cleanly handle http.MaxBytesError for enforcing req payload size

If a middleware applies `http.MaxBytesReader` and we encounter an
`*http.MaxBytesError` when reading the request body, return an HTTP 413
with a clean error.

Author: bgentry

apiendpoint/api_endpoint.go

@@ -139,6 +139,10 @@ func executeAPIEndpoint[TReq any, TResp any](w http.ResponseWriter, r *http.Requ
 		if r.Method != http.MethodGet {
 			reqData, err := io.ReadAll(r.Body)
 			if err != nil {
+				var maxBytesErr *http.MaxBytesError
+				if errors.As(err, &maxBytesErr) {
+					return apierror.NewRequestEntityTooLarge("Request entity too large.")
+				}
 				return fmt.Errorf("error reading request body: %w", err)
 			}
 

apiendpoint/api_endpoint_test.go

@@ -72,6 +72,19 @@ func TestMountAndServe(t *testing.T) {
 		requireStatusAndJSONResponse(t, http.StatusOK, &getResponse{Message: "Hello."}, bundle.recorder)
 	})
 
+	t.Run("MaxBytesErrorHandling", func(t *testing.T) {
+		t.Parallel()
+
+		mux, bundle := setup(t)
+
+		payload := mustMarshalJSON(t, &postRequest{Message: "Hello."})
+
+		req := httptest.NewRequest(http.MethodPost, "/api/post-endpoint/123", bytes.NewBuffer(payload))
+		req.Body = http.MaxBytesReader(bundle.recorder, io.NopCloser(bytes.NewReader(payload)), int64(len(payload)-1))
+		mux.ServeHTTP(bundle.recorder, req)
+		requireStatusAndJSONResponse(t, http.StatusRequestEntityTooLarge, &apierror.APIError{Message: "Request entity too large."}, bundle.recorder)
+	})
+
 	t.Run("MethodNotAllowed", func(t *testing.T) {
 		t.Parallel()
 
@@ -302,9 +315,10 @@ func (a *getEndpoint) Execute(_ context.Context, req *getRequest) (*getResponse,
 
 type postEndpoint struct {
 	Endpoint[postRequest, postResponse]
+	MaxBodyBytes int64
 }
 
-func (*postEndpoint) Meta() *EndpointMeta {
+func (a *postEndpoint) Meta() *EndpointMeta {
 	return &EndpointMeta{
 		Pattern:    "POST /api/post-endpoint/{id}",
 		StatusCode: http.StatusCreated,

apierror/api_error.go

@@ -131,6 +131,23 @@ func NewNotFoundf(format string, a ...any) *NotFound {
 	return NewNotFound(fmt.Sprintf(format, a...))
 }
 
+//
+// RequestEntityTooLarge
+//
+
+type RequestEntityTooLarge struct { //nolint:errname
+	APIError
+}
+
+func NewRequestEntityTooLarge(message string) *RequestEntityTooLarge {
+	return &RequestEntityTooLarge{
+		APIError: APIError{
+			Message:    message,
+			StatusCode: http.StatusRequestEntityTooLarge,
+		},
+	}
+}
+
 //
 // ServiceUnavailable
 //