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# util/queue.py
# Copyright (C) 2005-2021 the SQLAlchemy authors and contributors
# <see AUTHORS file>
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php
"""An adaptation of Py2.3/2.4's Queue module which supports reentrant
behavior, using RLock instead of Lock for its mutex object. The
Queue object is used exclusively by the sqlalchemy.pool.QueuePool
class.
This is to support the connection pool's usage of weakref callbacks to return
connections to the underlying Queue, which can in extremely
rare cases be invoked within the ``get()`` method of the Queue itself,
producing a ``put()`` inside the ``get()`` and therefore a reentrant
condition.
"""
from collections import deque
from time import time as _time
from .compat import threading
__all__ = ["Empty", "Full", "Queue"]
class Empty(Exception):
"Exception raised by Queue.get(block=0)/get_nowait()."
pass
class Full(Exception):
"Exception raised by Queue.put(block=0)/put_nowait()."
pass
class Queue:
def __init__(self, maxsize=0, use_lifo=False):
"""Initialize a queue object with a given maximum size.
If `maxsize` is <= 0, the queue size is infinite.
If `use_lifo` is True, this Queue acts like a Stack (LIFO).
"""
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 two 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)
# If this queue uses LIFO or FIFO
self.use_lifo = use_lifo
def qsize(self):
"""Return the approximate size of the queue (not reliable!)."""
self.mutex.acquire()
n = self._qsize()
self.mutex.release()
return n
def empty(self):
"""Return True if the queue is empty, False otherwise (not
reliable!)."""
self.mutex.acquire()
n = self._empty()
self.mutex.release()
return n
def full(self):
"""Return True if the queue is full, False otherwise (not
reliable!)."""
self.mutex.acquire()
n = self._full()
self.mutex.release()
return n
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 positive number, it blocks at
most `timeout` seconds and raises the ``Full`` 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 ``Full`` exception
(`timeout` is ignored in that case).
"""
self.not_full.acquire()
try:
if not block:
if self._full():
raise Full
elif timeout is None:
while self._full():
self.not_full.wait()
else:
if timeout < 0:
raise ValueError("'timeout' must be a positive number")
endtime = _time() + timeout
while self._full():
remaining = endtime - _time()
if remaining <= 0.0:
raise Full
self.not_full.wait(remaining)
self._put(item)
self.not_empty.notify()
finally:
self.not_full.release()
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 ``Full`` exception.
"""
return self.put(item, False)
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 positive number, it blocks at most `timeout`
seconds and raises the ``Empty`` exception if no item was
available within that time. Otherwise (`block` is false),
return an item if one is immediately available, else raise the
``Empty`` exception (`timeout` is ignored in that case).
"""
self.not_empty.acquire()
try:
if not block:
if self._empty():
raise Empty
elif timeout is None:
while self._empty():
self.not_empty.wait()
else:
if timeout < 0:
raise ValueError("'timeout' must be a positive number")
endtime = _time() + timeout
while self._empty():
remaining = endtime - _time()
if remaining <= 0.0:
raise Empty
self.not_empty.wait(remaining)
item = self._get()
self.not_full.notify()
return item
finally:
self.not_empty.release()
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 ``Empty`` exception.
"""
return self.get(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.maxsize = maxsize
self.queue = deque()
def _qsize(self):
return len(self.queue)
# Check whether the queue is empty
def _empty(self):
return not self.queue
# Check whether the queue is full
def _full(self):
return self.maxsize > 0 and len(self.queue) == self.maxsize
# Put a new item in the queue
def _put(self, item):
self.queue.append(item)
# Get an item from the queue
def _get(self):
if self.use_lifo:
# LIFO
return self.queue.pop()
else:
# FIFO
return self.queue.popleft()