test: unflake async hooks statwatcher test

Under high load 2 types of issues arise with this test.
* filesystem calls gets queued even when the 'sync' is used which leads
  to async_hooks being called with the events of tmpdir clean or
  initial file write after clean.
  This is solved by counting all 'change' calls while making sure there
  is no dependency of StatWatcher's on one another and the expected
  changes are waited for.

* some events are getting lost with the current
  clean->write->write_and_watch strategy. Specifically I observed the
  file size going from 0 to 5 entirely skipping 3 even though the write
  call was there (this happened reliably on -j128).
  So I've changed the strategy to avoid additional write considering
  this still tests the hooks correctly.
  This may indicate some sort of bug in async_hooks though I'm not sure.

Closes: nodejs#21425

PR-URL: nodejs#30362
Fixes: nodejs#21425
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>

test/async-hooks/async-hooks.status

@@ -5,8 +5,6 @@ prefix async-hooks
 # sample-test                        : PASS,FLAKY
 
 [true] # This section applies to all platforms
-# https://github.com/nodejs/node/issues/21425
-test-statwatcher: PASS,FLAKY
 
 [$system==win32]
 

test/async-hooks/test-statwatcher.js

@@ -15,68 +15,81 @@ tmpdir.refresh();
 
 const file1 = path.join(tmpdir.path, 'file1');
 const file2 = path.join(tmpdir.path, 'file2');
-fs.writeFileSync(file1, 'foo');
-fs.writeFileSync(file2, 'bar');
 
-const hooks = initHooks();
-hooks.enable();
-
-function onchange1(curr, prev) {
-  console.log('Watcher: w1');
+const onchangex = (x) => (curr, prev) => {
+  console.log(`Watcher: ${x}`);
   console.log('current stat data:', curr);
   console.log('previous stat data:', prev);
-}
-// Install first file watcher
-const w1 = fs.watchFile(file1, { interval: 10 }, onchange1);
+};
+
+const checkWatcherStart = (name, watcher) => {
+  assert.strictEqual(watcher.type, 'STATWATCHER');
+  assert.strictEqual(typeof watcher.uid, 'number');
+  assert.strictEqual(watcher.triggerAsyncId, 1);
+  checkInvocations(watcher, { init: 1 },
+                   `${name}: when started to watch file`);
+};
+
+const hooks = initHooks();
+hooks.enable();
 
+// Install first file watcher.
+const w1 = fs.watchFile(file1, { interval: 10 }, onchangex('w1'));
 let as = hooks.activitiesOfTypes('STATWATCHER');
 assert.strictEqual(as.length, 1);
+// Count all change events to account for all of them in checkInvocations.
+let w1HookCount = 0;
+w1.on('change', () => w1HookCount++);
 
 const statwatcher1 = as[0];
-assert.strictEqual(statwatcher1.type, 'STATWATCHER');
-assert.strictEqual(typeof statwatcher1.uid, 'number');
-assert.strictEqual(statwatcher1.triggerAsyncId, 1);
-checkInvocations(statwatcher1, { init: 1 },
-                 'watcher1: when started to watch file');
-
-function onchange2(curr, prev) {
-  console.log('Watcher: w2');
-  console.log('current stat data:', curr);
-  console.log('previous stat data:', prev);
-}
+checkWatcherStart('watcher1', statwatcher1);
 
 // Install second file watcher
-const w2 = fs.watchFile(file2, { interval: 10 }, onchange2);
+const w2 = fs.watchFile(file2, { interval: 10 }, onchangex('w2'));
 as = hooks.activitiesOfTypes('STATWATCHER');
 assert.strictEqual(as.length, 2);
+// Count all change events to account for all of them in checkInvocations.
+let w2HookCount = 0;
+w2.on('change', () => w2HookCount++);
 
 const statwatcher2 = as[1];
-assert.strictEqual(statwatcher2.type, 'STATWATCHER');
-assert.strictEqual(typeof statwatcher2.uid, 'number');
-assert.strictEqual(statwatcher2.triggerAsyncId, 1);
 checkInvocations(statwatcher1, { init: 1 },
                  'watcher1: when started to watch second file');
-checkInvocations(statwatcher2, { init: 1 },
-                 'watcher2: when started to watch second file');
+checkWatcherStart('watcher2', statwatcher2);
 
 setTimeout(() => fs.writeFileSync(file1, 'foo++'),
            common.platformTimeout(100));
-w1.once('change', common.mustCall((curr, prev) => {
-  console.log('w1 change', curr, prev);
+w1.on('change', common.mustCallAtLeast((curr, prev) => {
+  console.log('w1 change to', curr, 'from', prev);
+  // Wait until we get the write above.
+  if (prev.size !== 0 || curr.size !== 5)
+    return;
+  // Remove listeners to make w1HookCount final
+  w1.removeAllListeners('change');
+
   setImmediate(() => {
-    checkInvocations(statwatcher1, { init: 1, before: 1, after: 1 },
+    checkInvocations(statwatcher1,
+                     { init: 1, before: w1HookCount, after: w1HookCount },
                      'watcher1: when unwatched first file');
     checkInvocations(statwatcher2, { init: 1 },
                      'watcher2: when unwatched first file');
 
     setTimeout(() => fs.writeFileSync(file2, 'bar++'),
                common.platformTimeout(100));
-    w2.once('change', common.mustCall((curr, prev) => {
-      console.log('w2 change', curr, prev);
+    w2.on('change', common.mustCallAtLeast((curr, prev) => {
+      console.log('w2 change to', curr, 'from', prev);
+      // Wait until we get the write above.
+      if (prev.size !== 0 || curr.size !== 5)
+        return;
+      // Remove listeners to make w2HookCount final
+      w2.removeAllListeners('change');
+
       setImmediate(() => {
-        checkInvocations(statwatcher1, { init: 1, before: 1, after: 1 },
+        checkInvocations(statwatcher1,
+                         { init: 1, before: w1HookCount, after: w1HookCount },
                          'watcher1: when unwatched second file');
-        checkInvocations(statwatcher2, { init: 1, before: 1, after: 1 },
+        checkInvocations(statwatcher2,
+                         { init: 1, before: w2HookCount, after: w2HookCount },
                          'watcher2: when unwatched second file');
         fs.unwatchFile(file1);
         fs.unwatchFile(file2);
@@ -85,13 +98,13 @@ w1.once('change', common.mustCall((curr, prev) => {
   });
 }));
 
-process.on('exit', onexit);
-
-function onexit() {
+process.once('exit', () => {
   hooks.disable();
   hooks.sanityCheck('STATWATCHER');
-  checkInvocations(statwatcher1, { init: 1, before: 1, after: 1 },
+  checkInvocations(statwatcher1,
+                   { init: 1, before: w1HookCount, after: w1HookCount },
                    'watcher1: when process exits');
-  checkInvocations(statwatcher2, { init: 1, before: 1, after: 1 },
+  checkInvocations(statwatcher2,
+                   { init: 1, before: w2HookCount, after: w2HookCount },
                    'watcher2: when process exits');
-}
+});