The first Ready after RestartNode (with no snapshot) will have no
unstable entries, so we don't have the correct prevLastUnstablei
when Advance is called. This would cause raftLog.unstable to move
backwards and previously-stable entries would be returned to
the application again.
This should have been caught by the "unexpected Ready" portion of
TestNodeRestart, but it went unnoticed because the Node's goroutine
takes some time to read from advancec and prepare the write to read to
readyc. Added a small (1ms) delay to all such tests to ensure that the
goroutine has time to enter its select wait.
Node set the applied to committed right after it sends out Ready to application. This is not
correct since the application has not actually applied the entries at that point. We add a
Advance interface to Node. Application needs to call Advance to tell raft Node its progress.
Also this change can avoid unnecessary copying when application is still applying entires but
there are more entries to be applied.
Before this commit, compact always compact log at current appliedindex of raft.
This prevents us from doing non-blocking snapshot since we have to make snapshot
and compact atomically. To prepare for non-blocking snapshot, this commit make
compact supports index and nodes parameters. After completing snapshot, the applier
should call compact with the snapshot index and the nodes at snapshot index to do
a compaction at snapsohot index.
The usage of removed:
1. tell removed node about its removal explicitly using msgDenied
2. prevent removed node disrupt cluster progress by launching leader election
It is set when apply node removal, or receive msgDenied.
