topo: Add a test for full coverage of the ShardInfo.CheckServedTypesM…

…igration method.

go/vt/topo/shard.go

@@ -484,7 +484,7 @@ func (si *ShardInfo) CheckServedTypesMigration(tabletType topodatapb.TabletType,
 	// master is a special case with a few extra checks
 	if tabletType == topodatapb.TabletType_MASTER {
 		if len(cells) > 0 {
-			return fmt.Errorf("cannot migrate only some cells for master in shard %v/%v", si.keyspace, si.shardName)
+			return fmt.Errorf("cannot migrate only some cells for MASTER in shard %v/%v. Do not specify a list of cells", si.keyspace, si.shardName)
 		}
 		if remove && len(si.ServedTypes) > 1 {
 			// Log which types must be migrated first.

go/vt/topo/shard_test.go

@@ -337,6 +337,10 @@ func TestUpdateServedTypesMap(t *testing.T) {
 		t.Fatalf("remove master failed: %v", err)
 	}
 	// MASTER
+	// Migration fails if a list of cells is specified.
+	if err := si.UpdateServedTypesMap(topodatapb.TabletType_MASTER, []string{"first", "third"}, true); err == nil || err.Error() != "cannot migrate only some cells for MASTER in shard ks/sh. Do not specify a list of cells" {
+		t.Fatalf("remove master failed: %v", err)
+	}
 	if err := si.UpdateServedTypesMap(topodatapb.TabletType_MASTER, nil, true); err != nil {
 		t.Fatalf("remove master failed: %v", err)
 	}