Add support for special hostname <broadcast>

[jpbede]

Currently sendto would fail if you use the special hostname <broadcast>, which is a valid use according to the Python docs. This PR proposes to to "resolve" it to the broadcast address before passing it to uv_ip4_addr.

This resolves the issue #540

[fantix]

Thanks for the PR! This is almost a good one, except that __convert_pyaddr_to_sockaddr also affected getaddrinfo():

>>> import uvloop; loop=uvloop.new_event_loop(); loop.run_until_complete(loop.getaddrinfo('<broadcast>', 0, type=socket.SOCK_DGRAM))
[(<AddressFamily.AF_INET: 2>, <SocketKind.SOCK_DGRAM: 2>, 17, '', ('255.255.255.255', 0))]

while CPython won't return 255.255.255.255:

import socket
>>> socket.getaddrinfo('<broadcast>', 0, type=socket.SOCK_DGRAM)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.12/socket.py", line 964, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
socket.gaierror: [Errno -2] Name or service not known

Or asyncio:

>>> import asyncio
>>> loop=asyncio.new_event_loop()
>>> loop.run_until_complete(loop.getaddrinfo('<broadcast>',0,type=socket.SOCK_DGRAM))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/asyncio/base_events.py", line 901, in getaddrinfo
    return await self.run_in_executor(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/socket.py", line 964, in getaddrinfo
    for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
socket.gaierror: [Errno -2] Name or service not known

[jpbede]

Thanks for the review. I moved the "resolving" of <broadcast> to the UDP handle and added a test case for getaddrinfo

[fantix]

Thanks!

[rlippmann]

Any chance of this being merged?