test_runner: recalculate run duration on watch restart

lib/internal/test_runner/runner.js

@@ -502,7 +502,10 @@ function watchFiles(testFiles, opts) {
         return; // Avoid rerunning files when file deleted
       }
     }
-
+    // Reset the root start time to recalculate the duration
+    // of the run
+    opts.root.clearExecutionTime();
+    // Restart test files
     if (opts.isolation === 'none') {
       PromisePrototypeThen(restartTestFile(kIsolatedProcessName), undefined, (error) => {
         triggerUncaughtException(error, true /* fromPromise */);

lib/internal/test_runner/test.js

@@ -1335,6 +1335,11 @@ class Test extends AsyncResource {
     this.parent.reportStarted();
     this.reporter.start(this.nesting, this.loc, this.name);
   }
+
+  clearExecutionTime() {
+    this.startTime = hrtime();
+    this.endTime = null;
+  }
 }
 
 class TestHook extends Test {

test/parallel/test-runner-run-watch.mjs

@@ -189,6 +189,8 @@ async function testWatch(
   action === 'rename2' && await testRename();
   action === 'delete' && await testDelete();
   action === 'create' && await testCreate();
+
+  return runs;
 }
 
 describe('test runner watch mode', () => {
@@ -241,6 +243,20 @@ describe('test runner watch mode', () => {
       await testWatch({ action: 'create', fileToCreate: 'new-test-file.test.js' });
     });
 
+  // This test is flaky by its nature as it relies on the timing of 2 different runs
+  // considering the number of digits in the duration_ms is 9
+  // the chances of having the same duration_ms are very low
+  // but not impossible
+  // In case of costant failures, consider increasing the number of tests
+  it('should recalculate the run duration on a watch restart', async () => {
+    const testRuns = await testWatch({ file: 'test.js', fileToUpdate: 'test.js' });
+    const durations = testRuns.map((run) => {
+      const runDuration = run.match(/# duration_ms\s([\d.]+)/);
+      return runDuration;
+    });
+    assert.notDeepStrictEqual(durations[0][1], durations[1][1]);
+  });
+
   describe('test runner watch mode with different cwd', () => {
     it(
       'should execute run using a different cwd for the runner than the process cwd',