folly::ConcurrentHashMap crashes under high contention about folly

[instw]

Thanks for the repro! This issue seems to be due to ConcurrentHashMap not being able to reclaim deleted entries fast enough.
Each entry in the ConcurrentHashMap is referenced by it's predecessor via a linked list, and this reference is kept track of using Hazard pointer link counting logic. Effectively when an erase is performed, the older map entry doesn't get cleaned up immediately, but rather as part of a reclamation process that's usually asynchronous. If the rate at which keys are erased is considerably higher than the rate at which keys are reclaimed, certain internal counters (which are 16 bits in size) overflow. The first error you posted is related directly to the overflow.

For the second - It seems your benchmarking code most likely runs without debug compilation, which causes some invariant checks to be skipped, leading to double reclamation and the second error.