watch: allow listening for grouped changes

PR-URL: https://github.com/nodejs/node/pull/52722
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/watch_mode/files_watcher.js

@@ -19,7 +19,7 @@ const { addAbortListener } = require('internal/events/abort_listener');
 const { watch } = require('fs');
 const { fileURLToPath } = require('internal/url');
 const { resolve, dirname } = require('path');
-const { setTimeout } = require('timers');
+const { setTimeout, clearTimeout } = require('timers');
 
 const supportsRecursiveWatching = process.platform === 'win32' ||
   process.platform === 'darwin';
@@ -27,9 +27,10 @@ const supportsRecursiveWatching = process.platform === 'win32' ||
 class FilesWatcher extends EventEmitter {
   #watchers = new SafeMap();
   #filteredFiles = new SafeSet();
-  #debouncing = new SafeSet();
   #depencencyOwners = new SafeMap();
   #ownerDependencies = new SafeMap();
+  #debounceOwners = new SafeSet();
+  #debounceTimer;
   #debounce;
   #mode;
   #signal;
@@ -80,17 +81,20 @@ class FilesWatcher extends EventEmitter {
   }
 
   #onChange(trigger) {
-    if (this.#debouncing.has(trigger)) {
-      return;
-    }
     if (this.#mode === 'filter' && !this.#filteredFiles.has(trigger)) {
       return;
     }
-    this.#debouncing.add(trigger);
     const owners = this.#depencencyOwners.get(trigger);
-    setTimeout(() => {
-      this.#debouncing.delete(trigger);
-      this.emit('changed', { owners });
+    if (owners) {
+      for (const owner of owners) {
+        this.#debounceOwners.add(owner);
+      }
+    }
+    clearTimeout(this.#debounceTimer);
+    this.#debounceTimer = setTimeout(() => {
+      this.#debounceTimer = null;
+      this.emit('changed', { owners: this.#debounceOwners });
+      this.#debounceOwners.clear();
     }, this.#debounce).unref();
   }
 

test/parallel/test-watch-mode-files_watcher.mjs

@@ -70,6 +70,29 @@ describe('watch mode file watcher', () => {
     assert.ok(changesCount < 5);
   });
 
+  it('should debounce changes on multiple files', async () => {
+    const files = [];
+    for (let i = 0; i < 10; i++) {
+      const file = tmpdir.resolve(`file-debounced-${i}`);
+      writeFileSync(file, 'written');
+      watcher.filterFile(file);
+      files.push(file);
+    }
+
+    files.forEach((file) => writeFileSync(file, '1'));
+    files.forEach((file) => writeFileSync(file, '2'));
+    files.forEach((file) => writeFileSync(file, '3'));
+    files.forEach((file) => writeFileSync(file, '4'));
+
+    await setTimeout(200); // debounce * 2
+    files.forEach((file) => writeFileSync(file, '5'));
+    const changed = once(watcher, 'changed');
+    files.forEach((file) => writeFileSync(file, 'after'));
+    await changed;
+    // Unfortunately testing that changesCount === 2 is flaky
+    assert.ok(changesCount < 5);
+  });
+
   it('should ignore files in watched directory if they are not filtered',
      { skip: !supportsRecursiveWatching }, async () => {
        watcher.on('changed', common.mustNotCall());