Steps to reproduce the issue

This bug is not easy to reproduce, because it seems to be random at first. Best way to reproduce is have a user assigned to all the ACL grops in Joomla, and then open a few tabs in rapid succession with a subpage that performs a save on the user object.

Expected result

No #__user_usergroup_map records get lost

Actual result

User gets unsassigned from groups. If JTable::store() happens on the same user concurrently, it will eventually lead to loss of #__user_usergroup_map rows for that user. Because JTable::store() deletes everything from #__user_usergroup_map and then re-adds one by one, if that process happens multiple times in parallel for the same user, the inserts and deletes get mixed up.

System information (as much as possible)

Joomla 3.3.3

Additional comments

// TODO: This is a dumb way to handle the groups.

Among JomSocial developers issues caused by this implementation are known as "the bug from hell" that lead to users randomly losing priviledges on a dozen or so websites, it took us qite a while to nail it.

I wrote a fix for this, that instead of performing "delete and re-insert one by one", grabs the records from the database, loops through them, decides which should stay and which should go based on $this->groups and then adds new ones (if any) from $this->groups

http://pastebin.com/NJcmnaCT