Rocksdb: 2PC bug

[cooljiansir]

In 2PC transaction, after we write a prepare entry into WAL, we call a Flush before MarkLogAsContainingPrepSection, the log file containing the prepare entry will be deleted.
see https://github.com/facebook/rocksdb/pull/4612

[sagar0]

@maysamyabandeh could you take a look?

[maysamyabandeh]

Thanks @cooljiansir Seems to be a valid issue. One solution could be marking the prepared logs in the write thread via PreReleaseCallback. (We can then remove logs_with_prep_mutex_ as it would be already protected by mutex_.) The drawback would be longer write time which could have negative impact in performance but I do not see any way around it.
@yiwu-arbug @lth what do you think?

[lth]

That sounds reasonable to me, since we need some way of writing WAL and marking logs atomically.

We will have to extend the callback to include log_used though, so we just have to make sure that's available when we call the callback.

[yiwu-arbug]

+1 for marking log atomically during write. I'm not familiar with the MarkLogAsContainingPrepSection though. Not sure if it will really have impact on write time.