Add contrib/ui-api-proxy.go for testing UI features with live data.

ui-api-proxy is a reverse http proxy that allows the UI to be served from
a different host than the API. This allows testing new UI features served
from localhost but using live production data.

Change-Id: I1cef74c995ac21f4310c5ced4c042ece59a1a8e8

contrib/ui-api-proxy.go

@@ -0,0 +1,65 @@
+// ui-api-proxy is a reverse http proxy that allows the UI to be served from
+// a different host than the API. This allows testing new UI features served
+// from localhost but using live production data.
+//
+// Run the binary, download & install the Go tools available at
+// http://golang.org/doc/install . To run, execute `go run ui-api-proxy.go`.
+// For a description of the available flags, execute
+// `go run ui-api-proxy.go --help`.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"log"
+	"net"
+	"net/http"
+	"net/http/httputil"
+	"net/url"
+	"strings"
+	"time"
+)
+
+var (
+	ui   = flag.String("ui", "http://localhost:8080", "host to which ui requests will be forwarded")
+	api  = flag.String("api", "https://gerrit-review.googlesource.com", "host to which api requests will be forwarded")
+	port = flag.Int("port", 0, "port on which to run this server")
+)
+
+func main() {
+	flag.Parse()
+
+	uiURL, err := url.Parse(*ui)
+	if err != nil {
+		log.Fatalf("proxy: parsing ui addr %q failed: %v\n", *ui, err)
+	}
+	apiURL, err := url.Parse(*api)
+	if err != nil {
+		log.Fatalf("proxy: parsing api addr %q failed: %v\n", *api, err)
+	}
+
+	l, err := net.Listen("tcp", fmt.Sprintf("127.0.0.1:%v", *port))
+	if err != nil {
+		log.Fatalln("proxy: listen failed: ", err)
+	}
+	defer l.Close()
+	fmt.Printf("OK\nListening on http://%v/\n", l.Addr())
+
+	err = http.Serve(l, &httputil.ReverseProxy{
+		FlushInterval: 500 * time.Millisecond,
+		Director: func(r *http.Request) {
+			if strings.HasPrefix(r.URL.Path, "/changes/") || strings.HasPrefix(r.URL.Path, "/projects/") {
+				r.URL.Scheme, r.URL.Host = apiURL.Scheme, apiURL.Host
+			} else {
+				r.URL.Scheme, r.URL.Host = uiURL.Scheme, uiURL.Host
+			}
+			if r.URL.Scheme == "" {
+				r.URL.Scheme = "http"
+			}
+			r.Host, r.URL.Opaque, r.URL.RawQuery = r.URL.Host, r.RequestURI, ""
+		},
+	})
+	if err != nil {
+		log.Fatalln("proxy: serve failed: ", err)
+	}
+}