How to get a notification, or callback, when data arrives in a socket?

[ruipacheco]

In Boost Asio it's normal to register a callback for when data arrives in a socket using async_read which receives a buffer and a callback. When the buffer is full the callback is executed.

How do I do the same thing using Tokio for a long-lived connection?

[Kestrer]

As far as I can tell from the documentation the equivalent code with Tokio is:

use tokio::io::AsyncReadExt;

tokio::spawn(async move {
    let mut buf = [0; 256];
    reader.read_exact(&mut buf).await?;
    // the rest of the code here
});

You can look at the AsyncReadExt docs for other useful methods on an AsyncRead.

[ruipacheco]

Won't read_exact block on a long lived connection even after reading all data?

[kornel]

.await doesn't block threads. It only suspends execution of code until the data arrives. It behaves exactly like turning rest of that async block into a callback (behind the scenes it is a callback from a Waker given by read_exact to runtime that executes that async block).

[ruipacheco]

So this would always be called when data arrives in the socket or only once?

[kornel]

spawn runs code once. read_exact waits until it got exactly as many bytes as required, even if that took multiple syscalls.

[ruipacheco]

I know I've accepted a solution but just so I don't need to open a new thread, is there a way to always be notified whenever data arrives in the socket, assuming the connection will never be closed?

[kornel]

loop {
   let bytes_read = socket.read(&mut buf).await?;
}

[ruipacheco]

How would I make this live for the same amount of time as a struct? Can I turn it into a property?

[kornel]

Async code can be stored as an object — a Future, but it doesn't do anything while it's sitting in a struct. It needs to be spawned to run.

You can tie it to an object using channels:

let (sender, receiver) = tokio::sync::mpsc::channel(1);
// add your own error handling
spawn(async move {
   loop {
      let bytes_read = socket.read(&mut buf).await?;
      // it'll fail when receiver is dropped
      sender.send(buf[0..bytes_read].to_vec())?; 
   }
});

// store receiver in a struct, and later you can do:
somestruct.receiver.recv().await;

but that's a bit overcomplicated, because usually you'd store socket in a struct, and just read from it in async fn.

Callback-based design is rather different than async/future-based design, so I suspect you'll need to adjust your design to fit async code, instead of trying to make async functions work like callbacks.