feat(puppeteer): support convenience env variables (#3190)

We had (and still have) a ton of pull requests to support
PUPPETEER_EXECUTABLE_PATH and PUPPETEER_CHROMIUM_REVISION in puppeteer launcher.

We were hesitant before since env variables are not scoped
and thus don't make a good interface for a library. Now, since we
determined `puppeteer-core` as a library and `puppeteer` as our end-user
product, it's safe to satisfy our user needs.

This patch:
- teaches PUPPETEER_EXECUTABLE_PATH and PUPPETEER_CHROMIUM_REVISION
  env variables to control how Puppeteer launches browser
- makes sure these variables play no role in `puppeteer-core` package.

lib/Launcher.js

@@ -22,7 +22,7 @@ const {Connection} = require('./Connection');
 const {Browser} = require('./Browser');
 const readline = require('readline');
 const fs = require('fs');
-const {helper, assert, debugError} = require('./helper');
+const {helper, debugError} = require('./helper');
 const ChromiumRevision = require(path.join(helper.projectRoot(), 'package.json')).puppeteer.chromium_revision;
 const {TimeoutError} = require('./Errors');
 
@@ -95,10 +95,10 @@ class Launcher {
 
     let chromeExecutable = executablePath;
     if (!executablePath) {
-      const browserFetcher = new BrowserFetcher();
-      const revisionInfo = browserFetcher.revisionInfo(ChromiumRevision);
-      assert(revisionInfo.local, `Chromium revision is not downloaded. Run "npm install" or "yarn install"`);
-      chromeExecutable = revisionInfo.executablePath;
+      const {missingText, executablePath} = resolveExecutablePath();
+      if (missingText)
+        throw new Error(missingText);
+      chromeExecutable = executablePath;
     }
 
     const usePipe = chromeArguments.includes('--remote-debugging-pipe');
@@ -252,9 +252,7 @@ class Launcher {
    * @return {string}
    */
   static executablePath() {
-    const browserFetcher = new BrowserFetcher();
-    const revisionInfo = browserFetcher.revisionInfo(ChromiumRevision);
-    return revisionInfo.executablePath;
+    return resolveExecutablePath().executablePath;
   }
 
   /**
@@ -330,6 +328,27 @@ function waitForWSEndpoint(chromeProcess, timeout) {
   });
 }
 
+/**
+ * @return {{executablePath: string, missingText: ?string}}
+ */
+function resolveExecutablePath() {
+  const executablePath = helper.getEnv('PUPPETEER_EXECUTABLE_PATH');
+  if (executablePath) {
+    const missingText = !fs.existsSync(executablePath) ? 'Tried to use PUPPETEER_EXECUTABLE_PATH env variable to launch browser but did not find any executable at: ' + executablePath : null;
+    return { executablePath, missingText };
+  }
+  const browserFetcher = new BrowserFetcher();
+  const revision = helper.getEnv('PUPPETEER_CHROMIUM_REVISION');
+  if (revision) {
+    const revisionInfo = browserFetcher.revisionInfo(revision);
+    const missingText = !revisionInfo.local ? 'Tried to use PUPPETEER_CHROMIUM_REVISION env variable to launch browser but did not find executable at: ' + revisionInfo.executablePath : null;
+    return {executablePath: revisionInfo.executablePath, missingText};
+  }
+  const revisionInfo = browserFetcher.revisionInfo(ChromiumRevision);
+  const missingText = !revisionInfo.local ? `Chromium revision is not downloaded. Run "npm install" or "yarn install"` : null;
+  return {executablePath: revisionInfo.executablePath, missingText};
+}
+
 /**
  * @typedef {Object} ChromeArgOptions
  * @property {boolean=} headless

lib/helper.js

@@ -20,7 +20,11 @@ const {TimeoutError} = require('./Errors');
 const debugError = require('debug')(`puppeteer:error`);
 /** @type {?Map<string, boolean>} */
 let apiCoverage = null;
-let projectRoot = null;
+
+// Project root will be different for node6-transpiled code.
+const projectRoot = fs.existsSync(path.join(__dirname, '..', 'package.json')) ? path.join(__dirname, '..') : path.join(__dirname, '..', '..');
+const packageJson = require(path.join(projectRoot, 'package.json'));
+
 class Helper {
   /**
    * @param {Function|string} fun
@@ -45,14 +49,21 @@ class Helper {
     }
   }
 
+  /**
+   * @param {string} name
+   * @return {(string|undefined)}
+   */
+  static getEnv(name) {
+    // Ignore all PUPPETEER_* env variables in puppeteer-core package.
+    if (name.startsWith('PUPPETEER_') && packageJson.name === 'puppeteer-core')
+      return undefined;
+    return process.env[name];
+  }
+
   /**
    * @return {string}
    */
   static projectRoot() {
-    if (!projectRoot) {
-      // Project root will be different for node6-transpiled code.
-      projectRoot = fs.existsSync(path.join(__dirname, '..', 'package.json')) ? path.join(__dirname, '..') : path.join(__dirname, '..', '..');
-    }
     return projectRoot;
   }