Derived store is unsubscribed twice and started without any subscribers

[jrouleau]

Describe the bug

When a derived store is unsubscribed from another derived store sharing the same dependency, it is unsubscribed from twice and then started without any subscribers.

As far as I understand, here is a timeline of events for the following example:

1. store3 is subscribed
2. store3 subscribes to store1 as a dependency
3. store1 is true, so store3 subscribes to store2
4. store2 subscribes to store1 as a dependency
5. store1 is set to false
6. store3 receives update from store1 and unsubscribes from store2
7. store2 receives update from store1, its stop function is run again, and then it is started without any subscribers

Reproduction

  import { derived, writable } from 'svelte/store';

  const store1 = writable(true);

  const store2 = derived([store1], ([$store1], set) => {
    console.log('store2 started');
    let unsubscribed = false;
    return () => {
      console.log('store2 stopped');
      if (unsubscribed) console.error('already unsubscribed!');
      unsubscribed = true;
    };
  });

  const store3 = derived([store1], ([$store1], set) => {
    if ($store1) return store2.subscribe(() => {});
  });

  store3.subscribe(() => {});
  store1.set(false);

Logs

store2 started
store2 stopped
store2 stopped
already unsubscribed!
store2 started

System Info

npmPackages:
    svelte: ^3.55.1 => 3.55.1

Severity

annoyance

[Conduitry]

This should be fixed in 3.57.0.