Fix #159: LanguageServer.Shutdown hangs on flush

The change fixes an issue in the LanguageServer's Shutdown method where
buffered output in the OutputDebouncer gets flushed before shutting down.
The problem here is the Wait call which blocks the message dispatcher
thread from completing the OutputDebouncer.OnFlush method's SendEvent
call.  The fix is to change the Shutdown method to async so that the
Flush call can be awaited, unblocking the message dispatcher thread's
SynchronizationContext for the SendEvent call to complete.

Author: daviwil

src/PowerShellEditorServices.Protocol/Server/LanguageServer.cs

@@ -80,10 +80,10 @@ protected override void Initialize()
             this.SetRequestHandler(DebugAdapterMessages.EvaluateRequest.Type, this.HandleEvaluateRequest);
         }
 
-        protected override void Shutdown()
+        protected override async Task Shutdown()
         {
             // Make sure remaining output is flushed before exiting
-            this.outputDebouncer.Flush().Wait();
+            await this.outputDebouncer.Flush();
 
             Logger.Write(LogLevel.Normal, "Language service is shutting down...");
 

src/PowerShellEditorServices.Protocol/Server/LanguageServerBase.cs

@@ -34,11 +34,9 @@ protected override Task OnStart()
             return Task.FromResult(true);
         }
 
-        protected override Task OnStop()
+        protected override async Task OnStop()
         {
-            this.Shutdown();
-
-            return Task.FromResult(true);
+            await this.Shutdown();
         }
 
         /// <summary>
@@ -49,37 +47,37 @@ protected override Task OnStop()
 
         /// <summary>
         /// Can be overridden by the subclass to provide shutdown
-        /// logic before the server exits.
+        /// logic before the server exits.  Subclasses do not need
+        /// to invoke or return the value of the base implementation.
         /// </summary>
-        protected virtual void Shutdown()
+        protected virtual Task Shutdown()
         {
             // No default implementation yet.
+            return Task.FromResult(true);
         }
 
-        private Task HandleShutdownRequest(
+        private async Task HandleShutdownRequest(
             object shutdownParams,
             RequestContext<object> requestContext)
         {
             // Allow the implementor to shut down gracefully
-            this.Shutdown();
+            await this.Shutdown();
 
-            return requestContext.SendResult(new object());
+            await requestContext.SendResult(new object());
         }
 
-        private Task HandleExitNotification(
+        private async Task HandleExitNotification(
             object exitParams,
             EventContext eventContext)
         {
             // Stop the server channel
-            this.Stop();
+            await this.Stop();
 
             // Notify any waiter that the server has exited
             if (this.serverExitedTask != null)
             {
                 this.serverExitedTask.SetResult(true);
             }
-
-            return Task.FromResult(true);
         }
     }
 }