Add support for language server exit & shutdown.

src/VisualStudio/Core/Def/Implementation/LanguageClient/AbstractLanguageServerClient.cs

@@ -68,8 +68,6 @@ public AbstractLanguageServerClient(LanguageServerProtocol languageServerProtoco
 
         public Task<Connection> ActivateAsync(CancellationToken token)
         {
-            Contract.ThrowIfFalse(_languageServer == null, "This language server has already been initialized");
-
             var (clientStream, serverStream) = FullDuplexStream.CreatePair();
             _languageServer = new InProcLanguageServer(serverStream, serverStream, _languageServerProtocol, _workspace, _diagnosticService, clientName: _diagnosticsClientName);
             return Task.FromResult(new Connection(clientStream, clientStream));

src/VisualStudio/Core/Def/Implementation/LanguageClient/InProcLanguageServer.cs

@@ -100,7 +100,31 @@ public async Task InitializedAsync()
         }
 
         [JsonRpcMethod(Methods.ShutdownName)]
-        public object? Shutdown(CancellationToken _) => null;
+        public Task ShutdownAsync(CancellationToken _)
+        {
+            _diagnosticService.DiagnosticsUpdated -= DiagnosticService_DiagnosticsUpdated;
+
+            return Task.CompletedTask;
+        }
+
+        [JsonRpcMethod(Methods.ExitName)]
+        public Task ExitAsync(CancellationToken _)
+        {
+            try
+            {
+                if (!_jsonRpc.IsDisposed)
+                {
+                    _jsonRpc.Dispose();
+                }
+            }
+            catch (Exception)
+            {
+                // Swallow exceptions thrown by disposing our JsonRpc object. Disconnected events can potentially throw their own exceptions so
+                // we purposefully ignore all of those exceptions in an effort to shutdown gracefully.
+            }
+
+            return Task.CompletedTask;
+        }
 
         [JsonRpcMethod(Methods.ExitName)]
         public void Exit()