fix(Dgraph): fix bug when deleting and adding to a single UID predica…

…te in the same transaction. (#6431)

The iterate method does not account for delete postings in the same transaction
so they have to be preserved manually. Otherwise, if a deletion and an add operation
to the same uid happen in the same predicate, the deletion operation is not
applied.

Fixes DGRAPH-1309

posting/list.go

@@ -359,8 +359,17 @@ func (l *List) updateMutationLayer(mpost *pb.Posting, singleUidUpdate bool) erro
 		// The current value should be deleted in favor of this value. This needs to
 		// be done because the fingerprint for the value is not math.MaxUint64 as is
 		// the case with the rest of the scalar predicates.
-		plist := &pb.PostingList{}
-		plist.Postings = append(plist.Postings, mpost)
+		newPlist := &pb.PostingList{}
+		newPlist.Postings = append(newPlist.Postings, mpost)
+
+		// Add the deletions in the existing plist because those postings are not picked
+		// up by iterating. Not doing so would result in delete operations that are not
+		// applied when the transaction is committed.
+		for _, post := range plist.Postings {
+			if post.Op == Del && post.Uid != mpost.Uid {
+				newPlist.Postings = append(newPlist.Postings, post)
+			}
+		}
 
 		err := l.iterate(mpost.StartTs, 0, func(obj *pb.Posting) error {
 			// Ignore values which have the same uid as they will get replaced
@@ -374,7 +383,7 @@ func (l *List) updateMutationLayer(mpost *pb.Posting, singleUidUpdate bool) erro
 			// for the mutation stored in mpost.
 			objCopy := proto.Clone(obj).(*pb.Posting)
 			objCopy.Op = Del
-			plist.Postings = append(plist.Postings, objCopy)
+			newPlist.Postings = append(newPlist.Postings, objCopy)
 			return nil
 		})
 		if err != nil {
@@ -383,7 +392,7 @@ func (l *List) updateMutationLayer(mpost *pb.Posting, singleUidUpdate bool) erro
 
 		// Update the mutation map with the new plist. Return here since the code below
 		// does not apply for predicates of type uid.
-		l.mutationMap[mpost.StartTs] = plist
+		l.mutationMap[mpost.StartTs] = newPlist
 		return nil
 	}
 

systest/mutations_test.go

@@ -100,6 +100,7 @@ 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 across txns", wrap(OverwriteUidPredicatesMultipleTxn))
 	t.Run("overwrite uid predicates reverse index", wrap(OverwriteUidPredicatesReverse))
 	t.Run("delete and query same txn", wrap(DeleteAndQuerySameTxn))
 	t.Run("add and query zero datetime value", wrap(AddAndQueryZeroTimeValue))
@@ -2349,6 +2350,60 @@ func OverwriteUidPredicatesReverse(t *testing.T, c *dgo.Dgraph) {
 		string(resp.GetJson()))
 }
 
+func OverwriteUidPredicatesMultipleTxn(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 .
+		name: string @index(exact) .`,
+	}
+	err := c.Alter(ctx, op)
+	require.NoError(t, err)
+
+	resp, err := c.NewTxn().Mutate(context.Background(), &api.Mutation{
+		CommitNow: true,
+		SetNquads: []byte(`
+		_:alice <name> "Alice" .
+		_:bob <name> "Bob" .
+		_:alice <best_friend> _:bob .`),
+	})
+	require.NoError(t, err)
+
+	alice := resp.Uids["alice"]
+	bob := resp.Uids["bob"]
+
+	txn := c.NewTxn()
+	_, err = txn.Mutate(context.Background(), &api.Mutation{
+		DelNquads: []byte(fmt.Sprintf("<%s> <best_friend> <%s> .", alice, bob)),
+	})
+	require.NoError(t, err)
+
+	_, err = txn.Mutate(context.Background(), &api.Mutation{
+		SetNquads: []byte(fmt.Sprintf(`<%s> <best_friend> _:carl .
+		_:carl <name> "Carl" .`, alice)),
+	})
+	require.NoError(t, err)
+	err = txn.Commit(context.Background())
+	require.NoError(t, err)
+
+	query := fmt.Sprintf(`{
+		me(func:uid(%s)) {
+			name
+			best_friend {
+				name
+			}
+		}
+	}`, alice)
+
+	resp, err = c.NewReadOnlyTxn().Query(ctx, query)
+	require.NoError(t, err)
+	testutil.CompareJSON(t, `{"me":[{"name":"Alice","best_friend": {"name": "Carl"}}]}`,
+		string(resp.GetJson()))
+}
+
 func DeleteAndQuerySameTxn(t *testing.T, c *dgo.Dgraph) {
 	// Set the schema.
 	ctx := context.Background()