loqrecovery: fixup and comment on sep-raft-log

pkg/kv/kvserver/loqrecovery/apply.go

@@ -317,6 +317,8 @@ func applyReplicaUpdate(
 
 	hs.LeadEpoch = 0
 
+	// TODO(sep-raft-log): when raft and state machine engines are separated, this
+	// update must be written to the raft engine.
 	if err := sl.SetHardState(ctx, readWriter, hs); err != nil {
 		return PrepareReplicaReport{}, errors.Wrap(err, "setting HardState")
 	}

pkg/kv/kvserver/loqrecovery/collect.go

@@ -158,7 +158,10 @@ func CollectStoresReplicaInfo(
 			return loqrecoverypb.ClusterReplicaInfo{}, CollectionStats{}, errors.New("can't collect info from stored that belong to different clusters")
 		}
 		nodes[ident.NodeID] = struct{}{}
-		if err := visitStoreReplicas(ctx, reader, ident.StoreID, ident.NodeID, version,
+		// TODO(sep-raft-log): use different readers when the raft and state machine
+		// engines are separate. Since the engines are immutable in this path, there
+		// is no question whether to and in which order to grab engine snapshots.
+		if err := visitStoreReplicas(ctx, reader, reader, ident.StoreID, ident.NodeID,
 			func(info loqrecoverypb.ReplicaInfo) error {
 				replicas = append(replicas, info)
 				return nil
@@ -178,19 +181,21 @@ func CollectStoresReplicaInfo(
 
 func visitStoreReplicas(
 	ctx context.Context,
-	reader storage.Reader,
+	state, raft storage.Reader,
 	storeID roachpb.StoreID,
 	nodeID roachpb.NodeID,
-	targetVersion clusterversion.ClusterVersion,
 	send func(info loqrecoverypb.ReplicaInfo) error,
 ) error {
-	if err := kvstorage.IterateRangeDescriptorsFromDisk(ctx, reader, func(desc roachpb.RangeDescriptor) error {
+	if err := kvstorage.IterateRangeDescriptorsFromDisk(ctx, state, func(desc roachpb.RangeDescriptor) error {
 		rsl := stateloader.Make(desc.RangeID)
-		rstate, err := rsl.Load(ctx, reader, &desc)
+		rstate, err := rsl.Load(ctx, state, &desc)
 		if err != nil {
 			return err
 		}
-		hstate, err := rsl.LoadHardState(ctx, reader)
+		// TODO(pav-kv): the LoQ recovery flow uses only the applied index, and the
+		// HardState.Commit loaded here is unused. Consider removing. Make sure this
+		// doesn't break compatibility for ReplicaInfo unmarshalling.
+		hstate, err := rsl.LoadHardState(ctx, raft)
 		if err != nil {
 			return err
 		}
@@ -199,8 +204,12 @@ func visitStoreReplicas(
 		// at potentially uncommitted entries as we have no way to determine their
 		// outcome, and they will become committed as soon as the replica is
 		// designated as a survivor.
+		// TODO(sep-raft-log): decide which LogID to read from. If the raft and
+		// state machine readers are slightly out of sync, the LogIDs may mismatch.
+		// For the heuristics here, it would probably make sense to read from all
+		// LogIDs with unapplied entries.
 		rangeUpdates, err := GetDescriptorChangesFromRaftLog(
-			ctx, desc.RangeID, rstate.RaftAppliedIndex+1, math.MaxInt64, reader)
+			ctx, desc.RangeID, rstate.RaftAppliedIndex+1, math.MaxInt64, raft)
 		if err != nil {
 			return err
 		}

pkg/kv/kvserver/loqrecovery/server.go

@@ -121,7 +121,6 @@ func (s Server) ServeLocalReplicas(
 	_ *serverpb.RecoveryCollectLocalReplicaInfoRequest,
 	stream serverpb.Admin_RecoveryCollectLocalReplicaInfoServer,
 ) error {
-	v := s.settings.Version.ActiveVersion(ctx)
 	var stores []*kvserver.Store
 	if err := s.stores.VisitStores(func(s *kvserver.Store) error {
 		stores = append(stores, s)
@@ -135,9 +134,14 @@ func (s Server) ServeLocalReplicas(
 	for _, s := range stores {
 		s := s // copy for closure
 		g.Go(func() error {
+			// TODO(sep-raft-log): when raft and state machine engines are separate,
+			// we need two snapshots here. This path is online, so we should make sure
+			// these snapshots are consistent (in particular, the LogID must match
+			// across the two), or make sure that LogID mismatch is handled in
+			// visitStoreReplicas.
 			reader := s.TODOEngine().NewSnapshot()
 			defer reader.Close()
-			return visitStoreReplicas(ctx, reader, s.StoreID(), s.NodeID(), v,
+			return visitStoreReplicas(ctx, reader, reader, s.StoreID(), s.NodeID(),
 				func(info loqrecoverypb.ReplicaInfo) error {
 					return syncStream.Send(&serverpb.RecoveryCollectLocalReplicaInfoResponse{ReplicaInfo: &info})
 				})