# -*- coding: utf-8 -*-

#    Copyright (C) 2013 Yahoo! Inc. All Rights Reserved.
#
#    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.

import logging
import threading

from concurrent import futures

try:
    from eventlet.green import threading as green_threading
    from eventlet import greenpool
    from eventlet import patcher
    from eventlet import queue
    EVENTLET_AVAILABLE = True
except ImportError:
    EVENTLET_AVAILABLE = False


from taskflow.utils import lock_utils

LOG = logging.getLogger(__name__)

# NOTE(harlowja): this object signals to threads that they should stop
# working and rest in peace.
_TOMBSTONE = object()


class _WorkItem(object):
    def __init__(self, future, fn, args, kwargs):
        self.future = future
        self.fn = fn
        self.args = args
        self.kwargs = kwargs

    def run(self):
        if not self.future.set_running_or_notify_cancel():
            return
        try:
            result = self.fn(*self.args, **self.kwargs)
        except BaseException as e:
            self.future.set_exception(e)
        else:
            self.future.set_result(result)


class _Worker(object):
    def __init__(self, executor, work_queue, worker_id):
        self.executor = executor
        self.work_queue = work_queue
        self.worker_id = worker_id

    def __call__(self):
        try:
            while True:
                work = self.work_queue.get(block=True)
                if work is _TOMBSTONE:
                    # NOTE(harlowja): give notice to other workers (this is
                    # basically a chain of tombstone calls that will cause all
                    # the workers on the queue to eventually shut-down).
                    self.work_queue.put(_TOMBSTONE)
                    break
                else:
                    work.run()
        except BaseException:
            LOG.critical("Exception in worker %s of '%s'",
                         self.worker_id, self.executor, exc_info=True)


class GreenFuture(futures.Future):
    def __init__(self):
        super(GreenFuture, self).__init__()
        # NOTE(harlowja): replace the built-in condition with a greenthread
        # compatible one so that when getting the result of this future the
        # functions will correctly yield to eventlet. If this is not done then
        # waiting on the future never actually causes the greenthreads to run
        # and thus you wait for infinity.
        if not patcher.is_monkey_patched('threading'):
            self._condition = green_threading.Condition()


class GreenExecutor(futures.Executor):
    """A greenthread backed executor."""

    def __init__(self, max_workers=1000):
        assert EVENTLET_AVAILABLE, 'eventlet is needed to use GreenExecutor'
        assert int(max_workers) > 0, 'Max workers must be greater than zero'
        self._max_workers = int(max_workers)
        self._pool = greenpool.GreenPool(self._max_workers)
        self._work_queue = queue.LightQueue()
        self._shutdown_lock = threading.RLock()
        self._shutdown = False

    @lock_utils.locked(lock='_shutdown_lock')
    def submit(self, fn, *args, **kwargs):
        if self._shutdown:
            raise RuntimeError('cannot schedule new futures after shutdown')
        f = GreenFuture()
        w = _WorkItem(f, fn, args, kwargs)
        self._work_queue.put(w)
        # Spin up any new workers (since they are spun up on demand and
        # not at executor initialization).
        self._spin_up()
        return f

    def _spin_up(self):
        cur_am = (self._pool.running() + self._pool.waiting())
        if cur_am < self._max_workers and cur_am < self._work_queue.qsize():
            # Spin up a new worker to do the work as we are behind.
            worker = _Worker(self, self._work_queue, cur_am + 1)
            self._pool.spawn(worker)

    def shutdown(self, wait=True):
        with self._shutdown_lock:
            self._shutdown = True
            self._work_queue.put(_TOMBSTONE)
        if wait:
            self._pool.waitall()