CODEWITHSUNDEEP
rustasync-await
Eloff
Eloff

Reputation: 21656

Get all avalable items from a futures Stream (non-blocking)

I have a WebSocket connection which wraps a futures_core::stream::Stream (incoming) and Sink (outgoing).

I want to decode and process all available messages from the Stream without blocking. Clearly at the socket level it's a TCP/IP stream of bytes and there is going to be 0..N messages sitting in the socket receive buffer waiting for a call to read(). A non-blocking call to read could well read multiple pipelined websocket frames. At the level of the Rust abstraction this might be possible with fn poll_next(...):

The trait is modelled after Future, but allows poll_next to be called even after a value has been produced, yielding None once the stream has been fully exhausted.

However, I don't know how to use this poll method directly without the async/await syntax, and even if I can, I don't see how it solves the problem. If I call it in a loop while I get back Some(frame), collecting the frames in a Vec, it will still suspend the task when it runs out of buffered frames and return Poll::Pending - so I won't be able to do anything with the collected frames immediately anyway. Ideally I need to process the collected frames when I get Poll::Pending without suspending anything, and then call it again allowing it to suspend only the second time around, if need be. Is there a solution possible here that doesn't involve discarding all of the future abstractions and resorting to buffering and parsing web socket frames myself?

Upvotes: 2

Views: 650

Answers (1)

user1937198
user1937198

Reputation: 5348

You seem to have a misunderstanding of how suspensions work. When the parent function calls poll_next in a loop, it is not poll_next returning Poll::Pending that results in a suspension. Instead it is when the function containing the loop returns a Poll::Pending as a result of that. But there is nothing that says you have to do that immediatly. You are free to process the frames you have collected before returning to the executor.

Upvotes: 1

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