fix(Dgraph): update reverse index when updating single UID predicates. (

#5868) (#6006)

When updating a single uid predicate with a reverse index, the existing entry in the
reverse index should be deleted first.

Fixes DGRAPH-1738

(cherry picked from commit 5b70fe8)

posting/index.go

@@ -212,8 +212,37 @@ func (txn *Txn) addReverseAndCountMutation(ctx context.Context, t *pb.DirectedEd
 	if err != nil {
 		return err
 	}
+	if plist == nil {
+		return errors.Errorf("nil posting list for reverse key %s", hex.Dump(key))
+	}
+
+	// For single uid predicates, updating the reverse index requires that the existing
+	// entries for this key in the index are removed.
+	pred, ok := schema.State().Get(ctx, t.Attr)
+	isSingleUidUpdate := ok && !pred.GetList() && pred.GetValueType() == pb.Posting_UID &&
+		t.Op == pb.DirectedEdge_SET && t.ValueId != 0
+	if isSingleUidUpdate {
+		dataKey := x.DataKey(t.Attr, t.Entity)
+		dataList, err := getFn(dataKey)
+		if err != nil {
+			return errors.Wrapf(err, "cannot find single uid list to update with key %s",
+				hex.Dump(dataKey))
+		}
+		err = dataList.Iterate(txn.StartTs, 0, func(p *pb.Posting) error {
+			delEdge := &pb.DirectedEdge{
+				Entity:  t.Entity,
+				ValueId: p.Uid,
+				Attr:    t.Attr,
+				Op:      pb.DirectedEdge_DEL,
+			}
+			return txn.addReverseAndCountMutation(ctx, delEdge)
+		})
+		if err != nil {
+			return errors.Wrapf(err, "cannot remove existing reverse index entries for key %s",
+				hex.Dump(dataKey))
+		}
+	}
 
-	x.AssertTrue(plist != nil)
 	// We must create a copy here.
 	edge := &pb.DirectedEdge{
 		Entity:  t.ValueId,
@@ -234,6 +263,7 @@ func (txn *Txn) addReverseAndCountMutation(ctx context.Context, t *pb.DirectedEd
 			return err
 		}
 	}
+
 	return nil
 }
 
@@ -438,6 +468,15 @@ func (l *List) AddMutationWithIndex(ctx context.Context, edge *pb.DirectedEdge,
 
 	doUpdateIndex := pstore != nil && schema.State().IsIndexed(ctx, edge.Attr)
 	hasCountIndex := schema.State().HasCount(ctx, edge.Attr)
+
+	// Add reverse mutation irrespective of hasMutated, server crash can happen after
+	// mutation is synced and before reverse edge is synced
+	if (pstore != nil) && (edge.ValueId != 0) && schema.State().IsReversed(ctx, edge.Attr) {
+		if err := txn.addReverseAndCountMutation(ctx, edge); err != nil {
+			return err
+		}
+	}
+
 	val, found, cp, err := txn.addMutationHelper(ctx, l, doUpdateIndex, hasCountIndex, edge)
 	if err != nil {
 		return err
@@ -475,13 +514,6 @@ func (l *List) AddMutationWithIndex(ctx context.Context, edge *pb.DirectedEdge,
 			}
 		}
 	}
-	// Add reverse mutation irrespective of hasMutated, server crash can happen after
-	// mutation is synced and before reverse edge is synced
-	if (pstore != nil) && (edge.ValueId != 0) && schema.State().IsReversed(ctx, edge.Attr) {
-		if err := txn.addReverseAndCountMutation(ctx, edge); err != nil {
-			return err
-		}
-	}
 	return nil
 }
 

systest/mutations_test.go

@@ -98,6 +98,7 @@ func TestSystem(t *testing.T) {
 	t.Run("count index delete on non list predicate", wrap(CountIndexNonlistPredicateDelete))
 	t.Run("Reverse count index delete", wrap(ReverseCountIndexDelete))
 	t.Run("overwrite uid predicates", wrap(OverwriteUidPredicates))
+	t.Run("overwrite uid predicates reverse index", wrap(OverwriteUidPredicatesReverse))
 	t.Run("delete and query same txn", wrap(DeleteAndQuerySameTxn))
 }
 
@@ -2187,6 +2188,93 @@ func OverwriteUidPredicates(t *testing.T, c *dgo.Dgraph) {
 		string(resp.GetJson()))
 }
 
+func OverwriteUidPredicatesReverse(t *testing.T, c *dgo.Dgraph) {
+	ctx := context.Background()
+	op := &api.Operation{DropAll: true}
+	require.NoError(t, c.Alter(ctx, op))
+
+	op = &api.Operation{
+		Schema: `
+		best_friend: uid @reverse .
+		name: string @index(exact) .`,
+	}
+	err := c.Alter(ctx, op)
+	require.NoError(t, err)
+
+	txn := c.NewTxn()
+	_, err = txn.Mutate(context.Background(), &api.Mutation{
+		CommitNow: true,
+		SetNquads: []byte(`
+		_:alice <name> "Alice" .
+		_:bob <name> "Bob" .
+		_:alice <best_friend> _:bob .`),
+	})
+	require.NoError(t, err)
+
+	q := `{
+  me(func: eq(name, Alice)) {
+	name
+	best_friend {
+		name
+	}
+  }
+}`
+	resp, err := c.NewReadOnlyTxn().Query(ctx, q)
+	require.NoError(t, err)
+	testutil.CompareJSON(t, `{"me":[{"name":"Alice","best_friend": {"name": "Bob"}}]}`,
+		string(resp.GetJson()))
+
+	reverseQuery := `{
+		reverse(func: has(~best_friend)) {
+			name
+			~best_friend {
+				name
+			}
+		}}`
+	resp, err = c.NewReadOnlyTxn().Query(ctx, reverseQuery)
+	require.NoError(t, err)
+	testutil.CompareJSON(t, `{"reverse":[{"name":"Bob","~best_friend": [{"name": "Alice"}]}]}`,
+		string(resp.GetJson()))
+
+	upsertQuery := `query { alice as var(func: eq(name, Alice)) }`
+	upsertMutation := &api.Mutation{
+		SetNquads: []byte(`
+		_:carol <name> "Carol" .
+		uid(alice) <best_friend> _:carol .`),
+	}
+	req := &api.Request{
+		Query:     upsertQuery,
+		Mutations: []*api.Mutation{upsertMutation},
+		CommitNow: true,
+	}
+	_, err = c.NewTxn().Do(ctx, req)
+	require.NoError(t, err)
+
+	resp, err = c.NewReadOnlyTxn().Query(ctx, reverseQuery)
+	require.NoError(t, err)
+	testutil.CompareJSON(t, `{"reverse":[{"name":"Carol","~best_friend": [{"name": "Alice"}]}]}`,
+		string(resp.GetJson()))
+
+	// Delete the triples and verify the reverse edge is gone.
+	upsertMutation = &api.Mutation{
+		DelNquads: []byte(`
+		uid(alice) <best_friend> * .`),
+	}
+	req = &api.Request{
+		Query:     upsertQuery,
+		Mutations: []*api.Mutation{upsertMutation},
+		CommitNow: true,
+	}
+	_, err = c.NewTxn().Do(ctx, req)
+	require.NoError(t, err)
+
+	resp, err = c.NewReadOnlyTxn().Query(ctx, reverseQuery)
+	require.NoError(t, err)
+	testutil.CompareJSON(t, `{"reverse":[]}`,
+		string(resp.GetJson()))
+
+}
+
 func DeleteAndQuerySameTxn(t *testing.T, c *dgo.Dgraph) {
 	// Set the schema.
 	ctx := context.Background()