homepage

Intermittently, when using asyncio.wait_for() with asyncio.open_connection() to cancel the open_connection() task after a timeout period, an asyncio.futures.InvalidStateError is raised. It seems to be a race condition, if the open_connection() call succeeds roughly at the same time as the timeout.
In order to recreate this issue, I used a slightly-modified version of the example at https://docs.python.org/3.4/library/asyncio-stream.html?highlight=open_connection#example
==== example.py ====
import asyncio
import urllib.parse
import sys
@asyncio.coroutine
def print_http_headers(url, timeout):
 url = urllib.parse.urlsplit(url)
 try:
 reader, writer = yield from asyncio.wait_for(asyncio.open_connection(url.hostname, 80), timeout)
 except asyncio.TimeoutError:
 print('Timed out with {}-second timeout'.format(timeout))
 return
 query = ('HEAD {url.path} HTTP/1.0\r\n'
 'Host: {url.hostname}\r\n'
 '\r\n').format(url=url)
 writer.write(query.encode('latin-1'))
 while True:
 line = yield from reader.readline()
 if not line:
 break
 line = line.decode('latin1').rstrip()
 if line:
 print('HTTP header> %s' % line)
 print('Success with {}-second timeout'.format(timeout))
url = sys.argv[1]
loop = asyncio.get_event_loop()
for timeout in range(5, 0, -1):
 task = asyncio.async(print_http_headers(url, timeout/100))
 loop.run_until_complete(task)
loop.close()
==== Output of "./example.py http://www.yahoo.com/" after running multiple times ====
HTTP header> HTTP/1.0 301 Redirect
HTTP header> Date: 2014年5月06日 18:07:43 GMT
HTTP header> Connection: close
HTTP header> Via: http/1.1 ir12.fp.ne1.yahoo.com (ApacheTrafficServer/4.0.2)
HTTP header> Server: ATS
HTTP header> Cache-Control: no-store
HTTP header> Content-Type: text/html; charset=utf-8
HTTP header> Content-Language: en
HTTP header> Location: https://www.yahoo.com/
HTTP header> Content-Length: 214
Success with 0.04-second timeout
Timed out with 0.03-second timeout
Timed out with 0.02-second timeout
Exception in callback <bound method Future.set_result of Future<CANCELLED>>(None,)
handle: Handle(<bound method Future.set_result of Future<CANCELLED>>, (None,))
Traceback (most recent call last):
 File "/opt/python3/lib/python3.4/asyncio/events.py", line 39, in _run
 self._callback(*self._args)
 File "/opt/python3/lib/python3.4/asyncio/futures.py", line 298, in set_result
 raise InvalidStateError('{}: {!r}'.format(self._state, self))
asyncio.futures.InvalidStateError: CANCELLED: Future<CANCELLED>
Timed out with 0.01-second timeout