Potential UI race condition in bug subscriber list

[Gary Poster]

After branches for bug 772754 land.

Subscribers list UI on the bug page is vulnerable to the race condition in one special set of circumstances: one has to unsubscribe a person/team and then use "subscribe someone else" to subscribe the same person/team. If successful animations get timed such that subscriber name gets removed from the list while successful subscription animation is in progress (and before it completes), subscriber name might be removed and not readded. This is pretty complex set of circumstances so I didn't bother with catering for it.

Proper fix would have to be in BugSubscribersLoader (in lib/lp/bugs/javascript/subscribers_list.js) so it ensures one operation is cancelled when the overriding one happens. Some support code to check the state would be needed in SubscribersList JS class in the same file.

I was unable to trigger this either locally or on qastaging: all the calls happened too quickly for me to be able to time the two animations exactly right.