Ensure reserved predicates cannot be moved. (dgraph-io#3137)

This change ensures reserved predicates cannot be moved, either by calls
to the /movePred endpoint or during rebalancing of predicates.

dgraph/cmd/zero/http.go

@@ -155,6 +155,9 @@ func (st *state) moveTablet(w http.ResponseWriter, r *http.Request) {
 
 	groupId, ok := intFromQueryParam(w, r, "group")
 	if !ok {
+		w.WriteHeader(http.StatusBadRequest)
+		x.SetStatus(w, x.ErrorInvalidRequest, fmt.Sprintf(
+			"Query parameter 'group' should contain a valid integer."))
 		return
 	}
 	dstGroup := uint32(groupId)

dgraph/cmd/zero/tablet.go

@@ -89,6 +89,11 @@ func (s *Server) movePredicate(predicate string, srcGroup, dstGroup uint32) erro
 	ctx, span := otrace.StartSpan(ctx, "Zero.MovePredicate")
 	defer span.End()
 
+	// Ensure that reserved predicates cannot be moved.
+	if x.IsReservedPredicate(predicate) {
+		return x.Errorf("Unable to move reserved predicate %s", predicate)
+	}
+
 	// Ensure that I'm connected to the rest of the Zero group, and am the leader.
 	if _, err := s.latestMembershipState(ctx); err != nil {
 		return x.Errorf("Unable to reach quorum: %v", err)
@@ -216,6 +221,11 @@ func (s *Server) chooseTablet() (predicate string, srcGroup uint32, dstGroup uin
 		size := int64(0)
 		group := s.state.Groups[srcGroup]
 		for _, tab := range group.Tablets {
+			// Reserved predicates should always be in group 1 so do not re-balance them.
+			if x.IsReservedPredicate(tab.Predicate) {
+				continue
+			}
+
 			// Finds a tablet as big a possible such that on moving it dstGroup's size is
 			// less than or equal to srcGroup.
 			if tab.Space <= sizeDiff/2 && tab.Space > size {