raft: extract Progress update on MsgApp to a method

Previously, Progress update on MsgApp send was scattered across raft.go and
tracker/progress.go. This commit better encapsulates this logic in the Progress
type.

Signed-off-by: Pavel Kalinnikov <pavel@cockroachlabs.com>

raft/raft.go

@@ -472,20 +472,9 @@ func (r *raft) maybeSendAppend(to uint64, sendIfEmpty bool) bool {
 	}
 
 	// Send the actual MsgApp otherwise, and update the progress accordingly.
-	// TODO(pavelkalinnikov): factor out the Progress update to a method
 	next := pr.Next // save Next for later, as the progress update can change it
-	if n := len(ents); n != 0 {
-		switch pr.State {
-		// optimistically increase the next when in StateReplicate
-		case tracker.StateReplicate:
-			last := ents[n-1].Index
-			pr.OptimisticUpdate(last)
-			pr.Inflights.Add(last)
-		case tracker.StateProbe:
-			pr.ProbeSent = true
-		default:
-			r.logger.Panicf("%x is sending append in unhandled state %s", r.id, pr.State)
-		}
+	if err := pr.UpdateOnEntriesSend(len(ents), next); err != nil {
+		r.logger.Panicf("%x: %v", r.id, err)
 	}
 	r.send(pb.Message{
 		To:      to,

raft/tracker/progress.go

@@ -137,6 +137,29 @@ func (pr *Progress) BecomeSnapshot(snapshoti uint64) {
 	pr.PendingSnapshot = snapshoti
 }
 
+// UpdateOnEntriesSend updates the progress on the given number of consecutive
+// entries being sent in a MsgApp, appended at and after the given log index.
+func (pr *Progress) UpdateOnEntriesSend(entries int, nextIndex uint64) error {
+	switch pr.State {
+	case StateReplicate:
+		if entries > 0 {
+			last := nextIndex + uint64(entries) - 1
+			pr.OptimisticUpdate(last)
+			pr.Inflights.Add(last)
+		}
+	case StateProbe:
+		// TODO(pavelkalinnikov): this condition captures the previous behaviour,
+		// but we should set ProbeSent unconditionally for simplicity, because any
+		// MsgApp in StateProbe is a probe, not only non-empty ones.
+		if entries > 0 {
+			pr.ProbeSent = true
+		}
+	default:
+		return fmt.Errorf("sending append in unhandled state %s", pr.State)
+	}
+	return nil
+}
+
 // MaybeUpdate is called when an MsgAppResp arrives from the follower, with the
 // index acked by it. The method returns false if the given n index comes from
 // an outdated message. Otherwise it updates the progress and returns true.