Provide yes man endpoint for inflexible load balancers

Certain load balancers, such as Amazon's Elastic Load Balancer, have a very
limited notion of health. While a properly configured and operational registry
should always return a 401 when hitting "/v2/", such load balancers cannot be
configured to treat this response code as healthy. This changeset makes "/"
always return a 200 response, unless the health checks have failed.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/registry/main.go

@@ -72,8 +72,9 @@ func main() {
 	app := handlers.NewApp(ctx, *config)
 	app.RegisterHealthChecks()
 	handler := configureReporting(app)
-	handler = panicHandler(handler)
+	handler = alive("/", handler)
 	handler = health.Handler(handler)
+	handler = panicHandler(handler)
 	handler = gorhandlers.CombinedLoggingHandler(os.Stdout, handler)
 
 	if config.HTTP.Debug.Addr != "" {
@@ -313,3 +314,20 @@ func panicHandler(handler http.Handler) http.Handler {
 		handler.ServeHTTP(w, r)
 	})
 }
+
+// alive simply wraps the handler with a route that always returns an http 200
+// response when the path is matched. If the path is not matched, the request
+// is passed to the provided handler. There is no guarantee of anything but
+// that the server is up. Wrap with other handlers (such as health.Handler)
+// for greater affect.
+func alive(path string, handler http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path == path {
+			w.Header().Set("Cache-Control", "no-cache")
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+
+		handler.ServeHTTP(w, r)
+	})
+}