test: add checkIfCollectable to test/common/gc.js

PR-URL: nodejs#49671
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/common/gc.js

@@ -0,0 +1,70 @@
+'use strict';
+
+// TODO(joyeecheung): merge ongc.js and gcUntil from common/index.js
+// into this.
+
+// This function can be used to check if an object factor leaks or not,
+// but it needs to be used with care:
+// 1. The test should be set up with an ideally small
+//   --max-old-space-size or --max-heap-size, which combined with
+//   the maxCount parameter can reproduce a leak of the objects
+//   created by fn().
+// 2. This works under the assumption that if *none* of the objects
+//   created by fn() can be garbage-collected, the test would crash due
+//   to OOM.
+// 3. If *any* of the objects created by fn() can be garbage-collected,
+//   it is considered leak-free. The FinalizationRegistry is used to
+//   terminate the test early once we detect any of the object is
+//   garbage-collected to make the test less prone to false positives.
+//   This may be especially important for memory management relying on
+//   emphemeron GC which can be inefficient to deal with extremely fast
+//   heap growth.
+// Note that this can still produce false positives. When the test using
+// this function still crashes due to OOM, inspect the heap to confirm
+// if a leak is present (e.g. using heap snapshots).
+// The generateSnapshotAt parameter can be used to specify a count
+// interval to create the heap snapshot which may enforce a more thorough GC.
+// This can be tried for code paths that require it for the GC to catch up
+// with heap growth. However this type of forced GC can be in conflict with
+// other logic in V8 such as bytecode aging, and it can slow down the test
+// significantly, so it should be used scarcely and only as a last resort.
+async function checkIfCollectable(
+  fn, maxCount = 4096, generateSnapshotAt = Infinity, logEvery = 128) {
+  let anyFinalized = false;
+  let count = 0;
+
+  const f = new FinalizationRegistry(() => {
+    anyFinalized = true;
+  });
+
+  async function createObject() {
+    const obj = await fn();
+    f.register(obj);
+    if (count++ < maxCount && !anyFinalized) {
+      setImmediate(createObject, 1);
+    }
+    // This can force a more thorough GC, but can slow the test down
+    // significantly in a big heap. Use it with care.
+    if (count % generateSnapshotAt === 0) {
+      // XXX(joyeecheung): This itself can consume a bit of JS heap memory,
+      // but the other alternative writeHeapSnapshot can run into disk space
+      // not enough problems in the CI & be slower depending on file system.
+      // Just do this for now as long as it works and only invent some
+      // internal voodoo when we absolutely have no other choice.
+      require('v8').getHeapSnapshot().pause().read();
+      console.log(`Generated heap snapshot at ${count}`);
+    }
+    if (count % logEvery === 0) {
+      console.log(`Created ${count} objects`);
+    }
+    if (anyFinalized) {
+      console.log(`Found finalized object at ${count}, stop testing`);
+    }
+  }
+
+  createObject();
+}
+
+module.exports = {
+  checkIfCollectable,
+};