fix: deadlock when sending events

[Pickleboyonline]

Summary

In short, if an RCTEventDispatcher observer sends an event on the same thread that the observer was initially on, there will be a deadlock due to sendEvent already having the lock active on the _observers NSHashTable. An example where this occurred was when we had react-native-gesture-handler trigger an animated event, which then triggered an event on the underlying component being animated as a result of it being an observer on the animation event. Since this all occurred on the main thread, we ended up with a deadlock and the app froze.

To prevent this scenario, I used a NSRecursiveLock for _observersLock to be able to dispatch events on the same thread from observers.

@joebernard

Changelog

[iOS] [Fix] - Prevent deadlock when dispatching events from observers on the same thread.

Test Plan

Not sure if there are any tests present for sending events with RCTEventDispatcher already in place, but In regular app usage this solution has proved to be a viable and stable option so far and has prevented the deadlock from occurring.

This would still be thread-safe since we are now allowing the event to be sent through observers on the same thread the initial event was dispatched on. The only issue I could see here is the behavior of sending an event could be changed.

[facebook-github-bot]

@charlesbdudley has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @Pickleboyonline in 68fd1e5.

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