Summary:
This PR adds a `DB::WriteWithCallback` API that does the same things as `DB::Write` while takes an argument `UserWriteCallback` to execute custom callback functions during the write.
We currently support two types of callback functions: `OnWriteEnqueued` and `OnWalWriteFinish`. The former is invoked after the write is enqueued, and the later is invoked after WAL write finishes when applicable.
These callback functions are intended for users to use to improve synchronization between concurrent writes, their execution is on the write's critical path so it will impact the write's latency if not used properly. The documentation for the callback interface mentioned this and suggest user to keep these callback functions' implementation minimum.
Although transaction interfaces' writes doesn't yet allow user to specify such a user write callback argument, the `DBImpl::Write*` type of APIs do not differentiate between regular DB writes or writes coming from the transaction layer when it comes to supporting this `UserWriteCallback`. These callbacks works for all the write modes including: default write mode, Options.two_write_queues, Options.unordered_write, Options.enable_pipelined_write
Pull Request resolved: https://github.com/facebook/rocksdb/pull/12603
Test Plan: Added unit test in ./write_callback_test
Reviewed By: anand1976
Differential Revision: D58044638
Pulled By: jowlyzhang
fbshipit-source-id: 87a84a0221df8f589ec8fc4d74597e72ce97e4cd
