Silently closes UDP socket

• uvloop version: 0.14
• Python version: 3.8.2
• Platform: Linux
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: Yes
• Does uvloop behave differently from vanilla asyncio? How?: Yes. See below

uvloop silently closes UDP socket when sending data to incorrect destination.

Test program:

import asyncio
import uvloop

# uncomment this line to check it with uvloop
#uvloop.install()


async def main():
    loop = asyncio.get_event_loop()
    transport, proto = await loop.create_datagram_endpoint(
        asyncio.DatagramProtocol,
        local_addr=("192.168.0.2", 0),
    )
    print(transport.get_extra_info('sockname'))
    print("Before sending the message with None destination")
    transport.sendto(b"deadbeef")
    print("After sending the message with None destination")
    
    await asyncio.sleep(0.1)  # let asyncio closes internal socket

    print("Before sending the message with incorrect port destination")
    transport.sendto(b"deadbeef", ("45.83.128.251", 0))
    print("After sending the message with incorrect port destination")
    transport.close()


asyncio.get_event_loop().run_until_complete(main())

asyncio output:

uvloop output:

Then we can cut off first block with sending to None destination. And here we have NEW ONE differece in handling ip:port.

async def main():
    loop = asyncio.get_event_loop()
    transport, proto = await loop.create_datagram_endpoint(
        asyncio.DatagramProtocol,
        local_addr=("192.168.0.2", 0),
    )
    print(transport.get_extra_info('sockname'))
    print("Please use `netstat -au` to see this socket really listening. I will wait 10 sec")
    await asyncio.sleep(10)
    
    print("Before sending the message with incorrect port destination")
    transport.sendto(b"deadbeef", ("45.83.128.251", 0))
    print("After sending the message with incorrect port destination")

    print("Please use `netstat -au` to see this socket STILL really listening. I will wait 10 sec")
    await asyncio.sleep(10)

    transport.close()

When i run it with asyncio event loop all looks fine, socket still present.

When i run it with uvloop it silently closes, and also i can go into infinity wait on recv() call

As i know RFC describe that case as (https://tools.ietf.org/html/rfc8085#section-5.1):

A UDP sender SHOULD NOT use a source port value of zero.  A source
   port number that cannot be easily determined from the address or
   payload type provides protection at the receiver from data injection
   attacks by off-path devices.  A UDP receiver SHOULD NOT bind to port
   zero.

But i got real messages from the internet with that port. I will drop it in my application, but i think uvloop should not silently closes socket. It's very unexpected and differ from asyncio event loop.

Proof from Sentry (https://github.com/spumer/source-query-proxy):

next iteration of recv packet was failed, cause in previous we send response to zero port

Start by running the issue's UDP reproducer with uvloop and vanilla asyncio, focusing on create_datagram_endpoint and transport.sendto with destination port 0. Compare the socket state and subsequent recv behavior after the send. Done means an invalid UDP destination does not silently close the transport, and the behavior is documented by a regression test.