Such that any UDS socket pair is represented (and with the recent
updates to) a `USDAddress` via a similar pair-`tuple[str, int]` as TCP
sockets, a pair of the `.filepath: Path` & the peer proc's `.pid: int`
which we read from the underlying `socket.socket` using
`.set/getsockopt()` calls
Impl deats,
- using the Linux specific APIs, we add a `get_peer_info()` which reads
the `(pid, uid, gid)` using the `SOL_SOCKET` and `SOL_PEECRED` opts to
`sock.getsockopt()`.
|_ this presumes the client has been correspondingly configured to
deliver the creds via a `sock.setsockopt(SOL_SOCKET, SO_PASSCRED,
1)` call - this required us to override `trio.open_unix_socket()`.
- override `trio.open_unix_socket()` as per the above bullet to ensure
connecting peers always transmit "credentials" options info to the
listener.
- update `.get_stream_addrs()` to always call `get_peer_info()` and
extract the peer's pid for the `raddr` and use `os.getpid()` for
`laddr` (obvi).
|_ as part of the new impl also `log.info()` the creds-info deats and
socket-file path.
|_ handle the oddity where it depends which of `.getpeername()` or
`.getsockname()` will return the file-path; i think it's to do with
who is client vs. server?
Related refinements,
- set `.layer_key: int = 4` for the "transport layer" ;)
- tweak some typing and multi-line unpacking in `.ipc/_tcp`.
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