Support CONNECT over h2

[nox]

There is a h2 issue but even if support for CONNECT was perfect in h2, there are still details to sort out on hyper itself too. I'm filing this issue because I started working on that today and will probably need some help.

[nox]

AFAICT for the client part we mostly want to patch this match arm:

hyper/src/proto/h2/client.rs

Lines 217 to 290 in 117cc49

	Poll::Ready(Some((req, cb))) => {
	// check that future hasn't been canceled already
	if cb.is_canceled() {
	trace!("request callback is canceled");
	continue;
	}
	let (head, body) = req.into_parts();
	let mut req = ::http::Request::from_parts(head, ());
	super::strip_connection_headers(req.headers_mut(), true);
	if let Some(len) = body.size_hint().exact() {
	if len != 0 || headers::method_has_defined_payload_semantics(req.method()) {
	headers::set_content_length_if_missing(req.headers_mut(), len);
	}
	}
	let eos = body.is_end_stream();
	let (fut, body_tx) = match self.h2_tx.send_request(req, eos) {
	Ok(ok) => ok,
	Err(err) => {
	debug!("client send request error: {}", err);
	cb.send(Err((crate::Error::new_h2(err), None)));
	continue;
	}
	};
	let ping = self.ping.clone();
	if !eos {
	let mut pipe = Box::pin(PipeToSendStream::new(body, body_tx)).map(|res| {
	if let Err(e) = res {
	debug!("client request body error: {}", e);
	}
	});
	// eagerly see if the body pipe is ready and
	// can thus skip allocating in the executor
	match Pin::new(&mut pipe).poll(cx) {
	Poll::Ready(_) => (),
	Poll::Pending => {
	let conn_drop_ref = self.conn_drop_ref.clone();
	// keep the ping recorder's knowledge of an
	// "open stream" alive while this body is
	// still sending...
	let ping = ping.clone();
	let pipe = pipe.map(move |x| {
	drop(conn_drop_ref);
	drop(ping);
	x
	});
	self.executor.execute(pipe);
	}
	}
	}
	let fut = fut.map(move |result| match result {
	Ok(res) => {
	// record that we got the response headers
	ping.record_non_data();
	let content_length = decode_content_length(res.headers());
	let res = res.map(|stream| {
	let ping = ping.for_stream(&stream);
	crate::Body::h2(stream, content_length, ping)
	});
	Ok(res)
	}
	Err(err) => {
	ping.ensure_not_timed_out().map_err(|e| (e, None))?;
	debug!("client response error: {}", err);
	Err((crate::Error::new_h2(err), None))
	}
	});
	self.executor.execute(cb.send_when(fut));
	continue;
	}

Specifically, ultimately we want to patch this snippet so that it inserts an OnUpgrade extension in the returned response, with its inner oneshot channel already sent to with a Upgraded built from the SendStream<_> that was returned by self.h2_tx.send_request and the RecvStream returned from the ResponseFuture:

hyper/src/proto/h2/client.rs

Lines 270 to 280 in 117cc49

	Ok(res) => {
	// record that we got the response headers
	ping.record_non_data();
	let content_length = decode_content_length(res.headers());
	let res = res.map(|stream| {
	let ping = ping.for_stream(&stream);
	crate::Body::h2(stream, content_length, ping)
	});
	Ok(res)
	}

The first issue I'm experiencing is that the SendStream<_> is a SendStream<SendBuf<B as Body>::Data>> and we need this to be the AsyncWrite half of the Upgraded IO object and I have no damn clue how we can possibly do that.

[nox]

I guess I can change the struct SendBuf to an enum like this:

enum SendBuf<B> {
    Buf(B),
    Cursor(Cursor<Box<[u8]>>),
    None,
}

where SendBuf::Cursor is only ever used in the impl of AsyncWrite for the h2 upgraded stream.

[nox]

To implement AsyncRead over RecvStream, I need hyperium/h2#532.

[nox]

I'm also not exactly sure what poll_flush and poll_shutdown should do.

[nox]

AFAIK poll_shutdown can just send an empty data frame with the EOS flag set, but we definitely need something in h2 to properly implement poll_flush.

[nox]

So, I implemented poll_flush as a noop and tried adding upgrades to the server-side of Hyper, and I encountered a quite pesky blocker.

To implement Io over an H2 stream, I wrote this:

struct H2Upgraded<B>
where
    B: Buf,
{
    ping: Recorder,
    send_stream: SendStream<SendBuf<B>>,
    recv_stream: RecvStream,
    buf: Bytes,
}

impl<B> AsyncRead for H2Upgraded<B>
where
    B: Buf,
{
    fn poll_read(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        read_buf: &mut ReadBuf<'_>,
    ) -> Poll<Result<(), io::Error>> {
        let mut polled_stream = false;
        Poll::Ready(loop {
            if !self.buf.is_empty() {
                let cnt = std::cmp::min(self.buf.len(), read_buf.remaining());
                read_buf.put_slice(&self.buf[..cnt]);
                self.buf.advance(cnt);
                let _ = self.recv_stream.flow_control().release_capacity(cnt);
            }
            if polled_stream || read_buf.remaining() == 0 {
                break Ok(());
            }
            debug_assert!(!self.buf.is_empty());
            self.buf = match ready!(self.recv_stream.poll_data(cx)) {
                None => break Ok(()),
                Some(Ok(buf)) => {
                    self.ping.record_data(buf.len());
                    buf
                }
                Some(Err(e)) => {
                    return Poll::Ready(Err(h2_to_io_error(e)));
                }
            };
            polled_stream = true;
        })
    }
}

impl<B> AsyncWrite for H2Upgraded<B>
where
    B: Buf,
{
    fn poll_write(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8],
    ) -> Poll<Result<usize, io::Error>> {
        if buf.is_empty() {
            return Poll::Ready(Ok(0));
        }
        self.send_stream.reserve_capacity(buf.len());
        Poll::Ready(match ready!(self.send_stream.poll_capacity(cx)) {
            None => Ok(0),
            Some(Ok(cnt)) => self.write(&buf[..cnt], false).map(|()| cnt),
            Some(Err(e)) => Err(h2_to_io_error(e)),
        })
    }

    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
        Poll::Ready(Ok(()))
    }

    fn poll_shutdown(
        mut self: Pin<&mut Self>,
        _cx: &mut Context<'_>,
    ) -> Poll<Result<(), io::Error>> {
        Poll::Ready(self.write(&[], true))
    }
}

So far so good, right?

Then in the server code, I pass an H2Upgraded to Upgraded::new and this is where I am stuck.

Upgraded wants a Send I/O object, but all we get from the h2 crate is a SendStream<SendBuf<<B as Body>::Data>, and the system has no Send bound on B::Data. I'll try to propagate this bound everywhere but AFAIK that's a breaking change.

[nox]

Ah right, it also wants B::Data to be 'static.