[core, iOS] Fix MGLOfflinePack invalidate crash

[julianrex]

EDIT:

This PR fixes a crash in invalidate due to nil pointer access, and adds code to catch an unhandled exception that is yet to be fully investigated.

The issue of accessing MGLOfflineStorage and MGLOfflinePack from multiple threads is NOT covered by this PR, and will hopefully be addressed by a refactor.

Until that point, it remains our recommendation that MGLOfflineStorage not be called from a background thread.

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Original post:

WIP branch investigating issues around #15536.

The cause of the crash looks like a null pointer access in setOfflineRegionObserver in -[MGLOfflinePack invalidate] - triggered by 2 calls to invalidate, most likely on different threads.

This PR fixes the null pointer access, but in the process of testing this, and testing the scenario in #15536 an exception fired (I was also seeing what looked like FileSource crashes when the test fails - possibly when exit is called by the test runner).

This PR does NOT address any issues with accessing MGLOfflinePack and MGLOfflineStorage.packs from multiple threads.

Re: the exceptions, @tmpsantos as discussed earlier:

At a13dd1b - the test test15536RemovePacksWhileReloading will trigger an exception that's caught in CFRunLoopRun() (RunLoop::run()).

The exception is std::runtime_error("Malformed offline region definition"); thrown in offline.cpp. The region is "", i.e. an empty string. I do not know why.

What's going on here?

The subsequent commit 2143e77 adds a catch to OfflineDatabase::getRegionDefinition, and the test should complete.

[tmpsantos]

OfflineDatabase LGTM

[fabian-guerra]

Thank you for fixing this. Overall tests LGTM. Just a question regarding thread safeness.