fix(header): Allow header according to RFC 7231 (HTTP 405) (#3759)

Co-authored-by: Helios <i@shangyes.net>

gin.go

@@ -633,17 +633,25 @@ func (engine *Engine) handleHTTPRequest(c *Context) {
 	}
 
 	if engine.HandleMethodNotAllowed {
+		// According to RFC 7231 section 6.5.5, MUST generate an Allow header field in response
+		// containing a list of the target resource's currently supported methods.
+		allowed := make([]string, 0, len(t)-1)
 		for _, tree := range engine.trees {
 			if tree.method == httpMethod {
 				continue
 			}
 			if value := tree.root.getValue(rPath, nil, c.skippedNodes, unescape); value.handlers != nil {
+				allowed = append(allowed, tree.method)
+			}
+		}
+		if len(allowed) > 0 {
 			c.handlers = engine.allNoMethod
+			c.writermem.Header().Set("Allow", strings.Join(allowed, ", "))
 			serveError(c, http.StatusMethodNotAllowed, default405Body)
 			return
 		}
 	}
-	}
+
 	c.handlers = engine.allNoRoute
 	serveError(c, http.StatusNotFound, default404Body)
 }

routes_test.go

@@ -514,6 +514,18 @@ func TestRouteNotAllowedEnabled2(t *testing.T) {
 	assert.Equal(t, http.StatusMethodNotAllowed, w.Code)
 }
 
+func TestRouteNotAllowedEnabled3(t *testing.T) {
+	router := New()
+	router.HandleMethodNotAllowed = true
+	router.GET("/path", func(c *Context) {})
+	router.POST("/path", func(c *Context) {})
+	w := PerformRequest(router, http.MethodPut, "/path")
+	assert.Equal(t, http.StatusMethodNotAllowed, w.Code)
+	allowed := w.Header().Get("Allow")
+	assert.Contains(t, allowed, "GET")
+	assert.Contains(t, allowed, "POST")
+}
+
 func TestRouteNotAllowedDisabled(t *testing.T) {
 	router := New()
 	router.HandleMethodNotAllowed = false