fix: Make --inspect-port=0 work

Selecting port zero makes the inspector bind to a random port.

The inspector prints the URL it listens on to stderr.  Parse the
output from the child process to find the port number.

This commit coincidentally also makes `node inspect --port=0 x.js`
work and a regression test to that effect has been added.

Note that connecting to an existing process that listens on a
non-standard port with `node inspect -p <pid>` does not work
because then we never see the output from the inspector.

Fixes: nodejs/node#16469

lib/_inspect.js

@@ -42,18 +42,9 @@ const [ InspectClient, createRepl ] =
 
 const debuglog = util.debuglog('inspect');
 
-const DEBUG_PORT_PATTERN = /^--(?:debug|inspect)(?:-port|-brk)?=(\d{1,5})$/;
-function getDefaultPort() {
-  for (const arg of process.execArgv) {
-    const match = arg.match(DEBUG_PORT_PATTERN);
-    if (match) {
-      return +match[1];
-    }
-  }
-  return 9229;
-}
-
 function portIsFree(host, port, timeout = 2000) {
+  if (port === 0) return Promise.resolve();  // Binding to a random port.
+
   const retryDelay = 150;
   let didTimeOut = false;
 
@@ -110,9 +101,11 @@ function runScript(script, scriptArgs, inspectHost, inspectPort, childPrint) {
         let output = '';
         function waitForListenHint(text) {
           output += text;
-          if (/Debugger listening on/.test(output)) {
+          if (/Debugger listening on ws:\/\/\[?(.+?)\]?:(\d+)\//.test(output)) {
+            const host = RegExp.$1;
+            const port = Number.parseInt(RegExp.$2);
             child.stderr.removeListener('data', waitForListenHint);
-            resolve(child);
+            resolve([child, port, host]);
           }
         }
 
@@ -160,10 +153,11 @@ class NodeInspector {
                                        options.port,
                                        this.childPrint.bind(this));
     } else {
-      this._runScript = () => Promise.resolve(null);
+      this._runScript =
+          () => Promise.resolve([null, options.port, options.host]);
     }
 
-    this.client = new InspectClient(options.port, options.host);
+    this.client = new InspectClient();
 
     this.domainNames = ['Debugger', 'HeapProfiler', 'Profiler', 'Runtime'];
     this.domainNames.forEach((domain) => {
@@ -223,17 +217,16 @@ class NodeInspector {
 
   run() {
     this.killChild();
-    const { host, port } = this.options;
 
-    return this._runScript().then((child) => {
+    return this._runScript().then(([child, port, host]) => {
       this.child = child;
 
       let connectionAttempts = 0;
       const attemptConnect = () => {
         ++connectionAttempts;
         debuglog('connection attempt #%d', connectionAttempts);
         this.stdout.write('.');
-        return this.client.connect()
+        return this.client.connect(port, host)
           .then(() => {
             debuglog('connection established');
             this.stdout.write(' ok');
@@ -288,7 +281,7 @@ class NodeInspector {
 
 function parseArgv([target, ...args]) {
   let host = '127.0.0.1';
-  let port = getDefaultPort();
+  let port = 9229;
   let isRemote = false;
   let script = target;
   let scriptArgs = args;

lib/internal/inspect_client.js

@@ -164,12 +164,12 @@ function decodeFrameHybi17(data) {
 }
 
 class Client extends EventEmitter {
-  constructor(port, host) {
+  constructor() {
     super();
     this.handleChunk = this._handleChunk.bind(this);
 
-    this._port = port;
-    this._host = host;
+    this._port = undefined;
+    this._host = undefined;
 
     this.reset();
   }
@@ -284,7 +284,9 @@ class Client extends EventEmitter {
     });
   }
 
-  connect() {
+  connect(port, host) {
+    this._port = port;
+    this._host = host;
     return this._discoverWebsocketPath()
       .then((urlPath) => this._connectWebsocket(urlPath));
   }

test/cli/launch.test.js

@@ -26,6 +26,46 @@ test('custom port', (t) => {
     });
 });
 
+test('random port', (t) => {
+  const script = Path.join('examples', 'three-lines.js');
+
+  const cli = startCLI(['--port=0', script]);
+
+  return cli.waitForInitialBreak()
+    .then(() => cli.waitForPrompt())
+    .then(() => {
+      t.match(cli.output, 'debug>', 'prints a prompt');
+      t.match(
+        cli.output,
+        /< Debugger listening on /,
+        'forwards child output');
+    })
+    .then(() => cli.quit())
+    .then((code) => {
+      t.equal(code, 0, 'exits with success');
+    });
+});
+
+test('random port with --inspect-port=0', (t) => {
+  const script = Path.join('examples', 'three-lines.js');
+
+  const cli = startCLI([script], ['--inspect-port=0']);
+
+  return cli.waitForInitialBreak()
+    .then(() => cli.waitForPrompt())
+    .then(() => {
+      t.match(cli.output, 'debug>', 'prints a prompt');
+      t.match(
+        cli.output,
+        /< Debugger listening on /,
+        'forwards child output');
+    })
+    .then(() => cli.quit())
+    .then((code) => {
+      t.equal(code, 0, 'exits with success');
+    });
+});
+
 test('examples/three-lines.js', (t) => {
   const script = Path.join('examples', 'three-lines.js');
   const cli = startCLI([script]);

test/cli/start-cli.js

@@ -16,8 +16,8 @@ const BREAK_MESSAGE = new RegExp('(?:' + [
   'exception', 'other', 'promiseRejection',
 ].join('|') + ') in', 'i');
 
-function startCLI(args) {
-  const child = spawn(process.execPath, [CLI, ...args]);
+function startCLI(args, flags = []) {
+  const child = spawn(process.execPath, [...flags, CLI, ...args]);
   let isFirstStdoutChunk = true;
 
   const outputBuffer = [];