watchFile never fires when vfs uses a real dir

Author: Dhruv

lib/internal/vfs/providers/real.js

@@ -543,9 +543,9 @@ class RealFSProvider extends VirtualProvider {
     return fs.promises.watch(realPath, options);
   }
 
-  watchFile(vfsPath, options) {
+  watchFile(vfsPath, options, listener) {
     const realPath = this.#resolvePath(vfsPath);
-    return fs.watchFile(realPath, options, () => {});
+    return fs.watchFile(realPath, options, listener);
   }
 
   unwatchFile(vfsPath, listener) {

test/parallel/test-vfs-real-provider-watch.js

@@ -38,3 +38,19 @@ assert.strictEqual(myVfs.provider.supportsWatch, true);
   myVfs.watchFile('/wf.txt', { persistent: false }, listener);
   myVfs.unwatchFile('/wf.txt', listener);
 }
+
+// watchFile listener fires when the underlying file changes.
+// Refs: https://github.com/nodejs/node/issues/64536
+{
+  const target = path.join(root, 'wf-change.txt');
+  fs.writeFileSync(target, 'a');
+  let size = 2;
+  const rewrite = setInterval(() => {
+    fs.writeFileSync(target, 'x'.repeat(size++));
+  }, 100);
+  myVfs.watchFile('/wf-change.txt', { interval: 50 }, common.mustCall((curr, prev) => {
+    clearInterval(rewrite);
+    myVfs.unwatchFile('/wf-change.txt');
+    assert.notStrictEqual(curr.size, prev.size);
+  }));
+}