# Copyright 2009 Brian Quinlan. All Rights Reserved.
#   Licensed to PSF under a Contributor Agreement.
#
# Copyright (C) 2013  Google Inc.
#   Changes to this file are licensed under the same terms as the original file
#   (the Python Software Foundation License).


from __future__ import with_statement
import threading
import weakref
import sys

from concurrent.futures import _base

try:
    import queue
except ImportError:
    import Queue as queue


# This file provides an UnsafeThreadPoolExecutor, which operates exactly like
# the upstream Python version of ThreadPoolExecutor with one exception: it
# doesn't wait for worker threads to finish before shutting down the Python
# interpreter.
#
# This is dangerous for many workloads, but fine for some (like when threads
# only send network requests). The YCM workload is one of those workloads where
# it's safe (the aforementioned network requests case).

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:
            e = sys.exc_info()[1]
            self.future.set_exception(e)
        else:
            self.future.set_result(result)


def _worker(executor_reference, work_queue):
    try:
        while True:
            work_item = work_queue.get(block=True)
            if work_item is not None:
                work_item.run()
                continue
            executor = executor_reference()
            # Exit if:
            #   - The executor that owns the worker has been collected OR
            #   - The executor that owns the worker has been shutdown.
            if executor is None or executor._shutdown:
                # Notice other workers
                work_queue.put(None)
                return
            del executor
    except BaseException:
        _base.LOGGER.critical('Exception in worker', exc_info=True)


class UnsafeThreadPoolExecutor(_base.Executor):
    def __init__(self, max_workers):
        """Initializes a new ThreadPoolExecutor instance.

        Args:
            max_workers: The maximum number of threads that can be used to
                execute the given calls.
        """
        self._max_workers = max_workers
        self._work_queue = queue.Queue()
        self._threads = set()
        self._shutdown = False
        self._shutdown_lock = threading.Lock()

    def submit(self, fn, *args, **kwargs):
        with self._shutdown_lock:
            if self._shutdown:
                raise RuntimeError('cannot schedule new futures after shutdown')

            f = _base.Future()
            w = _WorkItem(f, fn, args, kwargs)

            self._work_queue.put(w)
            self._adjust_thread_count()
            return f
    submit.__doc__ = _base.Executor.submit.__doc__

    def _adjust_thread_count(self):
        # When the executor gets lost, the weakref callback will wake up
        # the worker threads.
        def weakref_cb(_, q=self._work_queue):
            q.put(None)
        # TODO(bquinlan): Should avoid creating new threads if there are more
        # idle threads than items in the work queue.
        if len(self._threads) < self._max_workers:
            t = threading.Thread(target=_worker,
                                 args=(weakref.ref(self, weakref_cb),
                                       self._work_queue))
            t.daemon = True
            t.start()
            self._threads.add(t)

    def shutdown(self, wait=True):
        with self._shutdown_lock:
            self._shutdown = True
            self._work_queue.put(None)
        if wait:
            for t in self._threads:
                t.join()
    shutdown.__doc__ = _base.Executor.shutdown.__doc__