raft: extract progressTracker and Progress

Mechanically extract `progressTracker`, `Progress`, and `inflights`
to their own package named `tracker`. This is done mechanically and
with as little manual intervention as possible. (In consequence, the
merit of this commit is not in the resulting aesthetics; those will
be cleaned up in the next commit).

This passes tests, but has various lint violations addressed in a
follow-up commit (if desired, they can be squashed after review).

The follow-up commit also improves documentation throughout.

etcdserver/raft.go

@@ -34,7 +34,6 @@ import (
 	"go.etcd.io/etcd/raft/raftpb"
 	"go.etcd.io/etcd/wal"
 	"go.etcd.io/etcd/wal/walpb"
-
 	"go.uber.org/zap"
 )
 
@@ -44,7 +43,7 @@ const (
 	maxSizePerMsg = 1 * 1024 * 1024
 	// Never overflow the rafthttp buffer, which is 4096.
 	// TODO: a better const?
-	maxInflightMsgs = 4096 / 8
+	MaxInflightMsgs = 4096 / 8
 )
 
 var (
@@ -464,7 +463,7 @@ func startNode(cfg ServerConfig, cl *membership.RaftCluster, ids []types.ID) (id
 		HeartbeatTick:   1,
 		Storage:         s,
 		MaxSizePerMsg:   maxSizePerMsg,
-		MaxInflightMsgs: maxInflightMsgs,
+		MaxInflightMsgs: MaxInflightMsgs,
 		CheckQuorum:     true,
 		PreVote:         cfg.PreVote,
 	}
@@ -518,7 +517,7 @@ func restartNode(cfg ServerConfig, snapshot *raftpb.Snapshot) (types.ID, *member
 		HeartbeatTick:   1,
 		Storage:         s,
 		MaxSizePerMsg:   maxSizePerMsg,
-		MaxInflightMsgs: maxInflightMsgs,
+		MaxInflightMsgs: MaxInflightMsgs,
 		CheckQuorum:     true,
 		PreVote:         cfg.PreVote,
 	}
@@ -615,7 +614,7 @@ func restartAsStandaloneNode(cfg ServerConfig, snapshot *raftpb.Snapshot) (types
 		HeartbeatTick:   1,
 		Storage:         s,
 		MaxSizePerMsg:   maxSizePerMsg,
-		MaxInflightMsgs: maxInflightMsgs,
+		MaxInflightMsgs: MaxInflightMsgs,
 		CheckQuorum:     true,
 		PreVote:         cfg.PreVote,
 	}

raft/node.go

@@ -353,15 +353,15 @@ func (n *node) run(r *raft) {
 			}
 		case m := <-n.recvc:
 			// filter out response message from unknown From.
-			if pr := r.prs.getProgress(m.From); pr != nil || !IsResponseMsg(m.Type) {
+			if pr := r.prs.GetProgress(m.From); pr != nil || !IsResponseMsg(m.Type) {
 				r.Step(m)
 			}
 		case cc := <-n.confc:
 			if cc.NodeID == None {
 				select {
 				case n.confstatec <- pb.ConfState{
-					Nodes:    r.prs.voterNodes(),
-					Learners: r.prs.learnerNodes()}:
+					Nodes:    r.prs.VoterNodes(),
+					Learners: r.prs.LearnerNodes()}:
 				case <-n.done:
 				}
 				break
@@ -384,8 +384,8 @@ func (n *node) run(r *raft) {
 			}
 			select {
 			case n.confstatec <- pb.ConfState{
-				Nodes:    r.prs.voterNodes(),
-				Learners: r.prs.learnerNodes()}:
+				Nodes:    r.prs.VoterNodes(),
+				Learners: r.prs.LearnerNodes()}:
 			case <-n.done:
 			}
 		case <-n.tickc: