Version

└── tokio v1.43.0
    └── tokio-macros v2.5.0 (proc-macro)

Platform
RHEL 8
x86_64
Intel CascadeLake

Description
The MPSC channel receiver is failing to receive messages when being repeatedly polled. I notice that it will receive exactly 128 elements (4 underlying blocks) before failing.

Here is a reproduction. Note that the select! macro simplifies the code here, but the issue also reproduces if the select! is replaced by a custom future that repeatedly invokes Future::poll on the Receiver object, verifying that the select! isn't a factor.

use tokio::{select, sync::mpsc};

#[tokio::main]
async fn main() {
    let (sender, mut receiver) = mpsc::channel(2);

    tokio::spawn(async move {
        for n in 0usize.. {
            sender.send(n).await.unwrap();
        }
    });

    loop {
        select! {
            msg = receiver.recv() => println!("Received {}", msg.unwrap()),
            () = std::future::ready(()) => {}
        }
    }
}

The output is as follows. No logging is observed after 128 elements have been received.

Received 0
Received 1
Received 2
... (lines omitted for brevity) ...
Received 127