Unit testing code with WeakRef/FinalizationRegistry, like this LinkedAbortController

[noseratio]

TLTR: is there a way to force GC in the current version of Node?

I'm looking at using AbortController for token-based cooperative cancellation, where it's common to have a hierarchy of linked token sources. That'd be child AbortController objects which would listen for abort event on their parent AbortController.signal. Here is a related WHATWG discussion thread.

So I'm experimenting with a version of LinkedAbortController that uses WeakRef and FinalizationRegistry (as described in V8's Weak references and finalizers), in attempt to help freeing the parentAbortSignal.removeEventListener('abort', listener) registrations for orphaned child AbortController instances. The cleanup should be taking place as a part of GC, when there is no more references left to a particular instance of LinkedAbortController, besides the weak ref from its internal listener for parent signal.

The full code gist is here. I'm struggling to come up with a deterministic test for this in Node. Even when I call global.gc(true) in a loop with a delay like this, I still need a final ~10s delay to see the finalizer callbacks invoked.

Is there a more predictable way of forcing GC in Node? I'm using the latest v15.4.0.

OTOH, I'm able to test it in the browser, the finalizers gets called when I click the Trash icon ("Collect garbage") in the Performance section of DevTools. So, for browsers, I should be able to automate this with Puppeteer.

Thanks for any help!

[jasnell]

Forcing? Not really. You could try the natives syntax function %CollectGarbage but I believe that has the same impact as calling global.gc()

[noseratio]

Forcing? Not really. You could try the natives syntax function %CollectGarbage but I believe that has the same impact as calling global.gc()

@jasnell This is brilliant, thanks! Unlike global.gc(true) , calling %CollectGarbage(true) triggers the GC right away, it's exactly what I was looking for.

To others who (like me) never heard about built-in functions before and might have ended up here, you need to run it like this to enable %CollectGarbage(true):

node --allow-natives-syntax LinkedAbortController.js

To enable global.gc(true):

node --expose-gc --trace-gc LinkedAbortController.js

[benjamingr]

See https://github.com/nodejs/node/pull/36607/files#diff-7177d219f295c5297f8e5d119cea177fc0b50c1df33161729ec0e1213f6c3b24R572-R577 as an example

[tantaman]

If anyone else ends up here, I had to:

1. Create my data
2. Enqueue a micro task (e.g. setImmediate) to run global.gc()
3. Check that weak refs were cleaned up after said task completed

Any other way and the weak refs were not cleaned up.

e.g.,

test('cleaned', async () => {
  const w = weak();
  await new Promise(resolve => setTimeout(resolve, 0));
  global.gc();
  expect(w.deref()).toEqual(null);
});

function weak() {
  return new WeakRef({});
}