fix: detect port-in-use before starting (#1234)

Add a port availability pre-check before starting the whistle server.
When the port is already in use, output a friendly error message to
stderr and exit, instead of crashing with an uncaught exception.

Compared to the previous attempt (PR #1313), this fix:
- Works for all startup modes, not just cluster mode
- Uses console.error + process.exit instead of throw in async callback
- Does not include unrelated changes (cluster mode, config loading, etc.)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: echopi

index.js

@@ -67,6 +67,15 @@ if (!net._normalizeConnectArgs) {
   };
 }
 
+function checkPortAvailable(port, host, cb) {
+  var server = net.createServer();
+  server.unref();
+  server.on('error', cb);
+  server.listen(port, host || '0.0.0.0', function() {
+    server.close(cb);
+  });
+}
+
 module.exports = function(options, callback) {
   if (typeof options === 'function') {
     callback = options;
@@ -75,6 +84,22 @@ module.exports = function(options, callback) {
   if (options && options.debugMode) {
     env.PFORK_MODE = 'bind';
   }
-  require('./lib/config').extend(options);
-  return require('./lib')(callback);
+  var conf = require('./lib/config').extend(options);
+  if (options && options.server) {
+    return require('./lib')(callback, options.server);
+  }
+  checkPortAvailable(conf.port, conf.host, function(err) {
+    if (err) {
+      if (err.code === 'EADDRINUSE') {
+        var addr = (conf.host ? conf.host + ':' : '') + conf.port;
+        console.error('[!] Port ' + addr + ' is already in use.');
+        console.error('[i] Use -p <port> to specify another port.');
+      } else {
+        console.error(err.stack || err.message || err);
+      }
+      process.exit(1);
+      return;
+    }
+    require('./lib')(callback);
+  });
 };