gc.js

'use strict';

const wait = require('timers/promises').setTimeout;

// 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();
}

// Repeat an operation and give GC some breathing room at every iteration.
async function runAndBreathe(fn, repeat, waitTime = 20) {
  for (let i = 0; i < repeat; i++) {
    await fn();
    await wait(waitTime);
  }
}

module.exports = {
  checkIfCollectable,
  runAndBreathe,
};