Fix shutting down a notebook when no session is running (#5560)

Addresses #4043.

### QA Notes

I don't know if we'll be able to repro this one, so we might just have
to review the code changes.

Author: seeM

extensions/positron-notebook-controllers/src/notebookSessionService.ts

@@ -221,10 +221,9 @@ export class NotebookSessionService implements vscode.Disposable {
 		// Construct a wrapping promise that resolves/rejects after the session maps have been updated.
 		const shutdownPromise = (async () => {
 			try {
-				const session = await this.doShutdownRuntimeSession(notebookUri);
+				await this.doShutdownRuntimeSession(notebookUri);
 				this._shuttingDownSessionsByNotebookUri.delete(notebookUri);
 				this.setNotebookSession(notebookUri, undefined);
-				log.info(`Session ${session.metadata.sessionId} is shutdown`);
 			} catch (err) {
 				this._startingSessionsByNotebookUri.delete(notebookUri);
 				throw err;
@@ -236,27 +235,12 @@ export class NotebookSessionService implements vscode.Disposable {
 		return shutdownPromise;
 	}
 
-	async doShutdownRuntimeSession(notebookUri: vscode.Uri): Promise<positron.LanguageRuntimeSession> {
+	async doShutdownRuntimeSession(notebookUri: vscode.Uri): Promise<void> {
 		// Get the notebook's session.
-		let session = this._notebookSessionsByNotebookUri.get(notebookUri);
-
+		const session = await this.getExistingOrPendingSession(notebookUri);
 		if (!session) {
-			// If the notebook's session is still starting, wait for it to finish.
-			const startingSessionPromise = this._startingSessionsByNotebookUri.get(notebookUri) ||
-				this._restartingSessionsByNotebookUri.get(notebookUri);
-			if (startingSessionPromise) {
-				try {
-					session = await startingSessionPromise;
-				} catch (err) {
-					log.error(`Waiting for notebook runtime to start before shutting down failed. Reason ${err}`);
-					throw err;
-				}
-			}
-		}
-
-		// Ensure that we have a session.
-		if (!session) {
-			throw new Error(`Tried to shutdown runtime for notebook without a running runtime: ${notebookUri.path}`);
+			log.debug(`Tried to shutdown runtime for notebook without a running or starting runtime: ${notebookUri.path}`);
+			return;
 		}
 
 		// Start the shutdown sequence.
@@ -288,7 +272,29 @@ export class NotebookSessionService implements vscode.Disposable {
 			throw err;
 		}
 
-		return session;
+		log.info(`Session ${session.metadata.sessionId} is shutdown`);
+	}
+
+	private async getExistingOrPendingSession(notebookUri: vscode.Uri): Promise<positron.LanguageRuntimeSession | undefined> {
+		// Check for an active session first.
+		const activeSession = this._notebookSessionsByNotebookUri.get(notebookUri);
+		if (activeSession) {
+			return activeSession;
+		}
+
+		// Check for a pending session.
+		const pendingSessionPromise = this._startingSessionsByNotebookUri.get(notebookUri) ||
+			this._restartingSessionsByNotebookUri.get(notebookUri);
+		if (pendingSessionPromise) {
+			try {
+				return await pendingSessionPromise;
+			} catch (err) {
+				// No need to log; the error will be handled elsewhere.
+			}
+		}
+
+		// There is no existing or pending session for the notebook.
+		return undefined;
 	}
 
 	/**