fix(router): ensure subgraph timeouts/nil responses are handled corre…

…ctly (#1449)

router-tests/header_propagation_test.go

@@ -178,6 +178,33 @@ func TestHeaderPropagation(t *testing.T) {
 			})
 		})
 
+		t.Run("works with unresponsive subgraph", func(t *testing.T) {
+			t.Parallel()
+			testenv.Run(t, &testenv.Config{
+				RouterOptions: global(config.ResponseHeaderRuleAlgorithmLastWrite, customHeader, ""),
+				Subgraphs: testenv.SubgraphsConfig{
+					Employees: testenv.SubgraphConfig{
+						Middleware: func(handler http.Handler) http.Handler {
+							return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+								w.Header().Set(customHeader, employeeVal)
+								handler.ServeHTTP(w, r)
+							})
+						},
+					},
+					Hobbies: testenv.SubgraphConfig{
+						CloseOnStart: true,
+					},
+				},
+			}, func(t *testing.T, xEnv *testenv.Environment) {
+				res := xEnv.MakeGraphQLRequestOK(testenv.GraphQLRequest{
+					Query: queryEmployeeWithHobby,
+				})
+				ch := strings.Join(res.Response.Header.Values(customHeader), ",")
+				require.Equal(t, employeeVal, ch)
+				require.Equal(t, `{"errors":[{"message":"Failed to fetch from Subgraph 'hobbies' at Path 'employee'."}],"data":{"employee":{"id":1,"hobbies":null}}}`, res.Body)
+			})
+		})
+
 		t.Run("local last write wins", func(t *testing.T) {
 			t.Parallel()
 			testenv.Run(t, &testenv.Config{

router/core/header_rule_engine.go

@@ -263,6 +263,11 @@ func (h *HeaderPropagation) OnOriginRequest(request *http.Request, ctx RequestCo
 }
 
 func (h *HeaderPropagation) OnOriginResponse(resp *http.Response, ctx RequestContext) *http.Response {
+	// In the case of an error response, it is possible that the response is nil
+	if resp == nil {
+		return nil
+	}
+
 	propagation := getResponseHeaderPropagation(resp.Request.Context())
 	if propagation == nil {
 		return resp

router/core/header_rule_engine_test.go

@@ -174,6 +174,22 @@ func TestPropagateHeaderRule(t *testing.T) {
 		assert.Equal(t, "test1", updatedClientReq.Header.Get("X-Test-1"))
 
 	})
+
+	t.Run("Should handle nil resonses", func(t *testing.T) {
+		ht, err := NewHeaderPropagation(&config.HeaderRules{
+			All: &config.GlobalHeaderRule{},
+		})
+		assert.Nil(t, err)
+
+		rr := httptest.NewRecorder()
+		resp := ht.OnOriginResponse(nil, &requestContext{
+			logger:           zap.NewNop(),
+			responseWriter:   rr,
+			operation:        &operationContext{},
+			subgraphResolver: NewSubgraphResolver(nil),
+		})
+		require.Nil(t, resp)
+	})
 }
 
 func TestRenamePropagateHeaderRule(t *testing.T) {