kvserver: do lazy map allocations in replicaFlowControl

These methods will typically return empty maps, so we can avoid the
allocations.

Informs #111561

Epic: none

Release note: None

pkg/kv/kvserver/flow_control_replica.go

@@ -60,12 +60,14 @@ func (rf *replicaFlowControl) getPausedFollowers() map[roachpb.ReplicaID]struct{
 
 func (rf *replicaFlowControl) getBehindFollowers() map[roachpb.ReplicaID]struct{} {
 	rf.assertLocked()
-	behindFollowers := make(map[roachpb.ReplicaID]struct{})
+	var behindFollowers map[roachpb.ReplicaID]struct{}
 	rf.mu.internalRaftGroup.WithProgress(func(id uint64, _ raft.ProgressType, progress rafttracker.Progress) {
 		if progress.State == rafttracker.StateReplicate {
 			return
 		}
-
+		if behindFollowers == nil {
+			behindFollowers = make(map[roachpb.ReplicaID]struct{})
+		}
 		replID := roachpb.ReplicaID(id)
 		behindFollowers[replID] = struct{}{}
 
@@ -90,12 +92,15 @@ func (rf *replicaFlowControl) getBehindFollowers() map[roachpb.ReplicaID]struct{
 
 func (rf *replicaFlowControl) getInactiveFollowers() map[roachpb.ReplicaID]struct{} {
 	rf.assertLocked()
-	inactiveFollowers := make(map[roachpb.ReplicaID]struct{})
+	var inactiveFollowers map[roachpb.ReplicaID]struct{}
 	for _, desc := range rf.getDescriptor().Replicas().Descriptors() {
 		if desc.ReplicaID == rf.getReplicaID() {
 			continue
 		}
 		if !rf.mu.lastUpdateTimes.isFollowerActiveSince(desc.ReplicaID, timeutil.Now(), rf.store.cfg.RangeLeaseDuration) {
+			if inactiveFollowers == nil {
+				inactiveFollowers = make(map[roachpb.ReplicaID]struct{})
+			}
 			inactiveFollowers[desc.ReplicaID] = struct{}{}
 		}
 	}
@@ -104,12 +109,15 @@ func (rf *replicaFlowControl) getInactiveFollowers() map[roachpb.ReplicaID]struc
 
 func (rf *replicaFlowControl) getDisconnectedFollowers() map[roachpb.ReplicaID]struct{} {
 	rf.assertLocked()
-	disconnectedFollowers := make(map[roachpb.ReplicaID]struct{})
+	var disconnectedFollowers map[roachpb.ReplicaID]struct{}
 	for _, desc := range rf.getDescriptor().Replicas().Descriptors() {
 		if desc.ReplicaID == rf.getReplicaID() {
 			continue
 		}
 		if !rf.store.raftTransportForFlowControl.isConnectedTo(desc.StoreID) {
+			if disconnectedFollowers == nil {
+				disconnectedFollowers = make(map[roachpb.ReplicaID]struct{})
+			}
 			disconnectedFollowers[desc.ReplicaID] = struct{}{}
 		}
 	}

pkg/kv/kvserver/flow_control_replica_integration.go

@@ -286,6 +286,9 @@ func (f *replicaFlowControlIntegrationImpl) handle() (kvflowcontrol.Handle, bool
 // reconnect previously disconnected streams if we're able.
 func (f *replicaFlowControlIntegrationImpl) refreshStreams(ctx context.Context, reason string) {
 	f.disconnectStreams(ctx, f.notActivelyReplicatingTo(), reason)
+	// TODO(sumeer): we call notActivelyReplicatingTo() again in tryReconnect(),
+	// which is wasteful, since refreshStreams is called on every raft tick.
+	// Simply pass the return value from the call above to the following method.
 	f.tryReconnect(ctx)
 }