Summary:
The `SharedState` constructor had an early return in case of
`-test_batches_snapshots=1`. This early return caused `num_bg_threads_`
to never be incremented. Consequently, the driver thread could cleanup
objects like the `SharedState` while BG threads were still running and
accessing it, leading to crash.
The fix is to move the logic for counting threads (both FG and BG) to
the place they are launched. That way we can be sure the counts are
consistent, at least for now.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9313
Test Plan:
below command used to fail, now it passes.
```
$ ./db_stress --db=./test-db/ --expected_values_dir=./test-db-expected/ --max_key=100000 --ops_per_thread=1000 --sync_fault_injection=1 --clear_column_family_one_in=0 --destroy_db_initially=0 -reopen=0 -test_batches_snapshots=1
```
Reviewed By: jay-zhuang
Differential Revision: D33198670
Pulled By: ajkr
fbshipit-source-id: 126592dc1eb31998bc8f82ffbf5a0d4eb8dec317
84228e21e8