Add simple logging to example-shttp-fileserver (#157)

Not entirely sure this still fits the scope of an "example".

Log requests requests in a simple format:

    <time> <remote address> "<request>" <status code> <size of reply>

for example

    2020/07/29 07:49:13 19-ffaa:1:106,127.0.0.1:3173 "GET /foo HTTP/3/SCION" 200 4

Author: matzf

_examples/shttp/fileserver/main.go

@@ -30,5 +30,38 @@ func main() {
 	flag.Parse()
 
 	handler := http.FileServer(http.Dir(""))
-	log.Fatal(shttp.ListenAndServe(fmt.Sprintf(":%d", *port), handler, nil))
+	log.Fatal(shttp.ListenAndServe(fmt.Sprintf(":%d", *port), withLogger(handler), nil))
+}
+
+// withLogger returns a handler that logs requests (after completion) in a simple format:
+//	  <time> <remote address> "<request>" <status code> <size of reply>
+func withLogger(h http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		wrec := &recordingResponseWriter{ResponseWriter: w}
+		h.ServeHTTP(wrec, r)
+
+		log.Printf("%s \"%s %s %s/SCION\" %d %d\n",
+			r.RemoteAddr,
+			r.Method, r.URL, r.Proto,
+			wrec.status, wrec.bytes)
+	})
+}
+
+type recordingResponseWriter struct {
+	http.ResponseWriter
+	status int
+	bytes  int
+}
+
+func (r *recordingResponseWriter) WriteHeader(statusCode int) {
+	r.status = statusCode
+	r.ResponseWriter.WriteHeader(statusCode)
+}
+
+func (r *recordingResponseWriter) Write(b []byte) (int, error) {
+	if r.status == 0 {
+		r.status = http.StatusOK
+	}
+	r.bytes += len(b)
+	return r.ResponseWriter.Write(b)
 }