RFC: UNIX server sockets

[Tronic]

Implement high level API for UNIX server sockets, based on #279

This needs some more work but I'd like to invite others for comments at this point.

[decentral1se]

Just tried to figure out how to serve a unix socket and ended up here, thanks for this effort ... I guess my only capacity to help this along is basically to bump the PR with this comment 😅 There is a whole bunch of stuff here I had no idea that should be taken care of when dealing with sockets...

[decentral1se]

Oh yeah, could also take it for a test drive! Saw the following warning when running serve_unix:

TrioDeprecationWarning: subclassing trio.SocketListener is deprecated since Trio 0.15.0; use composition or delegation instead (#1044) class UnixSocketListener(trio.SocketListener):

Seems to do the job 🌞

from functools import partial
from trio import open_nursery, open_unix_socket, run
from trio_socket import serve_unix  # gh:python-trio/trio/pull/1433

SOCK_PATH = "/tmp/temp.sock"

async def client():
    stream = await open_unix_socket(SOCK_PATH)
    async with stream:
        await stream.send_all(b"hello, world")

async def server(stream):
    async for data in stream:
        print(f"received -> {data}")

async def main():
    async with open_nursery() as server_nursery:
        await server_nursery.start(partial(serve_unix, server, SOCK_PATH))
        async with open_nursery() as client_nursery:
            client_nursery.start_soon(client)
        server_nursery.cancel_scope.cancel()

run(main)

[py-radicz]

I am also bumping this PR as I think it would be nice to have also UDS server support - thank you @Tronic + @decentral1se you saved me ton of time figuring this out

[GalaxySnail]

Why is it necessary here?

[CoolCat467]

Not sure, should we try removing this?

[codecov]

Codecov Report

Attention: 110 lines in your changes are missing coverage. Please review.

Comparison is base (78c55aa) 92.94% compared to head (99f17c5) 98.57%.

❗ Current head 99f17c5 differs from pull request most recent head 8e613a0. Consider uploading reports for the commit 8e613a0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1433      +/-   ##
==========================================
+ Coverage   92.94%   98.57%   +5.63%     
==========================================
  Files         117      117              
  Lines       17634    17774     +140     
  Branches     3172     3174       +2     
==========================================
+ Hits        16390    17521    +1131     
+ Misses       1154      204     -950     
+ Partials       90       49      -41     

Files	Coverage Δ
trio/__init__.py	100.00% <100.00%> (ø)
trio/_tests/test_highlevel_open_unix_listeners.py	38.46% <38.46%> (ø)
trio/_highlevel_open_unix_listeners.py	30.88% <30.88%> (ø)

... and 231 files with indirect coverage changes