsCompleted)
{
state.Registration.Dispose();
state.Timer.Dispose();
}

return state.Task;
Expand Down