Mutex is a variable, which means there needs to be only one mutex
value per scope. We don't need a separate mutex inside store struct,
**if we assume that `TxnPut` and `TxnRange` are called ONLY ONCE
per transaction (between `TxnBegin` and `TxnEnd`)**, as documented.
The point is to decouple the key-value storage layer and the
event notification layer clearly. It gives the watchableKV the
flexibility to define whatever event structure it wants without
breaking the ondisk format at key-value storage layer.
Changes:
1. change the format of key and value stored in backend
Store KeyValue struct instead of Event struct in backend value for
better abstraction as xiang suggests. And record the corresponded
action in the backend key.
2. Remove word 'event' from functions
One txn is treated as atomic, and might contain multiple Put/Delete/Range
operations. For now, between these operations, we might call forecCommit
to sync the change to disk, or backend may commit it in background.
Thus the snapshot state might contains an unfinished multiple objects
transaction, which is dangerous if database is restored from the snapshot.
This PR makes KV txn hold batchTx lock during the process and avoids
commit to happen.
When using Snapshot function, it is expected:
1. know the size of snapshot before writing data
2. split snapshot-ready phase and write-data phase. so we could cut
snapshot first and write data later.
Update its interface to fit the requirement of etcdserver.
Change to the function:
1. specify the meaning of startRev and endRev parameters
2. specify the meaning of returned nextRev
Moreover, it adds unit tests for the function.
If backend is closed, the operations on backend in compact
goroutine will panic. So this PR waits for compact goroutine to exit
before close backend.
This fixes the TestWorkflow failure too.
Snapshot takes an io.Writer and writes the entire backend data to
the given writer. Snapshot writes a consistent view and does not
block other storage operations.
Restore restores the in-memory states (index and book keeping) of
the storage from the backend data.
