net/http/httputil: Pass a Logger to ReverseProxy, allowing the user to control logging.

Fixes #8553.

LGTM=bradfitz
R=golang-codereviews, dave, bradfitz
CC=golang-codereviews
https://golang.org/cl/132750043

src/pkg/net/http/httputil/reverseproxy.go

@@ -40,6 +40,12 @@ type ReverseProxy struct {
 	// response body.
 	// If zero, no periodic flushing is done.
 	FlushInterval time.Duration
+
+	// ErrorLog specifies an optional logger for errors
+	// that occur when attempting to proxy the request.
+	// If nil, logging goes to os.Stderr via the log package's
+	// standard logger.
+	ErrorLog *log.Logger
 }
 
 func singleJoiningSlash(a, b string) string {
@@ -138,7 +144,7 @@ func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
 
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
-		log.Printf("http: proxy error: %v", err)
+		p.logf("http: proxy error: %v", err)
 		rw.WriteHeader(http.StatusInternalServerError)
 		return
 	}
@@ -171,6 +177,14 @@ func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
 	io.Copy(dst, src)
 }
 
+func (p *ReverseProxy) logf(format string, args ...interface{}) {
+	if p.ErrorLog != nil {
+		p.ErrorLog.Printf(format, args...)
+	} else {
+		log.Printf(format, args...)
+	}
+}
+
 type writeFlusher interface {
 	io.Writer
 	http.Flusher