Add force_send method to channel Sender

[bergkvist]

Add a force_push method to the Sender part of a channel.

In particular, this is useful for bounded channels of non-zero size.

Motivation:

• Lossy Channels #400

Design goals:

• It is fine to lose messages, but messages should never appear in a different order from which they were sent
• For bounded (non-zero-sized) channels, force_send should never fail (except when the receiver has been dropped)
• Should not be blocking

use crossbeam_channel::{bounded, ForceSendError};

let (s, r) = bounded(3);

assert_eq!(s.force_send(0), Ok(None));
assert_eq!(s.force_send(1), Ok(None));
assert_eq!(s.force_send(2), Ok(None));
assert_eq!(s.force_send(3), Ok(Some(0)));

assert_eq!(r.recv(), Ok(1));

assert_eq!(s.force_send(4), Ok(None));

assert_eq!(r.recv(), Ok(2));
assert_eq!(r.recv(), Ok(3));

assert_eq!(s.force_send(5), Ok(None));
assert_eq!(s.force_send(6), Ok(None));
assert_eq!(s.force_send(7), Ok(Some(4)));
assert_eq!(s.force_send(8), Ok(Some(5)));

assert_eq!(r.recv(), Ok(6));
assert_eq!(r.recv(), Ok(7));
assert_eq!(r.recv(), Ok(8));

drop(r);

assert_eq!(s.force_send(9), Err(ForceSendError(9)));

Closes #400

[proski]

It's not clear from the description what would happen if there is no receive operation. Would the call fail (despite what the start of the comment says)? Would the data be returned to the caller?

[proski]

The "lossy channel" proposal is about removal the oldest element rather than the latest:

By "lossy channel", I mean one that discards old data when the buffer is full

Removing the latest element makes it impossible to force send multiple messages unless the reader reads a message at the right time. That's very limiting and unreliable. The "force" is one element deep.

Figuratively speaking, users expect a bulldozer, and you give them a chisel.

[taiki-e]

Thanks for the PR. I would prefer that force_push in crossbeam_channel have the same semantics as force_push in crossbeam_queue::ArrayQueue (#789):

let q = crossbeam_queue::ArrayQueue::new(2);

assert_eq!(q.force_push(10), None);
assert_eq!(q.force_push(20), None);
assert_eq!(q.force_push(30), Some(10));

As said in #400 (comment), it should be able to be implemented by porting #789.

[bergkvist]

I agree that removing the oldest entry is more desirable than removing the most recent one.

I've been putting some more thought into this, and force_send seems to be tricky compared to try_send.

Consider the following, a bounded(6) channel (H=head (read index), T=tail (write index)):

step  next state        op
       
        H
                    T
1     [ 0, 1, 2, 3, _, _ ]   force_send <- 3
      
        H
                       T
2     [ 0, 1, 2, 3, 4, _ ]   force_send <- 4
      
           H
                       T
3     [ _, 1, 2, 3, 4, _ ]   recv -> 0
      
           H
        T              
4     [ _, 1, 2, 3, 4, 5 ]   force_send <- 1
              
           H
           T            
5     [ 6, 1, 2, 3, 4, 5 ]   force_send <- 6
              
              H
              T            
6     [ 6, 7, 2, 3, 4, 5 ]   force_send <- 7 (removes/returns 1)
      
                 H
              T            
7     [ 6, 7, _, 3, 4, 5 ]   recv -> 2

Notice that force_send only has different behavior from try_send in step 6 (where the channel is full), where try_send will fail, but force_send should overwrite the oldest value, updating both H and T indices at the same time.

And here lies the problem, H and T both are atomic usizes, meaning we can ensure atomicity when updating one of them using compare_exchange_weak(..), but since we need to update both of them at the same time, we have a problem (on x86 we have https://docs.rs/mwcas/latest/mwcas/).

[sbarral]

[...] but since we need to update both of them at the same time, we have a problem (on x86 we have https://docs.rs/mwcas/latest/mwcas/).

I had a cursory look at the implementation in #789 and I couldn't really see what makes this issue different. What mandates an atomic update of both head and tail for a channel when a non-atomic update is acceptable for ArrayQueue?