an attempt to impl zero-copy

[zhboner]

an attempt to impl zero-copy

[zhboner]

Zero copy would be a big improvement to the current implemention, but I don't think it is ready in tokio as mentioned in this issue tokio-rs/tokio#2438

[zephyrchien]

Yup, zero-copy is of much significance, it would become a regret if Realm does not support this feature.

Tokio has its own difficulties, we could not expect Tokio to provide zero-copy APIs in the short term. But in our case, the problem could be simplified, making it possible to write a Tokio-based zero-copy API on our own.

Our async task is just to proxy 2 TCP connection, we never use buffers across tasks. If we create a buffer(pipe) within the task, we could make sure that the pipe is always valid during the async operation. When the task is finished or cancelled, the pipe will be dropped as normal.

Moreover, we could count the left bytes in the pipe, and limit the max read/write bytes when calling splice. This ensures that the pipe will never block, and we no longer need to poll the pipe, waiting for its readable/writeable event.

Given the inner fd of tokio::TcpStream has been registered to the event loop, the only thing we need to do is:

1. wait the inner fd of TcpStream to become readable
2. splice fd1 -> pipe_write_half
3. wait the inner fd of another TcpStream to become writeable
4. splice pipe_read_half -> fd2

Actually I have did this a few days ago(0abbf2b bin). I added these codes to relay.rs directly. I think the better approach is to encapsulate the pipe into a struct, and impl Drop trait for it, so that the pipe could be dropped automatically.