test_runner: report changed file on watch restart

doc/api/test.md

@@ -3933,6 +3933,18 @@ Emitted when no more tests are queued for execution in watch mode.
 
 ### Event: `'test:watch:restarted'`
 
+<!-- YAML
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/64457
+    description: Added the `file` property to the event data.
+-->
+
+* `data` {Object}
+  * `file` {string|undefined} The path of the file whose change triggered the
+    restart. This value is `undefined` when the triggering file cannot be
+    determined.
+
 Emitted when one or more tests are restarted due to a file change in watch mode.
 
 ## `getTestContext()`

lib/internal/test_runner/runner.js

@@ -631,7 +631,7 @@ function watchFiles(testFiles, opts) {
   }
 
   // Watch for changes in current filtered files
-  const onChanged = ({ owners, eventType }) => {
+  const onChanged = ({ owners, eventType, trigger }) => {
     if (!opts.hasFiles && (eventType === 'rename' || eventType === 'change')) {
       const updatedTestFiles = createTestFileList(opts.globPatterns, opts.cwd);
       const newFileName = ArrayPrototypeFind(updatedTestFiles, (x) => !ArrayPrototypeIncludes(testFiles, x));
@@ -653,7 +653,7 @@ function watchFiles(testFiles, opts) {
     // Reset the root start time to recalculate the duration
     // of the run
     opts.root.clearExecutionTime();
-    opts.root.reporter[kEmitMessage]('test:watch:restarted');
+    opts.root.reporter[kEmitMessage]('test:watch:restarted', { __proto__: null, file: trigger });
 
     // Restart test files
     if (opts.isolation === 'none') {

test/test-runner/test-run-watch-restarted-file.mjs

@@ -0,0 +1,47 @@
+// Test run({ watch: true }) reports the triggering file path in the
+// test:watch:restarted event data.
+import * as common from '../common/index.mjs';
+import { run } from 'node:test';
+import assert from 'node:assert';
+import { writeFileSync } from 'node:fs';
+import { join } from 'node:path';
+import { once } from 'node:events';
+import tmpdir from '../common/tmpdir.js';
+import { refreshForTestRunnerWatch, skipIfNoWatch, fixtureContent } from '../common/watch.js';
+
+skipIfNoWatch();
+refreshForTestRunnerWatch();
+
+const changedFile = join(tmpdir.path, 'test.js');
+
+let alreadyDrained = false;
+const restartedFiles = [];
+const onRestarted = common.mustCall((file) => {
+  restartedFiles.push(file);
+}, 1);
+
+const controller = new AbortController();
+const stream = run({
+  cwd: tmpdir.path,
+  watch: true,
+  signal: controller.signal,
+}).on('data', function({ type, data }) {
+  if (type === 'test:watch:restarted') {
+    onRestarted(data.file);
+  }
+  if (type === 'test:watch:drained') {
+    if (alreadyDrained) {
+      controller.abort();
+    }
+    alreadyDrained = true;
+  }
+});
+
+await once(stream, 'test:watch:drained');
+
+writeFileSync(changedFile, fixtureContent['test.js']);
+
+// eslint-disable-next-line no-unused-vars
+for await (const _ of stream);
+
+assert.deepStrictEqual(restartedFiles, [changedFile]);