[server] How to support multiple interfaces at once?

[jamesmunns]

There is some interest for having servers being able to serve multiple interfaces, e.g. offering services on both a USB and SPI port at the same time. Right now, the server is built around listening to a single interface with the Wire(Tx/Rx/Spawn) traits. I've had a few private requests for this, and at least one in public.

We may want to offer the ability to find some way to expand this. Challenges I can think of offhand:

• How does the "plumbing" work for the server? e.g. how do we get the right WireTx for the given WireRx?
  • Right now the dispatch code only handles one of each.
  • How can we avoid "single interface" servers paying any overhead?
• Should all interfaces offer the same services?
  • Will users only want some services on some interfaces?
  • If all interfaces don't have the same services, how does this affect key collision avoidance?
• Should handlers be able to "know" which interface they are being called from? e.g. do they need to be passed some context?
• How do we handle "fairness" when it comes to servicing messages?
  • Do we always "round robin" the interfaces? like ABABABABAB?
  • Will users want to be able to have "priority" for interfaces? e.g. "always serve A if there is a pending message, then only serve B if A is idle"?
  • Some other way?
• How are topics_out publishes handled?
  • Are they sent to all interfaces?
  • Do we have separate Senders for each interface and the app has to know which to use?
• How do we behave when one interface is up, and the other is down?
  • e.g. SPI is always connected, but USB may not always be.
  • Does this change anything?