chore: allow reading stderr of the browser process (#9813)

Co-authored-by: Nikolay Vitkov <34244704+Lightning00Blade@users.noreply.github.com>

packages/browsers/src/launcher.ts

@@ -17,6 +17,7 @@
 import childProcess from 'child_process';
 import os from 'os';
 import path from 'path';
+import readline from 'readline';
 
 import {
   Browser,
@@ -88,6 +89,9 @@ export function launch(opts: LaunchOptions): Process {
   return new Process(opts);
 }
 
+export const CDP_WEBSOCKET_ENDPOINT_REGEX =
+  /^DevTools listening on (ws:\/\/.*)$/;
+
 class Process {
   #executablePath;
   #args: string[];
@@ -245,6 +249,66 @@ class Process {
     }
     this.#clearListeners();
   }
+
+  waitForLineOutput(regex: RegExp, timeout?: number): Promise<string> {
+    if (!this.#browserProcess.stderr) {
+      throw new Error('`browserProcess` does not have stderr.');
+    }
+    const rl = readline.createInterface(this.#browserProcess.stderr);
+    let stderr = '';
+
+    return new Promise((resolve, reject) => {
+      rl.on('line', onLine);
+      rl.on('close', onClose);
+      this.#browserProcess.on('exit', onClose);
+      this.#browserProcess.on('error', onClose);
+      const timeoutId = timeout ? setTimeout(onTimeout, timeout) : 0;
+
+      const cleanup = (): void => {
+        if (timeoutId) {
+          clearTimeout(timeoutId);
+        }
+        rl.off('line', onLine);
+        rl.off('close', onClose);
+        this.#browserProcess.off('exit', onClose);
+        this.#browserProcess.off('error', onClose);
+      };
+
+      function onClose(error?: Error): void {
+        cleanup();
+        reject(
+          new Error(
+            [
+              `Failed to launch the browser process!${
+                error ? ' ' + error.message : ''
+              }`,
+              stderr,
+            ].join('\n')
+          )
+        );
+      }
+
+      function onTimeout(): void {
+        cleanup();
+        reject(
+          new Error(
+            `Timed out after ${timeout} ms while waiting for the WS endpoint URL to appear in stdout!`
+          )
+        );
+      }
+
+      function onLine(line: string): void {
+        stderr += line + '\n';
+        const match = line.match(regex);
+        if (!match) {
+          return;
+        }
+        cleanup();
+        // The RegExp matches, so this will obviously exist.
+        resolve(match[1]!);
+      }
+    });
+  }
 }
 
 const PROCESS_ERROR_EXPLANATION = `Puppeteer was unable to kill the process which ran the browser binary.

packages/browsers/test/src/launcher.spec.ts

@@ -21,7 +21,11 @@ import path from 'path';
 
 import {Browser, BrowserPlatform} from '../../lib/cjs/browsers/browsers.js';
 import {fetch} from '../../lib/cjs/fetch.js';
-import {computeExecutablePath, launch} from '../../lib/cjs/launcher.js';
+import {
+  CDP_WEBSOCKET_ENDPOINT_REGEX,
+  computeExecutablePath,
+  launch,
+} from '../../lib/cjs/launcher.js';
 
 import {testChromeBuildId, testFirefoxBuildId} from './versions.js';
 
@@ -91,6 +95,7 @@ describe('launcher', () => {
       const process = launch({
         executablePath,
         args: [
+          '--headless=new',
           '--use-mock-keychain',
           '--disable-features=DialMediaRouteProvider',
           `--user-data-dir=${path.join(tmpDir, 'profile')}`,
@@ -98,6 +103,27 @@ describe('launcher', () => {
       });
       await process.close();
     });
+
+    it('should allow parsing stderr output of the browser process', async () => {
+      const executablePath = computeExecutablePath({
+        cacheDir: tmpDir,
+        browser: Browser.CHROME,
+        buildId: testChromeBuildId,
+      });
+      const process = launch({
+        executablePath,
+        args: [
+          '--headless=new',
+          '--use-mock-keychain',
+          '--disable-features=DialMediaRouteProvider',
+          '--remote-debugging-port=9222',
+          `--user-data-dir=${path.join(tmpDir, 'profile')}`,
+        ],
+      });
+      const url = await process.waitForLineOutput(CDP_WEBSOCKET_ENDPOINT_REGEX);
+      await process.close();
+      assert.ok(url.startsWith('ws://127.0.0.1:9222/devtools/browser'));
+    });
   });
 
   describe('Firefox', function () {