How to Log Real-Time HTTP Responses with `http.HandleFunc` and Middleware?

Logging Incoming HTTP Requests with http.HandleFunc

In our previous discussion, we explored inspecting HTTP responses using a fake request, a technique suitable for unit tests. However, we also seek a way to log real-time response data on a live server.

Middleware Chaining

A prevalent approach involves creating a middleware chain. Libraries like Negroni offer middleware functionality, where handlers are combined and executed sequentially. A minimal middleware implementation can be achieved using handler combinators:

<code class="go">func NewFooHandler(next http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Do pre-handling tasks
        next(w, r)
        // Do post-handling tasks
    }
}</code>

These combinators can then be chained to form a handler:

<code class="go">h := NewFooHandler(NewBarHandler(NewBazHandler(Sink)))</code>

Applying to HTTP.HandleFunc

To apply this technique to your problem, you can create a handler combinator:

<code class="go">func NewResponseLoggingHandler(next http.HandlerFunc) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Record the response
        c := httptest.NewRecorder()
        next(c, r)
        
        // Copy responses
        for k, v := range c.HeaderMap {
            w.Header()[k] = v
        }
        w.WriteHeader(c.Code)
        c.Body.WriteTo(w)
    }
}</code>

Now, you can create a default handler that combines custom handlers with response logging:

<code class="go">func NewDefaultHandler(next http.HandlerFunc) http.HandlerFunc {
    return NewResponseLoggingHandler(NewOtherStuffHandler(next))
}</code>

By using this default handler:

<code class="go">h := NewDefaultHandler(...)</code>

All subsequent handlers will automatically include response logging.

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