raft: compact takes index and nodes parameters

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.

etcdserver/server.go

@@ -229,6 +229,7 @@ func (s *EtcdServer) run() {
 	var syncC <-chan time.Time
 	// snapi indicates the index of the last submitted snapshot request
 	var snapi, appliedi int64
+	var nodes []int64
 	for {
 		select {
 		case <-s.ticker:
@@ -265,6 +266,19 @@ func (s *EtcdServer) run() {
 				appliedi = e.Index
 			}
 
+			if rd.SoftState != nil {
+				nodes = rd.SoftState.Nodes
+				if rd.RaftState == raft.StateLeader {
+					syncC = s.syncTicker
+				} else {
+					syncC = nil
+				}
+				if rd.SoftState.ShouldStop {
+					s.Stop()
+					return
+				}
+			}
+
 			if rd.Snapshot.Index > snapi {
 				snapi = rd.Snapshot.Index
 			}
@@ -278,21 +292,9 @@ func (s *EtcdServer) run() {
 			}
 
 			if appliedi-snapi > s.snapCount {
-				s.snapshot()
+				s.snapshot(appliedi, nodes)
 				snapi = appliedi
 			}
-
-			if rd.SoftState != nil {
-				if rd.RaftState == raft.StateLeader {
-					syncC = s.syncTicker
-				} else {
-					syncC = nil
-				}
-				if rd.SoftState.ShouldStop {
-					s.Stop()
-					return
-				}
-			}
 		case <-syncC:
 			s.sync(defaultSyncTimeout)
 		case <-s.done:
@@ -517,14 +519,14 @@ func (s *EtcdServer) apply(r pb.Request) Response {
 }
 
 // TODO: non-blocking snapshot
-func (s *EtcdServer) snapshot() {
+func (s *EtcdServer) snapshot(snapi int64, snapnodes []int64) {
 	d, err := s.store.Save()
 	// TODO: current store will never fail to do a snapshot
 	// what should we do if the store might fail?
 	if err != nil {
 		panic("TODO: this is bad, what do we do about it?")
 	}
-	s.node.Compact(d)
+	s.node.Compact(snapi, snapnodes, d)
 	s.storage.Cut()
 }