AsyncLocalStorage is captured on the first console.log which prevents garbage collection of objects in the storage

[Jamesernator]

Version

v20.3.0

Platform

Linux 5.19.0-46-generic #47~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Jun 21 15:35:31 UTC 2 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

async_hooks

What steps will reproduce the bug?

Run the following with node --expose-gc:

import { AsyncLocalStorage } from "node:async_hooks";

const storage = new AsyncLocalStorage();

const finalizer = new FinalizationRegistry((token) => {
    console.log("COLLECTED", token);
});

const wr = new WeakRef({});
finalizer.register(wr.deref(), "obj");

// Whether COLLECTED is printed depends on whether this is present or not
// console.log("OUTER LOG");

storage.run({ obj: wr.deref() }, () => {
    console.log(storage.getStore()?.obj);
});

await new Promise((resolve) => setTimeout(resolve, 1000));

gc();

How often does it reproduce? Is there a required condition?

This happens consistently.

What is the expected behavior? Why is that the expected behavior?

The object stored in the WeakRef should be collected when gc() is hit, but whether this happens depends on whether the line:

// console.log("OUTER LOG");

is commented or not.

What do you see instead?

The object is not collected if the first console.log happens under the storage.run callback.

Additional information

For whatever reason wherever the first console.log happens this is where the async resource for the TTY is made. This causes objects in the AsyncLocalStorage to be eternally referenced and never eligible for garbage collection.

To allow GC of values in AsyncLocalStorage, the fix here would be to simply create the TTY's async resource using the root's async scope rather than whatever happens to be active.

(I can't see large reason why the current behaviour would be desirable as it just makes GC of values dependent on first console.log. If anyone needed such behaviour legitimately it'd make more sense to expose an initTTY function or similar that captures the async scope at that time).

[legendecas]

I agree lasily initialized resources that are been kept alive by the runtime indefinitely should not capture the async context, instead the root async context should be used, like the global console (uses TTY internally). There are no means for user code to properly release them.