Fail runServer quickly if the port is not available

Summary:
Adds an early check that the Metro server port is available before trying to set up the bundler. This allows integrators to give faster feedback to the user in this case.

Changelog:

* **Fix**: Fail `runServer` quickly if the port is not available.

Reviewed By: GijsWeterings

Differential Revision: D35849394

fbshipit-source-id: ea01c47a0ff78783670113ede6d8e86b05e76b1e

Author: motiz88

packages/metro/src/index.js

@@ -37,6 +37,7 @@ const http = require('http');
 const https = require('https');
 const {getDefaultConfig, loadConfig, mergeConfig} = require('metro-config');
 const {InspectorProxy} = require('metro-inspector-proxy');
+const net = require('net');
 const {parse} = require('url');
 const ws = require('ws');
 
@@ -225,6 +226,8 @@ exports.runServer = async (
     websocketEndpoints = {},
   }: RunServerOptions,
 ): Promise<HttpServer | HttpsServer> => {
+  await earlyPortCheck(host, config.server.port);
+
   if (secure != null || secureCert != null || secureKey != null) {
     // eslint-disable-next-line no-console
     console.warn(
@@ -463,3 +466,17 @@ exports.attachMetroCli = function (
   }
   return yargs;
 };
+
+async function earlyPortCheck(host: void | string, port: number) {
+  const server = net.createServer(c => c.end());
+  try {
+    await new Promise((resolve, reject) => {
+      server.on('error', err => {
+        reject(err);
+      });
+      server.listen(port, host, undefined, () => resolve());
+    });
+  } finally {
+    await new Promise(resolve => server.close(() => resolve()));
+  }
+}