RuntimeError when reading from stream right after a cancellation

[vxgmichel]

The following code produces a RuntimeError if the first readline operation times out:

    try:
        return await asyncio.wait_for(reader.readline(), 1.)
    except asyncio.TimeoutError:
        print('Timeout!')
    # await asyncio.sleep(0)
    return await reader.readline()

The corresponding error:

File "/usr/local/lib/python3.5/asyncio/streams.py", line 419, in _wait_for_data
    'already waiting for incoming data' % func_name)
RuntimeError: readline() called while another coroutine is already waiting for 
incoming data

That doesn't happen if the await asyncio.sleep(0) line is uncommented.

That's because wait_for cancels the reader.readline() task but doesn't give the control back to the event loop. Then we try to read again and _wait_for_data check if reader._waiter is None. That fails because the finally clause of the previous _wait_for_data still hasn't cleared reader._waiter.

I guess this could be solved by either checking for cancelled _waiter at the beginning of _wait_for_data or giving the control back to the event loop after a failed wait_for.

[Javieracost]

I can confirm this behavior and I'd say it can be filed as a bug.