raft: never remove the last voter

Before this change, it was possible for a Raft group to apply a
configuration change that removes the last voter. Such a group
is stuck and it's unclear that this operation is ever useful.
On the flip side, it makes an awkward edge case to allow, one
that will be more awkward in the context of joint consensus.

Remove it and thereby reduce the mental overhead of reasoning
about configuration changes.

raft/raft_test.go

@@ -1139,10 +1139,16 @@ func TestCommit(t *testing.T) {
 		storage.Append(tt.logs)
 		storage.hardState = pb.HardState{Term: tt.smTerm}
 
-		sm := newTestRaft(1, []uint64{1}, 10, 2, storage)
-		sm.prs.RemoveAny(1)
+		var ids []uint64
 		for j := 0; j < len(tt.matches); j++ {
-			sm.prs.InitProgress(uint64(j)+1, tt.matches[j], tt.matches[j]+1, false)
+			ids = append(ids, uint64(j+1))
+		}
+		sm := newTestRaft(1, ids, 10, 2, storage)
+
+		for j := 0; j < len(tt.matches); j++ {
+			id := uint64(j + 1)
+			sm.prs.Progress[id].Match = tt.matches[j]
+			sm.prs.Progress[id].Next = tt.matches[j] + 1
 		}
 		sm.maybeCommit()
 		if g := sm.raftLog.committed; g != tt.w {
@@ -3142,9 +3148,8 @@ func TestRemoveNode(t *testing.T) {
 		t.Errorf("nodes = %v, want %v", g, w)
 	}
 
-	// remove all nodes from cluster
+	// The last remaining node will refuse to remove itself.
 	r.applyConfChange(pb.ConfChange{NodeID: 1, Type: pb.ConfChangeRemoveNode})
-	w = []uint64{}
 	if g := r.prs.VoterNodes(); !reflect.DeepEqual(g, w) {
 		t.Errorf("nodes = %v, want %v", g, w)
 	}
@@ -3160,14 +3165,13 @@ func TestRemoveLearner(t *testing.T) {
 		t.Errorf("nodes = %v, want %v", g, w)
 	}
 
-	w = []uint64{}
-	if g := r.prs.LearnerNodes(); !reflect.DeepEqual(g, w) {
+	if w, g := []uint64{}, r.prs.LearnerNodes(); !reflect.DeepEqual(g, w) {
 		t.Errorf("nodes = %v, want %v", g, w)
 	}
 
-	// remove all nodes from cluster
+	// The remaining voter will refuse to remove itself.
 	r.applyConfChange(pb.ConfChange{NodeID: 1, Type: pb.ConfChangeRemoveNode})
-	if g := r.prs.VoterNodes(); !reflect.DeepEqual(g, w) {
+	if w, g := []uint64{1}, r.prs.VoterNodes(); !reflect.DeepEqual(g, w) {
 		t.Errorf("nodes = %v, want %v", g, w)
 	}
 }

raft/tracker/tracker.go

@@ -157,6 +157,11 @@ func (p *ProgressTracker) RemoveAny(id uint64) {
 		panic(fmt.Sprintf("peer %x is both voter and learner", id))
 	}
 
+	if okV1 && len(p.Voters[0]) == 1 {
+		// Never remove the last voter.
+		return
+	}
+
 	delete(p.Voters[0], id)
 	delete(p.Voters[1], id)
 	delete(p.Learners, id)