• 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