Unsubscribe a listener inside a listener has side-effects on other listeners

[arian]

Unsubscribing a listener inside a listener, will splice the listeners array in the store of createStore, while looping over that array using forEach in dispatch.

For example the following program fails:

import assert from 'assert';
import {createStore} from 'redux';

var store = createStore(() => []);

var countA = 0;
var countB = 0;
var countC = 0;

var unsubA = store.subscribe(() => {
    countA++;
});

var unsubB = store.subscribe(() => {
    countB++;
    unsubB();
});

var unsubC = store.subscribe(() => {
    countC++;
});

store.dispatch({type: 'X'});
store.dispatch({type: 'Y'});

console.log(countA, countB, countC);

assert.equal(countA, 2, 'countA');
assert.equal(countB, 1, 'countB');
assert.equal(countC, 2, 'countC');

countC is 1, but it should be 2.

A fix for this problem would be to copy the array before doing the forEach in the dispatch function, for example:

listeners.slice().forEach(listener => listener());

[gaearon]

Thanks! Would you please add a failing test? (And maybe you'd like to contribute a fix too? ;-)
The branch for development is breaking-changes-1.0.

[gaearon]

Fixed by #462.

[xogeny]

I have a question about this. The idea is the copy the array so subscribes can't affect the loop. But if the first listener unsubscribes a later listener, that won't be handled correctly. The unsubscribed listener will be called one more time after it was unsubscribed.

Is that really the desired behavior? I agree it is a tricky issue and I'm not 100% certain how to address that. But before even trying to create a PR I wanted to take a step back and ask the "big picture" question of "When do we expect an unsubscribe to be applied?". Specifically, how should this case where one listener unsubscribes a later listener? (one of many potential corner cases worth considering)

Although not strictly relevant, I think it might be worth nothing that this is an issue with React because you can get into a situation (this is how I got here, in fact) where a component gets unsubscribed when it is unmounted. But the listener is invoke on more time after the unsubscribe/unmounting and the listener calls setState which leads to a nasty message on the console. I'm trying to avoid that situation. In my case, I want the unsubscribe to be effective immediately because otherwise the listener will be called after the component is unmounted.

Thoughts?