How to change the response code of http request using middleware?

Question content

I am trying to develop an api gateway using go-chi. I want to set the status code to 200 if the request contains "_always200=true" in the query. Here's what I've tried: custom_response_writer.go：

type CustomResponseWriter struct {
    http.ResponseWriter
    Buf         *bytes.Buffer
    StatusCode  int
    WroteHeader bool
}

func NewCustomResponseWriter(w http.ResponseWriter) *CustomResponseWriter {
    return &CustomResponseWriter{ResponseWriter: w, Buf: new(bytes.Buffer)}
}

func (c *CustomResponseWriter) WriteHeader(code int) {
    c.StatusCode = code
    c.ResponseWriter.WriteHeader(code)
}

func (c *CustomResponseWriter) Write(b []byte) (int, error) {
    return c.Buf.Write(b)
}

middleware:

func HeaderFilterMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        always200, _ := r.Context().Value(always200QueryKey).(bool)
        crw := NewCustomResponseWriter(w)
        next.ServeHTTP(crw, r)

        if always200 {
            crw.WriteHeader(http.StatusOK)
        }
    })
}

Always returns 404. Is there any way to solve this problem?

Correct answer

There are at least two problems in the code.

1. always200, _ := r.Context().Value(always200QueryKey).(bool) is not the correct way to get query parameters. I think you should use r.URL.Query().Get("_always200") to read it and compare the value with the string "true".

2. It is too late to call crw.WriteHeader(http.StatusOK) after next.ServeHTTP(crw, r). When I tested the code, it printed this warning message:

   http: superfluous response.WriteHeader call from main.(*CustomResponseWriter).WriteHeader

The following is a modified example to change the response status code:

<code>package main

import (
    "bytes"
    "net/http"
    "strings"

    "github.com/go-chi/chi/v5"
)

type CustomResponseWriter struct {
    http.ResponseWriter
    Buf                *bytes.Buffer
    OriginalStatusCode int
    WroteHeader        bool
    always200          bool
}

func NewCustomResponseWriter(w http.ResponseWriter, always200 bool) *CustomResponseWriter {
    return &CustomResponseWriter{
        ResponseWriter: w,
        Buf:            new(bytes.Buffer),
        always200:      always200,
    }
}

func (c *CustomResponseWriter) WriteHeader(code int) {
    c.OriginalStatusCode = code

    if c.always200 {
        code = http.StatusOK
    }
    c.ResponseWriter.WriteHeader(code)
}

func (c *CustomResponseWriter) Write(b []byte) (int, error) {
    return c.Buf.Write(b)
}

func HeaderFilterMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        always200 := r.URL.Query().Get("_always200")
        crw := NewCustomResponseWriter(w, strings.EqualFold(always200, "true"))
        next.ServeHTTP(crw, r)
    })
}

func main() {
    r := chi.NewRouter()

    r.Use(HeaderFilterMiddleware)

    r.Get("/", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("welcome"))
    })
    http.ListenAndServe(":3000", r)
}
</code>

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