Add support for language server exit & shutdown. (#44900)

- The language server platform is properly implementing solution close understanding for local language servers and I found that when testing their bits Roslyn would explode gloriously. Turns out shutdown and exit support just wasn't fully enabled yet.
- When the language server platform reboots language servers it re-invokes Activate and because of that Roslyn explodes because the language client has already been activated and there was previously a contract throws check on already being initialized.
- Implemented the `ShutdownAsync` target on the `InProcLanguageServer` to detach from the diagnostic service.
- Implemented the `ExitAsync` target on the `InProcLanguageServer` to properly dispose our `JsonRpc` object.

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

@@ -70,7 +70,7 @@ public AbstractLanguageServerClient(LanguageServerProtocol languageServerProtoco
 
         public Task<Connection> ActivateAsync(CancellationToken token)
         {
-            Contract.ThrowIfFalse(_languageServer == null, "This language server has already been initialized");
+            Contract.ThrowIfTrue(_languageServer?.Running == true, "The language server has not yet shutdown.");
 
             var (clientStream, serverStream) = FullDuplexStream.CreatePair();
             _languageServer = new InProcLanguageServer(serverStream, serverStream, _languageServerProtocol, _workspace,

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

@@ -40,6 +40,7 @@ internal class InProcLanguageServer
         private readonly CodeAnalysis.Workspace _workspace;
 
         private VSClientCapabilities _clientCapabilities;
+        private bool _shuttingDown;
 
         public InProcLanguageServer(Stream inputStream,
             Stream outputStream,
@@ -66,6 +67,8 @@ public InProcLanguageServer(Stream inputStream,
             _clientCapabilities = new VSClientCapabilities();
         }
 
+        public bool Running => !_shuttingDown && !_jsonRpc.IsDisposed;
+
         /// <summary>
         /// Handle the LSP initialize request by storing the client capabilities
         /// and responding with the server capabilities.
@@ -104,11 +107,32 @@ public async Task InitializedAsync()
         }
 
         [JsonRpcMethod(Methods.ShutdownName)]
-        public object? Shutdown(CancellationToken _) => null;
+        public Task ShutdownAsync(CancellationToken _)
+        {
+            Contract.ThrowIfTrue(_shuttingDown, "Shutdown has already been called.");
+
+            _shuttingDown = true;
+            _diagnosticService.DiagnosticsUpdated -= DiagnosticService_DiagnosticsUpdated;
+
+            return Task.CompletedTask;
+        }
 
         [JsonRpcMethod(Methods.ExitName)]
-        public void Exit()
+        public Task ExitAsync(CancellationToken _)
         {
+            Contract.ThrowIfFalse(_shuttingDown, "Shutdown has not been called yet.");
+
+            try
+            {
+                _jsonRpc.Dispose();
+            }
+            catch (Exception e) when (FatalError.ReportWithoutCrash(e))
+            {
+                // 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.TextDocumentDefinitionName, UseSingleObjectParameterDeserialization = true)]