leveldb: add a Sync method

[Stebalien]

Put + Sync is the equivalent of calling Put with WriteOptions.Sync.

We'd like this method for go-ipfs as we're adding a Sync(keyPrefix string) method to our datastore abstraction. We considered adding a Sync option to the Put method as leveldb does but:

• It's not always possible to know when a write is the "final" write before actually performing the write.
• We'd like to be able to "sync" different sub-datastores independently (i.e., sync everything under keyPrefix).

[lrita]

The Sync method should acquire writing-lock(writeLockC) to keep thread safe.

[Stebalien]

That depends on the guarantees of the Syncer interface. The current implementations (os.File, memWriter, fileWrap, writer) are all internally thread-safe and interleaving syncs with other writes shouldn't cause any problems.

I'm happy to take the lock anyways but I'd rather not stop the world unnecessarily.

[lrita]

Your patch will cause panic by getting nil of db.journalWriter without acquire writing-lock(writeLockC), it is easy the find.

[Stebalien]

Ah, you're right. I misunderstood how the journalWriter was used. I'm not sure if taking a lock is sufficient as the _previous_ journal may not have been synced. Is that the case?

[lrita]

goleveldb will auto rotate journalWriter internally.

If you want implement the sync barrier during a lot of async writing, you will do a lot modification, you need find all the un-compaction journal files, and sync each of them.

[Stebalien]

Yeah. This is going to need some discussion.