Solve "The Proxy Problem" of InstrumentHandler

InstrumentHandler provides proxy object over given http.ResponseWriter
- responseWriterDelegator that only implements the bare minimum
required of an http.ResponseWriter and doesn’t implement any interface
upgrades (http.Flusher, http.Hijacker, etc.). This commit fixes it by
providing fancyResponseWriterDelegator with all the fancy bells and
whistles if standard library http.ResponseWriter is detected.

Heavily inspired by Goji's middleware.

https://avtok.com/2014/11/05/interface-upgrades.html#the-proxy-problem

prometheus/http.go

@@ -14,6 +14,9 @@
 package prometheus
 
 import (
+	"bufio"
+	"io"
+	"net"
 	"net/http"
 	"strconv"
 	"strings"
@@ -141,7 +144,18 @@ func InstrumentHandlerFuncWithOpts(opts SummaryOpts, handlerFunc func(http.Respo
 			urlLen = len(r.URL.String())
 		}
 		go computeApproximateRequestSize(r, out, urlLen)
-		handlerFunc(delegate, r)
+
+		_, cn := w.(http.CloseNotifier)
+		_, fl := w.(http.Flusher)
+		_, hj := w.(http.Hijacker)
+		_, rf := w.(io.ReaderFrom)
+		var rw http.ResponseWriter
+		if cn && fl && hj && rf {
+			rw = &fancyResponseWriterDelegator{delegate}
+		} else {
+			rw = delegate
+		}
+		handlerFunc(rw, r)
 
 		elapsed := float64(time.Since(now)) / float64(time.Microsecond)
 
@@ -197,6 +211,31 @@ func (r *responseWriterDelegator) Write(b []byte) (int, error) {
 	return n, err
 }
 
+type fancyResponseWriterDelegator struct {
+	*responseWriterDelegator
+}
+
+func (f *fancyResponseWriterDelegator) CloseNotify() <-chan bool {
+	return f.ResponseWriter.(http.CloseNotifier).CloseNotify()
+}
+
+func (f *fancyResponseWriterDelegator) Flush() {
+	f.ResponseWriter.(http.Flusher).Flush()
+}
+
+func (f *fancyResponseWriterDelegator) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	return f.ResponseWriter.(http.Hijacker).Hijack()
+}
+
+func (f *fancyResponseWriterDelegator) ReadFrom(r io.Reader) (int64, error) {
+	if !f.wroteHeader {
+		f.WriteHeader(http.StatusOK)
+	}
+	n, err := f.ResponseWriter.(io.ReaderFrom).ReadFrom(r)
+	f.written += n
+	return n, err
+}
+
 func sanitizeMethod(m string) string {
 	switch m {
 	case "GET", "get":