net/http: ResponseWriter implementations don't always follow documentation

[blixt]

These are excerpts from the documentation for the ResponseWriter interface. For Write:

// If WriteHeader has not yet been called, Write calls
// WriteHeader(http.StatusOK) before writing the data.

For WriteHeader:

// If WriteHeader is not called explicitly, the first call to Write
// will trigger an implicit WriteHeader(http.StatusOK).

The code below never calls ResponseWriter.WriteHeader for simple requests. I had expected that the implementation of the ResponseWriter interface would follow it by calling out to the interface WriteHeader method instead of directly calling its internal writeHeader function.

package main

import (
    "fmt"
    "net/http"
)

func main() {
    http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
        fmt.Fprintln(w, "Hello World!")
    })
    http.ListenAndServe(":8080", Middleware(http.DefaultServeMux))
}

func Middleware(h http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        // Use a different ResponseWriter for this request...
        w2 := myResponseWriter{w}
        h.ServeHTTP(w2, r)
    })
}

type myResponseWriter struct {
    http.ResponseWriter
}

func (w myResponseWriter) WriteHeader(code int) {
    // This code is never called with the http.response implementation.
    w.ResponseWriter.WriteHeader(code)
    fmt.Println("Yay, I wrote a header!")
}

I also put this code on the Go Playground.

Go version that I'm using:

go version go1.7.3 darwin/amd64

[blixt]

Nevermind, I was thinking a bit oddly about this, that something else is causing WriteHeader to be called but if the concrete implementation does everything it can't know that it's been wrapped.

[blixt]

If someone else finds this issue because they were trying to do what I'm doing, I believe this is a valid base to start off a ResponseWriter wrapper from:

type MyResponseWriter struct {
    http.ResponseWriter
    wroteHeader bool
}

func (w *MyResponseWriter) Write(p []byte) (n int, err error) {
    if !w.wroteHeader {
        w.WriteHeader(http.StatusOK)
    }
    return w.ResponseWriter.Write(p)
}

func (w *MyResponseWriter) WriteHeader(code int) {
    w.ResponseWriter.WriteHeader(code)
    // Check after in case there's error handling in the wrapped ResponseWriter.
    if !w.wroteHeader {
        return
    }
    w.wroteHeader = true
    // Do other stuff here.
}