raft: simplify auto-leave joint config of entry application logic

This commit simplifies the logic added in 37c7e4d to auto-leave joint
configurations. It does so by making the following adjustments to the
code:

- remove the `oldApplied <= r.pendingConfIndex` condition. This does
  not seem necessary. When a node first attempts to auto-leave a joint
  config, it will bump `r.pendingConfIndex` when proposing. In cases
  where `oldApplied >= r.pendingConfIndex`, the proposal must have
  already been applied. Reviewers should double check this.
- use raft.Step instead of custom proposal code. This code was already
  present in stepLeader, so there was no reason to duplicate it. This
  would have avoided bugs like the one we fixed in etcd-io#14538.
- use `confChangeToMsg` to generate message, to centralize the creation
  of all `MsgProp{EntryConfChange}` messages.

raft/raft.go

@@ -548,26 +548,20 @@ func (r *raft) advance(rd Ready) {
 	// new Commit index, this does not mean that we're also applying
 	// all of the new entries due to commit pagination by size.
 	if newApplied := rd.appliedCursor(); newApplied > 0 {
-		oldApplied := r.raftLog.applied
 		r.raftLog.appliedTo(newApplied)
 
-		if r.prs.Config.AutoLeave && oldApplied <= r.pendingConfIndex && newApplied >= r.pendingConfIndex && r.state == StateLeader {
+		if r.prs.Config.AutoLeave && newApplied >= r.pendingConfIndex && r.state == StateLeader {
 			// If the current (and most recent, at least for this leader's term)
 			// configuration should be auto-left, initiate that now. We use a
 			// nil Data which unmarshals into an empty ConfChangeV2 and has the
 			// benefit that appendEntry can never refuse it based on its size
 			// (which registers as zero).
-			ent := pb.Entry{
-				Type: pb.EntryConfChangeV2,
-				Data: nil,
-			}
-			// There's no way in which this proposal should be able to be rejected.
-			if !r.appendEntry(ent) {
-				panic("refused un-refusable auto-leaving ConfChangeV2")
-			}
-			r.bcastAppend()
-			r.pendingConfIndex = r.raftLog.lastIndex()
 			r.logger.Infof("initiating automatic transition out of joint configuration %s", r.prs.Config)
+			m, err := confChangeToMsg(nil)
+			if err != nil {
+				panic(err)
+			}
+			_ = r.Step(m)
 		}
 	}
 

raft/raftpb/confchange.go

@@ -35,7 +35,13 @@ func MarshalConfChange(c ConfChangeI) (EntryType, []byte, error) {
 	var typ EntryType
 	var ccdata []byte
 	var err error
-	if ccv1, ok := c.AsV1(); ok {
+	if c == nil {
+		// A nil data unmarshals into an empty ConfChangeV2 and has the benefit
+		// that appendEntry can never refuse it based on its size (which
+		// registers as zero).
+		typ = EntryConfChangeV2
+		ccdata = nil
+	} else if ccv1, ok := c.AsV1(); ok {
 		typ = EntryConfChange
 		ccdata, err = ccv1.Marshal()
 	} else {