Throw if the user requests a port and it's already taken

packages/wmr/src/lib/net-utils.js

@@ -1,6 +1,28 @@
 import net from 'net';
 import os from 'os';
 
+/**
+ * Check if a port is free
+ * @param {number} port
+ * @returns {Promise<boolean>}
+ */
+export async function isPortFree(port) {
+	try {
+		await new Promise((resolve, reject) => {
+			const server = net.createServer();
+			server.unref();
+			server.on('error', reject);
+			server.listen({ port }, () => {
+				server.close(resolve);
+			});
+		});
+		return true;
+	} catch (err) {
+		if (err.code !== 'EADDRINUSE') throw err;
+		return false;
+	}
+}
+
 /**
  * Check if the requested port is free and increase port number
  * sequentially until we find a free port.
@@ -15,19 +37,7 @@ export async function getFreePort(port) {
 
 	// Limit to 20 attempts for now
 	while (!found && attempts <= 20) {
-		try {
-			await new Promise((resolve, reject) => {
-				const server = net.createServer();
-				server.unref();
-				server.on('error', reject);
-				server.listen({ port }, () => {
-					port = server.address().port;
-					found = true;
-					server.close(resolve);
-				});
-			});
-		} catch (err) {
-			if (err.code !== 'EADDRINUSE') throw err;
+		if (!isPortFree(port)) {
 			port++;
 			attempts++;
 		}

packages/wmr/src/start.js

@@ -1,6 +1,6 @@
 import server from './server.js';
 import wmrMiddleware from './wmr-middleware.js';
-import { getFreePort, getServerAddresses } from './lib/net-utils.js';
+import { getFreePort, getServerAddresses, isPortFree } from './lib/net-utils.js';
 import { normalizeOptions } from './lib/normalize-options.js';
 import { setCwd } from './plugins/npm-plugin/registry.js';
 import { formatBootMessage } from './lib/output-utils.js';
@@ -21,7 +21,19 @@ export default async function start(options = {}) {
 
 	options = await normalizeOptions(options, 'start');
 
-	options.port = await getFreePort(options.port || process.env.PORT || 8080);
+	// Don't use another free port if the user explicitely
+	// requested a specific one.
+	const userPort = options.port || process.env.PORT;
+	if (userPort !== undefined) {
+		if (await isPortFree(+userPort)) {
+			options.port = +userPort;
+		} else {
+			throw new Error(`Another process is already running on port ${userPort}. Please choose a different port.`);
+		}
+	} else {
+		options.port = await getFreePort(8080);
+	}
+
 	options.host = options.host || process.env.HOST;
 
 	options.middleware = [].concat(