Snapshots caveats

[ronag]

There are some caveats regarding snapshots and read/write ordering that are not entirely obvious.

e.g. in the following case:

const promise = db.put(...)
await db.get(...)
await promise

The put will not be visible for following reads until after the put has been completed, i.e. it is not enough to schedule the write for it to be visible in reads, it actually has to be completed first.

[ronag]

If napi_create_async_work is ordered we could actually do GetSnapshot() inside the worker queue in which case the following reads would immediately see the writes, i.e. the root problem here is that we defer writes to the worker queue while we read the snapshots immediately on the main thread. So the ordering gets inverted.

[vweevers]

Level/abstract-level#42