crash in metrics gesture tracking

[incanus]

Crashes are occurring inside of -[MGLMapView trackGestureEvent:forRecognizer:], probably compounded the extreme rapid-fire nature of them during pans and zooms.

This ticket tracks crashes related to mutation of objects during iteration.

• https://rink.hockeyapp.net/manage/apps/163490/app_versions/2/crash_reasons/31107650
• https://rink.hockeyapp.net/manage/apps/163490/app_versions/2/crash_reasons/31108916
• https://rink.hockeyapp.net/manage/apps/163490/app_versions/2/crash_reasons/31128721
• https://rink.hockeyapp.net/manage/apps/163490/app_versions/2/crash_reasons/31136788
• https://rink.hockeyapp.net/manage/apps/163490/app_versions/2/crash_reasons/31152675

[incanus]

There are other lingering crashes in -[MGLMapView trackGestureEvent:forRecognizer:] as well, but let's knock out the above, which looks to me like a clear sign of non-threadsafety, first.

[bleege]

The first 2 crash reports from HockeyApp both give the following as the crash point:

10 UIKit 0x000000018aa6ba40 -[UIGestureRecognizer locationInView:] + 264
10 UIKit 0x000000018b25ba40 -[UIGestureRecognizer locationInView:] + 264

The final 3 crash reports all report:

We have determined that the link register (lr) is very likely to contain the return address of frame #0's calling function, and have inserted it into the crashing thread's backtrace as frame #1 to aid in analysis. This determination was made by applying a heuristic to determine whether the crashing function was likely to have created a new stack frame at the time of the crash.

[incanus]

This just sounds like UI access not on the main thread to me.

[bleege]

Yep. https://github.com/mapbox/mapbox-gl-native/pull/1115/files#r27249748

[incanus]

Just about through with a large refactor here. The main cause of crashes here is that we are crossing access to _queue from both of our custom serial queues as well as the main UI thread.

[bleege]