Make Watchman existence check lazy and async

Summary:
(This is a change I originally made "upstream" to `jest-haste-map` in jestjs/jest#12675)

Currently, `metro-file-map` checks for Watchman existence at import time (top level) with a blocking `execSync` call. This has a few downsides:

 - Blocking at the top level holds up main thread (by 20-200ms) at a time when there's CPU work to be done.
 - `execSync` spawns a shell, which isn't necessary here - `execFile` is more efficient.
 - The exec happens totally unnecessarily even if the consumer specifies `useWatchman: false`.

This diff extracts this to an async utility function, executed lazily and memoized by the core class.

Reviewed By: huntie

Differential Revision: D39772133

fbshipit-source-id: 7f7d5acd3506cb0c27a8f67e8813e867f25a83ba

packages/metro-file-map/src/__tests__/index-test.js

@@ -17,9 +17,9 @@ function mockHashContents(contents) {
   return crypto.createHash('sha1').update(contents).digest('hex');
 }
 
-jest.mock('child_process', () => ({
-  // If this does not throw, we'll use the (mocked) watchman crawler
-  execSync() {},
+jest.mock('../lib/canUseWatchman', () => ({
+  __esModule: true,
+  default: async () => true,
 }));
 
 jest.mock('jest-worker', () => ({

packages/metro-file-map/src/index.js

@@ -36,6 +36,7 @@ import {DiskCacheManager} from './cache/DiskCacheManager';
 import H from './constants';
 import getMockName from './getMockName';
 import HasteFS from './HasteFS';
+import canUseWatchman from './lib/canUseWatchman';
 import deepCloneInternalData from './lib/deepCloneInternalData';
 import * as fastPath from './lib/fast_path';
 import getPlatformExtension from './lib/getPlatformExtension';
@@ -48,7 +49,6 @@ import NodeWatcher from './watchers/NodeWatcher';
 // $FlowFixMe[untyped-import] - WatchmanWatcher
 import WatchmanWatcher from './watchers/WatchmanWatcher';
 import {getSha1, worker} from './worker';
-import {execSync} from 'child_process';
 import EventEmitter from 'events';
 import invariant from 'invariant';
 // $FlowFixMe[untyped-import] - jest-regex-util
@@ -145,14 +145,6 @@ const VCS_DIRECTORIES = ['.git', '.hg']
   .map(vcs => escapePathForRegex(path.sep + vcs + path.sep))
   .join('|');
 
-const canUseWatchman = ((): boolean => {
-  try {
-    execSync('watchman --version', {stdio: ['ignore']});
-    return true;
-  } catch {}
-  return false;
-})();
-
 /**
  * HasteMap is a JavaScript implementation of Facebook's haste module system.
  *
@@ -233,6 +225,7 @@ const canUseWatchman = ((): boolean => {
 export default class HasteMap extends EventEmitter {
   _buildPromise: ?Promise<InternalDataObject>;
   _cachePath: Path;
+  _canUseWatchmanPromise: Promise<boolean>;
   _changeInterval: ?IntervalID;
   _console: Console;
   _options: InternalOptions;
@@ -787,7 +780,7 @@ export default class HasteMap extends EventEmitter {
     return this._worker;
   }
 
-  _crawl(hasteMap: InternalData): Promise<?(
+  async _crawl(hasteMap: InternalData): Promise<?(
     | Promise<{
         changedFiles?: FileData,
         hasteMap: InternalData,
@@ -798,8 +791,7 @@ export default class HasteMap extends EventEmitter {
     this._options.perfLogger?.point('crawl_start');
     const options = this._options;
     const ignore = (filePath: string) => this._ignore(filePath);
-    const crawl =
-      canUseWatchman && this._options.useWatchman ? watchmanCrawl : nodeCrawl;
+    const crawl = (await this._shouldUseWatchman()) ? watchmanCrawl : nodeCrawl;
     const crawlerOptions: CrawlerOptions = {
       abortSignal: this._crawlerAbortController.signal,
       computeSha1: options.computeSha1,
@@ -851,11 +843,11 @@ export default class HasteMap extends EventEmitter {
   /**
    * Watch mode
    */
-  _watch(hasteMap: InternalData): Promise<void> {
+  async _watch(hasteMap: InternalData): Promise<void> {
     this._options.perfLogger?.point('watch_start');
     if (!this._options.watch) {
       this._options.perfLogger?.point('watch_end');
-      return Promise.resolve();
+      return;
     }
 
     // In watch mode, we'll only warn about module collisions and we'll retain
@@ -864,12 +856,11 @@ export default class HasteMap extends EventEmitter {
     this._options.retainAllFiles = true;
 
     // WatchmanWatcher > FSEventsWatcher > sane.NodeWatcher
-    const WatcherImpl =
-      canUseWatchman && this._options.useWatchman
-        ? WatchmanWatcher
-        : FSEventsWatcher.isSupported()
-        ? FSEventsWatcher
-        : NodeWatcher;
+    const WatcherImpl = (await this._shouldUseWatchman())
+      ? WatchmanWatcher
+      : FSEventsWatcher.isSupported()
+      ? FSEventsWatcher
+      : NodeWatcher;
 
     const extensions = this._options.extensions;
     const ignorePattern = this._options.ignorePattern;
@@ -1067,12 +1058,10 @@ export default class HasteMap extends EventEmitter {
 
     this._changeInterval = setInterval(emitChange, CHANGE_INTERVAL);
 
-    return Promise.all(this._options.roots.map(createWatcher)).then(
-      watchers => {
-        this._watchers = watchers;
-        this._options.perfLogger?.point('watch_end');
-      },
-    );
+    await Promise.all(this._options.roots.map(createWatcher)).then(watchers => {
+      this._watchers = watchers;
+      this._options.perfLogger?.point('watch_end');
+    });
   }
 
   /**
@@ -1163,6 +1152,16 @@ export default class HasteMap extends EventEmitter {
     );
   }
 
+  async _shouldUseWatchman(): Promise<boolean> {
+    if (!this._options.useWatchman) {
+      return false;
+    }
+    if (!this._canUseWatchmanPromise) {
+      this._canUseWatchmanPromise = canUseWatchman();
+    }
+    return this._canUseWatchmanPromise;
+  }
+
   _createEmptyMap(): InternalData {
     return {
       clocks: new Map(),

packages/metro-file-map/src/lib/__tests__/canUseWatchman-test.js

@@ -0,0 +1,44 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow strict
+ */
+
+import canUseWatchman from '../canUseWatchman';
+
+const mockExecFile = jest.fn();
+jest.mock('child_process', () => ({
+  execFile: (...args) => mockExecFile(...args),
+}));
+
+describe('canUseWatchman', () => {
+  beforeEach(() => {
+    jest.clearAllMocks();
+  });
+
+  it('executes watchman --version and returns true on success', async () => {
+    mockExecFile.mockImplementation((file, args, cb) => {
+      expect(file).toBe('watchman');
+      expect(args).toStrictEqual(['--version']);
+      cb(null, {stdout: 'v123'});
+    });
+    expect(await canUseWatchman()).toBe(true);
+    expect(mockExecFile).toHaveBeenCalledWith(
+      'watchman',
+      ['--version'],
+      expect.any(Function),
+    );
+  });
+
+  it('returns false when execFile fails', async () => {
+    mockExecFile.mockImplementation((file, args, cb) => {
+      cb(new Error());
+    });
+    expect(await canUseWatchman()).toBe(false);
+    expect(mockExecFile).toHaveBeenCalled();
+  });
+});

packages/metro-file-map/src/lib/canUseWatchman.js

@@ -0,0 +1,21 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow strict
+ */
+
+import {execFile} from 'child_process';
+import {promisify} from 'util';
+
+export default async function canUseWatchman(): Promise<boolean> {
+  try {
+    await promisify(execFile)('watchman', ['--version']);
+    return true;
+  } catch {
+    return false;
+  }
+}