Atomically draining & dropping a consumer of Channel / Oneshot about futures-rs HOT 5 CLOSED

I don't think these'd be too difficult to implement, but @carllerche do you have a use case in mind as well? It might be good to have one just to help designing the APIs to work with it.

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Off the top of my head, a Task is managing a Tcp connection for a client. The client handle sends requests to the connection task using a channel. The connection errors. It would be nice for the channel that sends requests to be closed cleanly. Aka, the consuming end can drop knowing that it received all pending requests and completed them w/ an error and that the client handle will not be able to send any further requests to the dead connection.

Hope this makes sense.

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I've run into the exact problem @carllerche describes with mpsc::channel. Atomic close and drain from the receiver side would be nice to have here.

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Ok I think that makes sense yeah. I think this is going to need coordination/APIs on both ends. That is, the producer half will need a Result of some form to hand back the message or at least indicate that delivery failed. The production half probably just needs a try_release method of some form then that attempts to shut down the channel but fails if there's items in flight.

I think for stream::channel we have a vector to return the item with a result, but for oneshot we do not. This can likely be solved by just returning a result from complete, akin to SendError in mpsc

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added to channel in #245

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