Add a Sync barrier method

[Stebalien]

Proposal

Add a Sync method to sync all records written before Sync was called to disk.

Motivation

Currently, it's possible to synchronously write individual records. However, synchronously writing a record doesn't guarantee that previously written (non-sync) records will be synced (they could be in a different journal).

When writing collections of objects, it's useful to be able to:

• Write a collection of objects.
• Sync the collection of objects.
• Synchronously update an index over those objects.

Importantly, the index shouldn't be updated until the objects have been synced.

goleveldb currently provides batches and transactions but neither seem to serve this purpose. The best external solution I can think of is to write a wrapper with an in-memory write buffer. However, I'd prefer to just write to disk and let the OS decide when to sync and/or when the buffer is full.

Implementation

Unfortunately, my understanding of leveldb and goleveldb is limited. My first attempt was #310 but I didn't realize journals were rotated. Given that journals are rotated, the only solution I can think of is:

• When closing a journal, asynchronously fsync it before closing.
• When the user calls Sync, sync the current journal, wait for all asynchronously syncing journals to finish syncing, then fsync the directory.
• When opening a "finished" journal for the first time, fsync it in the background (in the program crashed while data was still being written to disk).

Does this sound like it'll work?

[Stebalien]

Note: This is a very low priority feature request. I'm not going to have time to implement it any time soon and I don't really expect anyone else to get to it. I'm just leaving this here in case anyone is interested.

Feel free to close this as out of scope and/or "not worth the effort".