Pool.close() does not close the pool gracefully

[tomalexander]

• asyncpg version: 0.15.0
• 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 local
• Python version: 3.6.5
• Platform: Arch Linux
• 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?: Yes

The documentation for Pool.close() says that it "Gracefully close all connections in the pool." however if any connection is open then it throws an exception which seems not graceful. I've pasted a minimal example below demonstrating the issue but I believe its caused by Pool.close() setting self._closed immediately, which makes any call to Pool.release() fail because it immediately calls _check_init() which checks the value of the self._closed boolean and throws an exception.

Minimal Example

#!/usr/bin/env python3
#

import asyncio

from asyncpg import create_pool


async def get_pool():
    new_pool = await create_pool(
        'postgresql://localhost:5432/postgres',
        min_size=1,
        max_size=4,
        loop=loop,
    )
    return new_pool


async def hold_connection_open():
    async with pool.acquire() as connection:
        async with connection.transaction():
            connection_has_been_opened.set_result(True)
            asyncio.sleep(3)


async def wait_then_close():
    await connection_has_been_opened
    await pool.close()


loop = asyncio.get_event_loop()
pool = loop.run_until_complete(get_pool())
connection_has_been_opened = asyncio.Future()

asyncio.ensure_future(hold_connection_open())
loop.run_until_complete(wait_then_close())

Output from running example

$ ./bug.py
Task exception was never retrieved
future: <Task finished coro=<hold_connection_open() done, defined at ./bug.py:21> exception=InterfaceError('pool is closed',)>
Traceback (most recent call last):
  File "./bug.py", line 25, in hold_connection_open
    asyncio.sleep(3)
  File "/home/talexander/.virtualenvs/dynamic_async_postgres/lib/python3.6/site-packages/asyncpg/transaction.py", line 91, in __aexit__
    await self.__commit()
  File "/home/talexander/.virtualenvs/dynamic_async_postgres/lib/python3.6/site-packages/asyncpg/transaction.py", line 179, in __commit
    await self._connection.execute(query)
  File "/home/talexander/.virtualenvs/dynamic_async_postgres/lib/python3.6/site-packages/asyncpg/connection.py", line 238, in execute
    return await self._protocol.query(query, timeout)
  File "asyncpg/protocol/protocol.pyx", line 319, in query
  File "asyncpg/protocol/protocol.pyx", line 796, in asyncpg.protocol.protocol.BaseProtocol._dispatch_result
asyncpg.exceptions._base.InternalClientError: got result for unknown protocol state 3

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./bug.py", line 25, in hold_connection_open
    asyncio.sleep(3)
  File "/home/talexander/.virtualenvs/dynamic_async_postgres/lib/python3.6/site-packages/asyncpg/pool.py", line 583, in __aexit__
    await self.pool.release(con)
  File "/home/talexander/.virtualenvs/dynamic_async_postgres/lib/python3.6/site-packages/asyncpg/pool.py", line 497, in release
    self._check_init()
  File "/home/talexander/.virtualenvs/dynamic_async_postgres/lib/python3.6/site-packages/asyncpg/pool.py", line 544, in _check_init
    raise exceptions.InterfaceError('pool is closed')
asyncpg.exceptions._base.InterfaceError: pool is closed

[elprans]

I think close() should wait for all connections to be release()-d before shutting down the pool. Violent shutdown should be done with Pool.terminate().