Logging responses to incoming HTTP requests inside http.HandleFunc

Question

This is a follow-up question to In go, how to inspect the http response that is written to http.ResponseWriter? since the solution there requires faking a request, which wor

Answer 1

Implementing Mat Ryer's approach with logging request id based on httptest.ResponseRecorder

Disadvantages of using httptest.ResponseRecorder:

• HTTP/1.1 only
• Doesn't supports ReadFrom(), Hijack(), Flush() maybe something else
• Headers like Content-Length and Date are not available in recorder

Code:

import (
    "fmt"
    "github.com/google/uuid"
    "log"
    "net/http"
    "net/http/httptest"
    "net/http/httputil"
    "strings"
)

func main() {
    logger := log.New(os.Stdout, "server: ", log.Lshortfile)
    http.HandleFunc("/api/smth", Adapt(smth, httpLogger(quips.logger)))
    panic(http.ListenAndServe(":8080", nil))
}

type Adapter func(http.HandlerFunc) http.HandlerFunc

func Adapt(h http.HandlerFunc, adapters ...Adapter) http.HandlerFunc {
    for _, adapter := range adapters {
        h = adapter(h)
    }
    return h
}

func httpLogger(logger *log.Logger) Adapter {
    return func(h http.HandlerFunc) http.HandlerFunc {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            dumpBody := true
            if r.ContentLength > 1024 {
                dumpBody = false
            }
            dump, err := httputil.DumpRequest(r, dumpBody)
            if err != nil {
                http.Error(w, fmt.Sprint(err), http.StatusInternalServerError)
                return
            }

            reqId, err := uuid.NewRandom()
            if err != nil {
                http.Error(w, fmt.Sprint(err), http.StatusInternalServerError)
                return
            }

            logger.Printf("<<<<< Request %s\n%s\n<<<<<", reqId.String(), string(dump))

            recorder := httptest.NewRecorder()
            defer func() {
                var sb strings.Builder
                fmt.Fprintf(&sb, "%s %d\n", recorder.Result().Proto, recorder.Result().StatusCode)

                for h, v := range recorder.Result().Header {
                    w.Header()[h] = v
                    for _, headerValue := range v {
                        fmt.Fprintf(&sb, "%s: %s\n", h, headerValue)
                    }
                }
                w.Header().Set("X-Request-Id", reqId.String())
                fmt.Fprintf(&sb, "X-Request-Id: %s\n", reqId.String())
                fmt.Fprintf(&sb, "Content-Length: %d\n", recorder.Body.Len())
                fmt.Fprint(&sb, "\n")
                sb.Write(recorder.Body.Bytes())

                logger.Printf(">>>>> Response %s\n%s\n>>>>>", reqId.String(), sb.String())

                w.WriteHeader(recorder.Result().StatusCode)
                recorder.Body.WriteTo(w)
            }()
            h.ServeHTTP(recorder, r)
        })
    }
}