mpsc::send returns error once, then success

[jsgf]

If you close the rx side of a channel pair before sending to it, then the first send will fail with a SendError as expected, but a second send will return Ok(()).

use std::sync::mpsc::{channel, SendError};

#[test]
fn doublesend() {
    let (tx, _) = channel();

    assert_eq!(tx.send(123), Ok(()));
    assert_eq!(tx.send(123), Err(SendError(123)));
    assert_eq!(tx.send(123), Err(SendError(123)));
}

Fails with:

thread 'doublesend' panicked at 'assertion failed: (left == right) (left: Err("SendError(..)"), right: Ok(()))', :7

However, if something is successfully sent before the receiver is dropped (even if it never receives the value), then it works as expected:

use std::sync::mpsc::{channel, SendError};
use std::mem;

#[test]
fn doublesend() {
    let (tx, rx) = channel();

    assert_eq!(tx.send(123), Ok(()));
    mem::drop(rx);
    assert_eq!(tx.send(123), Err(SendError(123)));
    assert_eq!(tx.send(123), Err(SendError(123)));
}

[alexcrichton]

I may be a bit confused, but it looks like everything is working as intended here? In the first example the receiver is immediately dropped (binding via _), and the first send fails (line 7) with an error appropriately because there is no receiver to receive the message.

The second example also looks correct as when the first message is sent the receiver is still active (could possibly acquire the message), but afterwards it's guaranteed to never be received so the next two sends are errors.

[bluss]

The code here demonstrates the problem https://users.rust-lang.org/t/mpsc-sender-send-only-returns-senderror-once-then-succeeds/4894

[alexcrichton]

Aha! So to inline the problem:

use std::sync::mpsc::{channel, SendError};

fn main() {
    let (tx, _) = channel();

    let _ = tx.send(123);
    assert_eq!(tx.send(123), Err(SendError(123)));
}

Yes, this is indeed unexpected behavior! It's probably something to do with the upgrade logic in channels.

[apasel422]

I think the issue here is that oneshot::Packet::send unconditionally sets the state field to DATA, even when the subsequent match indicates that the state was DISCONNECTED. This causes the first send to error correctly, but causes the second to succeed.