fix: entity null fetch produces resolver error

v2/pkg/engine/resolve/loader.go

@@ -542,6 +542,12 @@ func (l *Loader) mergeResult(fetchItem *FetchItem, res *result, items []*astjson
 
 	// Check if data needs processing.
 	if res.postProcessing.SelectResponseDataPath != nil && astjson.ValueIsNull(responseData) {
+		// First check if this is actually an entity null fetch, instead of a data null fetch.
+		// In this case we return early to avoid adding subgraph errors or merging this into items.
+		if isNullEntityFetch(res.postProcessing.SelectResponseDataPath, response) {
+			return nil
+		}
+
 		// When:
 		// - No errors or data are present
 		// - Status code is not within the 2XX range
@@ -637,6 +643,27 @@ func (l *Loader) mergeResult(fetchItem *FetchItem, res *result, items []*astjson
 	return nil
 }
 
+// isNullEntityFetch returns true, if the fetch at dataPath is null
+// but it's part of an actual array response, i.e.
+// { "data": { "_entities": [null] } }
+// instead of
+// { "data": null }
+// The latter will return false.
+func isNullEntityFetch(dataPath []string, response *astjson.Value) bool {
+	if len(dataPath) > 1 && dataPath[len(dataPath)-2] == "_entities" {
+		// data path is an entity array
+		// check that the entity fetch's parent array exists
+		parentPath := dataPath[:len(dataPath)-1]
+		parentData := response.Get(parentPath...)
+
+		if astjson.ValueIsNonNull(parentData) && parentData.Type() == astjson.TypeArray {
+			return true
+		}
+	}
+
+	return false
+}
+
 var (
 	errorsInvalidInputHeader = []byte(`{"errors":[{"message":"Failed to render Fetch Input","path":[`)
 	errorsInvalidInputFooter = []byte(`]}]}`)

v2/pkg/engine/resolve/resolve_test.go

@@ -2141,6 +2141,78 @@ func TestResolver_ResolveGraphQLResponse(t *testing.T) {
 			},
 		}, *NewContext(context.Background()), `{"errors":[{"message":"Failed to fetch from Subgraph 'Users' at Path 'query'."}],"data":null}`
 	}))
+	t.Run("fetch with null entity and non-nullable root field", testFn(func(t *testing.T, ctrl *gomock.Controller) (node *GraphQLResponse, ctx Context, expectedOutput string) {
+		mockDataSource := NewMockDataSource(ctrl)
+		mockDataSource.EXPECT().
+			Load(gomock.Any(), gomock.Any(), gomock.Any()).
+			DoAndReturn(func(ctx context.Context, headers http.Header, input []byte) ([]byte, error) {
+				// Subgraph returns null for the entity
+				return []byte(`{"data":{"_entities":[null]}}`), nil
+			})
+		return &GraphQLResponse{
+			Fetches: SingleWithPath(&SingleFetch{
+				FetchConfiguration: FetchConfiguration{
+					DataSource: mockDataSource,
+					PostProcessing: PostProcessingConfiguration{
+						SelectResponseDataPath:   []string{"data", "_entities", "0"},
+						SelectResponseErrorsPath: []string{"errors"},
+					},
+				},
+				Info: &FetchInfo{
+					DataSourceID:   "Users",
+					DataSourceName: "Users",
+				},
+			}, "query"),
+			Data: &Object{
+				Nullable: false,
+				Fields: []*Field{
+					{
+						Name: []byte("name"),
+						Value: &String{
+							Path:     []string{"name"},
+							Nullable: false,
+						},
+					},
+				},
+			},
+		}, *NewContext(context.Background()), `{"errors":[{"message":"Cannot return null for non-nullable field 'Query.name'.","path":["name"]}],"data":null}`
+	}))
+	t.Run("fetch with null entity and nullable root field", testFn(func(t *testing.T, ctrl *gomock.Controller) (node *GraphQLResponse, ctx Context, expectedOutput string) {
+		mockDataSource := NewMockDataSource(ctrl)
+		mockDataSource.EXPECT().
+			Load(gomock.Any(), gomock.Any(), gomock.Any()).
+			DoAndReturn(func(ctx context.Context, headers http.Header, input []byte) ([]byte, error) {
+				// Subgraph returns null for the entity
+				return []byte(`{"data":{"_entities":[null]}}`), nil
+			})
+		return &GraphQLResponse{
+			Fetches: SingleWithPath(&SingleFetch{
+				FetchConfiguration: FetchConfiguration{
+					DataSource: mockDataSource,
+					PostProcessing: PostProcessingConfiguration{
+						SelectResponseDataPath:   []string{"data", "_entities", "0"},
+						SelectResponseErrorsPath: []string{"errors"},
+					},
+				},
+				Info: &FetchInfo{
+					DataSourceID:   "Users",
+					DataSourceName: "Users",
+				},
+			}, "query"),
+			Data: &Object{
+				Nullable: false,
+				Fields: []*Field{
+					{
+						Name: []byte("name"),
+						Value: &String{
+							Path:     []string{"name"},
+							Nullable: true,
+						},
+					},
+				},
+			},
+		}, *NewContext(context.Background()), `{"data":{"name":null}}`
+	}))
 	t.Run("root field with nested non-nullable fields returns null", testFn(func(t *testing.T, ctrl *gomock.Controller) (node *GraphQLResponse, ctx Context, expectedOutput string) {
 		return &GraphQLResponse{
 			Fetches: Single(&SingleFetch{