InterfaceError On Exception Inside PoolAcquireContext But Outside Transaction #232

• asyncpg version: 0.13.0
• PostgreSQL version: 9.6.3
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?: Using local install.
• Python version: 3.6.2
• Platform: Client on macOS 10.12.6, PostgreSQL inside linux x86_64 docker container.
• Do you use pgbouncer?: No.
• Did you install asyncpg with pip?: Yes.
• If you built asyncpg locally, which version of Cython did you use?: Not built locally.
• Can the issue be reproduced under both asyncio and
  uvloop?: Unsure.

Unexpected Behaviour Example

If an exception occurs outside of a transaction block but within a pool acquire block, an InterfaceError is raised.

import asyncpg
import asyncio

loop = asyncio.get_event_loop()

async def iterate(con):
    async with con.transaction():
        async for record in con.cursor("SELECT 1"):
            yield record

async def run():
    pool = await asyncpg.create_pool(user='postgres')
    async with pool.acquire() as con:
        async for _ in iterate(con):
            raise Exception()

loop.run_until_complete(run())

This will cause asyncpg.exceptions._base.InterfaceError: cannot perform operation: another operation is in progress which I did not expect.

Expected Behaviour Example

If the transaction block is moved to encompass the exception, this behaves as expected.

import asyncpg
import asyncio

loop = asyncio.get_event_loop()

async def iterate(con):
    async for record in con.cursor("SELECT 1"):
        yield record

async def run():
    pool = await asyncpg.create_pool(user='postgres')
    async with pool.acquire() as con:
        async with con.transaction():
            async for _ in iterate(con):
                raise Exception()

loop.run_until_complete(run())

This will cause Exception as I expected.