fix(updatenotification): don't spawn a child process when running under PM2 (#4166)

Previously, `nodeRestart()` would spawn a detached child and exit. Under
PM2 that's a problem: PM2 also respawns on exit, so both race to bind
the same port.

The fix: When `process.env.pm_id` is set, just exit and let PM2 handle
the restart.

The spawn logic is moved into its own method so it can be tested
cleanly.

Partially fixes #4165

defaultmodules/updatenotification/update_helper.js

@@ -130,21 +130,43 @@ class Updater {
 		});
 	}
 
-	// restart MagicMirror with the same start command as the current process
+	/**
+	 * Restart the current MagicMirror process after a successful auto-update.
+	 * Under PM2 just exit and let PM2 respawn — spawning a child here would
+	 * race against PM2 and cause an EADDRINUSE conflict (see issue #4165).
+	 * Standalone: spawn a detached clone of the current process, then exit.
+	 */
 	nodeRestart () {
 		Log.info("Restarting MagicMirror...");
-		const out = process.stdout;
-		const err = process.stderr;
 
-		// Restart with the same binary and arguments as the current process
-		const binary = process.argv[0];
-		const args = process.argv.slice(1);
-		const options = { cwd: this.root_path, detached: true, stdio: ["ignore", out, err] };
-		const subprocess = Spawn(binary, args, options);
-		subprocess.unref(); // allow the current process to exit without waiting for the subprocess
+		const isManagedByPm2 = process.env.pm_id !== undefined;
+		if (isManagedByPm2) {
+			Log.info("Running under PM2 — exiting for PM2 to respawn.");
+			process.exit(0);
+			return;
+		}
+
+		this._spawnDetachedSelf();
 		process.exit();
 	}
 
+	/**
+	 * Spawn a detached clone of the current Node process (same binary + argv),
+	 * wired to the parent's stdout/stderr.
+	 */
+	_spawnDetachedSelf () {
+		const nodeBinary = process.argv[0];
+		const nodeArgs = process.argv.slice(1);
+		const spawnOptions = {
+			cwd: this.root_path,
+			detached: true,
+			stdio: ["ignore", process.stdout, process.stderr]
+		};
+
+		const child = Spawn(nodeBinary, nodeArgs, spawnOptions);
+		child.unref();
+	}
+
 	// check if module is MagicMirror
 	isMagicMirror (module) {
 		if (module === "MagicMirror") return true;

tests/unit/functions/update_helper_spec.js

@@ -87,4 +87,33 @@ describe("UpdateHelper", () => {
 		vi.advanceTimersByTime(3000);
 		expect(nodeRestartSpy).toHaveBeenCalledTimes(1);
 	});
+
+	describe("nodeRestart", () => {
+		it("exits without spawning when running under PM2", async () => {
+			process.env.pm_id = "0";
+
+			const updater = await createUpdater();
+			const exitSpy = vi.spyOn(process, "exit").mockImplementation(() => {});
+			const spawnSpy = vi.spyOn(updater, "_spawnDetachedSelf").mockImplementation(() => {});
+
+			updater.nodeRestart();
+
+			expect(exitSpy).toHaveBeenCalledWith(0);
+			expect(spawnSpy).not.toHaveBeenCalled();
+		});
+
+		it("spawns a detached child process when not running under PM2", async () => {
+			delete process.env.pm_id;
+
+			const updater = await createUpdater();
+			const exitSpy = vi.spyOn(process, "exit").mockImplementation(() => {});
+			const spawnSpy = vi.spyOn(updater, "_spawnDetachedSelf").mockImplementation(() => {});
+
+			updater.nodeRestart();
+
+			expect(spawnSpy).toHaveBeenCalledOnce();
+			expect(exitSpy).toHaveBeenCalledOnce();
+			expect(exitSpy).not.toHaveBeenCalledWith(0);
+		});
+	});
 });