Memory leak 'by default' with expired clients.

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If a user doesn't manually remove disconnected clients they'll continue to accumulate in Room.clients. To me this is unacceptable. I think this justifies reviving an old idea of mine, to have an optional output channel for expired clients.

Disconnected clients wouldn't be stored inside Room at all; depending on Option they would be either Dropped or emitted on a disconnected_client_tx channel.

I've been trying to figure out what features should go on Room directly. A related idea is to have a client_insertion_rx channel for asynchronously adding clients #14 . These two channels would start to really up the complexity of Room.

Design exploration

• Room
  • Room removes from Room.clients immediately when disconnect is detected.
  • impl Stream for Room emits a disconnected Client as Err
• As-yet-unnamed type we'll call ChannelledRoom.
  • Presumably provides the same Future-creating methods as Room. A trait could enforce this. The various future types could be parametric over that trait if the different logic could be kept to impl Sink + Stream for ChannelledRoom.
  • impl Sink + Stream for ChannelledRoom takes Err(Client)s and emits them on disconnected_client_tx. I can drop irritating Error: Clone requirements if this then returns something less precise in Err.

Tradeoffs

• The memory leak must go. The library must be safe by default.
• We still need to get ahold of disconnected clients, as many network services will want to reestablish connections/do logic with that.
• This ChannelledRoom is only strictly necessary for easy combinator code. Custom futures can simply insert new clients and trivially use returned errors.
• ChannelledRoom would make the library more complicated. It leads in a promising direction, as a generalised ReconnectingRoom might then be possible, but we're talking quite complex genericism here.
• The value of non-custom-futures code is largely appealing/simple examples. I don't think it would be sensible very often to directly use Room.

Decisions

• Room must stop storing disconnected Client. Remove that immediately. Stop storing disconnected clients in Room. #21
• ChannelledRoom is a nice-to-have. Something to think about after a v0.1.3.

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Fixing the memory leak in #21 led to a major reworking, taking out things like Status that had seemed less useful as time went on.

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This issue mainly relates to the memory leak. Closing in favor of doing ChannelledRoom in #14 .