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

首先，使用 uvloop 和 vanilla asyncio 运行 issue 的 UDP 复现程序，重点关注目标端口为 0 时的 create_datagram_endpoint 和 transport.sendto。比较发送后的 socket 状态以及后续的 recv 行为。完成的标准是：无效的 UDP 目标不会静默关闭 transport，并且该行为由回归测试记录。