add fixed complexity limit

graphql/handler/extension/complexity.go

@@ -15,6 +15,13 @@ type ComplexityLimit func(ctx context.Context, rc *graphql.OperationContext) int
 
 var _ graphql.OperationContextMutator = ComplexityLimit(func(ctx context.Context, rc *graphql.OperationContext) int { return 0 })
 
+// FixedComplexityLimit sets a complexity limit that does not change
+func FixedComplexityLimit(limit int) graphql.HandlerExtension {
+	return ComplexityLimit(func(ctx context.Context, rc *graphql.OperationContext) int {
+		return limit
+	})
+}
+
 func (c ComplexityLimit) MutateOperationContext(ctx context.Context, rc *graphql.OperationContext) *gqlerror.Error {
 	es := graphql.GetServerContext(ctx)
 	op := rc.Doc.Operations.ForName(rc.OperationName)

graphql/handler/extension/complexity_test.go

@@ -46,6 +46,26 @@ func TestHandlerComplexity(t *testing.T) {
 	})
 }
 
+func TestFixedComplexity(t *testing.T) {
+	h := testserver.New()
+	h.Use(extension.FixedComplexityLimit(2))
+	h.AddTransport(&transport.POST{})
+
+	t.Run("below complexity limit", func(t *testing.T) {
+		h.SetCalculatedComplexity(2)
+		resp := doRequest(h, "POST", "/graphql", `{"query":"{ name }"}`)
+		require.Equal(t, http.StatusOK, resp.Code, resp.Body.String())
+		require.Equal(t, `{"data":{"name":"test"}}`, resp.Body.String())
+	})
+
+	t.Run("above complexity limit", func(t *testing.T) {
+		h.SetCalculatedComplexity(4)
+		resp := doRequest(h, "POST", "/graphql", `{"query":"{ name }"}`)
+		require.Equal(t, http.StatusUnprocessableEntity, resp.Code, resp.Body.String())
+		require.Equal(t, `{"errors":[{"message":"operation has complexity 4, which exceeds the limit of 2"}],"data":null}`, resp.Body.String())
+	})
+}
+
 func doRequest(handler http.Handler, method string, target string, body string) *httptest.ResponseRecorder {
 	r := httptest.NewRequest(method, target, strings.NewReader(body))
 	r.Header.Set("Content-Type", "application/json")