Summary:
Unfortunately, it seems like mysqld reuses xids across machine restarts. When that happens, we could have something like the following happening:
```
BEGIN_PREPARE(unprepared) Put(a) END_PREPARE(xid = 1)
-- crash and recover with Put(a) rolled back as it was not prepared
BEGIN_PREPARE(prepared) Put(b) END_PREPARE(xid = 1)
COMMIT(xid = 1)
-- crash and recover with both a, b
```
To solve this, we will have to log the rollback batch into the WAL during recovery.
WritePrepared already logs the rollback batch into the WAL, if a rollback happens after prepare, so there is no problem there.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6418
Differential Revision: D19896151
Pulled By: lth
fbshipit-source-id: 2ff65ddc5fe75efd57736fed4b7cd7a109d26609
fb571509a7