Recover from crash using undo log

[arajkumar]

This commit remembers the file write position of a last successful transaction in a undo file. If there's a crash and we need to restart, we use this undo file to get rid of any partial work from the last transaction that wasn't finished.

TODO: Can we use the undo log mechanism to even for graceful exit? This would simplify the code and make it uniform for both graceful and ungraceful exits/crashes. The idea is to undo the changes made by the last incomplete transaction by reading the undo log file and write other messages like ENDPOS.

[arajkumar]

I think implementing a proper undo log will become tricky to unless we resort to a transactional file write. Maybe opening the latest file and appending a ROLLBACK(if last txn is incomplete) will be much easier to implement.

[dimitri]

I think implementing a proper undo log will become tricky to unless we resort to a transactional file write. Maybe opening the latest file and appending a ROLLBACK(if last txn is incomplete) will be much easier to implement.

What if the latest/current transaction is incomplete and started several JSON files ago?

[arajkumar]

I think implementing a proper undo log will become tricky to unless we resort to a transactional file write. Maybe opening the latest file and appending a ROLLBACK(if last txn is incomplete) will be much easier to implement.

What if the latest/current transaction is incomplete and started several JSON files ago?

It doesn't matter, all we do is appending a rollback message to latest file if we find the last statement is part of the incomplete txn. It is same as what we do for graceful exit.