Every 32 messages is_empty() on Receiver and UnboundedReceiver returns false even though len == 0

[sjoubert49]

Version
tokio v1.37.0

Platform
Windows 10, 64 bits

Description

This issue was first mentionned in a stackoverflow question
Every 32 messages, after reading all messages from the Receiver, the call to receiver.is_empty() returns false, even though the reported len is 0.

The issue can easily be reproduced with

use tokio::sync::mpsc;

#[tokio::main]
async fn main() {
    let (sender, mut receiver) = mpsc::channel(33);

    for value in 1..257 {
        sender.send(value).await.unwrap();
        receiver.recv().await.unwrap();
        if !receiver.is_empty() {
            // This should not be called
            println!("{value}. len: {}", receiver.len());
        }
    }
}

There are some lines printed, even though is_empty() should always be true after receiving the last value.

32. len: 0
64. len: 0
96. len: 0
128. len: 0
160. len: 0
192. len: 0
224. len: 0
256. len: 0

NB: 32 is BLOCK_CAP so I'm guessing the issue is removing a value from the last slot in a Block, but I don't understand the code well enough so far to reach a better conclusion.

Also tested

The same issue is seen when:

• using an unbounded_channel instead of an channel
• with a capacity of 1
• sending a non zero-sized value (value from the loop)
• in a single thread synchronous code with the blocking versions of the calls to send and recv

[Darksonn]

Thank you for reporting this issue. I have filed #6603 to fix this.