Implement Weak version of mpsc::Sender

[b-naber]

Attempt to fix #4023

[Darksonn]

Seems like a relatively simple implementation.

[b-naber]

Looks like we can't use loom's Arc version either, since it doesn't have a downgrade assoc function. Is it a problem using std::Arc or shall we add downgrade to loom::Arc? I guess this is blocked on loom having downgrade and a Weak version (I can open a PR for that if we want to do that).

[Darksonn]

I believe that it should be possible to get this work using the normal Arc because we count the number of senders separately in tx_count.

[b-naber]

I think this would require us to manually call decrement_strong_count though on downcast, otherwise the channel would never be dropped, wouldn't it? Which isn't inherently bad, but we would basically manually mirror Weak then afaict. Using Weak seems like the better option, also seems straightforward to add to loom.

[Darksonn]

Well, it's true that this means that the Chan struct may stay alive until all Weak handles are dropped, but its unclear whether that's a problem.

[b-naber]

Ok, I thought that would be a no-go. If that's fine then I'll gladly change the PR.

Probably should mention in the docs for WeakSender, that all outstanding instances need to be dropped for the memory of the underlying channel to be freed, shouldn't we?

[Darksonn]

I mean, that's true for the weak version of Arc too. I think the bigger question is when pending messages are dropped if there are some left in the channel at the end.

[b-naber]

I mean, that's true for the weak version of Arc too.

Is it? I think inner of the Arc is dropped once the strong count reaches 0.

I think the bigger question is when pending messages are dropped if there are some left in the channel at the end.

hm I think they should. When the last (strong) Sender is dropped, we'll put a 'closed' message on the list and any message that is put onto the list after that should be dropped imo. What do you think?

[Darksonn]

Is it? I think inner of the Arc is dropped once the strong count reaches 0.

The value in the Arc is dropped, yes, but the allocation containing it stays around.

hm I think they should. When the last (strong) Sender is dropped, we'll put a 'closed' message on the list and any message that is put onto the list after that should be dropped imo. What do you think?

Messages can't be put on the list after dropping the last sender?

Perhaps you mean that we should drop pending messages when dropping the receiver. I'm happy with doing that.

[b-naber]

Maybe I'm misunderstanding, so just to clarify: The only way for messages to be pending is if they're awaiting being granted a permit or are itself futures that are awaiting, right? And once a permit is given out the message is put onto the list. We now want to forbid those messages, which were pending, to be put onto the list after the Receiver was dropped?

[Darksonn]

I believe that we already prevent sending messages after the Receiver is dropped. The question is what happens to those sent before the Receiver was dropped.

let (send, recv) = mpsc::unbounded();
send.send("hello");
drop(recv);
// is "hello" dropped now?

[Darksonn]

Besides these comments, I think this looks good.

[Darksonn]

Please click "resolve" on the things you have fixed.

[Darksonn]

Looks good to me.