Store's user-supplied unsubscribe function not called on unsubscribe

Describe the bug

If a store is created with a function as a second argument, that function is called when the subscriber count goes from zero to one. In theory, the function returned from that function is called when the subscriber count goes from one to zero. It doesn’t.

To Reproduce

https://svelte.dev/repl/77e4730d8b744fa4ab047a8354b9aedd?version=3.12.1

Expected behavior

When visible becomes false, an alert should appear.

Severity

It’s not great, though in practice it probably only means the occasional memory leak.

[ghost]

Hi Rich, there’s a double subscribe in your REPL which is causing the issue.

Looking at the debugger, the auto-subscribe fires first, which drops into this code correctly:

if (subscribers.length === 1) {
   stop = start(set) || noop;
}

However, you have another subscribe inside that start which bumps the subscribers length to 2, which causes this code fragment to be skipped on destroy_component:

if (subscribers.length === 0) {
	stop();
	stop = null;
}

Here’s the debugger state:

Right before the component context is wiped out, we can see the store still has a subscriber with the proper stop function stored in it.

I can see the usefulness of protecting against accidental repeated subscribes, though we’d have to add some tracking information so we tell if all subscriptions originate from same component. If they all have the same origin then we’d need to call the stop() function regardless of subscription count.

[Conduitry]

I don’t think there’s a bug here. As others have noted, the on-first-subscriber function is adding its own first subscriber, which is not being removed until the on-removal-of-last-subscriber function is called, which will be never, as there’s still that extra subscriber hanging. Without that extra subscription, this works fine.