AttributeError: 'NoneType' object has no attribute 'cancelled'

• asyncpg version: 0.14.0
• PostgreSQL version: 9.6.6
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: I can't reproduce
• Python version: 3.6.4
• Platform: Alpine Linux
• Do you use pgbouncer?: Yes
• Did you install asyncpg with pip?: Yes
• If you built asyncpg locally, which version of Cython did you use?: 0.27.3
• Can the issue be reproduced under both asyncio and
  uvloop?: No

Same query sometimes breaks with this, meaning the exact query other times works flawlessly, any ideas?

[06:06:23][ERROR] robbie.http.microservice.middleware.errors api.py:__call__:242 | ("Non SQL error happened: 'NoneType' object has no attribute 'cancelled'", AttributeError("'NoneType' object has no attribute 'cancelled'",))
Traceback (most recent call last):
  File "robbie/sql/postgresql/utils.py", line 119, in robbie.sql.postgresql.utils.sql_exceptions_handler.wrapper
  File "robbie/sql/postgresql/client.py", line 119, in fetch
  File "/usr/local/lib/python3.6/site-packages/asyncpg/pool.py", line 405, in fetch
    return await con.fetch(query, *args, timeout=timeout)
  File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 345, in fetch
    return await self._execute(query, args, 0, timeout)
  File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 1289, in _execute
    query, args, limit, timeout, return_status=return_status)
  File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 1297, in __execute
    return await self._do_execute(query, executor, timeout)
  File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 1312, in _do_execute
    stmt = await self._get_statement(query, timeout)
  File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 286, in _get_statement
    statement = await self._protocol.prepare(stmt_name, query, timeout)
  File "asyncpg/protocol/protocol.pyx", line 168, in prepare
  File "asyncpg/protocol/coreproto.pyx", line 122, in asyncpg.protocol.protocol.CoreProtocol._read_server_messages
  File "asyncpg/protocol/coreproto.pyx", line 625, in asyncpg.protocol.protocol.CoreProtocol._push_result
  File "asyncpg/protocol/protocol.pyx", line 818, in asyncpg.protocol.protocol.BaseProtocol._on_result
  File "asyncpg/protocol/protocol.pyx", line 745, in asyncpg.protocol.protocol.BaseProtocol._dispatch_result
AttributeError: 'NoneType' object has no attribute 'cancelled'

Bắt đầu bằng việc lần theo traceback qua asyncpg/pool.py, connection.py và các tệp protocol, tập trung vào đường dẫn phân phối kết quả nơi cancelled được truy cập. Vì báo cáo không có cách tái hiện, trước tiên hãy thu hẹp các điều kiện tạo ra giá trị None dưới asyncio và uvloop, sau đó thêm một regression test cho thấy truy vấn hoàn tất mà không có AttributeError này.