[RuntimeAsync] Runtime Async may not honor ExecutionContext.SuppressFlow().

[VSadov]

Flowing execution context when it is suppressed is observable. One purpose for suppressing could be hiding secrets, although most commonly the flow is suppressed for performance reasons.

Not reacting to suppression may result in failures like:

    System.Net.Sockets.Tests.ExecutionContextFlowTest.SocketAsyncEventArgs_ExecutionContextFlowsAcrossSendAsyncOperation(suppressContext: True, sendMode: 2) [FAIL]
      Assert.Equal() Failure: Values differ
      Expected: 0
      Actual:   42
      Stack Trace:
        /_/src/libraries/System.Net.Sockets/tests/FunctionalTests/ExecutionContextFlowTest.cs(345,0): at System.Net.Sockets.Tests.ExecutionContextFlowTest.SocketAsyncEventArgs_ExecutionContextFlowsAcrossSendAsyncOperation(Boolean suppressContext, Int32 sendMode)
        /_/src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncHelpers.CoreCLR.cs(478,0): at System.Runtime.CompilerServices.AsyncHelpers.RuntimeAsyncTaskCore.DispatchContinuations[T,TOps](T task)
        --- End of stack trace from previous location ---
        --- End of stack trace from previous location ---
  Finished:    System.Net.Sockets.Tests

[VSadov]

I think this can be fixed by simply checking for IsFlowSuppressed in RestoreContextsOnSuspension and not capturing exec context, but need to be sure.

Possible Fix: 2982687

== Edit:
RestoreContextsOnSuspension may be used for synchronous restore (i.e. before calling into notifier). CaptureExecutionContext could be a more appropriate place.

[davidwrighton]

This is not fixed if DOTNET_JitOptimizeAwait is set to 0.

[VSadov]

This is not fixed if DOTNET_JitOptimizeAwait is set to 0.

The unoptimized path uses the actual awaiter, thus we typically do not have compat issues there.
When there is no Task/awaiter - that is when we get into all kind of trouble and need to simulate what awaiter would do.

It is possible though that somehow we still have an issue or something regressed.

How did it reproduce? Just via running some existing tests with set DOTNET_JitOptimizeAwait=0 ?