[issue31096] asyncio.stream.FlowControlMixin._drain_helper may lead to a blocking behavior

[Dmitry Malinovsky]

New submission from Dmitry Malinovsky:

When there is a huge amount of `writer.write` calls followed by `await writer.drain()` on a non-paused channel, and there are no other coroutine switches, `await writer.drain()` immediately returns without a switch. This is because `asyncio.stream.FlowControlMixin._drain_helper` do not `yield` or `yield from` on a non-paused stream.

Use-case: AMQP basic.publish method, for which the broker (rabbitmq) do not send any replies back. Trying to publish 4k messages results in the following warnings (PYTHONASYNCIODEBUG env variable is set):
`Executing <Handle <TaskWakeupMethWrapper object at 0x1106fde28>(<Future finis...events.py:275>) created at /Users/malinoff/Projects/ideas/amqproto/amqproto/channel.py:85> took 2.371 seconds`
2.371 seconds is the time spent on 4k `basic_publish` calls.

You can find the test itself on github: https://github.com/malinoff/amqproto/blob/master/tests/stress/test_4k_msgs.py#L11-L12

An easy fix would be to replace return (https://github.com/python/cpython/blob/master/Lib/asyncio/streams.py#L206) with yield (and but the code below under the else clause; I'm willing to prepare a pull request), but maybe I'm missing something and such behavior is intentional?

----------
components: asyncio
messages: 299610
nosy: Dmitry Malinovsky, yselivanov
priority: normal
severity: normal
status: open
title: asyncio.stream.FlowControlMixin._drain_helper may lead to a blocking behavior
versions: Python 3.5, Python 3.6, Python 3.7

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue31096>
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