server: return authoritative span statistics for db details endpoint

Resolves: cockroachdb#96163

This change makes the admin API endpoint getting database statistics
scan KV for span statistics instead of using the range descriptor cache.
This provides authoritative output, helping deflake
`TestMultiRegionDatabaseStats`.

Release note (sql change): admin API database details endpoint now
returns authoritative range statistics.

pkg/server/admin.go

@@ -67,6 +67,7 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/util/mon"
 	"github.com/cockroachdb/cockroach/pkg/util/protoutil"
 	"github.com/cockroachdb/cockroach/pkg/util/quotapool"
+	"github.com/cockroachdb/cockroach/pkg/util/rangedesc"
 	"github.com/cockroachdb/cockroach/pkg/util/safesql"
 	"github.com/cockroachdb/cockroach/pkg/util/stop"
 	"github.com/cockroachdb/cockroach/pkg/util/timeutil"
@@ -1336,7 +1337,7 @@ func (s *adminServer) statsForSpan(
 	// Get a list of nodeIDs, range counts, and replica counts per node
 	// for the specified span.
 	nodeIDs, rangeCount, replCounts, err := getNodeIDsRangeCountReplCountForSpan(
-		ctx, s.distSender, rSpan,
+		ctx, s.db, rSpan, s.st,
 	)
 	if err != nil {
 		return nil, err
@@ -1451,23 +1452,29 @@ func (s *adminServer) statsForSpan(
 // Returns the list of node ids, range count,
 // and replica count for the specified span.
 func getNodeIDsRangeCountReplCountForSpan(
-	ctx context.Context, ds *kvcoord.DistSender, rSpan roachpb.RSpan,
+	ctx context.Context, db *kv.DB, rSpan roachpb.RSpan, st *cluster.Settings,
 ) (nodeIDList []roachpb.NodeID, rangeCount int64, replCounts map[roachpb.NodeID]int64, _ error) {
 	nodeIDs := make(map[roachpb.NodeID]struct{})
 	replCountForNodeID := make(map[roachpb.NodeID]int64)
-	ri := kvcoord.MakeRangeIterator(ds)
-	ri.Seek(ctx, rSpan.Key, kvcoord.Ascending)
-	for ; ri.Valid(); ri.Next(ctx) {
-		rangeCount++
-		for _, repl := range ri.Desc().Replicas().Descriptors() {
-			replCountForNodeID[repl.NodeID]++
-			nodeIDs[repl.NodeID] = struct{}{}
-		}
-		if !ri.NeedAnother(rSpan) {
-			break
+	scanner := rangedesc.NewScanner(db)
+	pageSize := int(roachpb.RangeDescPageSize.Get(&st.SV))
+	err := scanner.Scan(ctx, pageSize, func() {
+		// If the underlying txn fails and needs to be retried,
+		// clear the nodeIDs we've collected so far.
+		nodeIDs = map[roachpb.NodeID]struct{}{}
+		replCountForNodeID = map[roachpb.NodeID]int64{}
+		rangeCount = 0
+	}, rSpan.AsRawSpanWithNoLocals(), func(scanned ...roachpb.RangeDescriptor) error {
+		for _, desc := range scanned {
+			rangeCount++
+			for _, repl := range desc.Replicas().Descriptors() {
+				nodeIDs[repl.NodeID] = struct{}{}
+				replCountForNodeID[repl.NodeID]++
+			}
 		}
-	}
-	if err := ri.Error(); err != nil {
+		return nil
+	})
+	if err != nil {
 		return nil, 0, nil, err
 	}