[Backport 4.9] OF-2921: Prevent deadlock by not broadcasting synchronously #2647
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When a client drops offline, a presence update is broadcast on behalf of that client. We've observed deadlocks occurring when this happens (as the broadcast itself, especially when it occurs in context in of MUC rooms, will acquire mutexes). There appears to be little reason for this broadcast to happen synchronous to the process of session termination process. To reduce the likelihood of deadlocks, this commit makes the broadcast happen in an asynchronous process. (cherry picked from commit b98a83d)
When stream management is enabled, deliver and record stanzas under a mutex. If it's not enabled, don't acquire the lock to reduce lock contention. (cherry picked from commit debf053)
When a websocket error is raised while holding another mutex (eg: when processing a MUC stanza) then having _another_ mutex introduces a deadlock risk. This has been observed to happen in the wild. In this commit, the mutex held when processing a websocket error is removed. (cherry picked from commit 6428f38)
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Description
Backport of #2635 to
4.9.