Allow schema updates to reserved preds if the update is the same.

edgraph/server.go

@@ -351,8 +351,9 @@ func (s *Server) Alter(ctx context.Context, op *api.Operation) (*api.Payload, er
 	}
 
 	for _, update := range result.Schemas {
-		// Reserved predicates cannot be altered.
-		if x.IsReservedPredicate(update.Predicate) {
+		// Reserved predicates cannot be altered but let the update go through
+		// if the update is equal to the existing one.
+		if schema.IsReservedPredicateChanged(update.Predicate, update) {
 			err := fmt.Errorf("predicate %s is reserved and is not allowed to be modified",
 				update.Predicate)
 			return nil, err

ee/acl/acl_test.go

@@ -173,7 +173,15 @@ var query = fmt.Sprintf(`
 
 func alterReservedPredicates(t *testing.T, dg *dgo.Dgraph) {
 	ctx := context.Background()
+
+	// Test that alter requests are allowed if the new update is the same as
+	// the initial update for a reserved predicate.
 	err := dg.Alter(ctx, &api.Operation{
+		Schema: "dgraph.xid: string @index(exact) @upsert .",
+	})
+	require.NoError(t, err)
+
+	err = dg.Alter(ctx, &api.Operation{
 		Schema: "dgraph.xid: int .",
 	})
 	require.Error(t, err)

schema/schema.go

@@ -23,6 +23,7 @@ import (
 
 	"github.com/dgraph-io/badger"
 	"github.com/golang/glog"
+	"github.com/golang/protobuf/proto"
 	"golang.org/x/net/trace"
 
 	"github.com/dgraph-io/dgraph/protos/pb"
@@ -458,6 +459,27 @@ func InitialSchema() []*pb.SchemaUpdate {
 	return initialSchema
 }
 
+// IsReservedPredicateChanged returns true if the initial update for the reserved
+// predicate pred is different than the passed update.
+func IsReservedPredicateChanged(pred string, update *pb.SchemaUpdate) bool {
+	// Return false for non-reserved predicates.
+	if !x.IsReservedPredicate(pred) {
+		return false
+	}
+
+	initialSchema := InitialSchema()
+	for _, original := range initialSchema {
+		if original.Predicate != pred {
+			continue
+		}
+		if proto.Equal(original, update) {
+			return false
+		}
+		return true
+	}
+	return true
+}
+
 func reset() {
 	pstate = new(state)
 	pstate.init()