While trying to break some code in a project using asyncio, I found that under certain circumstances, asyncio.StreamWriter.drain raises an AssertionError.

1. There must be a lot of concurrent uses of "await writer.drain()"
2. The server on which we send data must be public, no AssertionError occurs while connected to 127.0.0.1

Task exception was never retrieved
future: <Task finished coro=<flooding() done, defined at client.py:10> exception=AssertionError()>
Traceback (most recent call last):
  File "client.py", line 12, in flooding
    await writer.drain()
  File "/usr/local/lib/python3.6/asyncio/streams.py", line 333, in drain
    yield from self._protocol._drain_helper()
  File "/usr/local/lib/python3.6/asyncio/streams.py", line 208, in _drain_helper
    assert waiter is None or waiter.cancelled()
AssertionError

I don't know much about how the drain function is working or how networking is handled by the OS, but I'm assuming that I'v reached some OS limitation which trigger this AssertionError.

I'm not sure how I'm supposed to handle that. Am I supposed to add some throttling because I should not send too much data concurrently? Is this considered as a bug? Any explanations are welcome.

Here some minimal client and server exemples if you want to try to reproduce it:
- Server: https://pastebin.com/SED89pwB
- Client: https://pastebin.com/ikJKHxi9

Also, I don't think this is limited to python 3.6, I'v found this old issue on the aaugustin's websockets repo which looks the same: https://github.com/aaugustin/websockets/issues/16