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queue.py
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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()