ConnectionDoesNotExistError when releasing

• asyncpg version: 0.21.0
• PostgreSQL version: 10.7
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: no
• Python version: 3.7
• Platform: debian (docker image python:3.7.9-slim)
• Do you use pgbouncer?: no
• Did you install asyncpg with pip?: yes
• If you built asyncpg locally, which version of Cython did you use?: no
• Can the issue be reproduced under both asyncio and
  uvloop?: only tried asyncio

I've found the following exception in our logs:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 214, in release
    self._con.terminate()
AttributeError: 'NoneType' object has no attribute 'terminate'

During handling of the above exception, another exception occurred:
File "/usr/local/lib/python3.7/site-packages/my_code_calling_release.py", line 40, in acquire
     await self.pool.release(con)
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 654, in release
     return await asyncio.shield(ch.release(timeout))   
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 216, in release
     raise ex   
File "/usr/local/lib/python3.7/site-packages/asyncpg/pool.py", line 206, in release
     await self._con.reset(timeout=budget)
File "/usr/local/lib/python3.7/site-packages/asyncpg/connection.py", line 1137, in reset
     await self.execute(reset_query, timeout=timeout)
File "/usr/local/lib/python3.7/site-packages/asyncpg/connection.py", line 295, in execute
     return await self._protocol.query(query, timeout)
File "asyncpg/protocol/protocol.pyx", line 316, in query

From what I can see, the call at self._con.reset is raising a ConnectionDoesNotExistError.
Should the exception be reraised at line 218 if the exception is an ConnectionDoesNotExistError (we know it's already closed)? It shouldn't even try to self._con.terminate() because self._con is already None (see the exception).

Not sure if the comments in that code still apply for that type of exception

Lee la ruta de release alrededor de las líneas 206-218 de asyncpg/pool.py y la ruta de reset en asyncpg/connection.py, centrándote en cómo ConnectionDoesNotExistError deja self._con como None. Reproduce o añade un caso de regresión para liberar una conexión ya cerrada y, después, verifica que release no intente terminar una conexión inexistente y preserve el comportamiento de excepción previsto.