`on_url` not called intermittently?

Reproduce the intermittent failure using the script in test_httptools.py and the ab benchmark described in the report. Start at HttpRequestParser and parser.feed_data, then trace how request data reaches Request.on_url. Done means determining why a valid request can complete without setting on_url_called and adding or updating coverage for that behavior.

From time to time, my server output an error that let me think that Request.on_url is not called for the current stream.

Here is a small script to reproduce the behaviour:

import asyncio

from httptools import HttpRequestParser


class Request:

    def __init__(self):
        self.EOF = False

    def on_url(self, url: bytes):
        self.on_url_called = True

    def on_message_complete(self):
        self.EOF = True


async def serve(reader, writer):
    chunks = 2 ** 16
    req = Request()
    parser = HttpRequestParser(req)
    while True:
        data = await reader.read(chunks)
        parser.feed_data(data)
        if not data or req.EOF:
            break
    assert req.on_url_called
    writer.write(b'HTTP/1.1 200 OK\r\nContent-Length: 2\r\n\r\nOK')
    writer.write_eof()


if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    coro = loop.create_task(asyncio.start_server(serve, '127.0.0.1', 8080))
    server = loop.run_until_complete(coro)
    try:
        loop.run_forever()
    except KeyboardInterrupt:
        print('Bye.')
    finally:
        server.close()
        loop.run_until_complete(server.wait_closed())
        loop.close()

To be run with python file.py

And then benchmark it with something like ab -n 10000 -c 8 'http://127.0.0.1:8080/'

Once every, say, 10000 requests, I get this error:

Task exception was never retrieved
future: <Task finished coro=<serve() done, defined at test_httptools.py:18> exception=AttributeError("'Request' object has no attribute 'on_url_called'",)>
Traceback (most recent call last):
  File "test_httptools.py", line 27, in serve
    assert req.on_url_called
AttributeError: 'Request' object has no attribute 'on_url_called'

Any hint? :)

Thanks!