A weird log line from the asyncio default exception handler: "Resetting connection with an active transaction"

Sometimes our production component logs the following error
21:05:49.897964000 [Error ] [asyncio] Resetting connection with an active transaction <asyncpg.connection.Connection object at 0x7fb1502561f8>

We create a connection pool first.
Then we use async_timeout, prepared statements, explicit transaction and a cursor to get data.
A coroutine that has this logic may be cancelled sometimes, and this is when we may see the error above. However not every
cancellation produces the error. Cancellation happens oftens but we see this error extremely rarely. I wasn't able to build a
minimal example to reproduce it.

We use a connection pool.
Later, in a different coroutine, we use async_timeout, prepared statements, explicit transaction and a cursor.
A coroutine that has this logic may be cancelled sometimes, and this is when we may see the error above. However not every
cancellation produces the error. Cancellation happens oftens but we see this error extremely rarely. I wasn't able to build a
minimal example to reproduce it.

`
pool = await asyncpg.create_pool(...)

        async with async_timeout.timeout(global_delay_sec) as cm:
            async with pool.acquire() as conn:
                stmt = await conn.prepare(sql)
                columns = stmt.get_attributes()
                ...
                async with conn.transaction():
                    cursor = await stmt.cursor()
                    raw_values = await cursor.fetch(max_rows + 1)
                    if len(raw_values) == max_rows + 1:
                        raise Exception(f"A total number of rows exceeded the allowed maximum of {max_rows} rows")

`

• asyncpg version:
  0.20.1

• PostgreSQL version:
  Reproduced with both postgres-11 and 12

• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?:
  No we don't use PostgreSQL SaaS
  I wasn't able to reproce it the issue locally.

• Python version:
  3.6.3

• Platform:
  lsb_release -d
  Description: CentOS Linux release 7.6.1810 (Core)

• Do you use pgbouncer?:
  no

• Did you install asyncpg with pip?:
  yes

• If you built asyncpg locally, which version of Cython did you use?:
  We do not build it locally

• Can the issue be reproduced under both asyncio and
  uvloop?:
  I do use uvloop, I couldn't reproduce it locally. It seems to be an extremely rare issue, happens in our production only once in a month

報告されている pool.acquire、prepared-statement、明示的トランザクション、カーソル、async_timeout のフローから始め、示されている操作中のキャンセルに焦点を当てます。記載されている PostgreSQL のバージョンを使って asyncio と uvloop での挙動を比較し、本番環境でのみ発生するログを説明すべき症状として使用します。まれな reset を再現するか、確認済みの原因と解決策を確立できれば完了です。