Watcher backends: merge 'add' and 'change' events into 'touch'

Summary:
Refactor watcher backends so that they're not required to disambiguate "add" file events from "change" file events, instead emitting ambiguous "touch" events.

This distinction isn't typically available from the host OS (`fsevents`, `ReadDirectoryChangesW`, `inotify`), so for the backends to supply it, they must track all files to know whether they existed prior to the OS event. Watchman does this by design anyway, and typically only once per session, but both our `FSEventsWatcher` and `NodeWatcher` currently crawl the whole directory tree and keep an in-memory list of files, and must do so on each Metro start.

We don't need this information from the watchers anyway, because we can already infer new vs modified according to whether the file is present in `TreeFS` (ie, from the crawl, or subsequent event). Therefore we *don't* need to reduce the granularity of `metro-file-map`'s *public* events, only the internal ones.

By simplifying this, we can follow up by taking a huge burden off the watcher backends.

Changelog: Internal

Differential Revision: D67579233

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

@@ -22,7 +22,7 @@ import type {AbortSignal} from 'node-abort-controller';
 
 import nodeCrawl from './crawlers/node';
 import watchmanCrawl from './crawlers/watchman';
-import {ADD_EVENT, CHANGE_EVENT} from './watchers/common';
+import {TOUCH_EVENT} from './watchers/common';
 import FSEventsWatcher from './watchers/FSEventsWatcher';
 import NodeWatcher from './watchers/NodeWatcher';
 import WatchmanWatcher from './watchers/WatchmanWatcher';
@@ -210,7 +210,7 @@ export class Watcher extends EventEmitter {
           watcher.on('all', (change: WatcherBackendChangeEvent) => {
             const basename = path.basename(change.relativePath);
             if (basename.startsWith(this._options.healthCheckFilePrefix)) {
-              if (change.event === ADD_EVENT || change.event === CHANGE_EVENT) {
+              if (change.event === TOUCH_EVENT) {
                 debug(
                   'Observed possible health check cookie: %s in %s',
                   change.relativePath,

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

@@ -1621,13 +1621,13 @@ describe('FileMap', () => {
       `;
       const e = mockEmitters[path.join('/', 'project', 'fruits')];
       e.emit('all', {
-        event: 'add',
+        event: 'touch',
         relativePath: 'Tomato.js',
         root: path.join('/', 'project', 'fruits'),
         metadata: MOCK_CHANGE_FILE,
       });
       e.emit('all', {
-        event: 'change',
+        event: 'touch',
         relativePath: 'Pear.js',
         root: path.join('/', 'project', 'fruits'),
         metadata: MOCK_CHANGE_FILE,
@@ -1662,13 +1662,13 @@ describe('FileMap', () => {
         // Tomato!
       `;
       e.emit('all', {
-        event: 'change',
+        event: 'touch',
         relativePath: 'Tomato.js',
         root: path.join('/', 'project', 'fruits'),
         metadata: MOCK_CHANGE_FILE,
       });
       e.emit('all', {
-        event: 'change',
+        event: 'touch',
         relativePath: 'Tomato.js',
         root: path.join('/', 'project', 'fruits'),
         metadata: MOCK_CHANGE_FILE,
@@ -1683,25 +1683,22 @@ describe('FileMap', () => {
         const {fileSystem} = await hm.build();
         const fruitsRoot = path.join('/', 'project', 'fruits');
         const e = mockEmitters[fruitsRoot];
-        mockFs[path.join(fruitsRoot, 'Tomato.js')] = `
-        // Tomato!
-      `;
         e.emit('all', {
-          event: 'change',
-          relativePath: 'Tomato.js',
+          event: 'touch',
+          relativePath: 'Strawberry.js',
           root: fruitsRoot,
           metadata: MOCK_CHANGE_FILE,
         });
         e.emit('all', {
-          event: 'change',
+          event: 'touch',
           relativePath: 'LinkToStrawberry.js',
           root: fruitsRoot,
           metadata: MOCK_CHANGE_LINK,
         });
         const {eventsQueue} = await waitForItToChange(hm);
         expect(eventsQueue).toEqual([
           {
-            filePath: path.join(fruitsRoot, 'Tomato.js'),
+            filePath: path.join(fruitsRoot, 'Strawberry.js'),
             metadata: MOCK_CHANGE_FILE,
             type: 'change',
           },
@@ -1718,25 +1715,22 @@ describe('FileMap', () => {
         const {fileSystem} = await hm.build();
         const fruitsRoot = path.join('/', 'project', 'fruits');
         const e = mockEmitters[fruitsRoot];
-        mockFs[path.join(fruitsRoot, 'Tomato.js')] = `
-        // Tomato!
-      `;
         e.emit('all', {
-          event: 'change',
-          relativePath: 'Tomato.js',
+          event: 'touch',
+          relativePath: 'Strawberry.js',
           root: fruitsRoot,
           metadata: MOCK_CHANGE_FILE,
         });
         e.emit('all', {
-          event: 'change',
+          event: 'touch',
           relativePath: 'LinkToStrawberry.js',
           root: fruitsRoot,
           metadata: MOCK_CHANGE_LINK,
         });
         const {eventsQueue} = await waitForItToChange(hm);
         expect(eventsQueue).toEqual([
           {
-            filePath: path.join(fruitsRoot, 'Tomato.js'),
+            filePath: path.join(fruitsRoot, 'Strawberry.js'),
             metadata: MOCK_CHANGE_FILE,
             type: 'change',
           },
@@ -1759,7 +1753,7 @@ describe('FileMap', () => {
         const {fileSystem} = await hm.build();
         const e = mockEmitters[path.join('/', 'project', 'fruits')];
         e.emit('all', {
-          event: 'add',
+          event: 'touch',
           relativePath: 'apple.js',
           root: path.join('/', 'project', 'fruits', 'node_modules', ''),
           metadata: MOCK_CHANGE_FILE,
@@ -1788,13 +1782,13 @@ describe('FileMap', () => {
 
         const e = mockEmitters[path.join('/', 'project', 'fruits')];
         e.emit('all', {
-          event: 'add',
+          event: 'touch',
           relativePath: 'Banana.js',
           root: path.join('/', 'project', 'fruits', ''),
           metadata: MOCK_CHANGE_FILE,
         });
         e.emit('all', {
-          event: 'add',
+          event: 'touch',
           relativePath: 'Banana.unwatched',
           root: path.join('/', 'project', 'fruits', ''),
           metadata: MOCK_CHANGE_FILE,
@@ -1803,7 +1797,7 @@ describe('FileMap', () => {
         const filePath = path.join('/', 'project', 'fruits', 'Banana.js');
         expect(eventsQueue).toHaveLength(1);
         expect(eventsQueue).toEqual([
-          {filePath, metadata: MOCK_CHANGE_FILE, type: 'add'},
+          {filePath, metadata: MOCK_CHANGE_FILE, type: 'change'},
         ]);
         expect(fileSystem.getModuleName(filePath)).toBeDefined();
       },
@@ -1844,7 +1838,7 @@ describe('FileMap', () => {
           link: 'Strawberry.js',
         };
         e.emit('all', {
-          event: 'add',
+          event: 'touch',
           relativePath: 'LinkToStrawberry.ext',
           root: path.join('/', 'project', 'fruits', ''),
           metadata: MOCK_CHANGE_LINK,
@@ -1921,13 +1915,13 @@ describe('FileMap', () => {
         expect(hasteMap.getModule('Orange', 'android')).toBeTruthy();
         const e = mockEmitters[path.join('/', 'project', 'fruits')];
         e.emit('all', {
-          event: 'change',
+          event: 'touch',
           relativePath: 'Orange.ios.js',
           root: path.join('/', 'project', 'fruits'),
           metadata: MOCK_CHANGE_FILE,
         });
         e.emit('all', {
-          event: 'change',
+          event: 'touch',
           relativePath: 'Orange.android.js',
           root: path.join('/', 'project', 'fruits'),
           metadata: MOCK_CHANGE_FILE,
@@ -1994,7 +1988,7 @@ describe('FileMap', () => {
         root: path.join('/', 'project', 'vegetables'),
       });
       mockEmitters[path.join('/', 'project', 'fruits')].emit('all', {
-        event: 'add',
+        event: 'touch',
         relativePath: 'Melon.js',
         root: path.join('/', 'project', 'fruits'),
         metadata: MOCK_CHANGE_FILE,
@@ -2033,13 +2027,13 @@ describe('FileMap', () => {
         `;
         const e = mockEmitters[path.join('/', 'project', 'fruits')];
         e.emit('all', {
-          event: 'change',
+          event: 'touch',
           relativePath: 'Pear.js',
           root: path.join('/', 'project', 'fruits'),
           metadata: MOCK_CHANGE_FILE,
         });
         e.emit('all', {
-          event: 'add',
+          event: 'touch',
           relativePath: 'Pear.js',
           root: path.join('/', 'project', 'fruits', 'another'),
           metadata: MOCK_CHANGE_FILE,
@@ -2081,13 +2075,13 @@ describe('FileMap', () => {
           const {fileSystem} = await hm.build();
           const e = mockEmitters[path.join('/', 'project', 'fruits')];
           e.emit('all', {
-            event: 'change',
+            event: 'touch',
             relativePath: 'Pear.js',
             root: path.join('/', 'project', 'fruits'),
             metadata: MOCK_CHANGE_FILE,
           });
           e.emit('all', {
-            event: 'add',
+            event: 'touch',
             relativePath: 'Pear.js',
             root: path.join('/', 'project', 'fruits', 'another'),
             metadata: MOCK_CHANGE_FILE,
@@ -2139,7 +2133,7 @@ describe('FileMap', () => {
             metadata: MOCK_CHANGE_FILE,
           });
           e.emit('all', {
-            event: 'add',
+            event: 'touch',
             relativePath: 'Pear2.js',
             root: path.join('/', 'project', 'fruits'),
             metadata: MOCK_CHANGE_FILE,
@@ -2164,7 +2158,7 @@ describe('FileMap', () => {
         `;
         const e = mockEmitters[path.join('/', 'project', 'fruits')];
         e.emit('all', {
-          event: 'add',
+          event: 'touch',
           relativePath: 'Pear2.js',
           root: path.join('/', 'project', 'fruits', 'another'),
           metadata: MOCK_CHANGE_FILE,
@@ -2189,13 +2183,13 @@ describe('FileMap', () => {
         // Tomato!
       `;
         e.emit('all', {
-          event: 'change',
+          event: 'touch',
           relativePath: 'tomato.js',
           root: path.join('/', 'project', 'fruits'),
           metadata: MOCK_CHANGE_FOLDER,
         });
         e.emit('all', {
-          event: 'change',
+          event: 'touch',
           relativePath: path.join('tomato.js', 'index.js'),
           root: path.join('/', 'project', 'fruits'),
           metadata: MOCK_CHANGE_FILE,

packages/metro-file-map/src/flow-types.js

@@ -324,7 +324,7 @@ export type ReadOnlyRawMockMap = $ReadOnly<{
 
 export type WatcherBackendChangeEvent =
   | $ReadOnly<{
-      event: 'change' | 'add',
+      event: 'touch',
       relativePath: string,
       root: string,
       metadata: ChangeEventMetadata,

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

@@ -898,14 +898,23 @@ export default class FileMap extends EventEmitter {
       // null, then it is assumed that the watcher does not have capabilities
       // to detect modified time, and change processing proceeds.
       if (
-        change.event === 'change' &&
+        change.event === 'touch' &&
         linkStats != null &&
         change.metadata.modifiedTime != null &&
         linkStats.modifiedTime === change.metadata.modifiedTime
       ) {
         return;
       }
 
+      // Emitted events, unlike memoryless backend events, specify 'add' or
+      // 'change' instead of 'touch'.
+      const eventTypeToEmit =
+        change.event === 'touch'
+          ? linkStats == null
+            ? 'add'
+            : 'change'
+          : 'delete';
+
       const onChangeStartTime = performance.timeOrigin + performance.now();
 
       changeQueue = changeQueue
@@ -915,7 +924,7 @@ export default class FileMap extends EventEmitter {
             nextEmit != null &&
             nextEmit.eventsQueue.find(
               event =>
-                event.type === change.event &&
+                event.type === eventTypeToEmit &&
                 event.filePath === absoluteFilePath &&
                 ((!event.metadata && !change.metadata) ||
                   (event.metadata &&
@@ -935,7 +944,7 @@ export default class FileMap extends EventEmitter {
             const event = {
               filePath: absoluteFilePath,
               metadata,
-              type: change.event,
+              type: eventTypeToEmit,
             };
             if (nextEmit == null) {
               nextEmit = {
@@ -960,9 +969,9 @@ export default class FileMap extends EventEmitter {
             );
           }
 
-          // If the file was added or changed,
+          // If the file was added or modified,
           // parse it and update the haste map.
-          if (change.event === 'add' || change.event === 'change') {
+          if (change.event === 'touch') {
             invariant(
               change.metadata.size != null,
               'since the file exists or changed, it should have known size',

packages/metro-file-map/src/watchers/FSEventsWatcher.js

@@ -33,9 +33,8 @@ try {
   // Optional dependency, only supported on Darwin.
 }
 
-const CHANGE_EVENT = 'change';
+const TOUCH_EVENT = 'touch';
 const DELETE_EVENT = 'delete';
-const ADD_EVENT = 'add';
 const ALL_EVENT = 'all';
 
 /**
@@ -94,7 +93,8 @@ export default class FSEventsWatcher extends EventEmitter {
       });
     });
 
-    debug(`Watching ${this.root}`);
+    const startTime = performance.now();
+    debug('Watching %s', this.root);
 
     this._tracked = new Set();
     const trackPath = (filePath: string) => {
@@ -108,6 +108,11 @@ export default class FSEventsWatcher extends EventEmitter {
         trackPath,
         () => {
           this.emit('ready');
+          debug(
+            'Scanned %s in %d',
+            this.root,
+            (performance.now() - startTime) / 1000,
+          );
           resolve();
         },
         (...args) => {
@@ -141,6 +146,7 @@ export default class FSEventsWatcher extends EventEmitter {
 
   async _handleEvent(filepath: string) {
     const relativePath = path.relative(this.root, filepath);
+    debug('Handling event on %s (root: %s)', relativePath, this.root);
 
     try {
       const stat = await fsPromises.lstat(filepath);
@@ -161,12 +167,8 @@ export default class FSEventsWatcher extends EventEmitter {
         size: stat.size,
       };
 
-      if (this._tracked.has(filepath)) {
-        this._emit({event: CHANGE_EVENT, relativePath, metadata});
-      } else {
-        this._tracked.add(filepath);
-        this._emit({event: ADD_EVENT, relativePath, metadata});
-      }
+      this._emit({event: TOUCH_EVENT, relativePath, metadata});
+      this._tracked.add(filepath);
     } catch (error) {
       if (error?.code !== 'ENOENT') {
         this.emit('error', error);