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@ciaranarcher
Created July 27, 2014 06:53
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Wrapping a ResponseWriter to capture the status code
// Create our own MyResponseWriter to wrap a standard http.ResponseWriter
// so we can store the status code.
type MyResponseWriter struct {
status int
http.ResponseWriter
}
func NewMyResponseWriter(res http.ResponseWriter) *MyResponseWriter {
// Default the status code to 200
return &MyResponseWriter{200, res}
}
// Give a way to get the status
func (w MyResponseWriter) Status() int {
return w.status
}
// Satisfy the http.ResponseWriter interface
func (w MyResponseWriter) Header() http.Header {
return w.ResponseWriter.Header()
}
func (w MyResponseWriter) Write(data []byte) (int, error) {
return w.ResponseWriter.Write(data)
}
func (w MyResponseWriter) WriteHeader(statusCode int) {
// Store the status code
w.status = statusCode
// Write the status code onward.
w.ResponseWriter.WriteHeader(statusCode)
}
@deankarn
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love the Gist, but there is one issue the struct functions should be defined as reference

// MyResponseWriter should be by reference
func (w *MyResponseWriter)

with this gist it is not apparent but if you look here: https://gist.github.com/joeybloggs/ccb8f57b46f770f8a10a
if it wasn't by reference then the size variable would be zero every time the Write function was called.

@stevenwilkin
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Embedding the http.ResponseWriter should mean that MyResponseWriter already satisfies the interface?

@blixt
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blixt commented Oct 28, 2016

I found this on a Google search so I thought I'd point out a few issues with the code in this gist for future visitors:

  • If the header is being written by the wrapped ResponseWriter implementation, it won't know to call your WriteHeader method so you have to call it yourself
  • The function receivers are not pointers so the status update won't actually be persisted on the original MyResponseWriter instance
  • (Minor) There's no need to manually forward the methods of embedded types (you don't need define the Header method)

Here's the fixed up code which should work in all cases:

type MyResponseWriter struct {
    http.ResponseWriter
    status      int
    wroteHeader bool
}

func NewMyResponseWriter(w http.ResponseWriter) *MyResponseWriter {
  return &MyResponseWriter{ResponseWriter: w}
}

func (w *MyResponseWriter) Status() int {
    return w.status
}

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.status = code
    w.wroteHeader = true
}

@felixge
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felixge commented Nov 12, 2016

I've created a package called called httpsnoop which fixes multiple problems with the original gist as well as the improved solution below.

The problems with the existing solutions are described here: https://github.com/felixge/httpsnoop#why-this-package-exists

@renthraysk
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http.Pusher interface can be implemented, as the documentation states

// Push returns ErrNotSupported if the client has disabled push or if push
// is not supported on the underlying connection.

func (w *MyResponseWriter) Push(target string, opts *http.PushOptions) error {
	if p, ok := w.ResponseWriter.(http.Pusher); ok {
		return p.Push(target, opts)
	}
	return http.ErrNotSupported
}

http.Hijacker interface isn't implemented in http2 connections.

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