Fix a couple of issues related to Process.WaitForExit on Unix

[stephentoub]

Fixes #85717

• Calling WaitForExit(timeout), if the timeout occurred before the background work item launch, WaitForExit would throw an AggregateException wrapping an OperationCanceledException, rather than just returning false.
• In rare race conditions, the background worker could end up overwriting with null the Task being used to track that there was a background operation. This could lead to there being a background Task in flight but it not actually being tracked anywhere, which then meant that we could end up with multiple polling loops all running concurrently. There was also an erroneous assert as a result (if WaitForExitAsync was called as part of a continuation, the lock that was supposed to be protecting it wouldn't be held).

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Issue Details

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Fixes #85717

• Calling WaitForExit(timeout), if the timeout occurred before the background work item launch, WaitForExit would throw an AggregateException wrapping an OperationCanceledException, rather than just returning false.
• In rare race conditions, the background worker could end up overwriting with null the Task being used to track that there was a background operation. This could lead to there being a background Task in flight but it not actually being tracked anywhere, which then meant that we could end up with multiple polling loops all running concurrently. There was also an erroneous assert as a result (if WaitForExitAsync was called as part of a continuation, the lock that was supposed to be protecting it wouldn't be held).

Author:	stephentoub
Assignees:	-
Labels:	area-System.Diagnostics.Process
Milestone:	-

[skyoxZ]

runtime/src/libraries/System.Diagnostics.Process/src/System/Diagnostics/ProcessWaitState.Unix.cs

Lines 203 to 204 in 977b6ee

	/// <summary>If a wait operation is in progress, the Task that represents it; otherwise, null.</summary>
	private Task? _waitInProgress;

The comments need to be updated.

[tmds]

@stephentoub I'll take a closer look tomorrow.

[tmds]

I'm unable to close the comments that have been resolved: #87174 (review), #87174 (review) and #87174 (review). Do you get a button for closing those? If so, you can close them?

I've added one new comment: #87174 (review).

Besides that suggestion, this looks good to me.

[stephentoub]

Feedback addressed.

[jozkee]

LGTM, thanks.