How to Effectively Control HTTP Headers in Go Middleware?

Controlling HTTP Headers with External Go Middleware

Issue: Overriding Server Headers in Middleware

You're facing a scenario where you have middleware in Go that attempts to set a custom "Server" header. However, existing "Server" headers set by the application or other middleware persist, resulting in multiple "Server" headers in the response.

Disallowed Header Modification

As per HTTP semantics, modifying response headers after the ServeHTTP method returns is prohibited. This restriction aims to establish clear boundaries between request handling and response finalization.

Proposed Solutions

1. Custom ResponseWriter:

Define a custom ResponseWriter wrapper that intercepts header modification operations. Before writing any headers, the wrapper inserts your custom "Server" header. This approach adds an extra layer of indirection but provides fine-grained control over header management.

Here's an example implementation of a custom ResponseWriter:

type serverWriter struct {
    w           http.ResponseWriter
    name        string
    wroteHeader bool
}

func (s serverWriter) WriteHeader(code int) {
    if s.wroteHeader == false {
        s.w.Header().Set("Server", s.name)
        s.wroteHeader = true
    }
    s.w.WriteHeader(code)
}

func (s serverWriter) Write(b []byte) (int, error) {
    return s.w.Write(b)
}

func (s serverWriter) Header() http.Header {
    return s.w.Header()
}

In the middleware, you can use this custom ResponseWriter to control header insertion:

func Server(h http.Handler, serverName string) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        sw := serverWriter{
            w:           w,
            name:        serverName,
            wroteHeader: false,
        }
        h.ServeHTTP(sw, r)
    })
}

2. Middleware Order Reversal:

An alternative approach is to reverse the order of middleware such that your "Server" header-setting middleware is executed after all other middleware. This ensures that your header modification is the last operation before the response is finalized.

3. Inner-Most Middleware:

If possible, place your "Server" header-setting middleware as the innermost layer of middleware. This eliminates the possibility of outer middleware modifying the "Server" header after you set it.

4. Response Modification:

As a last resort, you can consider using a custom http.Handler that intercepts the response and modifies the "Server" header accordingly. This approach requires careful handling to avoid breaking other functionality in your application.

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