Fix Post on disposed mailbox (#17849) (#17922)

* nullify disposed waithandle

* add test

* rns

---------

Co-authored-by: Tomas Grosup <[email protected]>

Author: majocha

docs/release-notes/.FSharp.Core/9.0.200.md

@@ -0,0 +1,9 @@
+### Fixed
+
+* Fix exception on Post after MailboxProcessor was disposed ([Issue #17849](https://github.com/dotnet/fsharp/issues/17849), [PR #17922](https://github.com/dotnet/fsharp/pull/17922))
+
+### Added
+
+### Changed
+
+### Breaking Changes

src/FSharp.Core/mailbox.fs

@@ -340,10 +340,11 @@ type Mailbox<'Msg>(cancellationSupported: bool, isThrowExceptionAfterDisposed: b
                     inboxStore.Clear()
 
                 arrivals.Clear()
-                isDisposed <- true)
+                isDisposed <- true
 
-            if isNotNull pulse then
-                (pulse :> IDisposable).Dispose()
+                if isNotNull pulse then
+                    (pulse :> IDisposable).Dispose()
+                    pulse <- null)
 
 #if DEBUG
     member x.UnsafeContents = (x.inbox, arrivals, pulse, savedCont) |> box

tests/FSharp.Core.UnitTests/FSharp.Core/Microsoft.FSharp.Control/MailboxProcessorType.fs

@@ -557,3 +557,42 @@ module MailboxProcessorType =
             }
 
         Assert.True(iteration.Result > 1, "TryScan did not timeout")
+
+    let ordered() =
+        let mutable current = 1
+        fun n ->
+            if n < current then failwith $"step {n} already happened"
+            SpinWait.SpinUntil(fun () -> n = current)
+            current <- n + 1
+
+    // See https://github.com/dotnet/fsharp/issues/17849
+    [<Fact>]
+    let ``Disposed MailboxProcessor does not throw on Post`` () =
+        task {
+            let step = ordered()
+      
+            let cts = new CancellationTokenSource()
+            let mb =
+                MailboxProcessor.Start( (fun inbox ->
+                    async {
+                        step 1
+                        do! inbox.Receive()
+                        do! inbox.Receive()
+                        return ()
+                    }),
+                    cancellationToken = cts.Token
+                )
+
+            step 2
+            // ensure pulse gets created
+            do! Task.Delay 100
+            mb.Post()
+
+            mb.Dispose()
+
+            do! Task.Delay 100
+
+            mb.Post()
+
+            cts.Cancel()
+        }