kvserver: campaign when leader is demoted to learner

This patch campaigns when the leader demotes itself to a learner during
an atomic conf change, instead of waiting until it is completely removed
from the range. Relying on the old leader to commit the final conf
change while it's a learner appears unfortunate, and is not compatible
with an upcoming etcd/raft change that makes the learner step down in
this case.

This is backwards compatible with 23.1, because the same follower
replica is responsible for campaigning, and it does not matter if it
campaigns during the demotion or removal -- in particular because it
forces an immediate election via `forceCampaignLocked()` which
immediately bumps the term.

Epic: none
Release note: None

Author: erikgrinaker

pkg/kv/kvserver/replica_raft.go

@@ -2603,9 +2603,9 @@ func ComputeRaftLogSize(
 }
 
 // shouldCampaignAfterConfChange returns true if the current replica should
-// campaign after a conf change. If the leader replica is removed, the
-// leaseholder should campaign. We don't want to campaign on multiple replicas,
-// since that would cause ties.
+// campaign after a conf change. If the leader replica is demoted or removed,
+// the leaseholder should campaign. We don't want to campaign on multiple
+// replicas, since that would cause ties.
 //
 // If there is no current leaseholder we'll have to wait out the election
 // timeout before someone campaigns, but that's ok -- either we'll have to wait
@@ -2622,7 +2622,7 @@ func shouldCampaignAfterConfChange(
 	raftStatus raft.BasicStatus,
 	leaseStatus kvserverpb.LeaseStatus,
 ) bool {
-	if raftStatus.Lead == 0 {
+	if raftStatus.Lead == raft.None {
 		// Leader unknown. We can't know if it was removed by the conf change, and
 		// because we force an election without prevote we don't want to risk
 		// throwing spurious elections.
@@ -2637,9 +2637,11 @@ func shouldCampaignAfterConfChange(
 		// don't expect to hit this, but let's be defensive.
 		return false
 	}
-	if _, ok := desc.GetReplicaDescriptorByID(roachpb.ReplicaID(raftStatus.Lead)); ok {
-		// The leader is still in the descriptor.
-		return false
+	if replDesc, ok := desc.GetReplicaDescriptorByID(roachpb.ReplicaID(raftStatus.Lead)); ok {
+		if replDesc.IsAnyVoter() {
+			// The leader is still a voter in the descriptor.
+			return false
+		}
 	}
 	// Prior to 23.2, the first voter in the descriptor campaigned, so we do
 	// the same in mixed-version clusters to avoid ties.