httputil: add ModifyRequest hook

Adds a ModifyRequest hook (field) to the ReverseProxy struct.

It aims to be a simplified and more recommended hook that you would want
to use if you plan to retain default reverse proxy behavior from Director,
but further extend it for specific cases a la vhost reverse proxying.

src/net/http/httputil/reverseproxy.go

@@ -30,9 +30,17 @@ type ReverseProxy struct {
 	// using Transport. Its response is then copied
 	// back to the original client unmodified.
 	// Director must not access the provided Request
-	// after returning.
+	// after returning. Default Director implements standard reverse-proxy
+	// behavior that retains scheme, host, path and query params from target URL.
+	// Also retains User-Agent header if set.
 	Director func(*http.Request)
 
+	// ModifyRequest must be a function which is called by Director
+	// that modifies the request into a new request to be sent using Transport.
+	// If you plan to retain default reverse proxy behavior from Director,
+	// but further extend it for specific cases a la vhost reverse proxying.
+	ModifyRequest func(*http.Request)
+
 	// The transport used to perform proxy requests.
 	// If nil, http.DefaultTransport is used.
 	Transport http.RoundTripper
@@ -107,7 +115,8 @@ func singleJoiningSlash(a, b string) string {
 // Director policy.
 func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
 	targetQuery := target.RawQuery
-	director := func(req *http.Request) {
+	proxy := &ReverseProxy{}
+	proxy.Director = func(req *http.Request) {
 		req.URL.Scheme = target.Scheme
 		req.URL.Host = target.Host
 		req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
@@ -120,8 +129,12 @@ func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
 			// explicitly disable User-Agent so it's not set to default value
 			req.Header.Set("User-Agent", "")
 		}
+
+		if proxy.ModifyRequest != nil {
+			proxy.ModifyRequest(req)
+		}
 	}
-	return &ReverseProxy{Director: director}
+	return proxy
 }
 
 func copyHeader(dst, src http.Header) {

src/net/http/httputil/reverseproxy_test.go

@@ -896,6 +896,39 @@ func TestClonesRequestHeaders(t *testing.T) {
 	if req.Header.Get("X-Forwarded-For") != "" {
 		t.Error("X-Forward-For header mutation modified caller's request")
 	}
+}
+
+func TestModifyRequest(t *testing.T) {
+	log.SetOutput(ioutil.Discard)
+	defer log.SetOutput(os.Stderr)
+	req, _ := http.NewRequest("GET", "http://foo.tld/", nil)
+	req.RemoteAddr = "1.2.3.4:56789"
+	url, err := url.Parse("http://foo.tld/")
+	if err != nil {
+		t.Error(err)
+	}
+
+	rp := NewSingleHostReverseProxy(url)
+	rp.ModifyRequest = func(req *http.Request) {
+		req.Header.Set("From-Modify-Request", "1")
+	}
+
+	rp.Transport = roundTripperFunc(func(req *http.Request) (*http.Response, error) {
+		if v := req.Header.Get("From-Modify-Request"); v != "1" {
+			t.Errorf("From-Modify-Request value = %q; want 1", v)
+		}
+		return nil, io.EOF
+	})
+
+	rp.ServeHTTP(httptest.NewRecorder(), req)
+
+	if req.Header.Get("From-Modify-Request") == "1" {
+		t.Error("ModifyRequest header mutation modified caller's request")
+	}
+
+	if req.Header.Get("X-Forwarded-For") != "" {
+		t.Error("X-Forward-For header mutation modified caller's request")
+	}
 
 }