ioloop.py

#!/usr/bin/env python
#
# Copyright 2009 Facebook
#
# Licensed under the Apache License, Version 2.0 (the "License"); you may
# not use this file except in compliance with the License. You may obtain
# a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
# License for the specific language governing permissions and limitations
# under the License.

"""An I/O event loop for non-blocking sockets.

Typical applications will use a single `IOLoop` object, in the
`IOLoop.instance` singleton.  The `IOLoop.start` method should usually
be called at the end of the ``main()`` function.  Atypical applications may
use more than one `IOLoop`, such as one `IOLoop` per thread, or per `unittest`
case.

In addition to I/O events, the `IOLoop` can also schedule time-based events.
`IOLoop.add_timeout` is a non-blocking alternative to `time.sleep`.
"""

from __future__ import with_statement

import datetime
import errno
import heapq
import os
import logging
import select
import thread
import threading
import time
import traceback

import stack_context

try:
    import signal
except ImportError:
    signal = None

from platform.auto import set_close_exec, Waker


class IOLoop(object):
    """A level-triggered I/O loop.

    We use epoll (Linux) or kqueue (BSD and Mac OS X; requires python
    2.6+) if they are available, or else we fall back on select(). If
    you are implementing a system that needs to handle thousands of
    simultaneous connections, you should use a system that supports either
    epoll or queue.

    Example usage for a simple TCP server::

        import errno
        import functools
        import ioloop
        import socket

        def connection_ready(sock, fd, events):
            while True:
                try:
                    connection, address = sock.accept()
                except socket.error, e:
                    if e.args[0] not in (errno.EWOULDBLOCK, errno.EAGAIN):
                        raise
                    return
                connection.setblocking(0)
                handle_connection(connection, address)

        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM, 0)
        sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        sock.setblocking(0)
        sock.bind(("", port))
        sock.listen(128)

        io_loop = ioloop.IOLoop.instance()
        callback = functools.partial(connection_ready, sock)
        io_loop.add_handler(sock.fileno(), callback, io_loop.READ)
        io_loop.start()

    """
    # Constants from the epoll module
    _EPOLLIN = 0x001
    _EPOLLPRI = 0x002
    _EPOLLOUT = 0x004
    _EPOLLERR = 0x008
    _EPOLLHUP = 0x010
    _EPOLLRDHUP = 0x2000
    _EPOLLONESHOT = (1 << 30)
    _EPOLLET = (1 << 31)

    # Our events map exactly to the epoll events
    NONE = 0
    READ = _EPOLLIN
    WRITE = _EPOLLOUT
    ERROR = _EPOLLERR | _EPOLLHUP | _EPOLLRDHUP

    def __init__(self, impl=None):
        self._impl = impl or _poll()
        if hasattr(self._impl, 'fileno'):
            set_close_exec(self._impl.fileno())
        self._handlers = {}
        self._events = {}
        self._callbacks = []
        self._callback_lock = threading.Lock()
        self._timeouts = []
        self._running = False
        self._stopped = False
        self._thread_ident = None
        self._blocking_signal_threshold = None

        # Create a pipe that we send bogus data to when we want to wake
        # the I/O loop when it is idle
        self._waker = Waker()
        self.add_handler(self._waker.fileno(),
                         lambda fd, events: self._waker.consume(),
                         self.READ)

    @staticmethod
    def instance():
        """Returns a global IOLoop instance.

        Most single-threaded applications have a single, global IOLoop.
        Use this method instead of passing around IOLoop instances
        throughout your code.

        A common pattern for classes that depend on IOLoops is to use
        a default argument to enable programs with multiple IOLoops
        but not require the argument for simpler applications::

            class MyClass(object):
                def __init__(self, io_loop=None):
                    self.io_loop = io_loop or IOLoop.instance()
        """
        if not hasattr(IOLoop, "_instance"):
            IOLoop._instance = IOLoop()
        return IOLoop._instance

    @staticmethod
    def initialized():
        """Returns true if the singleton instance has been created."""
        return hasattr(IOLoop, "_instance")

    def install(self):
        """Installs this IOloop object as the singleton instance.

        This is normally not necessary as `instance()` will create
        an IOLoop on demand, but you may want to call `install` to use
        a custom subclass of IOLoop.
        """
        assert not IOLoop.initialized()
        IOLoop._instance = self

    def close(self, all_fds=False):
        """Closes the IOLoop, freeing any resources used.

        If ``all_fds`` is true, all file descriptors registered on the
        IOLoop will be closed (not just the ones created by the IOLoop itself.
        """
        self.remove_handler(self._waker.fileno())
        if all_fds:
            for fd in self._handlers.keys()[:]:
                try:
                    os.close(fd)
                except Exception:
                    logging.debug("error closing fd %d", fd, exc_info=True)
        self._waker.close()
        self._impl.close()

    def add_handler(self, fd, handler, events):
        """Registers the given handler to receive the given events for fd."""
        self._handlers[fd] = stack_context.wrap(handler)
        self._impl.register(fd, events | self.ERROR)

    def update_handler(self, fd, events):
        """Changes the events we listen for fd."""
        self._impl.modify(fd, events | self.ERROR)

    def remove_handler(self, fd):
        """Stop listening for events on fd."""
        self._handlers.pop(fd, None)
        self._events.pop(fd, None)
        try:
            self._impl.unregister(fd)
        except (OSError, IOError):
            logging.debug("Error deleting fd from IOLoop", exc_info=True)

    def set_blocking_signal_threshold(self, seconds, action):
        """Sends a signal if the ioloop is blocked for more than s seconds.

        Pass seconds=None to disable.  Requires python 2.6 on a unixy
        platform.

        The action parameter is a python signal handler.  Read the
        documentation for the python 'signal' module for more information.
        If action is None, the process will be killed if it is blocked for
        too long.
        """
        if not hasattr(signal, "setitimer"):
            logging.error("set_blocking_signal_threshold requires a signal module "
                       "with the setitimer method")
            return
        self._blocking_signal_threshold = seconds
        if seconds is not None:
            signal.signal(signal.SIGALRM,
                          action if action is not None else signal.SIG_DFL)

    def set_blocking_log_threshold(self, seconds):
        """Logs a stack trace if the ioloop is blocked for more than s seconds.
        Equivalent to set_blocking_signal_threshold(seconds, self.log_stack)
        """
        self.set_blocking_signal_threshold(seconds, self.log_stack)

    def log_stack(self, signal, frame):
        """Signal handler to log the stack trace of the current thread.

        For use with set_blocking_signal_threshold.
        """
        logging.warning('IOLoop blocked for %f seconds in\n%s',
                        self._blocking_signal_threshold,
                        ''.join(traceback.format_stack(frame)))

    def start(self):
        """Starts the I/O loop.

        The loop will run until one of the I/O handlers calls stop(), which
        will make the loop stop after the current event iteration completes.
        """
        if self._stopped:
            self._stopped = False
            return
        self._thread_ident = thread.get_ident()
        self._running = True
        while True:
            # Never use an infinite timeout here - it can stall epoll
            poll_timeout = 0.2

            # Prevent IO event starvation by delaying new callbacks
            # to the next iteration of the event loop.
            with self._callback_lock:
                callbacks = self._callbacks
                self._callbacks = []
            for callback in callbacks:
                self._run_callback(callback)

            if self._timeouts:
                now = time.time()
                while self._timeouts:
                    if self._timeouts[0].callback is None:
                        # the timeout was cancelled
                        heapq.heappop(self._timeouts)
                    elif self._timeouts[0].deadline <= now:
                        timeout = heapq.heappop(self._timeouts)
                        self._run_callback(timeout.callback)
                    else:
                        milliseconds = self._timeouts[0].deadline - now
                        poll_timeout = min(milliseconds, poll_timeout)
                        break

            if self._callbacks:
                # If any callbacks or timeouts called add_callback,
                # we don't want to wait in poll() before we run them.
                poll_timeout = 0.0

            if not self._running:
                break

            if self._blocking_signal_threshold is not None:
                # clear alarm so it doesn't fire while poll is waiting for
                # events.
                signal.setitimer(signal.ITIMER_REAL, 0, 0)

            try:
                event_pairs = self._impl.poll(poll_timeout)
            except Exception, e:
                # Depending on python version and IOLoop implementation,
                # different exception types may be thrown and there are
                # two ways EINTR might be signaled:
                # * e.errno == errno.EINTR
                # * e.args is like (errno.EINTR, 'Interrupted system call')
                if (getattr(e, 'errno', None) == errno.EINTR or
                    (isinstance(getattr(e, 'args', None), tuple) and
                     len(e.args) == 2 and e.args[0] == errno.EINTR)):
                    continue
                else:
                    raise

            if self._blocking_signal_threshold is not None:
                signal.setitimer(signal.ITIMER_REAL,
                                 self._blocking_signal_threshold, 0)

            # Pop one fd at a time from the set of pending fds and run
            # its handler. Since that handler may perform actions on
            # other file descriptors, there may be reentrant calls to
            # this IOLoop that update self._events
            self._events.update(event_pairs)
            while self._events:
                fd, events = self._events.popitem()
                try:
                    self._handlers[fd](fd, events)
                except (OSError, IOError), e:
                    if e.args[0] == errno.EPIPE:
                        # Happens when the client closes the connection
                        pass
                    else:
                        logging.error("Exception in I/O handler for fd %d",
                                      fd, exc_info=True)
                except Exception:
                    logging.error("Exception in I/O handler for fd %d",
                                  fd, exc_info=True)
        # reset the stopped flag so another start/stop pair can be issued
        self._stopped = False
        if self._blocking_signal_threshold is not None:
            signal.setitimer(signal.ITIMER_REAL, 0, 0)

    def stop(self):
        """Stop the loop after the current event loop iteration is complete.
        If the event loop is not currently running, the next call to start()
        will return immediately.

        To use asynchronous methods from otherwise-synchronous code (such as
        unit tests), you can start and stop the event loop like this::

          ioloop = IOLoop()
          async_method(ioloop=ioloop, callback=ioloop.stop)
          ioloop.start()

        ioloop.start() will return after async_method has run its callback,
        whether that callback was invoked before or after ioloop.start.
        """
        self._running = False
        self._stopped = True
        self._waker.wake()

    def running(self):
        """Returns true if this IOLoop is currently running."""
        return self._running

    def add_timeout(self, deadline, callback):
        """Calls the given callback at the time deadline from the I/O loop.

        Returns a handle that may be passed to remove_timeout to cancel.

        ``deadline`` may be a number denoting a unix timestamp (as returned
        by ``time.time()`` or a ``datetime.timedelta`` object for a deadline
        relative to the current time.
        """
        timeout = _Timeout(deadline, stack_context.wrap(callback))
        heapq.heappush(self._timeouts, timeout)
        return timeout

    def remove_timeout(self, timeout):
        """Cancels a pending timeout.

        The argument is a handle as returned by add_timeout.
        """
        # Removing from a heap is complicated, so just leave the defunct
        # timeout object in the queue (see discussion in
        # http://docs.python.org/library/heapq.html).
        # If this turns out to be a problem, we could add a garbage
        # collection pass whenever there are too many dead timeouts.
        timeout.callback = None

    def add_callback(self, callback):
        """Calls the given callback on the next I/O loop iteration.

        It is safe to call this method from any thread at any time.
        Note that this is the *only* method in IOLoop that makes this
        guarantee; all other interaction with the IOLoop must be done
        from that IOLoop's thread.  add_callback() may be used to transfer
        control from other threads to the IOLoop's thread.
        """
        with self._callback_lock:
            list_empty = not self._callbacks
            self._callbacks.append(stack_context.wrap(callback))
        if list_empty and thread.get_ident() != self._thread_ident:
            # If we're in the IOLoop's thread, we know it's not currently
            # polling.  If we're not, and we added the first callback to an
            # empty list, we may need to wake it up (it may wake up on its
            # own, but an occasional extra wake is harmless).  Waking
            # up a polling IOLoop is relatively expensive, so we try to
            # avoid it when we can.
            self._waker.wake()

    def _run_callback(self, callback):
        try:
            callback()
        except Exception:
            self.handle_callback_exception(callback)

    def handle_callback_exception(self, callback):
        """This method is called whenever a callback run by the IOLoop
        throws an exception.

        By default simply logs the exception as an error.  Subclasses
        may override this method to customize reporting of exceptions.

        The exception itself is not passed explicitly, but is available
        in sys.exc_info.
        """
        logging.error("Exception in callback %r", callback, exc_info=True)


class _Timeout(object):
    """An IOLoop timeout, a UNIX timestamp and a callback"""

    # Reduce memory overhead when there are lots of pending callbacks
    __slots__ = ['deadline', 'callback']

    def __init__(self, deadline, callback):
        if isinstance(deadline, (int, long, float)):
            self.deadline = deadline
        elif isinstance(deadline, datetime.timedelta):
            self.deadline = time.time() + _Timeout.timedelta_to_seconds(deadline)
        else:
            raise TypeError("Unsupported deadline %r" % deadline)
        self.callback = callback

    @staticmethod
    def timedelta_to_seconds(td):
        """Equivalent to td.total_seconds() (introduced in python 2.7)."""
        return (td.microseconds + (td.seconds + td.days * 24 * 3600) * 10**6) / float(10**6)

    # Comparison methods to sort by deadline, with object id as a tiebreaker
    # to guarantee a consistent ordering.  The heapq module uses __le__
    # in python2.5, and __lt__ in 2.6+ (sort() and most other comparisons
    # use __lt__).
    def __lt__(self, other):
        return ((self.deadline, id(self)) <
                (other.deadline, id(other)))

    def __le__(self, other):
        return ((self.deadline, id(self)) <=
                (other.deadline, id(other)))


class PeriodicCallback(object):
    """Schedules the given callback to be called periodically.

    The callback is called every callback_time milliseconds.

    `start` must be called after the PeriodicCallback is created.
    """
    def __init__(self, callback, callback_time, io_loop=None):
        self.callback = callback
        self.callback_time = callback_time
        self.io_loop = io_loop or IOLoop.instance()
        self._running = False

    def start(self):
        """Starts the timer."""
        self._running = True
        self._next_timeout = time.time()
        self._schedule_next()

    def stop(self):
        """Stops the timer."""
        self._running = False

    def _run(self):
        if not self._running: return
        try:
            self.callback()
        except Exception:
            logging.error("Error in periodic callback", exc_info=True)
        self._schedule_next()

    def _schedule_next(self):
        if self._running:
            current_time = time.time()
            while self._next_timeout < current_time:
                self._next_timeout += self.callback_time / 1000.0
            self.io_loop.add_timeout(self._next_timeout, self._run)


class _EPoll(object):
    """An epoll-based event loop using our C module for Python 2.5 systems"""
    _EPOLL_CTL_ADD = 1
    _EPOLL_CTL_DEL = 2
    _EPOLL_CTL_MOD = 3

    def __init__(self):
        self._epoll_fd = epoll.epoll_create()

    def fileno(self):
        return self._epoll_fd

    def close(self):
        os.close(self._epoll_fd)

    def register(self, fd, events):
        epoll.epoll_ctl(self._epoll_fd, self._EPOLL_CTL_ADD, fd, events)

    def modify(self, fd, events):
        epoll.epoll_ctl(self._epoll_fd, self._EPOLL_CTL_MOD, fd, events)

    def unregister(self, fd):
        epoll.epoll_ctl(self._epoll_fd, self._EPOLL_CTL_DEL, fd, 0)

    def poll(self, timeout):
        return epoll.epoll_wait(self._epoll_fd, int(timeout * 1000))


class _KQueue(object):
    """A kqueue-based event loop for BSD/Mac systems."""
    def __init__(self):
        self._kqueue = select.kqueue()
        self._active = {}

    def fileno(self):
        return self._kqueue.fileno()

    def close(self):
        self._kqueue.close()

    def register(self, fd, events):
        self._control(fd, events, select.KQ_EV_ADD)
        self._active[fd] = events

    def modify(self, fd, events):
        self.unregister(fd)
        self.register(fd, events)

    def unregister(self, fd):
        events = self._active.pop(fd)
        self._control(fd, events, select.KQ_EV_DELETE)

    def _control(self, fd, events, flags):
        kevents = []
        if events & IOLoop.WRITE:
            kevents.append(select.kevent(
                    fd, filter=select.KQ_FILTER_WRITE, flags=flags))
        if events & IOLoop.READ or not kevents:
            # Always read when there is not a write
            kevents.append(select.kevent(
                    fd, filter=select.KQ_FILTER_READ, flags=flags))
        # Even though control() takes a list, it seems to return EINVAL
        # on Mac OS X (10.6) when there is more than one event in the list.
        for kevent in kevents:
            self._kqueue.control([kevent], 0)

    def poll(self, timeout):
        kevents = self._kqueue.control(None, 1000, timeout)
        events = {}
        for kevent in kevents:
            fd = kevent.ident
            if kevent.filter == select.KQ_FILTER_READ:
                events[fd] = events.get(fd, 0) | IOLoop.READ
            if kevent.filter == select.KQ_FILTER_WRITE:
                if kevent.flags & select.KQ_EV_EOF:
                    # If an asynchronous connection is refused, kqueue
                    # returns a write event with the EOF flag set.
                    # Turn this into an error for consistency with the
                    # other IOLoop implementations.
                    # Note that for read events, EOF may be returned before
                    # all data has been consumed from the socket buffer,
                    # so we only check for EOF on write events.
                    events[fd] = IOLoop.ERROR
                else:
                    events[fd] = events.get(fd, 0) | IOLoop.WRITE
            if kevent.flags & select.KQ_EV_ERROR:
                events[fd] = events.get(fd, 0) | IOLoop.ERROR
        return events.items()


class _Select(object):
    """A simple, select()-based IOLoop implementation for non-Linux systems"""
    def __init__(self):
        self.read_fds = set()
        self.write_fds = set()
        self.error_fds = set()
        self.fd_sets = (self.read_fds, self.write_fds, self.error_fds)

    def close(self):
        pass

    def register(self, fd, events):
        if events & IOLoop.READ: self.read_fds.add(fd)
        if events & IOLoop.WRITE: self.write_fds.add(fd)
        if events & IOLoop.ERROR:
            self.error_fds.add(fd)
            # Closed connections are reported as errors by epoll and kqueue,
            # but as zero-byte reads by select, so when errors are requested
            # we need to listen for both read and error.
            self.read_fds.add(fd)

    def modify(self, fd, events):
        self.unregister(fd)
        self.register(fd, events)

    def unregister(self, fd):
        self.read_fds.discard(fd)
        self.write_fds.discard(fd)
        self.error_fds.discard(fd)

    def poll(self, timeout):
        readable, writeable, errors = select.select(
            self.read_fds, self.write_fds, self.error_fds, timeout)
        events = {}
        for fd in readable:
            events[fd] = events.get(fd, 0) | IOLoop.READ
        for fd in writeable:
            events[fd] = events.get(fd, 0) | IOLoop.WRITE
        for fd in errors:
            events[fd] = events.get(fd, 0) | IOLoop.ERROR
        return events.items()


# Choose a poll implementation. Use epoll if it is available, fall back to
# select() for non-Linux platforms
if hasattr(select, "epoll"):
    # Python 2.6+ on Linux
    _poll = select.epoll
elif hasattr(select, "kqueue"):
    # Python 2.6+ on BSD or Mac
    _poll = _KQueue
else:
    try:
        # Linux systems with our C module installed
        import epoll
        _poll = _EPoll
    except Exception:
        # All other systems
        import sys
        if "linux" in sys.platform:
            logging.warning("epoll module not found; using select()")
        _poll = _Select