Add support for optional request logging

This is a debugging aid only.  The log format is not stable (and thus
not documented).

config.go

@@ -25,6 +25,7 @@ type Config struct {
 	UsernameHeader string
 	GroupsHeader   string
 	Timeout        time.Duration
+	LogRequests    bool
 }
 
 func ParseConfig() (*Config, error) {
@@ -85,6 +86,7 @@ func ParseConfig() (*Config, error) {
 	c.UsernameHeader = *rc.UsernameHeader
 	c.GroupsHeader = *rc.GroupsHeader
 	c.Timeout = time.Duration(*rc.Timeout) * time.Second
+	c.LogRequests = *rc.LogRequests
 
 	c.CookieSecret = uuid.New()
 
@@ -103,6 +105,7 @@ type rawConfig struct {
 	UsernameHeader *string
 	GroupsHeader   *string
 	Timeout        *int
+	LogRequests    *bool
 }
 
 func parseRawConfig() *rawConfig {
@@ -118,6 +121,7 @@ func parseRawConfig() *rawConfig {
 		UsernameHeader: flag.String("username-header", "Discourse-User-Name", "Request header to pass authenticated username into"),
 		GroupsHeader:   flag.String("groups-header", "Discourse-User-Groups", "Request header to pass authenticated groups into"),
 		Timeout:        flag.Int("timeout", 10, "Read/write timeout"),
+		LogRequests:    flag.Bool("log-requests", false, "log all requests to standard error"),
 	}
 	flag.Parse()
 	return c

logging.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"log"
+	"net/http"
+)
+
+type loggableResponseWriter struct {
+	StatusCode int
+
+	next http.ResponseWriter
+}
+
+func (w *loggableResponseWriter) Header() http.Header {
+	return w.next.Header()
+}
+
+func (w *loggableResponseWriter) Write(b []byte) (int, error) {
+	return w.next.Write(b)
+}
+
+func (w *loggableResponseWriter) WriteHeader(statusCode int) {
+	w.StatusCode = statusCode
+	w.next.WriteHeader(statusCode)
+}
+
+func (w *loggableResponseWriter) Flush() {
+	if flusher, ok := w.next.(http.Flusher); ok {
+		flusher.Flush()
+	}
+}
+
+func logHandler(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		lw := &loggableResponseWriter{next: w}
+		next.ServeHTTP(lw, r)
+		log.Printf("%s %s %s %d", r.RemoteAddr, r.Method, r.URL, lw.StatusCode)
+	})
+}

main.go

@@ -43,6 +43,10 @@ func main() {
 
 	handler := authProxyHandler(proxy, config)
 
+	if config.LogRequests {
+		handler = logHandler(handler)
+	}
+
 	server := &http.Server{
 		Addr:           config.ListenAddr,
 		Handler:        handler,