connection.close is throwing protocol error

まだ誰も着手していません。

asyncpg version: 0.18.1
PostgreSQL version: 9.5.10
Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?: using google cloud sql pg instance
Python version: 3.6.5
Platform: Ubuntu
Do you use pgbouncer?: No
Did you install asyncpg with pip?: Yes
If you built asyncpg locally, which version of Cython did you use?:
Can the issue be reproduced under both asyncio and
uvloop?: No

I have been facing this error for quite sometime intermittently `asyncpg.exceptions._base.InternalClientError: got result for unknown protocol state 0`

The entire stack trace:
```
File "/repo/server/schema_db.py", line 345, in read
await connection.close()

File "/usr/local/lib/python3.6/dist-packages/asyncpg/connection.py", line 1082, in close
await self._protocol.close(timeout)

File "asyncpg/protocol/protocol.pyx", line 560, in close

File "asyncpg/protocol/protocol.pyx", line 814, in asyncpg.protocol.protocol.BaseProtocol._dispatch_result

asyncpg.exceptions._base.InternalClientError: got result for unknown protocol state 0
```

Can someone explain when this exception is thrown and for what reason?
If i know the reason this exception is thrown, i can figure out the issue myself.
I'm not able to reproduce locally or in my stage environment.

schema_db.py:345 で失敗している await connection.close() から始め、報告されたスタックトレースとバージョンの詳細を使って、asyncpg/protocol/protocol.pyx の close と BaseProtocol._dispatch_result を調べます。Google Cloud SQL PostgreSQL インスタンスで不明なプロトコル状態を引き起こす断続的な条件を特定します。原因と、信頼できる再現または検証の手順が確立されれば、issue は完了です。

索引モデルが issue の本文から書いたものです。