Fix UnobservedTaskException from SemaphoreSlim.WaitAsync

[stephentoub]

If a SemaphoreSlim.WaitAsync times out, it correctly returns false, but it also results in TaskScheduler.UnobservedTaskException being raised unexpectedly, due to internal use of a faulted task whose exception isn't observed. This fixes that by marking any such exceptions as having been observed.

Fixes#60856

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Issue Details

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If a SemaphoreSlim.WaitAsync times out, it correctly returns false, but it also results in TaskScheduler.UnobservedTaskException being raised unexpectedly, due to internal use of a faulted task whose exception isn't observed. This fixes that by marking any such exceptions as having been observed.

Fixes #60856

Author:	stephentoub
Assignees:	-
Labels:	area-System.Threading.Tasks
Milestone:	7.0.0

[adamsitnik]

@stephentoub should we backport it to 6.0.1?

[stephentoub]

It'd be fine to propose doing so if you think it's worthwhile.

[adamsitnik]

It'd be fine to propose doing so if you think it's worthwhile.

I think that it's worthwhile. It's quite common to use TaskScheduler.UnobservedTaskException and it should be safe to assume that the event is raised for unobserved exceptions. And if exceptions are not used for flow control , it means that there is an actual problem when the even is raised . So if there is not too much risk in the fix, we should consider backporting it.

[stephentoub]

So if there is not too much risk in the fix, we should consider backporting it.

It should be low risk; it's just setting a flag that says "treat this as already observed". We could let it bake in main for a bit first if desired, or not.

[stephentoub]

/backport to release/6.0

[github-actions]

Started backporting to release/6.0: https://github.com/dotnet/runtime/actions/runs/1451429820