Proper way to shutdown asyncio tasks

Question

I am writing a tool which connects to X number of UNIX sockets, sends a command and saves the output in the local file-system. It runs this every X seconds. In order to perform some cleanup when the tool receives termination signals I register a function(shutdown) to signal.SIGHUP and signal.SIGTERM signals. This function cancels all tasks and then closes the event loop.

My problem is that I get

RuntimeError: Event loop stopped before Future completed

when I send signal.SIGTERM(kill ‘pid’). I have read the documentation about canceling tasks twice but I haven’t spot what I am doing wrong here.

I also noticed something strange, when I send the termination signal the program is in sleep mode and I see in the log that it wakes up the pull_stats() coroutine, you can see this in th first 2 lines of the log.

Log:

21:53:44,194 [23031] [MainThread:supervisor  ] DEBUG    **sleeping for 9.805s secs**
21:53:45,857 [23031] [MainThread:pull_stats  ] INFO     pull statistics
21:53:45,858 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin1.sock
21:53:45,858 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin4.sock
21:53:45,858 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin3.sock
21:53:45,858 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin3.sock
21:53:45,858 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin2.sock
21:53:45,858 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin2.sock
21:53:45,858 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin4.sock
21:53:45,859 [23031] [MainThread:get         ] DEBUG    connecting to UNIX socket /run/haproxy/admin1.sock
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     received stop signal, cancelling tasks...
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,859 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,860 [23031] [MainThread:shutdown    ] INFO     True
21:53:45,860 [23031] [MainThread:shutdown    ] INFO     stopping event loop
21:53:45,860 [23031] [MainThread:shutdown    ] INFO     bye, exiting...
Traceback (most recent call last):
  File "./pull.py", line 249, in <module>
    main()
  File "./pull.py", line 245, in main
    supervisor(loop, config)
  File "./pull.py", line 161, in supervisor
    config['pull']['socket-dir'], storage_dir, loop))
  File "/usr/lib/python3.4/asyncio/base_events.py", line 274, in run_until_complete
    raise RuntimeError('Event loop stopped before Future completed.')
RuntimeError: Event loop stopped before Future completed.

Here is the code:

def shutdown(loop):
    LOGGER.info('received stop signal, cancelling tasks...')
    for task in asyncio.Task.all_tasks():
        LOGGER.info(task.cancel())
    LOGGER.info('stopping event loop')
    loop.stop()
    LOGGER.info('bye, exiting...')


def write_file(filename, data):
    try:
        with open(filename, 'w') as file_handle:
            file_handle.write(data.decode())
    except OSError as exc:
        return False
    else:
        return True


@asyncio.coroutine
def get(socket_file, cmd, storage_dir, loop):
    connect = asyncio.open_unix_connection(socket_file)
    reader, writer = yield from asyncio.wait_for(connect, 1)

    writer.write('{c}n'.format(c=cmd).encode())
    data = yield from reader.read()
    writer.close()

    filename = os.path.basename(socket_file) + '_' + cmd.split()[1]
    filename = os.path.join(storage_dir, filename)
    result = yield from loop.run_in_executor(None, write_file, filename, data)

    return result


@asyncio.coroutine
def pull_stats(socket_dir, storage_dir, loop):
    socket_files = glob.glob(socket_dir + '/*sock*')
    coroutines = [get(socket_file, cmd, storage_dir, loop)
                  for socket_file in socket_files
                  for cmd in CMDS]
    status = yield from asyncio.gather(*coroutines)

    if len(set(status)) == 1 and True in set(status):
        return True
    else:
        return False


def supervisor(loop, config):
    dst_dir = config.get('pull', 'dst-dir')
    tmp_dst_dir = config.get('pull', 'tmp-dst-dir')

    while True:
        start_time = int(time.time())
        storage_dir = os.path.join(tmp_dst_dir, str(start_time))

        try:
            os.makedirs(storage_dir)
        except OSError as exc:
            msg = "failed to create directory {d}:{e}".format(d=storage_dir,
                                                              e=exc)
            LOGGER.critical(msg)

        # Launch all connections.
        result = loop.run_until_complete(pull_stats(
            config['pull']['socket-dir'], storage_dir, loop))

        if result:
            try:
                shutil.move(storage_dir, dst_dir)
            except OSError as exc:
                LOGGER.critical("failed to move %s to %s: %s", storage_dir,
                                dst_dir, exc)
                break
            else:
                LOGGER.info('statistics are saved in %s', os.path.join(
                    dst_dir, os.path.basename(storage_dir)))
        else:
            LOGGER.critical('failed to pull stats')
            shutil.rmtree(storage_dir)

        sleep = config.getint('pull', 'pull-interval') - (time.time() -
                                                          start_time)
        if 0 < sleep < config.getint('pull', 'pull-interval'):
            time.sleep(sleep)
    loop.close()
    sys.exit(1)


def main():
    args = docopt(__doc__, version=VERSION)
    config = ConfigParser(interpolation=ExtendedInterpolation())
    config.read_dict(copy.copy(DEFAULT_OPTIONS))
    config.read(args['--file'])

    loop = asyncio.get_event_loop()

    loop.add_signal_handler(signal.SIGHUP, functools.partial(shutdown, loop))
    loop.add_signal_handler(signal.SIGTERM, functools.partial(shutdown, loop))

    num_level = getattr(logging, config.get('pull', 'loglevel').upper(), None)
    LOGGER.setLevel(num_level)

    supervisor(loop, config)

# This is the standard boilerplate that calls the main() function.
if __name__ == '__main__':
    main()

Asked By: Pavlos Parissis

||

Source

Answer 1

The cancellation is not immediate and requires running ioloop to be resolved with exception CancelledError. Remove ioloop.stop from shutdown and handle exception in supervisor, to make things work. Below simplified example.

Important is, however you can cancel Task, it only stops watching/waiting for end/results and loop won’t handle further events for it. But the underneath request/pipe will not be stopped.

Simplified example:

import asyncio
import functools
import logging
import signal
import sys
from concurrent.futures import CancelledError


def shutdown(loop):
    logging.info('received stop signal, cancelling tasks...')
    for task in asyncio.Task.all_tasks():
        task.cancel()
    logging.info('bye, exiting in a minute...')    


@asyncio.coroutine
def get(i):
    logging.info('sleep for %d', i)
    yield from asyncio.sleep(i)    


@asyncio.coroutine
def pull_stats():
    coroutines = [get(i) for i in range(10,20)]
    status = yield from asyncio.gather(*coroutines)


def supervisor(loop):
    try:
        while True:
            result = loop.run_until_complete(pull_stats())
    except CancelledError:
        logging.info('CancelledError')
    loop.close()
    sys.exit(1)


def main():
    logging.getLogger().setLevel(logging.INFO)
    loop = asyncio.get_event_loop()
    loop.add_signal_handler(signal.SIGHUP, functools.partial(shutdown, loop))
    loop.add_signal_handler(signal.SIGTERM, functools.partial(shutdown, loop))
    supervisor(loop)


if __name__ == '__main__':
    main()

Note, that if you cancel only gather's Future, all children will be set as cancelled as well.

And the sleep thing

Any receipt of a signal or interrupt causes the program to resume execution. So when the process receive SIGTERM and handler is set, python allows you to handle it, to do this thread is resumed and sighandler is called. Due to implementation of ioloop and its signal handling, it keeps running after wake.

Answered By: kwarunek

Answer 2

Update: Code works as expected on python 3.4.4, see my comment below.
@kwarunek, when you mentioned on your last comment about ioloop keeps running I didn’t quite got it as my code worked, killing the process sends a cancellation to all tasks which are wake up. But, now I see your point because canceling the tasks is not triggered with 3.4.4, with 3.4.2 is fine.

21:28:09,004 [59441] [MainThread:supervisor] CRITICAL failed to pull   stats
<killing process>
21:28:11,826 [59441] [MainThread:supervisor] INFO     starting while loop
21:28:11,827 [59441] [MainThread:supervisor] INFO     launch the delegating coroutine
21:28:11,828 [59441] [MainThread:shutdown] INFO     received stop signal
21:28:11,828 [59441] [MainThread:shutdown] INFO     <Task finished coro=<pull_stats() done, defined at /opt/blue-python/3.4/lib/python3.4/site-packages/haproxystats/pull.py:124> result=False>
21:28:11,829 [59441] [MainThread:shutdown] INFO     cancelling task
21:28:11,829 [59441] [MainThread:supervisor] INFO     delegating coroutine finished
21:28:11,829 [59441] [MainThread:supervisor] CRITICAL failed to pull stats
21:28:21,009 [59441] [MainThread:supervisor] INFO     starting while loop
21:28:21,010 [59441] [MainThread:supervisor] INFO     launch the delegating coroutine
21:28:21,011 [59441] [MainThread:supervisor] INFO     delegating coroutine finished
2016-01-30 21:28:21,011 [59441] [MainThread:supervisor] CRITICAL failed to pull stats

while in python 3.4.2

 21:23:51,015 [10219] [MainThread:supervisor] CRITICAL failed to pull stats
 <killing process>
 21:23:55,737 [10219] [MainThread:supervisor] INFO     starting while loop
 21:23:55,737 [10219] [MainThread:supervisor] INFO     launch the delegating coroutine
 21:23:55,740 [10219] [MainThread:shutdown] INFO     received stop signal
 21:23:55,740 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,740 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,740 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,741 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,741 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,741 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,741 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,741 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,741 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,741 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,742 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,742 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,742 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,742 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,742 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,742 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,742 [10219] [MainThread:shutdown] INFO     <Task finished coro=<pull_stats() done, defined at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:124> result=False>
 21:23:55,743 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,743 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future pending cb=[Task._wakeup()]> cb=[gather.<locals>._done_callback(0)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,743 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,743 [10219] [MainThread:shutdown] INFO     <Task finished coro=<pull_stats() done, defined at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:124> result=False>
 21:23:55,744 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,744 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future pending cb=[Task._wakeup()]> cb=[gather.<locals>._done_callback(7)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,744 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,744 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future pending cb=[Task._wakeup()]> cb=[gather.<locals>._done_callback(4)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,745 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,745 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future pending cb=[Task._wakeup()]> cb=[gather.<locals>._done_callback(5)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,745 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,745 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future pendi...sk._wakeup()]>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,746 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,746 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future pending cb=[Task._wakeup()]> cb=[gather.<locals>._done_callback(3)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,746 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,746 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future pendi...sk._wakeup()]>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,747 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,747 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future pending cb=[Task._wakeup()]> cb=[gather.<locals>._done_callback(6)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,747 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,747 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future pendi...sk._wakeup()]>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,747 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,747 [10219] [MainThread:shutdown] INFO     <Task pending coro=<pull_stats() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:150> wait_for=<_GatheringFuture pending cb=[Task._wakeup()]> cb=[_raise_stop_error() at /usr/lib/python3.4/asyncio/base_events.py:101]>
 21:23:55,748 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,748 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future cancelled> cb=[gather.<locals>._done_callback(2)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,748 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,748 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future cancelled>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,748 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,749 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,749 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,749 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,749 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,749 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future cancelled>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,750 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,750 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,750 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,750 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,750 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,751 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future cancelled>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,751 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,751 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,751 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,751 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,751 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,752 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future cancelled>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,752 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,752 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,752 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,752 [10219] [MainThread:shutdown] INFO     <Task finished coro=<open_unix_connection() done, defined at /usr/lib/python3.4/asyncio/streams.py:107> exception=ConnectionRefusedError(111, 'Connection refused')>
 21:23:55,752 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,752 [10219] [MainThread:shutdown] INFO     <Task pending coro=<get() running at /home/pparissis/.virtualenvs/python3/lib/python3.4/site-packages/haproxystats/pull.py:93> wait_for=<Future cancelled> cb=[gather.<locals>._done_callback(1)() at /usr/lib/python3.4/asyncio/tasks.py:582]>
 21:23:55,753 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,753 [10219] [MainThread:shutdown] INFO     <Task pending coro=<open_unix_connection() running at /usr/lib/python3.4/asyncio/streams.py:107> cb=[_release_waiter(<Future cancelled>)() at /usr/lib/python3.4/asyncio/tasks.py:334]>
 21:23:55,753 [10219] [MainThread:shutdown] INFO     cancelling task
 21:23:55,754 [10219] [MainThread:supervisor] INFO     Received CancelledError exception
 21:23:55,754 [10219] [MainThread:supervisor] INFO     waiting for threads to finish any pending IO tasks
 21:23:55,754 [10219] [MainThread:supervisor] INFO     closing our asyncio loop
 21:23:55,755 [10219] [MainThread:supervisor] INFO     exiting with status 0

The main difference is when shutdown() sends the cancellation there are no tasks waken up and as a result the while loop isn’t stopped by the try catch block which handles the cancellation. How do I resolve this now?!

here is the code

def shutdown():
    """Performs a clean shutdown"""
    log.info('received stop signal')
    for task in asyncio.Task.all_tasks():
        log.info(task)
        log.info('cancelling task')
        task.cancel()


def write_file(filename, data):
    """Writes data to a file.

    Returns:
        True if succeeds False otherwise.
    """
    try:
        with open(filename, 'w') as file_handle:
            file_handle.write(data.decode())
    except OSError as exc:
        log.critical('failed to write data %s', exc)
        return False
    else:
        log.debug('data saved in %s', filename)
        return True


@asyncio.coroutine
def get(socket_file, cmd, storage_dir, loop, executor, timeout):
    """Fetches data from a UNIX socket.

    Sends a command to HAProxy over UNIX socket, reads the response and then
    offloads the writing of the received data to a thread, so we don't block
    this coroutine.

    Arguments:
        socket_file (str): The full path of the UNIX socket file to connect to.
        cmd (str): The command to send.
        storage_dir (str): The full path of the directory to save the response.
        loop (obj): A base event loop from asyncio module.
        executor (obj): A Threader executor to execute calls asynchronously.
        timeout (int): Timeout for the connection to the socket.

    Returns:
        True if statistics from a UNIX sockets are save False otherwise.
    """
    # try to connect to the UNIX socket
    connect = asyncio.open_unix_connection(socket_file)
    log.debug('connecting to UNIX socket %s', socket_file)
    try:
        reader, writer = yield from asyncio.wait_for(connect, timeout)
    except (ConnectionRefusedError, PermissionError, OSError) as exc:
        log.critical(exc)
        return False
    else:
        log.debug('connection established to UNIX socket %s', socket_file)

    log.debug('sending command "%s" to UNIX socket %s', cmd, socket_file)
    writer.write('{c}n'.format(c=cmd).encode())
    data = yield from reader.read()
    writer.close()

    if len(data) == 0:
        log.critical('received zero data')
        return False

    log.debug('received data from UNIX socket %s', socket_file)

    suffix = CMD_SUFFIX_MAP.get(cmd.split()[1])
    filename = os.path.basename(socket_file) + suffix
    filename = os.path.join(storage_dir, filename)
    log.debug('going to save data to %s', filename)
    # Offload the writing to a thread so we don't block ourselves.
    result = yield from loop.run_in_executor(executor,
                                             write_file,
                                             filename,
                                             data)

    return result


@asyncio.coroutine
def pull_stats(config, storage_dir, loop, executor):
    """Launches coroutines for pulling statistics from UNIX sockets.

    This a delegating routine.

    Arguments:
        config (obj): A configParser object which holds configuration.
        storage_dir (str): The absolute directory path to save the statistics.
        loop (obj): A base event loop.
        executor(obj): A ThreadPoolExecutor object.

    Returns:
        True if statistics from *all* UNIX sockets are fetched False otherwise.
    """
    # absolute directory path which contains UNIX socket files.
    socket_dir = config.get('pull', 'socket-dir')
    timeout = config.getint('pull', 'timeout')
    socket_files = [f for f in glob.glob(socket_dir + '/*')
                    if is_unix_socket(f)]

    log.debug('pull statistics')
    coroutines = [get(socket_file, cmd, storage_dir, loop, executor, timeout)
                  for socket_file in socket_files
                  for cmd in CMDS]
    # Launch all connections.
    status = yield from asyncio.gather(*coroutines)

    return len(set(status)) == 1 and True in set(status)


def supervisor(loop, config):
    """Coordinates the pulling of HAProxy statistics from UNIX sockets.

    This is the client routine which launches requests to all HAProxy
    UNIX sockets for retrieving statistics and save them to file-system.
    It runs indefinitely until main program is terminated.

    Arguments:
        loop (obj): A base event loop from asyncio module.
        config (obj): A configParser object which holds configuration.
    """
    dst_dir = config.get('pull', 'dst-dir')
    tmp_dst_dir = config.get('pull', 'tmp-dst-dir')
    executor = ThreadPoolExecutor(max_workers=config.getint('pull', 'workers'))
    exit_code = 1

    while True:
        log.info('starting while loop')
        start_time = int(time.time())
        # HAProxy statistics are stored in a directory and we use retrieval
        # time(seconds since the Epoch) as a name of the directory.
        # We first store them in a temporary place until we receive statistics
        # from all UNIX sockets.
        storage_dir = os.path.join(tmp_dst_dir, str(start_time))

        # If our storage directory can't be created we can't do much, thus
        # abort main program.
        try:
            os.makedirs(storage_dir)
        except OSError as exc:
            msg = "failed to make directory {d}:{e}".format(d=storage_dir,
                                                            e=exc)
            log.critical(msg)
            log.critical('a fatal error has occurred, exiting..')
            break

        try:
            log.info('launch the delegating coroutine')
            result = loop.run_until_complete(pull_stats(config, storage_dir,
                                                        loop, executor))
            log.info('delegating coroutine finished')
        except asyncio.CancelledError:
            log.info('Received CancelledError exception')
            exit_code = 0
            break

        # if and only if we received statistics from all sockets then move
        # statistics to the permanent directory.
        # NOTE: when temporary and permanent storage directory are on the same
        # file-system the move is actual a rename, which is an atomic
        # operation.
        if result:
            log.debug('move %s to %s', storage_dir, dst_dir)
            try:
                shutil.move(storage_dir, dst_dir)
            except OSError as exc:
                log.critical("failed to move %s to %s: %s", storage_dir,
                             dst_dir, exc)
                log.critical('a fatal error has occurred, exiting..')
                break
            else:
                log.info('statistics are stored in %s', os.path.join(
                    dst_dir, os.path.basename(storage_dir)))
        else:
            log.critical('failed to pull stats')
            log.debug('removing temporary directory %s', storage_dir)
            shutil.rmtree(storage_dir)

        # calculate sleep time which is interval minus elapsed time.
        sleep = config.getint('pull', 'pull-interval') - (time.time() -
                                                          start_time)
        if 0 < sleep < config.getint('pull', 'pull-interval'):
            log.debug('sleeping for %.3fs secs', sleep)
            time.sleep(sleep)

    # It is very unlikely that threads haven't finished their job by now, but
    # they perform disk IO operations which can take some time in certain
    # situations, thus we want to wait for them in order to perform a clean
    # shutdown.
    log.info('waiting for threads to finish any pending IO tasks')
    executor.shutdown(wait=True)
    log.info('closing our asyncio loop')
    loop.close()
    log.info('exiting with status %s', exit_code)
    sys.exit(exit_code)


def main():
    """Parses CLI arguments and launches main program."""
    args = docopt(__doc__, version=VERSION)

    config = ConfigParser(interpolation=ExtendedInterpolation())
    # Set defaults for all sections
    config.read_dict(copy.copy(DEFAULT_OPTIONS))
    # Load configuration from a file. NOTE: ConfigParser doesn't warn if user
    # sets a filename which doesn't exist, in this case defaults will be used.
    config.read(args['--file'])

    if args['--print']:
        for section in sorted(DEFAULT_OPTIONS):
            print("[{}]".format(section))
            for key, value in sorted(DEFAULT_OPTIONS[section].items()):
                print("{k} = {v}".format(k=key, v=value))
            print()
        sys.exit(0)
    if args['--print-conf']:
        for section in sorted(config):
            print("[{}]".format(section))
            for key, value in sorted(config[section].items()):
                print("{k} = {v}".format(k=key, v=value))
            print()
        sys.exit(0)

    log.setLevel(getattr(logging, config.get('pull', 'loglevel').upper(),
                         None))
    # Setup our event loop
    loop = asyncio.get_event_loop()

    # Register shutdown to signals
    loop.add_signal_handler(signal.SIGHUP, shutdown)
    loop.add_signal_handler(signal.SIGTERM, shutdown)

    # a temporary directory to store fetched data
    tmp_dst_dir = config['pull']['tmp-dst-dir']
    # a permanent directory to move data from the temporary directory. Data are
    # picked up by the process daemon from that directory.
    dst_dir = config['pull']['dst-dir']
    for directory in dst_dir, tmp_dst_dir:
        try:
            os.makedirs(directory)
        except OSError as exc:
            # errno 17 => file exists
            if exc.errno != 17:
                sys.exit("failed to make directory {d}:{e}".format(d=directory,
                                                                   e=exc))
    supervisor(loop, config)

# This is the standard boilerplate that calls the main() function.
if __name__ == '__main__':
    main()

Answered By: Pavlos Parissis

Answer 3

As signals are not available on Windows, the examples showing loop.add_signal_handler(...) will not work on that platform. Below, I provide a skeleton Python 3 script for doing a clean shutdown of asyncio Tasks that should work on any platform.

import sys
import traceback
import asyncio
_verbose = ("--verbose" in sys.argv)
opts = None

# Main
async def main():
    if _verbose: print(f"main() called. opts: {opts}")

    print("... add code here ...")


# Initialization and Startup
def boot(argv):
    global opts
    if _verbose: print(f"boot() called. argv: {argv}")
    # process cli args here
    opts = {"opt1": "val1", "verbose": _verbose}

    # Call main
    try :
        asyncio.get_event_loop().run_until_complete(main())
    
    except Exception as e:
        print(f"Fatal exception: {_get_last_exc()}")
        _exit_program(1)

    if _verbose: print("Script exiting cleanly")
    _exit_program(0)


# Utilities
def _get_last_exc():
    exc_type, exc_value, exc_traceback = sys.exc_info()
    sTB = 'n'.join(traceback.format_tb(exc_traceback))
    return f"{exc_type}n - msg: {exc_value}n stack: {sTB}"


def _exit_program(code=1):
    if _verbose: 
        print(f"_exit_program() called code = {code}")

    # kill all active asyncio Tasks
    if asyncio.Task:
        for task in asyncio.Task.all_tasks():
            if _verbose: print(f'cancelling task: {task}')
            try:
                task.cancel()
            except Exception as ex:
                if _verbose: print(f"Task kill failed: {_get_last_exc()}")
                pass

    # flush stderr and stdout
    sys.stdout.flush()
    sys.stderr.flush()

    # Shut down
    if _verbose: print(f"exiting with code {code}")
    sys.exit(code)


if __name__ == '__main__':
    boot(sys.argv[1:])

Answered By: Timothy C. Quinn

Proper way to shutdown asyncio tasks

Question:

Answers: