# Source code for tornado.ioloop

    #!/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 absolute_import, division, print_function

    import collections
    import datetime
    import errno
    import functools
    import heapq
    import itertools
    import logging
    import numbers
    import os
    import select
    import sys
    import threading
    import time
    import traceback
    import math

    from tornado4.concurrent import TracebackFuture, is_future
    from tornado4.log import app_log, gen_log
    from tornado4.platform.auto import set_close_exec, Waker
    from tornado4 import stack_context
    from tornado4.util import PY3, Configurable, errno_from_exception, timedelta_to_seconds

    try:
        import signal
    except ImportError:
        signal = None

    if PY3:
        import _thread as thread
    else:
        import thread

    _POLL_TIMEOUT = 3600.0

    class TimeoutError(Exception):
        pass

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

        We use ``epoll`` (Linux) or ``kqueue`` (BSD and Mac OS X) 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 ``kqueue``.

        Example usage for a simple TCP server:

        .. testcode::

            import errno
            import functools
            import tornado4.ioloop
            import socket

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

            if __name__ == '__main__':
                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 = tornado4.ioloop.IOLoop.current()
                callback = functools.partial(connection_ready, sock)
                io_loop.add_handler(sock.fileno(), callback, io_loop.READ)
                io_loop.start()

        .. testoutput::
           :hide:

        By default, a newly-constructed `IOLoop` becomes the thread's current
        `IOLoop`, unless there already is a current `IOLoop`. This behavior
        can be controlled with the ``make_current`` argument to the `IOLoop`
        constructor: if ``make_current=True``, the new `IOLoop` will always
        try to become current and it raises an error if there is already a
        current instance. If ``make_current=False``, the new `IOLoop` will
        not try to become current.

        .. versionchanged:: 4.2
           Added the ``make_current`` keyword argument to the `IOLoop`
           constructor.
        """
        # 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

        # Global lock for creating global IOLoop instance
        _instance_lock = threading.Lock()

        _current = threading.local()

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

            Most applications have a single, global `IOLoop` running on the
            main thread.  Use this method to get this instance from
            another thread.  In most other cases, it is better to use `current()`
            to get the current thread's `IOLoop`.
            """
            if not hasattr(IOLoop, "_instance"):
                with IOLoop._instance_lock:
                    if not hasattr(IOLoop, "_instance"):
                        # New instance after double check
                        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`.

            When using an `IOLoop` subclass, `install` must be called prior
            to creating any objects that implicitly create their own
            `IOLoop` (e.g., :class:`tornado4.httpclient.AsyncHTTPClient`).
            """
            assert not IOLoop.initialized()
            IOLoop._instance = self

        @staticmethod
        def clear_instance():
            """Clear the global `IOLoop` instance.

            .. versionadded:: 4.0
            """
            if hasattr(IOLoop, "_instance"):
                del IOLoop._instance

        @staticmethod
        def current(instance=True):
            """Returns the current thread's `IOLoop`.

            If an `IOLoop` is currently running or has been marked as
            current by `make_current`, returns that instance.  If there is
            no current `IOLoop`, returns `IOLoop.instance()` (i.e. the
            main thread's `IOLoop`, creating one if necessary) if ``instance``
            is true.

            In general you should use `IOLoop.current` as the default when
            constructing an asynchronous object, and use `IOLoop.instance`
            when you mean to communicate to the main thread from a different
            one.

            .. versionchanged:: 4.1
               Added ``instance`` argument to control the fallback to
               `IOLoop.instance()`.
            """
            current = getattr(IOLoop._current, "instance", None)
            if current is None and instance:
                return IOLoop.instance()
            return current

        def make_current(self):
            """Makes this the `IOLoop` for the current thread.

            An `IOLoop` automatically becomes current for its thread
            when it is started, but it is sometimes useful to call
            `make_current` explicitly before starting the `IOLoop`,
            so that code run at startup time can find the right
            instance.

            .. versionchanged:: 4.1
               An `IOLoop` created while there is no current `IOLoop`
               will automatically become current.
            """
            IOLoop._current.instance = self

        @staticmethod
        def clear_current():
            IOLoop._current.instance = None

        @classmethod
        def configurable_base(cls):
            return IOLoop

        @classmethod
        def configurable_default(cls):
            if hasattr(select, "epoll"):
                from tornado4.platform.epoll import EPollIOLoop
                return EPollIOLoop
            if hasattr(select, "kqueue"):
                # Python 2.6+ on BSD or Mac
                from tornado4.platform.kqueue import KQueueIOLoop
                return KQueueIOLoop
            from tornado4.platform.select import SelectIOLoop
            return SelectIOLoop

        def initialize(self, make_current=None):
            if make_current is None:
                if IOLoop.current(instance=False) is None:
                    self.make_current()
            elif make_current:
                if IOLoop.current(instance=False) is not None:
                    raise RuntimeError("current IOLoop already exists")
                self.make_current()

        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).

            Many applications will only use a single `IOLoop` that runs for the
            entire lifetime of the process.  In that case closing the `IOLoop`
            is not necessary since everything will be cleaned up when the
            process exits.  `IOLoop.close` is provided mainly for scenarios
            such as unit tests, which create and destroy a large number of
            ``IOLoops``.

            An `IOLoop` must be completely stopped before it can be closed.  This
            means that `IOLoop.stop()` must be called *and* `IOLoop.start()` must
            be allowed to return before attempting to call `IOLoop.close()`.
            Therefore the call to `close` will usually appear just after
            the call to `start` rather than near the call to `stop`.

            .. versionchanged:: 3.1
               If the `IOLoop` implementation supports non-integer objects
               for "file descriptors", those objects will have their
               ``close`` method when ``all_fds`` is true.
            """
            raise NotImplementedError()

        def add_handler(self, fd, handler, events):
            """Registers the given handler to receive the given events for ``fd``.

            The ``fd`` argument may either be an integer file descriptor or
            a file-like object with a ``fileno()`` method (and optionally a
            ``close()`` method, which may be called when the `IOLoop` is shut
            down).

            The ``events`` argument is a bitwise or of the constants
            ``IOLoop.READ``, ``IOLoop.WRITE``, and ``IOLoop.ERROR``.

            When an event occurs, ``handler(fd, events)`` will be run.

            .. versionchanged:: 4.0
               Added the ability to pass file-like objects in addition to
               raw file descriptors.
            """
            raise NotImplementedError()

        def update_handler(self, fd, events):
            """Changes the events we listen for ``fd``.

            .. versionchanged:: 4.0
               Added the ability to pass file-like objects in addition to
               raw file descriptors.
            """
            raise NotImplementedError()

        def remove_handler(self, fd):
            """Stop listening for events on ``fd``.

            .. versionchanged:: 4.0
               Added the ability to pass file-like objects in addition to
               raw file descriptors.
            """
            raise NotImplementedError()

        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 `signal` module for more information.
            If ``action`` is None, the process will be killed if it is
            blocked for too long.
            """
            raise NotImplementedError()

        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`.
            """
            gen_log.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 callbacks calls `stop()`, which
            will make the loop stop after the current event iteration completes.
            """
            raise NotImplementedError()

        def _setup_logging(self):
            """The IOLoop catches and logs exceptions, so it's
            important that log output be visible.  However, python's
            default behavior for non-root loggers (prior to python
            3.2) is to print an unhelpful "no handlers could be
            found" message rather than the actual log entry, so we
            must explicitly configure logging if we've made it this
            far without anything.

            This method should be called from start() in subclasses.
            """
            if not any([logging.getLogger().handlers,
                        logging.getLogger('tornado').handlers,
                        logging.getLogger('tornado4.application').handlers]):
                logging.basicConfig()

        def stop(self):
            """Stop the I/O loop.

            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``.

            Note that even after `stop` has been called, the `IOLoop` is not
            completely stopped until `IOLoop.start` has also returned.
            Some work that was scheduled before the call to `stop` may still
            be run before the `IOLoop` shuts down.
            """
            raise NotImplementedError()

        def run_sync(self, func, timeout=None):
            """Starts the `IOLoop`, runs the given function, and stops the loop.

            The function must return either a yieldable object or
            ``None``. If the function returns a yieldable object, the
            `IOLoop` will run until the yieldable is resolved (and
            `run_sync()` will return the yieldable's result). If it raises
            an exception, the `IOLoop` will stop and the exception will be
            re-raised to the caller.

            The keyword-only argument ``timeout`` may be used to set
            a maximum duration for the function.  If the timeout expires,
            a `TimeoutError` is raised.

            This method is useful in conjunction with `tornado4.gen.coroutine`
            to allow asynchronous calls in a ``main()`` function::

                @gen.coroutine
                def main():
                    # do stuff...

                if __name__ == '__main__':
                    IOLoop.current().run_sync(main)

            .. versionchanged:: 4.3
               Returning a non-``None``, non-yieldable value is now an error.
            """
            future_cell = [None]

            def run():
                try:
                    result = func()
                    if result is not None:
                        from tornado4.gen import convert_yielded
                        result = convert_yielded(result)
                except Exception:
                    future_cell[0] = TracebackFuture()
                    future_cell[0].set_exc_info(sys.exc_info())
                else:
                    if is_future(result):
                        future_cell[0] = result
                    else:
                        future_cell[0] = TracebackFuture()
                        future_cell[0].set_result(result)
                self.add_future(future_cell[0], lambda future: self.stop())
            self.add_callback(run)
            if timeout is not None:
                timeout_handle = self.add_timeout(self.time() + timeout, self.stop)
            self.start()
            if timeout is not None:
                self.remove_timeout(timeout_handle)
            if not future_cell[0].done():
                raise TimeoutError('Operation timed out after %s seconds' % timeout)
            return future_cell[0].result()

        def time(self):
            """Returns the current time according to the `IOLoop`'s clock.

            The return value is a floating-point number relative to an
            unspecified time in the past.

            By default, the `IOLoop`'s time function is `time.time`.  However,
            it may be configured to use e.g. `time.monotonic` instead.
            Calls to `add_timeout` that pass a number instead of a
            `datetime.timedelta` should use this function to compute the
            appropriate time, so they can work no matter what time function
            is chosen.
            """
            return time.time()

        def add_timeout(self, deadline, callback, *args, **kwargs):
            """Runs the ``callback`` at the time ``deadline`` from the I/O loop.

            Returns an opaque handle that may be passed to
            `remove_timeout` to cancel.

            ``deadline`` may be a number denoting a time (on the same
            scale as `IOLoop.time`, normally `time.time`), or a
            `datetime.timedelta` object for a deadline relative to the
            current time.  Since Tornado 4.0, `call_later` is a more
            convenient alternative for the relative case since it does not
            require a timedelta object.

            Note that it is not safe to call `add_timeout` from other threads.
            Instead, you must use `add_callback` to transfer control to the
            `IOLoop`'s thread, and then call `add_timeout` from there.

            Subclasses of IOLoop must implement either `add_timeout` or
            `call_at`; the default implementations of each will call
            the other.  `call_at` is usually easier to implement, but
            subclasses that wish to maintain compatibility with Tornado
            versions prior to 4.0 must use `add_timeout` instead.

            .. versionchanged:: 4.0
               Now passes through ``*args`` and ``**kwargs`` to the callback.
            """
            if isinstance(deadline, numbers.Real):
                return self.call_at(deadline, callback, *args, **kwargs)
            elif isinstance(deadline, datetime.timedelta):
                return self.call_at(self.time() + timedelta_to_seconds(deadline),
                                    callback, *args, **kwargs)
            else:
                raise TypeError("Unsupported deadline %r" % deadline)

        def call_later(self, delay, callback, *args, **kwargs):
            """Runs the ``callback`` after ``delay`` seconds have passed.

            Returns an opaque handle that may be passed to `remove_timeout`
            to cancel.  Note that unlike the `asyncio` method of the same
            name, the returned object does not have a ``cancel()`` method.

            See `add_timeout` for comments on thread-safety and subclassing.

            .. versionadded:: 4.0
            """
            return self.call_at(self.time() + delay, callback, *args, **kwargs)

        def call_at(self, when, callback, *args, **kwargs):
            """Runs the ``callback`` at the absolute time designated by ``when``.

            ``when`` must be a number using the same reference point as
            `IOLoop.time`.

            Returns an opaque handle that may be passed to `remove_timeout`
            to cancel.  Note that unlike the `asyncio` method of the same
            name, the returned object does not have a ``cancel()`` method.

            See `add_timeout` for comments on thread-safety and subclassing.

            .. versionadded:: 4.0
            """
            return self.add_timeout(when, callback, *args, **kwargs)

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

            The argument is a handle as returned by `add_timeout`.  It is
            safe to call `remove_timeout` even if the callback has already
            been run.
            """
            raise NotImplementedError()

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

            It is safe to call this method from any thread at any time,
            except from a signal handler.  Note that this is the **only**
            method in `IOLoop` that makes this thread-safety 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.

            To add a callback from a signal handler, see
            `add_callback_from_signal`.
            """
            raise NotImplementedError()

        def add_callback_from_signal(self, callback, *args, **kwargs):
            """Calls the given callback on the next I/O loop iteration.

            Safe for use from a Python signal handler; should not be used
            otherwise.

            Callbacks added with this method will be run without any
            `.stack_context`, to avoid picking up the context of the function
            that was interrupted by the signal.
            """
            raise NotImplementedError()

        def spawn_callback(self, callback, *args, **kwargs):
            """Calls the given callback on the next IOLoop iteration.

            Unlike all other callback-related methods on IOLoop,
            ``spawn_callback`` does not associate the callback with its caller's
            ``stack_context``, so it is suitable for fire-and-forget callbacks
            that should not interfere with the caller.

            .. versionadded:: 4.0
            """
            with stack_context.NullContext():
                self.add_callback(callback, *args, **kwargs)

        def add_future(self, future, callback):
            """Schedules a callback on the ``IOLoop`` when the given
            `.Future` is finished.

            The callback is invoked with one argument, the
            `.Future`.
            """
            assert is_future(future)
            callback = stack_context.wrap(callback)
            future.add_done_callback(
                lambda future: self.add_callback(callback, future))

        def _run_callback(self, callback):
            """Runs a callback with error handling.

            For use in subclasses.
            """
            try:
                ret = callback()
                if ret is not None:
                    from tornado4 import gen
                    # Functions that return Futures typically swallow all
                    # exceptions and store them in the Future.  If a Future
                    # makes it out to the IOLoop, ensure its exception (if any)
                    # gets logged too.
                    try:
                        ret = gen.convert_yielded(ret)
                    except gen.BadYieldError:
                        # It's not unusual for add_callback to be used with
                        # methods returning a non-None and non-yieldable
                        # result, which should just be ignored.
                        pass
                    else:
                        self.add_future(ret, self._discard_future_result)
            except Exception:
                self.handle_callback_exception(callback)

        def _discard_future_result(self, future):
            """Avoid unhandled-exception warnings from spawned coroutines."""
            future.result()

        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`.
            """
            app_log.error("Exception in callback %r", callback, exc_info=True)

        def split_fd(self, fd):
            """Returns an (fd, obj) pair from an ``fd`` parameter.

            We accept both raw file descriptors and file-like objects as
            input to `add_handler` and related methods.  When a file-like
            object is passed, we must retain the object itself so we can
            close it correctly when the `IOLoop` shuts down, but the
            poller interfaces favor file descriptors (they will accept
            file-like objects and call ``fileno()`` for you, but they
            always return the descriptor itself).

            This method is provided for use by `IOLoop` subclasses and should
            not generally be used by application code.

            .. versionadded:: 4.0
            """
            try:
                return fd.fileno(), fd
            except AttributeError:
                return fd, fd

        def close_fd(self, fd):
            """Utility method to close an ``fd``.

            If ``fd`` is a file-like object, we close it directly; otherwise
            we use `os.close`.

            This method is provided for use by `IOLoop` subclasses (in
            implementations of ``IOLoop.close(all_fds=True)`` and should
            not generally be used by application code.

            .. versionadded:: 4.0
            """
            try:
                try:
                    fd.close()
                except AttributeError:
                    os.close(fd)
            except OSError:
                pass

    class PollIOLoop(IOLoop):
        """Base class for IOLoops built around a select-like function.

        For concrete implementations, see `tornado4.platform.epoll.EPollIOLoop`
        (Linux), `tornado4.platform.kqueue.KQueueIOLoop` (BSD and Mac), or
        `tornado4.platform.select.SelectIOLoop` (all platforms).
        """
        def initialize(self, impl, time_func=None, **kwargs):
            super(PollIOLoop, self).initialize(**kwargs)
            self._impl = impl
            if hasattr(self._impl, 'fileno'):
                set_close_exec(self._impl.fileno())
            self.time_func = time_func or time.time
            self._handlers = {}
            self._events = {}
            self._callbacks = collections.deque()
            self._timeouts = []
            self._cancellations = 0
            self._running = False
            self._stopped = False
            self._closing = False
            self._thread_ident = None
            self._blocking_signal_threshold = None
            self._timeout_counter = itertools.count()

            # 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)

        def close(self, all_fds=False):
            self._closing = True
            self.remove_handler(self._waker.fileno())
            if all_fds:
                for fd, handler in list(self._handlers.values()):
                    self.close_fd(fd)
            self._waker.close()
            self._impl.close()
            self._callbacks = None
            self._timeouts = None

        def add_handler(self, fd, handler, events):
            fd, obj = self.split_fd(fd)
            self._handlers[fd] = (obj, stack_context.wrap(handler))
            self._impl.register(fd, events | self.ERROR)

        def update_handler(self, fd, events):
            fd, obj = self.split_fd(fd)
            self._impl.modify(fd, events | self.ERROR)

        def remove_handler(self, fd):
            fd, obj = self.split_fd(fd)
            self._handlers.pop(fd, None)
            self._events.pop(fd, None)
            try:
                self._impl.unregister(fd)
            except Exception:
                gen_log.debug("Error deleting fd from IOLoop", exc_info=True)

        def set_blocking_signal_threshold(self, seconds, action):
            if not hasattr(signal, "setitimer"):
                gen_log.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 start(self):
            if self._running:
                raise RuntimeError("IOLoop is already running")
            self._setup_logging()
            if self._stopped:
                self._stopped = False
                return
            old_current = getattr(IOLoop._current, "instance", None)
            IOLoop._current.instance = self
            self._thread_ident = thread.get_ident()
            self._running = True

            # signal.set_wakeup_fd closes a race condition in event loops:
            # a signal may arrive at the beginning of select/poll/etc
            # before it goes into its interruptible sleep, so the signal
            # will be consumed without waking the select.  The solution is
            # for the (C, synchronous) signal handler to write to a pipe,
            # which will then be seen by select.
            #
            # In python's signal handling semantics, this only matters on the
            # main thread (fortunately, set_wakeup_fd only works on the main
            # thread and will raise a ValueError otherwise).
            #
            # If someone has already set a wakeup fd, we don't want to
            # disturb it.  This is an issue for twisted, which does its
            # SIGCHLD processing in response to its own wakeup fd being
            # written to.  As long as the wakeup fd is registered on the IOLoop,
            # the loop will still wake up and everything should work.
            old_wakeup_fd = None
            if hasattr(signal, 'set_wakeup_fd') and os.name == 'posix':
                # requires python 2.6+, unix.  set_wakeup_fd exists but crashes
                # the python process on windows.
                try:
                    old_wakeup_fd = signal.set_wakeup_fd(self._waker.write_fileno())
                    if old_wakeup_fd != -1:
                        # Already set, restore previous value.  This is a little racy,
                        # but there's no clean get_wakeup_fd and in real use the
                        # IOLoop is just started once at the beginning.
                        signal.set_wakeup_fd(old_wakeup_fd)
                        old_wakeup_fd = None
                except ValueError:
                    # Non-main thread, or the previous value of wakeup_fd
                    # is no longer valid.
                    old_wakeup_fd = None

            try:
                while True:
                    # Prevent IO event starvation by delaying new callbacks
                    # to the next iteration of the event loop.
                    ncallbacks = len(self._callbacks)

                    # Add any timeouts that have come due to the callback list.
                    # Do not run anything until we have determined which ones
                    # are ready, so timeouts that call add_timeout cannot
                    # schedule anything in this iteration.
                    due_timeouts = []
                    if self._timeouts:
                        now = self.time()
                        while self._timeouts:
                            if self._timeouts[0].callback is None:
                                # The timeout was cancelled.  Note that the
                                # cancellation check is repeated below for timeouts
                                # that are cancelled by another timeout or callback.
                                heapq.heappop(self._timeouts)
                                self._cancellations -= 1
                            elif self._timeouts[0].deadline <= now:
                                due_timeouts.append(heapq.heappop(self._timeouts))
                            else:
                                break
                        if (self._cancellations > 512 and
                                self._cancellations > (len(self._timeouts) >> 1)):
                            # Clean up the timeout queue when it gets large and it's
                            # more than half cancellations.
                            self._cancellations = 0
                            self._timeouts = [x for x in self._timeouts
                                              if x.callback is not None]
                            heapq.heapify(self._timeouts)

                    for i in range(ncallbacks):
                        self._run_callback(self._callbacks.popleft())
                    for timeout in due_timeouts:
                        if timeout.callback is not None:
                            self._run_callback(timeout.callback)
                    # Closures may be holding on to a lot of memory, so allow
                    # them to be freed before we go into our poll wait.
                    due_timeouts = timeout = None

                    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
                    elif self._timeouts:
                        # If there are any timeouts, schedule the first one.
                        # Use self.time() instead of 'now' to account for time
                        # spent running callbacks.
                        poll_timeout = self._timeouts[0].deadline - self.time()
                        poll_timeout = max(0, min(poll_timeout, _POLL_TIMEOUT))
                    else:
                        # No timeouts and no callbacks, so use the default.
                        poll_timeout = _POLL_TIMEOUT

                    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 as 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 errno_from_exception(e) == 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 modify self._events
                    self._events.update(event_pairs)
                    while self._events:
                        fd, events = self._events.popitem()
                        try:
                            fd_obj, handler_func = self._handlers[fd]
                            handler_func(fd_obj, events)
                        except (OSError, IOError) as e:
                            if errno_from_exception(e) == errno.EPIPE:
                                # Happens when the client closes the connection
                                pass
                            else:
                                self.handle_callback_exception(self._handlers.get(fd))
                        except Exception:
                            self.handle_callback_exception(self._handlers.get(fd))
                    fd_obj = handler_func = None

            finally:
                # 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)
                IOLoop._current.instance = old_current
                if old_wakeup_fd is not None:
                    signal.set_wakeup_fd(old_wakeup_fd)

        def stop(self):
            self._running = False
            self._stopped = True
            self._waker.wake()

        def time(self):
            return self.time_func()

        def call_at(self, deadline, callback, *args, **kwargs):
            timeout = _Timeout(
                deadline,
                functools.partial(stack_context.wrap(callback), *args, **kwargs),
                self)
            heapq.heappush(self._timeouts, timeout)
            return timeout

        def remove_timeout(self, 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
            self._cancellations += 1

        def add_callback(self, callback, *args, **kwargs):
            if self._closing:
                return
            # Blindly insert into self._callbacks. This is safe even
            # from signal handlers because deque.append is atomic.
            self._callbacks.append(functools.partial(
                stack_context.wrap(callback), *args, **kwargs))
            if thread.get_ident() != self._thread_ident:
                # This will write one byte but Waker.consume() reads many
                # at once, so it's ok to write even when not strictly
                # necessary.
                self._waker.wake()
            else:
                # If we're on the IOLoop's thread, we don't need to wake anyone.
                pass

        def add_callback_from_signal(self, callback, *args, **kwargs):
            with stack_context.NullContext():
                self.add_callback(callback, *args, **kwargs)

    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', 'tdeadline']

        def __init__(self, deadline, callback, io_loop):
            if not isinstance(deadline, numbers.Real):
                raise TypeError("Unsupported deadline %r" % deadline)
            self.deadline = deadline
            self.callback = callback
            self.tdeadline = (deadline, next(io_loop._timeout_counter))

        # 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.tdeadline < other.tdeadline

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

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

        The callback is called every ``callback_time`` milliseconds.
        Note that the timeout is given in milliseconds, while most other
        time-related functions in Tornado use seconds.

        If the callback runs for longer than ``callback_time`` milliseconds,
        subsequent invocations will be skipped to get back on schedule.

        `start` must be called after the `PeriodicCallback` is created.

        .. versionchanged:: 4.1
           The ``io_loop`` argument is deprecated.
        """
        def __init__(self, callback, callback_time, io_loop=None):
            self.callback = callback
            if callback_time <= 0:
                raise ValueError("Periodic callback must have a positive callback_time")
            self.callback_time = callback_time
            self.io_loop = io_loop or IOLoop.current()
            self._running = False
            self._timeout = None

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

        def stop(self):
            """Stops the timer."""
            self._running = False
            if self._timeout is not None:
                self.io_loop.remove_timeout(self._timeout)
                self._timeout = None

        def is_running(self):
            """Return True if this `.PeriodicCallback` has been started.

            .. versionadded:: 4.1
            """
            return self._running

        def _run(self):
            if not self._running:
                return
            try:
                return self.callback()
            except Exception:
                self.io_loop.handle_callback_exception(self.callback)
            finally:
                self._schedule_next()

        def _schedule_next(self):
            if self._running:
                current_time = self.io_loop.time()

                if self._next_timeout <= current_time:
                    callback_time_sec = self.callback_time / 1000.0
                    self._next_timeout += (math.floor((current_time - self._next_timeout) /
                                                      callback_time_sec) + 1) * callback_time_sec

                self._timeout = self.io_loop.add_timeout(self._next_timeout, self._run)