raft: return error when ConfChange is rejected due to pending change

raft.go

@@ -1298,6 +1298,10 @@ func stepLeader(r *raft, m pb.Message) error {
 			return ErrProposalDropped
 		}
 
+		// Track if any ConfChange was rejected so we can return an error
+		// while still appending the noop to maintain log consistency.
+		var confChangeRejected bool
+
 		for i := range m.Entries {
 			e := &m.Entries[i]
 			var cc pb.ConfChangeI
@@ -1330,7 +1334,10 @@ func stepLeader(r *raft, m pb.Message) error {
 
 				if failedCheck != "" && !r.disableConfChangeValidation {
 					r.logger.Infof("%x ignoring conf change %v at config %s: %s", r.id, cc, r.trk.Config, failedCheck)
+					// Convert to noop to maintain log consistency, but mark as rejected
+					// so we can return an error to notify the caller.
 					m.Entries[i] = pb.Entry{Type: pb.EntryNormal}
+					confChangeRejected = true
 				} else {
 					r.pendingConfIndex = r.raftLog.lastIndex() + uint64(i) + 1
 					traceChangeConfEvent(cc, r)
@@ -1342,6 +1349,11 @@ func stepLeader(r *raft, m pb.Message) error {
 			return ErrProposalDropped
 		}
 		r.bcastAppend()
+		// Return error if any ConfChange was rejected, so the caller knows
+		// the config change didn't take effect (even though a noop was appended).
+		if confChangeRejected {
+			return ErrProposalDropped
+		}
 		return nil
 	case pb.MsgReadIndex:
 		// only one voting member (the leader) in the cluster

raft_test.go

@@ -2687,7 +2687,7 @@ func TestStepConfig(t *testing.T) {
 
 // TestStepIgnoreConfig tests that if raft step the second msgProp in
 // EntryConfChange type when the first one is uncommitted, the node will set
-// the proposal to noop and keep its original state.
+// the proposal to noop, keep its original state, and return an error.
 func TestStepIgnoreConfig(t *testing.T) {
 	// a raft that cannot make progress
 	r := newTestRaft(1, 10, 1, newTestMemoryStorage(withPeers(1, 2)))
@@ -2696,7 +2696,10 @@ func TestStepIgnoreConfig(t *testing.T) {
 	r.Step(pb.Message{From: 1, To: 1, Type: pb.MsgProp, Entries: []pb.Entry{{Type: pb.EntryConfChange}}})
 	index := r.raftLog.lastIndex()
 	pendingConfIndex := r.pendingConfIndex
-	r.Step(pb.Message{From: 1, To: 1, Type: pb.MsgProp, Entries: []pb.Entry{{Type: pb.EntryConfChange}}})
+	// Second ConfChange should be converted to noop but return an error
+	err := r.Step(pb.Message{From: 1, To: 1, Type: pb.MsgProp, Entries: []pb.Entry{{Type: pb.EntryConfChange}}})
+	assert.Equal(t, ErrProposalDropped, err)
+	// A noop entry should still be appended (for log consistency)
 	wents := []pb.Entry{{Type: pb.EntryNormal, Term: 1, Index: 3, Data: nil}}
 	ents, err := r.raftLog.entries(index+1, noLimit)
 	require.NoError(t, err)

testdata/confchange_v2_add_single_explicit.txt

@@ -103,6 +103,7 @@ propose-conf-change 1
 v3 v4 v5
 ----
 INFO 1 ignoring conf change {ConfChangeTransitionAuto [{ConfChangeAddNode 3} {ConfChangeAddNode 4} {ConfChangeAddNode 5}] []} at config voters=(1 2)&&(1): must transition out of joint config first
+raft proposal dropped
 
 # Propose a transition out of the joint config. We'll see this at index 6 below.
 propose-conf-change 1
@@ -165,6 +166,7 @@ stabilize
 propose-conf-change 1
 ----
 INFO 1 ignoring conf change {ConfChangeTransitionAuto [] []} at config voters=(1 2): not in joint state; refusing empty conf change
+raft proposal dropped
 
 # Finishes work for the empty entry we just proposed.
 stabilize