warning header were already written wanted to override status code 400 with 200问题排查

• 问题

在测试gin框架时遇到了一个问题，在调用

c.JSON(http.StatusOK,...)

时warning

warning header were already written wanted to override status code 400 with 200

• 排查过程：
  在handler里打点，查看在哪一个点出现了覆写问题。例如：

r.GET("/", func(c *gin.Context) {
    fmt.Println("Handler: written =", c.Writer.Written())
    c.JSON(200, gin.H{"message": "Hello"})
})

然后发现在代码中有一行的前后打点

fmt.Println("Handler: written =", c.Writer.Written())
c.BindJSON(&greq)
fmt.Println("Handler: written =", c.Writer.Written())

输出结果是

Handler: written = false
Handler: written = true

然后查golang接口手册

func (*Context) MustBindWith ¶
added in v1.3.0
func (c *Context) MustBindWith(obj any, b binding.Binding) error
MustBindWith binds the passed struct pointer using the specified binding engine. It will abort the request with HTTP 400 if any error occurs. See the binding package.

因此如果这一调用失败，已经写成400了，再覆写就会失效。