Request hangs with no feedback if server connection is refused

If the server is not listening for requests and the client attempts to make a request, the client will hang with no feedback on the failed request. Reproduce by:

1. make one basic grpc request to verify all is ok
2. kill / stop grpc server (envoy, node proxy, etc.)
3. attempt to make another grpc request

In the console (at least under chrome) you’ll see something similar to:

POST http://SOME_HOST/XXX/SOME_METHOD_NAME net::ERR_CONNECTION_REFUSED

The on('error'...) handler is not invoked and you have no way of knowing that the request failed.

I believe this is occurring because when XHR state is being inspected in the COMPLETE handler, the XHR status of 0 is not being checked. XHR status of 0 means that there was some problem that prevented the request from being sent (https://stackoverflow.com/questions/3825581/does-an-http-status-code-of-0-have-any-meaning/#answer-26451773)

Relevant code: https://github.com/grpc/grpc-web/blob/master/javascript/net/grpc/web/grpcwebclientreadablestream.js#L196-L214

I added a check for 0 and was then notified of the failed requests:

--- a/javascript/net/grpc/web/grpcwebclientreadablestream.js
+++ b/javascript/net/grpc/web/grpcwebclientreadablestream.js
@@ -196,7 +196,7 @@ const GrpcWebClientReadableStream = function(genericTransportInterface) {
   events.listen(this.xhr_, EventType.COMPLETE, function(e) {
     if (!self.onErrorCallback_) return;
     var lastErrorCode = self.xhr_.getLastErrorCode();
-    if (lastErrorCode != ErrorCode.NO_ERROR) {
+    if (lastErrorCode != ErrorCode.NO_ERROR || (self.xhr_.getStatus() == 0)) {
       self.onErrorCallback_({
         code: StatusCode.UNAVAILABLE,
         message: ErrorCode.getDebugMessage(lastErrorCode)

Please let me know if you want a PR or have thoughts on why my findings are invalid.

[zsluedem]

I met some similar problem like this.

I send a request to server which is down I can only get console error and not trigger the error hanler.

I think it is possible that grpc-web is only handling error from the grpc and eat all the errors in http request to the proxy.

[stanley-cheung]

This should be fixed recently. This is a unit test to show that if there’s an underlying xhr error, it should be returned as the err of the main callback. For streaming, it will be return as .on('error', ...).