Emit event when code in vm is fully executed

[mehmetb]

Is your feature request related to a problem? Please describe.
There is no way to know if the script running in vm is fully executed or not. Of course, all synchronous tasks are immediately executed when runInContext is called. But since all contexts share the event loop, we cannot tell if there are any more callbacks (of the vm code) in the event loop. (e.g. a setTimeout callback)

Describe the solution you'd like
vm.Script should use a separate event loop and emit an event when there are no more callbacks left.

Describe alternatives you've considered
As bnoordhuis stated, we can add bookkeeping to every call into the VM.

[bnoordhuis]

There is no way to know if the script running in vm is fully executed or not.

There is really no way to know short of solving the Halting Problem.

Example: a promise handler can always enqueue a new promise, which in turn can enqueue a new promise, etc.

It might be possible to implement some kind of "kill switch" that forcibly terminates the script and all outstanding microtasks/handles/requests/etc., but that's about it. But then you might as well use workers.

vm.Script should use a separate event loop

That's probably impossible because you can't nest event loops for technical reasons.

Example: on some platforms where node uses kqueue for its event loop (macos, the BSDs), adding a kqueue fd to another kqueue fd doesn't work because the nested fd doesn't generate events.

It also doesn't really solve the issue of microtasks/promises.

[mehmetb]

It also doesn't really solve the issue of microtasks/promises.

I was afraid you'd say that :/

But then you might as well use workers.

Yes, this will work. Sharing code between worker threads is harder but I now can tell that changing how VM works is harder.

Thank you for the thorough explanation @bnoordhuis!

[bnoordhuis]

I'll go ahead and close this out then. Thanks anyway for opening the issue!