Summary:
The core issue solved in D21389371 was that erased elements of a TinyMap were being iterated over, because TinyMap has somewhat-complicated logic around cleaning out the underlying vector. In some very marginal cases, vectors were not being cleaned and an iterator pointing at erased elements was being returned.
The diff prevents some possible crashes in `begin()` and `find()` while making it much less likely to iterate over erased elements.
We also add a unit test to catch the case fixed in D21389371, in particular.
We also are keeping the code added in D21389371 (for now) since it's a cheap check, and will be a safeguard until we have rigorous testing around TinyMap. To be clear that logic should noop currently, but will prevent crashes in case guarantees around TinyMap change in the future.
Currently there is only one line of code that actually uses the TinyMap iterator, so this should be safe.
Reviewed By: shergin
Differential Revision: D21392762
fbshipit-source-id: 36dc998958c230fad01af93338974f8889cbcf55
bb5d04366a