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A fork of Python 3.6's stdlib queue (found in Pythons 'cpython/Lib/queue.py')
with Lock swapped out for RLock to avoid a deadlock while garbage collecting.
https://github.com/python/cpython/blob/v3.6.12/Lib/queue.py
https://codewithoutrules.com/2017/08/16/concurrency-python/
https://bugs.python.org/issue14976
https://github.com/sqlalchemy/sqlalchemy/blob/4eb747b61f0c1b1c25bdee3856d7195d10a0c227/lib/sqlalchemy/queue.py#L1
We also vendor the code to evade eventlet's broken monkeypatching, see
https://github.com/getsentry/sentry-python/pull/484
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import threading
from collections import deque
from time import time
from typing import TYPE_CHECKING
if TYPE_CHECKING:
    from typing import Any
__all__ = ["EmptyError", "FullError", "Queue"]
class EmptyError(Exception):
    "Exception raised by Queue.get(block=0)/get_nowait()."
class FullError(Exception):
    "Exception raised by Queue.put(block=0)/put_nowait()."
class Queue:
    """Create a queue object with a given maximum size.
    If maxsize is <= 0, the queue size is infinite.
    def __init__(self, maxsize=0):
        self.maxsize = maxsize
        self._init(maxsize)
        # mutex must be held whenever the queue is mutating.  All methods
        # that acquire mutex must release it before returning.  mutex
        # is shared between the three conditions, so acquiring and
        # releasing the conditions also acquires and releases mutex.
        self.mutex = threading.RLock()
        # Notify not_empty whenever an item is added to the queue; a
        # thread waiting to get is notified then.
        self.not_empty = threading.Condition(self.mutex)
        # Notify not_full whenever an item is removed from the queue;
        # a thread waiting to put is notified then.
        self.not_full = threading.Condition(self.mutex)
        # Notify all_tasks_done whenever the number of unfinished tasks
        # drops to zero; thread waiting to join() is notified to resume
        self.all_tasks_done = threading.Condition(self.mutex)
        self.unfinished_tasks = 0
    def task_done(self):
        """Indicate that a formerly enqueued task is complete.
        Used by Queue consumer threads.  For each get() used to fetch a task,
        a subsequent call to task_done() tells the queue that the processing
        on the task is complete.
        If a join() is currently blocking, it will resume when all items
        have been processed (meaning that a task_done() call was received
        for every item that had been put() into the queue).
        Raises a ValueError if called more times than there were items
        placed in the queue.
        """
        with self.all_tasks_done:
            unfinished = self.unfinished_tasks - 1
            if unfinished <= 0:
                if unfinished < 0:
                    raise ValueError("task_done() called too many times")
                self.all_tasks_done.notify_all()
            self.unfinished_tasks = unfinished
    def join(self):
        """Blocks until all items in the Queue have been gotten and processed.
        The count of unfinished tasks goes up whenever an item is added to the
        queue. The count goes down whenever a consumer thread calls task_done()
        to indicate the item was retrieved and all work on it is complete.
        When the count of unfinished tasks drops to zero, join() unblocks.
        """
        with self.all_tasks_done:
            while self.unfinished_tasks:
                self.all_tasks_done.wait()
    def qsize(self):
        """Return the approximate size of the queue (not reliable!)."""
        with self.mutex:
            return self._qsize()
    def empty(self):
        """Return True if the queue is empty, False otherwise (not reliable!).
        This method is likely to be removed at some point.  Use qsize() == 0
        as a direct substitute, but be aware that either approach risks a race
        condition where a queue can grow before the result of empty() or
        qsize() can be used.
        To create code that needs to wait for all queued tasks to be
        completed, the preferred technique is to use the join() method.
        """
        with self.mutex:
            return not self._qsize()
    def full(self):
        """Return True if the queue is full, False otherwise (not reliable!).
        This method is likely to be removed at some point.  Use qsize() >= n
        as a direct substitute, but be aware that either approach risks a race
        condition where a queue can shrink before the result of full() or
        qsize() can be used.
        """
        with self.mutex:
            return 0 < self.maxsize <= self._qsize()
    def put(self, item, block=True, timeout=None):
        """Put an item into the queue.
        If optional args 'block' is true and 'timeout' is None (the default),
        block if necessary until a free slot is available. If 'timeout' is
        a non-negative number, it blocks at most 'timeout' seconds and raises
        the FullError exception if no free slot was available within that time.
        Otherwise ('block' is false), put an item on the queue if a free slot
        is immediately available, else raise the FullError exception ('timeout'
        is ignored in that case).
        """
        with self.not_full:
            if self.maxsize > 0:
                if not block:
                    if self._qsize() >= self.maxsize:
                        raise FullError()
                elif timeout is None:
                    while self._qsize() >= self.maxsize:
                        self.not_full.wait()
                elif timeout < 0:
                    raise ValueError("'timeout' must be a non-negative number")
                else:
                    endtime = time() + timeout
                    while self._qsize() >= self.maxsize:
                        remaining = endtime - time()
                        if remaining <= 0.0:
                            raise FullError()
                        self.not_full.wait(remaining)
            self._put(item)
            self.unfinished_tasks += 1
            self.not_empty.notify()
    def get(self, block=True, timeout=None):
        """Remove and return an item from the queue.
        If optional args 'block' is true and 'timeout' is None (the default),
        block if necessary until an item is available. If 'timeout' is
        a non-negative number, it blocks at most 'timeout' seconds and raises
        the EmptyError exception if no item was available within that time.
        Otherwise ('block' is false), return an item if one is immediately
        available, else raise the EmptyError exception ('timeout' is ignored
        in that case).
        """
        with self.not_empty:
            if not block:
                if not self._qsize():
                    raise EmptyError()
            elif timeout is None:
                while not self._qsize():
                    self.not_empty.wait()
            elif timeout < 0:
                raise ValueError("'timeout' must be a non-negative number")
            else:
                endtime = time() + timeout
                while not self._qsize():
                    remaining = endtime - time()
                    if remaining <= 0.0:
                        raise EmptyError()
                    self.not_empty.wait(remaining)
            item = self._get()
            self.not_full.notify()
            return item
    def put_nowait(self, item):
        """Put an item into the queue without blocking.
        Only enqueue the item if a free slot is immediately available.
        Otherwise raise the FullError exception.
        """
        return self.put(item, block=False)
    def get_nowait(self):
        """Remove and return an item from the queue without blocking.
        Only get an item if one is immediately available. Otherwise
        raise the EmptyError exception.
        """
        return self.get(block=False)
    # Override these methods to implement other queue organizations
    # (e.g. stack or priority queue).
    # These will only be called with appropriate locks held
    # Initialize the queue representation
    def _init(self, maxsize):
        self.queue = deque()  # type: Any
    def _qsize(self):
        return len(self.queue)
    # Put a new item in the queue
    def _put(self, item):
        self.queue.append(item)
    # Get an item from the queue
    def _get(self):
        return self.queue.popleft()
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