Fetching from pgpool-II hangs indefinitely

• asyncpg version: 0.20.1
• PostgreSQL version: 12.2
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: No.
• Python version: Python 3.7.7 (default, Apr 23 2020, 14:43:26)
• Platform: [GCC 8.3.0] on linux
• Do you use pgbouncer?: No, I use pgpool-II 4.1.1
• Did you install asyncpg with pip?: No, with poetry
• If you built asyncpg locally, which version of Cython did you use?: No
• Can the issue be reproduced under both asyncio and
  uvloop?: Yes

When querying the database through pgpool-II it hangs indefinitely, without raising any kind of exception. Connecting to database seems to be working, since for instance invalid password raises an exception:

asyncpg.exceptions.InternalServerError: md5 authentication failed
DETAIL:  password does not match

Steps to reproduce:

import asyncio
import asyncpg

async def run():
    conn = await asyncpg.connect(user='user', password='password',
                                 database='database', host='127.0.0.1') # connect to pgpool-II
    values = await conn.fetch('''SELECT 1''') # hangs here indefinitely
    await conn.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(run())

When connecting to the PostgreSQL directly, the example above works flawlessly.

Log from pgpool-II:

May 13 20:56:00 pg01 pgpool[11982]: 2020-05-13 20:56:00: pid 13189: ERROR:  unable to read data from frontend
May 13 20:56:00 pg01 pgpool[11982]: 2020-05-13 20:56:00: pid 13189: DETAIL:  EOF encountered with frontend

从提供的 Python 复现开始，分别通过 pgpool-II 和直接对 PostgreSQL 运行它，以确认行为在哪些地方不同。以 pgpool-II 日志为依据，调查 asyncpg connection 和 conn.fetch 路径；当 SELECT 1 完成，或不再无限期挂起而是抛出有用的 exception 时，即视为完成。