Multi-root discovery: pragmatic, simple consensus.

[goodboy]

This issue is a more practical and immediate equivalent of the discussion in #184.

From the start of the project a hacky naive approach to finding the addresses of other "actors" has been this idea of an arbiter which we implement as a "special actor" with methods for looking up (socket) addresses by name. This is of course not an ideal system since there will always be a race during a multi-tree startup for the "arbiter" address as well no flexible consensus system for how that position can be transferred to another tree / root actor when the first is torn down / fails. The fragility is further emphasized in how root actors "check" for the registry (arbiter) existing which is simply do a fast TCP connect and drop on the supposed arbiter socket address.

Summarizing the current naive/questionable design for an address registry:

• a single socket address is allocated to some root actor designated the "arbiter" (aka a registry actor) and this address is passed to other python programs which would like to search for actors also using this same registry
• the way to "check" if the arbiter "exists" is to do a nasty TCP connect/drop which results in us having to specially handle and remap trio.BrokenResourceErrors to an internal TransportClosed error which is ignore silently
• there is no mechanism for fail-over, arbiter re-election, transfer of the registry between trees

---

Digging into "why" this is in the code:

This "arbiter" idea was originally adopted from other "actor system" projects:

• examples

Places to start some research

• gossip protocol
• matrix "federation replication" api
• raft

WIP, will come back.

[goodboy]

resource dump, will do writeup later.

internal existing issues:
#184

resources to work toward our own discovery sys:
https://en.wikipedia.org/wiki/Fallacies_of_distributed_computing
https://spec.matrix.org/unstable/server-server-api/#room-state-resolution
https://spec.matrix.org/unstable/rooms/v2/
https://en.wikipedia.org/wiki/Multicast_DNS
https://en.wikipedia.org/wiki/Name_Service_Switch
https://en.wikipedia.org/wiki/Link-Local_Multicast_Name_Resolution
https://en.wikipedia.org/wiki/Zero-configuration_networking#Name_service_discovery
https://en.wikipedia.org/wiki/Service_Location_Protocol
https://en.wikipedia.org/wiki/Peer_Name_Resolution_Protocol
https://en.wikipedia.org/wiki/Name_Service_Switch
https://raft.github.io/raft.pdf