• uvloop version: 0.14.0, reproducible on master (35f8250)
• Python version: 3.8
• Platform: Linux
• Can you reproduce the bug with PYTHONASYNCIODEBUG in env?: Yes
• Does uvloop behave differently from vanilla asyncio? How?: Yes:

import asyncio
import contextvars

import aiohttp
import aiohttp.web
import uvloop


var = contextvars.ContextVar('uvloop is awesome', default=None)


async def handle(request):
    assert var.get() is not None
    return aiohttp.web.Response(text='OK!')


async def client():
    await asyncio.sleep(1)

    async with aiohttp.ClientSession() as session:
        async with session.get('http://localhost:12345/') as response:
            text = await response.text()
            assert text == 'OK!'


async def setup(app):
    var.set('It truly is!')


async def create_app():
    app = aiohttp.web.Application()
    app.on_startup.append(setup)
    app.add_routes([aiohttp.web.get('/', handle)])

    asyncio.create_task(client())

    return app

if __name__ == '__main__':
    asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
    aiohttp.web.run_app(create_app(), port=12345)

Code works as expected on asyncio loop (var has value in handler) but fails with uvloop. I tracked difference up to connection_made():

  File "uvloop/cbhandles.pyx", line 73, in uvloop.loop.Handle._run
  File "uvloop/handles/tcp.pyx", line 153, in uvloop.loop.TCPTransport._call_connection_made
  File "uvloop/handles/basetransport.pyx", line 134, in uvloop.loop.UVBaseTransport._call_connection_made
  File "uvloop/handles/basetransport.pyx", line 131, in uvloop.loop.UVBaseTransport._call_connection_made
  File "aiohttp/web_protocol.py", line 223, in connection_made

There is similar difference in UVStreamHandler._on_listen:

protocol_handler() is being called without context activation, while with asyncio it receives context. I'm not sure if this can be the root cause.