fix: propagate LanguagServerWrapper startup exception to consumers

If there is an exception in the startup of the LanguageServerWrapper,
consumers get a cancellation exception rather than the actual exception
that occurred.
With this commit, this behavior is changed. As a consequence of the
change, calls to LanguageServerWrapper.start() will do nothing after a
failure. To test for this case (and attempt a restart() if feasible),
the startupFailed() method is added.

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

@@ -241,8 +241,8 @@ private List<WorkspaceFolder> getRelevantWorkspaceFolders() {
 	}
 
 	/**
-	 * Starts a language server and triggers initialization. If language server is
-	 * started and active, does nothing. If language server is inactive, restart it.
+	 * Starts a language server and triggers initialization. If language server has been started
+	 * before, does nothing.
 	 */
 	public synchronized void start() {
 		start(false);
@@ -278,7 +278,7 @@ private synchronized void start(boolean forceRestart) {
 				stop();
 			}
 		}
-		if (this.initializeFuture == null) {
+		if (this.initializeFuture == null || forceRestart) {
 			final URI rootURI = getRootURI();
 			final Job job = createInitializeLanguageServerJob();
 			this.launcherFuture = new CompletableFuture<>();
@@ -356,7 +356,7 @@ private synchronized void start(boolean forceRestart) {
 				FileBuffers.getTextFileBufferManager().addFileBufferListener(fileBufferListener);
 				advanceInitializeFutureMonitor();
 			}).exceptionally(e -> {
-				stop();
+				shutdown();
 				final Throwable cause = e.getCause();
 				if (cause instanceof CancellationException c) {
 					throw c;
@@ -486,6 +486,13 @@ public synchronized boolean isActive() {
 		return this.launcherFuture != null && !this.launcherFuture.isDone() && !this.launcherFuture.isCancelled();
 	}
 
+	/**
+	 * @return whether the last startup attempt has failed
+	 */
+	public synchronized boolean startupFailed() {
+		return this.initializeFuture != null && this.initializeFuture.isCompletedExceptionally();
+	}
+
 	private void removeStopTimerTask() {
 		synchronized (timer) {
 			if (stopTimerTask != null) {
@@ -521,6 +528,14 @@ boolean isWrapperFor(LanguageServer server) {
 	}
 
 	public synchronized void stop() {
+		if (this.initializeFuture != null) {
+			initializeFuture.cancel(true);
+			this.initializeFuture= null;
+		}
+		shutdown();
+	}
+
+	private void shutdown() {
 		final boolean alreadyStopping = this.stopping.getAndSet(true);
 		if (alreadyStopping) {
 			return;
@@ -531,11 +546,6 @@ public synchronized void stop() {
 			this.languageClient.dispose();
 		}
 
-		if (this.initializeFuture != null) {
-			this.initializeFuture.cancel(true);
-			this.initializeFuture = null;
-		}
-
 		this.serverCapabilities = null;
 		this.dynamicRegistrations.clear();