Some AsyncMemoize fixes

[majocha]

Description

AsyncMemoize tests became flaky again because of recent changes, for example: https://dev.azure.com/dnceng-public/public/_build/results?buildId=881078&view=ms.vss-test-web.build-test-results-tab&runId=23026540&resultId=100189&paneView=debug

The problem was a not atomic handling of request count. It was possible to cancel the request mid-flight in such a way that the finally block decreasing request count never run.

Checklist

• Verified locally by running thousands of iterations concurrently.

[github-actions]

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cc @dotnet/fsharp-team-msft

[psfinaki]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[majocha]

Yeah, this returns an async computation that decreases request count in finally. But the increase is outside of the try block now.

fsharp/src/Compiler/Facilities/AsyncMemoize.fs

Lines 72 to 76 in 95fe045

	| Initial computation ->
	let cts = new CancellationTokenSource()
	let work = Async.StartAsTask(computation, cancellationToken = cts.Token)
	state <- Running (computation, work, cts, 1)
	detachable work

Benign in real usage but can be tripped in tests causing job cancellation to not happen.

[psfinaki]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[psfinaki]

Thank you :)

[0101]

Got a stress test failure with this fix already included :/

https://dev.azure.com/dnceng-public/public/_build/results?buildId=883746&view=ms.vss-test-web.build-test-results-tab&runId=23109844&resultId=100187&paneView=debug
( Test timed out - most likely deadlocked)

[majocha]

I'll try to stress test the stress test locally and reproduce it. The timeout in stress-test is very generous, it doesn't look like a fluke.

[majocha]

I can't repro the deadlock but after some 1.5k iterations I got once:

    ---- System.Exception : Seed 57146328 failed on iteration 30: System.AggregateException: One or more errors occurred. ---> System.AggregateException: One or more errors occurred. ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
   --- End of inner exception stack trace ---
   at <StartupCode$FSharp-Compiler-Service>.$AsyncMemoize.catchHandler@1[t](Exception exn) in E:\repos\fsharp\src\Compiler\Facilities\AsyncMemoize.fs:line 54
   at <StartupCode$FSharp-Compiler-Service>.$AsyncMemoize.clo@42-31.Invoke(Exception x)
   at Microsoft.FSharp.Control.AsyncPrimitives.CallFilterThenInvoke[T](AsyncActivation`1 ctxt, FSharpFunc`2 filterFunction, ExceptionDispatchInfo edi) in E:\repos\fsharp\src\FSharp.Core\async.fs:line 547
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in E:\repos\fsharp\src\FSharp.Core\async.fs:line 114
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at CompilerService.AsyncMemoize.loop@323-8.MoveNext() in E:\repos\fsharp\tests\FSharp.Compiler.ComponentTests\CompilerService\AsyncMemoize.fs:line 338
---> (Inner Exception #0) System.AggregateException: One or more errors occurred. ---> System.Threading.Tasks.TaskCanceledException: A task was canceled.
   --- End of inner exception stack trace ---
   at System.Threading.Tasks.Task`1.GetResultCore(Boolean waitCompletionNotification)
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
---> (Inner Exception #0) System.Threading.Tasks.TaskCanceledException: A task was canceled.<---
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