[Runtime Async] do not hold to continuation state when completing reusable valuetask sources

[VSadov]

Not exactly in the Runtime Async implementation, but the issue was found while testing Runtime Async in Libraries.

There is a scenario where ValueTaskSource upon completion invokes stashed {_continuation, _continuationState}. It is a one-shot invocation. Neither of {_continuation, _continuationState} are needed after that. However, while we naturally unroot _continuation by replacing it with a sentinel, the _continuationState will stay rooted until the VTS is reused.

As a result, if continuation expects that some state (like Http3ReadStream) gets finalized, it may not see that no matter how long it waits/checks/retries.

The retention was observable in intermittent failures in:

System.Net.Http.Functional.Tests.SocketsHttpHandler_HttpClientHandler_Finalization_Http3.IncompleteResponseStream_ResponseDropped_CancelsRequestToServer

(scenario may depend on mutual timings between Server and Client and whether sync/async paths are taken)

The whole purpose of the test is to see that if client drops the connection on the floor, it becomes unreachable reasonably soon and Finalization logic closes the connection.

What we see instead is the test waits for a long time and eventually times out.

A similar code pattern was observed in Threading/Channels/AsyncOperation.
Although I did not see any failures due to that, I applied the same fix of zeroing out the _continuationState before invoking the continuation.

Same applies to the _capturedContext. No need to hold to that for longer than needed.

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[jakobbotsch]

LGTM, but not my usual area of expertise.

[VSadov]

/ba-g mono interpreter failure is #124525

[VSadov]

LGTM, but not my usual area of expertise.

It is a fairly trivial change. If we do not keep the continuation delegate after calling it, there is no need to keep the arguments for the call.

Thanks!!