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test: add checkIfCollectable to test/common/gc.js
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PR-URL: nodejs#49671
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Yagiz Nizipli <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
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joyeecheung committed Sep 28, 2023
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'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,
};

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