How is order determined when priorities match?

[matanmarkind]

If multiple items are sent to the channel with the same priority, is there any promise about the order they will be received? In particular I want to know if sending order will be preserved.

So if I do the following:

channel.send(Item { priority: 1, val: X });
channel.send(Item { priority: 1, val: Y });

Is there any guarantee that X will be received before Y?

[rmcgibbo]

The internal data structure is a BinaryHeap, so the current behavior probably depends on that. But there's no guarantee.

[cameroncros]

This is a pretty major break in the definition of a "channel", and should be highlighted in your documentation.

All data sent on the Sender will become available on the Receiver in the same order as it was sent

https://doc.rust-lang.org/stable/std/sync/mpsc/fn.channel.html

The current impl definitely does not guarentee order:

use async_priority_channel::unbounded;

#[tokio::main]
async fn main() {
    let (tx, rx) = unbounded();

    for i in 0..10 {
        tx.send(i, 0).await.unwrap();
    }

    loop {
        let i = rx.recv().await.unwrap();
        println!("Got: [{i:?}]");
    }
}

Results in:

Got: [(0, 0)]
Got: [(2, 0)]
Got: [(6, 0)]
Got: [(9, 0)]
Got: [(8, 0)]
Got: [(5, 0)]
Got: [(7, 0)]
Got: [(4, 0)]
Got: [(1, 0)]
Got: [(3, 0)]