fixes to the migration code

pkg/sql/catalog/bootstrap/initial_values.go

@@ -7,6 +7,7 @@ package bootstrap
 
 import (
 	"bytes"
+	"context"
 	"crypto/sha256"
 	_ "embed"
 	"encoding/hex"
@@ -16,7 +17,11 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/clusterversion"
 	"github.com/cockroachdb/cockroach/pkg/config/zonepb"
 	"github.com/cockroachdb/cockroach/pkg/keys"
+	"github.com/cockroachdb/cockroach/pkg/kv"
 	"github.com/cockroachdb/cockroach/pkg/roachpb"
+
+	"github.com/cockroachdb/cockroach/pkg/sql/sem/catconstants"
+	"github.com/cockroachdb/cockroach/pkg/util/log"
 	"github.com/cockroachdb/errors"
 )
 
@@ -87,6 +92,74 @@ func buildLatestInitialValues(
 	return kvs, splits, nil
 }
 
+func CopySystemTableKVs(
+	ctx context.Context, txn *kv.Txn, targetCodec keys.SQLCodec, descsIDMap map[string]uint32,
+) ([]roachpb.KeyValue, error) {
+	sourceCodec := keys.SystemSQLCodec
+	tables := []catconstants.SystemTableName{
+		catconstants.ZonesTableName,
+		catconstants.TenantsTableName,
+		catconstants.TenantSettingsTableName,
+	}
+
+	batch := txn.NewBatch()
+	for _, table := range tables {
+		descID, ok := descsIDMap[string(table)]
+		if !ok {
+			log.Ops.Errorf(ctx, "descID not found for : %s", table)
+			return nil, errors.Errorf("descID not found for : %s", table)
+		}
+		span := sourceCodec.TableSpan(descID)
+		batch.Scan(span.Key, span.EndKey)
+	}
+
+	if err := txn.Run(ctx, batch); err != nil {
+		return nil, err
+	}
+
+	if len(batch.Results) != len(tables) {
+		return nil, errors.AssertionFailedf(
+			"unexpected batch result count, expected: %d, found: %d",
+			len(tables),
+			len(batch.Results))
+	}
+
+	ret := make([]roachpb.KeyValue, 0)
+	for i, result := range batch.Results {
+		if err := result.Err; err != nil {
+			return nil, err
+		}
+		rows := result.Rows
+		log.Ops.Infof(ctx, "rows count: %d, table id: %d", len(rows), descsIDMap[string(tables[i])])
+
+		tablePrefix := targetCodec.TablePrefix(descsIDMap[string(tables[i])])
+		kvs := make([]roachpb.KeyValue, 0, len(rows))
+		for _, row := range rows {
+			key, err := keys.StripTablePrefix(row.Key)
+			if err != nil {
+				return nil, err
+			}
+
+			// tablePrefix is a slice and reusing it will modify the underlying array in undesirable
+			// ways. Build a newKey to avoid that.
+			// TODO(chandrat) is it possible to avoid this garbage?
+			newKey := make([]byte, len(tablePrefix)+len(key))
+			newKey = append(newKey, tablePrefix...)
+			newKey = append(newKey, key...)
+
+			v := roachpb.KeyValue{
+				Key:   newKey,
+				Value: *row.Value,
+			}
+			v.Value.ClearChecksum()
+			kvs = append(kvs, v)
+		}
+		ret = append(ret, kvs...)
+	}
+
+	return ret, nil
+}
+
 // hardCodedInitialValues defines an initialValuesFactoryFn using
 // hard-coded values.
 type hardCodedInitialValues struct {