prevent endless async loop on Receive (#3498)

* prevent endless async loop on Receive

* don't run update loop on ui thread forever

* Update LanguageService.fs

src/fsharp/FSharp.Core/control.fs

@@ -2084,9 +2084,8 @@ namespace Microsoft.FSharp.Control
                             // If it isn't, then create it, and go back to the start to 
                             // check arrivals again.
                             match pulse with
-                            | null -> 
-                                if timeout >= 0 || cancellationSupported then 
-                                    ensurePulse() |> ignore
+                            | null when timeout >= 0 || cancellationSupported ->
+                                ensurePulse() |> ignore
                                 return! processFirstArrival()
                             | _ -> 
                                 // Wait until we have been notified about a message. When that happens, rescan the arrivals

vsintegration/src/FSharp.Editor/LanguageService/LanguageService.fs

@@ -352,7 +352,9 @@ and
                         while true do
                             let! siteProvider = inbox.Receive()
                             do! Async.SwitchToContext ctx
-                            this.SetupProjectFile(siteProvider, this.Workspace, "SetupProjectsAfterSolutionOpen") }
+                            this.SetupProjectFile(siteProvider, this.Workspace, "SetupProjectsAfterSolutionOpen")
+                            do! Async.SwitchToThreadPool()
+                    }
 
                 use _ = Events.SolutionEvents.OnAfterOpenProject |> Observable.subscribe ( fun args ->
                     match args.Hierarchy with