[release/6.0] Fix race condition with cancellation tokens in Read/Flush operations #59189
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…- There's a tight race where UnsafeRegister can fire inline and then ReadAsync never completes. This is because the cancellation token property has not been assigned yet so the callback noops. This change checks the result of the UnsafeRegister operation to see if ran synchronously and throws if it did. We also move any state transitions to after these checks to make sure the PipeAwaitable state doesn't change before throwing.
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This fix is incomplete, and the original bug reactivated. |
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Backport of #59090 to release/6.0
/cc @davidfowl
Customer Impact
@AArnott reported a race condition with PipeReader where the ReadAsync operation could hang in some hard to reproduce cases when used with a cancellation token. The race resulting in hangs in various usages of pipelines in https://github.com/AArnott/Nerdbank.Streams (which is used in VS).
Testing
Manual testing. It's very hard to write a unit test for this scenario.
Risk
Low. The change was made to only affect a very narrow case (synchronous completion). Asserts were added to make sure existing invariants weren't broken.