Async tests with cancellation are flaky

[brettfo]

The following tests are flaky:

fsharp/tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/AsyncModule.fs

Lines 276 to 339 in 402e1d3

	[<Fact>]
	member this.``OnCancel.RaceBetweenCancellationHandlerAndDisposingHandlerRegistration``() =
	let test() =
	use flag = new ManualResetEvent(false)
	use cancelHandlerRegistered = new ManualResetEvent(false)
	let cts = new System.Threading.CancellationTokenSource()
	let go = async {
	use! holder = Async.OnCancel(fun() -> lock flag (fun() -> flag.Set()) |> ignore)
	let _ = cancelHandlerRegistered.Set()
	while true do
	do! Async.Sleep 50
	}
	Async.Start (go, cancellationToken = cts.Token)
	//wait until we are sure the Async.OnCancel has run:
	Assert.True(cancelHandlerRegistered.WaitOne(TimeSpan.FromSeconds 5.))
	//now cancel:
	cts.Cancel()
	//cancel handler should have run:
	Assert.True(flag.WaitOne(TimeSpan.FromSeconds 5.))
	for _i = 1 to 300 do test()
	[<Fact>]
	member this.``OnCancel.RaceBetweenCancellationAndDispose``() =
	let flag = ref 0
	let cts = new System.Threading.CancellationTokenSource()
	let go = async {
	use disp =
	cts.Cancel()
	{ new IDisposable with
	override _.Dispose() = incr flag }
	while true do
	do! Async.Sleep 50
	}
	try
	Async.RunSynchronously (go, cancellationToken = cts.Token)
	with
	:? System.OperationCanceledException -> ()
	Assert.AreEqual(1, !flag)
	[<Fact>]
	member this.``OnCancel.CancelThatWasSignalledBeforeRunningTheComputation``() =
	let test() =
	let cts = new System.Threading.CancellationTokenSource()
	let go e (flag : bool ref) = async {
	let! _ = Async.AwaitWaitHandle e
	use! _holder = Async.OnCancel(fun () -> flag := true)
	while true do
	do! Async.Sleep 100
	}
	let evt = new System.Threading.ManualResetEvent(false)
	let finish = new System.Threading.ManualResetEvent(false)
	let cancelledWasCalled = ref false
	Async.StartWithContinuations(go evt cancelledWasCalled, ignore, ignore, (fun _ -> finish.Set() |> ignore), cancellationToken = cts.Token)
	evt.Set() |> ignore
	cts.Cancel()
	let ok = wait finish 3000
	Assert.True(ok, "Computation should be completed")
	Assert.False(!cancelledWasCalled, "Cancellation handler should not be called")
	for _i = 1 to 3 do test()

The signed build can fail with stack traces that look like:

The active test run was aborted. Reason: Test host process crashed : go

Unhandled exception. System.Threading.Tasks.TaskCanceledException: A task was canceled.

   at Microsoft.FSharp.Control.AsyncPrimitives.Start@893-1.Invoke(ExceptionDispatchInfo edi)
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 104
   at Microsoft.FSharp.Control.AsyncPrimitives.continuation@951-2(AsyncActivation`1 ctxt, Boolean useCcontForTaskCancellation, Task completedTask) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 952
   at Microsoft.FSharp.Control.AsyncPrimitives.taskContinueWithUnit(Task task, AsyncActivation`1 ctxt, Boolean useCcontForTaskCancellation) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 966
   at <StartupCode$FSharp-Core>.$Async.AwaitTask@1708-2.Invoke(AsyncActivation`1 ctxt) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 1708
   at Microsoft.FSharp.Control.AsyncPrimitives.ProtectedCode[T](AsyncActivation`1 ctxt, FSharpFunc`2 userCode) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 453
   at <StartupCode$FSharp-Core>.$Async.OnCancel@1690-5.Invoke(AsyncActivation`1 ctxt) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 1690
   at Microsoft.FSharp.Control.Trampoline.Execute(FSharpFunc`2 firstAction) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 104
   at <StartupCode$FSharp-Core>.$Async.-ctor@155-1.Invoke(Object o) in /Users/runner/work/1/s/src/fsharp/FSharp.Core/async.fs:line 157
   at System.Threading.ThreadPoolWorkQueue.Dispatch()

[psfinaki]

Hey those are still flaky and not running. Related: #17662.