fix(kit/feature): Ensure host is overridden as a workaround for stdlib bug.

kit/feature/http_proxy.go

@@ -49,7 +49,15 @@ func newReverseProxy(dest *url.URL, enablerKey string) *httputil.ReverseProxy {
 	defaultDirector := proxy.Director
 	proxy.Director = func(r *http.Request) {
 		defaultDirector(r)
+
 		r.Header.Set(headerProxyFlag, enablerKey)
+
+		// Override r.Host to prevent us sending this request back to ourselves.
+		// A bug in the stdlib causes this value to be preferred over the
+		// r.URL.Host (which is set in the default Director) if r.Host isn't
+		// empty (which it isn't).
+		// https://github.com/golang/go/issues/28168
+		r.Host = dest.Host
 	}
 	proxy.ModifyResponse = func(r *http.Response) error {
 		r.Header.Set(headerProxyFlag, enablerKey)

kit/feature/http_proxy_test.go

@@ -88,7 +88,7 @@ func TestHTTPProxy_RequestHeader(t *testing.T) {
 	proxy := NewHTTPProxy(sURL, logger, en)
 
 	w := httptest.NewRecorder()
-	r := httptest.NewRequest("GET", "http://example.com/foo", nil)
+	r := httptest.NewRequest(http.MethodGet, "http://example.com/foo", nil)
 	srcHandler(proxy)(w, r)
 }
 
@@ -104,7 +104,7 @@ func testHTTPProxy(logger *zap.Logger, enabler ProxyEnabler) (*http.Response, er
 	proxy := NewHTTPProxy(sURL, logger, enabler)
 
 	w := httptest.NewRecorder()
-	r := httptest.NewRequest("GET", "http://example.com/foo", nil)
+	r := httptest.NewRequest(http.MethodGet, "http://example.com/foo", nil)
 	srcHandler(proxy)(w, r)
 
 	return w.Result(), nil