watch: allow listening for grouped changes

lib/internal/watch_mode/files_watcher.js

@@ -17,17 +17,18 @@ 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';
 
 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;
@@ -75,17 +76,27 @@ 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);
+    if (owners) {
+      for (const owner of owners) {
+        this.#debounceOwners.add(owner);
+      }
+    }
+
+    clearTimeout(this.#debounceTimer);
+    this.#debounceTimer = setTimeout(() => {
+      this.#debounceTimer = null;
+      // In order to allow async processing of the 'changed' event, let's
+      // make sure the Set emitted is a new instance. We could emit using a copy
+      // of #debounceOwners, then clear #debounceOwners, but that would be
+      // slower than just replacing #debounceOwners with an empty Set.
+      const owners = this.#debounceOwners;
       this.emit('changed', { owners });
+      this.#debounceOwners = new SafeSet();
     }, this.#debounce).unref();
   }