Sending Poll::Pending appears to hang

[jr200]

Hi,

I'm not sure if I'm using this library correctly, so my apologies upfront...

In the server part of the test-suite, if I change the body of the implementation of the Stream trait to include a Poll::Pending return value, the client indefinitely hangs.

impl Stream for MessageStream {
    type Item = Result<EchoResponse, Status>;

    fn poll_next(mut self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        self.count += 1;
        if self.count < 3 {
            Poll::Pending
        } else if self.count < 6 {
            Poll::Ready(Some(Ok(EchoResponse {
                message: format!("echo({})", self.message),
            })))

        } else {
            Poll::Ready(None)
        }
    }
}

My use-case is my MessageStream wraps another which might return Poll::Pending. I should add I am also fairly new to rust.

Thanks!

[devashishdxt]

I this particular example, you need to arrange wake() call for the future so that the runtime knows when to poll the future again.

Read Rust async book for better understanding.

[devashishdxt]

@jr200 Can you confirm if your issue is resolved? If yes, we can close this issue. If not, can you provide more details of what you're trying to do?

[jr200]

Hi @devashishdxt, I've read the async book now (thanks for the recommendation!). I believe the simple test-case works, but I haven't managed to get my use-case working yet, where I am wrapping a Receiver in a Stream. Although the browser no longer hangs when a Poll::Pending is processed, either the messages do not make it through or I appear to get many more messages than I am expecting.

I am almost definitely still doing something wrong, so I'll try and put together a minimal example and debug it this weekend.

[jr200]

Had an initial look, and it seems that the server has to send all messages followed by a Poll::Ready(None) (close stream) before the client will begin to process the received messages. So in my case where I have essentially an infinite-stream, the client (browser) never begins to process the first message.

[teddyrendahl]

I'm seeing this as well. My client does not get the stream response until the stream has closed. My server works fine with the Javascript bindings so I don't think the issue is there.

[devashishdxt]

@teddyrendahl Can you post some sample JS code and your server code for my reference?

[jr200]

I was also able to get server-side streaming to work using rust/tonic-web and a JS grpc-web implementation. However, this was using grpcwebtext. The grpc-web page says Content-type: application/grpc-web+proto only supports unary calls, while Content-type: application/grpc-web-text supports both unary and server-side streaming calls.

Perhaps I was trying to do something that was never possible. Can you confirm this to be the case?

[devashishdxt]

@jr200 application/grpc-web-text is not yet supported on this crate. But, I'd like to implement support for this. For that, can you please share some sample code (server and js-client) that you're using so that I can test this particular usecase?

[jr200]

Sure - I will create a minimal example tonight and share it here.

But with regards to my original issue, can you confirm that this crate does not support streaming with Content-type: application/grpc-web+proto?

[jr200]

Hi @devashishdxt, I've put together a minimal example of a rust server exposing a streaming API over gRPC-web, and a with a JS/react frontend consuming the stream (Content-type: application/grpc-web-text).

Code is here: https://github.com/jr200/grpc-web-example.

Please let me know if are any issues...

[devashishdxt]

Hi @devashishdxt, I've put together a minimal example of a rust server exposing a streaming API over gRPC-web, and a with a JS/react frontend consuming the stream (Content-type: application/grpc-web-text).

Code is here: https://github.com/jr200/grpc-web-example.

Please let me know if are any issues...

Thanks for the example. I'll try and add support for this over the weekend.

[wackazong]

I think I can confirm this. When using the routeguide example from tonic and putting a small delay into list_features on the server side I see the messages incoming regularly.

When I do the same with this repo I see the results coming in from the server regularly in the network tab of the dev tools.

However, the messages are only processed after all have been received.

            // this let only returns after all messages have been received
            let mut stream = client
                .many_slow_hello(request)
                .await
                .expect("request successful")
                .into_inner();
            // this loop waiting for messages starts only once the stream has ended            
            loop {
                match stream.message().await {
                    Ok(Some(message)) => {
                        info!("many_slow_hello: {:?}", message.message);
                        received.write().push(message);
                    }
                    Ok(None) => {
                        info!("Stream ended");
                        break;
                    }
                    Err(status) => {
                        error!("Stream error: {}", status);
                    }
                }
            }

I am new to tonic and wasm but I have the hunch this could be the same issue.

Thanks in advance for your support! 🙏

[wackazong]

AFAICS this seems to be impossible in WASM. Getting the chunks of a HTTP response as they come in is not supported in reqwest and also not in other WASM-compatible HTTP clients.

[wngr]

Oh, what a bummer! What makes you think that?

Seems https://github.com/seanmonstar/reqwest/pull/1576 would provide that functionality, so in theory it looks like it's possible.

Edit: I just tried https://github.com/titanous/grpc-web-client/pull/13 as a drop-in-replacement for this crate. Server streaming just works.

[wackazong]

Seems seanmonstar/reqwest#1576 would provide that functionality, so in theory it looks like it's possible.

Yes, I think that is what would need to be enabled.

Edit: I just tried titanous/grpc-web-client#13 as a drop-in-replacement for this crate. Server streaming just works.

Great to hear, thanks!

Edit: grpc-web-client uses web-sys directly, way to go.

Edit: I can confirm, the drop-in replacement works for server streaming.

[wngr]

I think the way forward would be to re-use the whole parsing logic from tonic, and either use web-sys directly or via reqwest with added stream support.

[devashishdxt]

I'm already working on this in next branch of this repo. Will finish it in a few days.

[wngr]

Do you plan on re-using the decoding logic from tonic or stick to rolling your own?

[devashishdxt]

Released a new version. Hopefully it should solve the issues.

[wngr]

I can confirm it works. Thanks for this!