Investigage full-duplex request/response streaming

[gterzian]

See the discussions over at whatwg/fetch#425 (comment)

The idea is, I think, that the body of a request would be transmitted over the network, and at the same time the body of the response could already be read by script as a stream.

From the linked-to discussion, it appears web developers want this, and the spec sort of allows for it(?), however current implementations have a hard time changing their networking stack model to allow for this.

I think we should be able to do it, since #25873 has landed, and I don't think there is anything in the structure of net that would prevent this.

However first we need to investigate #26904, and then I think it would be interesting to see if Servo could support full-duplex streaming.

[gterzian]

A few pointers:

In obtain_response, which returns a future for the actual http response, we:

1. First setup an IPC route that will be used to receive chunks from the body from script, and transmit them over the network, at https://github.com/servo/servo/blob/73760ea59434971d24e6aac7e5fe3c79c1ba5bf6/components/net/http_loader.rs
2. Call request on the Hyper client, to get a future for the response, at https://github.com/servo/servo/blob/73760ea59434971d24e6aac7e5fe3c79c1ba5bf6/components/net/http_loader.rs

Then later, this future is used to:

1. Wait on it, to get the response headers, at https://github.com/servo/servo/blob/73760ea59434971d24e6aac7e5fe3c79c1ba5bf6/components/net/http_loader.rs
2. Used to get the rest of the data at: https://github.com/servo/servo/blob/73760ea59434971d24e6aac7e5fe3c79c1ba5bf6/components/net/http_loader.rs

I guess the question is really whether Hyper gives us the headers, and data, for the response, while the request stream is still being read from...

[gterzian]

Looking at https://docs.rs/hyper/0.13.6/src/hyper/client/mod.rs.html#245

I have a feeling using the response future requires first completing the request feature, meaning it would not work...