Fix CloseNotifier handling and avoid "the ResponseWriter does not implement CloseNotifier" warnings in logs

A change in #763 to address review comments caused problems. Originally,
instrumentedResponseWriter implemented the CloseNotifier interface, and
would panic if it was wrapping something that did not implement that
interface. This was split into a separate instrumentedResponseWriterCN
type that implements CloseNotifier, so there's a fallback if
instrumentedResponseWriter ever needs to wrap something that does not
implement this interface.

instrumentedResponseWriter's Value method would end up upcasting either
type back to instrumentedResponseWriter, which does not implement the
interface. In effect, instrumentedResponseWriterCN was never visible to
the handler.

This fixes the problem by implementing a wrapper Value method for
instrumentedResponseWriterCN.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

context/http.go

@@ -352,3 +352,13 @@ func (irw *instrumentedResponseWriter) Value(key interface{}) interface{} {
 fallback:
 	return irw.Context.Value(key)
 }
+
+func (irw *instrumentedResponseWriterCN) Value(key interface{}) interface{} {
+	if keyStr, ok := key.(string); ok {
+		if keyStr == "http.response" {
+			return irw
+		}
+	}
+
+	return irw.instrumentedResponseWriter.Value(key)
+}

registry/handlers/helpers.go

@@ -29,7 +29,7 @@ func copyFullPayload(responseWriter http.ResponseWriter, r *http.Request, destWr
 	if notifier, ok := responseWriter.(http.CloseNotifier); ok {
 		clientClosed = notifier.CloseNotify()
 	} else {
-		ctxu.GetLogger(context).Warn("the ResponseWriter does not implement CloseNotifier")
+		ctxu.GetLogger(context).Warnf("the ResponseWriter does not implement CloseNotifier (type: %T)", responseWriter)
 	}
 
 	// Read in the data, if any.