raft: add raft learner

raft/node.go

@@ -319,7 +319,7 @@ func (n *node) run(r *raft) {
 			r.Step(m)
 		case m := <-n.recvc:
 			// filter out response message from unknown From.
-			if _, ok := r.prs[m.From]; ok || !IsResponseMsg(m.Type) {
+			if pr := r.getProgress(m.From); pr != nil || !IsResponseMsg(m.Type) {
 				r.Step(m) // raft never returns an error
 			}
 		case cc := <-n.confc:
@@ -334,6 +334,8 @@ func (n *node) run(r *raft) {
 			switch cc.Type {
 			case pb.ConfChangeAddNode:
 				r.addNode(cc.NodeID)
+			case pb.ConfChangeAddLearnerNode:
+				r.addLearner(cc.NodeID)
 			case pb.ConfChangeRemoveNode:
 				// block incoming proposal when local node is
 				// removed

raft/progress.go

@@ -48,6 +48,7 @@ type Progress struct {
 	// When in ProgressStateSnapshot, leader should have sent out snapshot
 	// before and stops sending any replication message.
 	State ProgressStateType
+
 	// Paused is used in ProgressStateProbe.
 	// When Paused is true, raft should pause sending replication message to this peer.
 	Paused bool
@@ -76,6 +77,9 @@ type Progress struct {
 	// be freed by calling inflights.freeTo with the index of the last
 	// received entry.
 	ins *inflights
+
+	// IsLearner is true if this progress is tracked for a learner.
+	IsLearner bool
 }
 
 func (pr *Progress) resetState(state ProgressStateType) {