Perform updates to Stop Language Server Timer synchronously. (#670)

Multiple Timer threads can be left running if
LanguageServerWrapper.startStopTimer() runs in parallel with
LanguageServerWrapper.removeStopTimer() or another startStopTimer(). We
prevent this issue by reducing the Timer threads created down to one,
and then only scheduling or cancelling TimerTasks on that Timer thread
synchronously.

This should fix issue #644.

org.eclipse.lsp4e/src/org/eclipse/lsp4e/LanguageServerWrapper.java

@@ -156,7 +156,8 @@ public void dirtyStateChanged(IFileBuffer buffer, boolean isDirty) {
 	private LanguageServer languageServer;
 	private LanguageClientImpl languageClient;
 	private ServerCapabilities serverCapabilities;
-	private Timer timer;
+	private final Timer timer = new Timer("Stop Language Server Task Processor"); //$NON-NLS-1$
+	private TimerTask stopTimerTask;
 	private AtomicBoolean stopping = new AtomicBoolean(false);
 
 	private final ExecutorService dispatcher;
@@ -392,22 +393,28 @@ public boolean isActive() {
 		return this.launcherFuture != null && !this.launcherFuture.isDone() && !this.launcherFuture.isCancelled();
 	}
 
-	private void removeStopTimer() {
-		if (timer != null) {
-			timer.cancel();
-			timer = null;
+	private void removeStopTimerTask() {
+		synchronized (timer) {
+			if (stopTimerTask != null) {
+				stopTimerTask.cancel();
+				stopTimerTask = null;
+			}
 		}
 	}
 
-	private void startStopTimer() {
-		timer = new Timer("Stop Language Server Timer"); //$NON-NLS-1$
-
-		timer.schedule(new TimerTask() {
-			@Override
-			public void run() {
-				stop();
+	private void startStopTimerTask() {
+		synchronized (timer) {
+			if (stopTimerTask != null) {
+				stopTimerTask.cancel();
 			}
-		}, TimeUnit.SECONDS.toMillis(this.serverDefinition.lastDocumentDisconnectedTimeout));
+			stopTimerTask = new TimerTask() {
+				@Override
+				public void run() {
+					stop();
+				}
+			};
+			timer.schedule(stopTimerTask, TimeUnit.SECONDS.toMillis(this.serverDefinition.lastDocumentDisconnectedTimeout));
+		}
 	}
 
 	/**
@@ -425,7 +432,7 @@ public synchronized void stop() {
 		if (alreadyStopping) {
 			return;
 		}
-		removeStopTimer();
+		removeStopTimerTask();
 		if (this.initializeFuture != null) {
 			this.initializeFuture.cancel(true);
 			this.initializeFuture = null;
@@ -558,7 +565,7 @@ private boolean supportsWorkspaceFolderCapability() {
 	 * @noreference internal so far
 	 */
 	private @Nullable CompletableFuture<@NonNull LanguageServerWrapper> connect(@NonNull URI uri, IDocument document) throws IOException {
-		removeStopTimer();
+		removeStopTimerTask();
 		if (this.connectedDocuments.containsKey(uri)) {
 			return CompletableFuture.completedFuture(this);
 		}
@@ -601,8 +608,7 @@ public CompletableFuture<Void> disconnect(URI uri) {
 		}
 		if (this.connectedDocuments.isEmpty()) {
 			if (this.serverDefinition.lastDocumentDisconnectedTimeout != 0) {
-				removeStopTimer();
-				startStopTimer();
+				startStopTimerTask();
 			} else {
 				stop();
 			}