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Bug #788520: Queue class has logic error when non-blocking 

I don't agree it had a bug (see the report), so this is *not* a candidate
for backporting, but the docs were confusing and the Queue implementation
was old enough to vote.

Rewrote put/put_nowait/get/get_nowait from scratch, to use a pair of
Conditions (not_full and not_empty), sharing a common mutex.  The code
is 1/4 the size now, and 6.25x easier to understand.  For blocking
with timeout, we also get to reuse (indirectly) the tedious timeout
code from threading.Condition.  The Full and Empty exceptions raised
by non-blocking calls are now easy (instead of nearly impossible) to
explain truthfully:  Full is raised if and only if the Queue truly
is full when the non-blocking put call checks the queue size, and
similarly for Empty versus non-blocking get.

What I don't know is whether the new implementation is slower (or
faster) than the old one.  I don't really care.  Anyone who cares
a lot is encouraged to check that.
This commit is contained in:
Tim Peters 2004-07-12 00:45:14 +00:00
parent 183dabcd73
commit 5af0e41482
3 changed files with 82 additions and 94 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Doc/lib/libqueue.tex
View File

@ -26,13 +26,13 @@ zero, the queue size is infinite.
\begin{excdesc}{Empty}
Exception raised when non-blocking \method{get()} (or
\method{get_nowait()}) is called on a \class{Queue} object which is
empty or locked.
empty.
\end{excdesc}
\begin{excdesc}{Full}
Exception raised when non-blocking \method{put()} (or
\method{put_nowait()}) is called on a \class{Queue} object which is
full or locked.
full.
\end{excdesc}
\subsection{Queue Objects}
@ -51,7 +51,7 @@ semantics, this number is not reliable.
\begin{methoddesc}{empty}{}
Return \code{True} if the queue is empty, \code{False} otherwise.
Becauseof multithreading semantics, this is not reliable.
Because of multithreading semantics, this is not reliable.
\end{methoddesc}
\begin{methoddesc}{full}{}

159
Lib/Queue.py
View File

@ -1,6 +1,6 @@
"""A multi-producer, multi-consumer queue."""
from time import time as _time, sleep as _sleep
from time import time as _time
from collections import deque
__all__ = ['Empty', 'Full', 'Queue']
@ -20,14 +20,21 @@ class Queue:
If maxsize is <= 0, the queue size is infinite.
"""
try:
import thread
import threading
except ImportError:
import dummy_thread as thread
import dummy_threading as threading
self._init(maxsize)
self.mutex = thread.allocate_lock()
self.esema = thread.allocate_lock()
self.esema.acquire()
self.fsema = thread.allocate_lock()
# 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.Lock()
# 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)
def qsize(self):
"""Return the approximate size of the queue (not reliable!)."""
@ -61,51 +68,26 @@ class Queue:
is immediately available, else raise the Full exception ('timeout'
is ignored in that case).
"""
if block:
if timeout is None:
# blocking, w/o timeout, i.e. forever
self.fsema.acquire()
elif timeout >= 0:
# waiting max. 'timeout' seconds.
# this code snipped is from threading.py: _Event.wait():
# Balancing act: We can't afford a pure busy loop, so we
# have to sleep; but if we sleep the whole timeout time,
# we'll be unresponsive. The scheme here sleeps very
# little at first, longer as time goes on, but never longer
# than 20 times per second (or the timeout time remaining).
delay = 0.0005 # 500 us -> initial delay of 1 ms
endtime = _time() + timeout
while True:
if self.fsema.acquire(0):
break
remaining = endtime - _time()
if remaining <= 0: #time is over and no slot was free
raise Full
delay = min(delay * 2, remaining, .05)
_sleep(delay) #reduce CPU usage by using a sleep
else:
raise ValueError("'timeout' must be a positive number")
elif not self.fsema.acquire(0):
raise Full
self.mutex.acquire()
release_fsema = True
if not block:
return self.put_nowait(item)
self.not_full.acquire()
try:
was_empty = self._empty()
if 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)
# If we fail before here, the empty state has
# not changed, so we can skip the release of esema
if was_empty:
self.esema.release()
# If we fail before here, the queue can not be full, so
# release_full_sema remains True
release_fsema = not self._full()
self.not_empty.notify()
finally:
# Catching system level exceptions here (RecursionDepth,
# OutOfMemory, etc) - so do as little as possible in terms
# of Python calls.
if release_fsema:
self.fsema.release()
self.mutex.release()
self.not_full.release()
def put_nowait(self, item):
"""Put an item into the queue without blocking.
@ -113,7 +95,15 @@ class Queue:
Only enqueue the item if a free slot is immediately available.
Otherwise raise the Full exception.
"""
return self.put(item, False)
self.not_full.acquire()
try:
if self._full():
raise Full
else:
self._put(item)
self.not_empty.notify()
finally:
self.not_full.release()
def get(self, block=True, timeout=None):
"""Remove and return an item from the queue.
@ -126,49 +116,27 @@ class Queue:
available, else raise the Empty exception ('timeout' is ignored
in that case).
"""
if block:
if timeout is None:
# blocking, w/o timeout, i.e. forever
self.esema.acquire()
elif timeout >= 0:
# waiting max. 'timeout' seconds.
# this code snipped is from threading.py: _Event.wait():
# Balancing act: We can't afford a pure busy loop, so we
# have to sleep; but if we sleep the whole timeout time,
# we'll be unresponsive. The scheme here sleeps very
# little at first, longer as time goes on, but never longer
# than 20 times per second (or the timeout time remaining).
delay = 0.0005 # 500 us -> initial delay of 1 ms
endtime = _time() + timeout
while 1:
if self.esema.acquire(0):
break
remaining = endtime - _time()
if remaining <= 0: #time is over and no element arrived
raise Empty
delay = min(delay * 2, remaining, .05)
_sleep(delay) #reduce CPU usage by using a sleep
else:
raise ValueError("'timeout' must be a positive number")
elif not self.esema.acquire(0):
raise Empty
self.mutex.acquire()
release_esema = True
if not block:
return self.get_nowait()
self.not_empty.acquire()
try:
was_full = self._full()
if 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()
# If we fail before here, the full state has
# not changed, so we can skip the release of fsema
if was_full:
self.fsema.release()
# Failure means empty state also unchanged - release_esema
# remains True.
release_esema = not self._empty()
self.not_full.notify()
return item
finally:
if release_esema:
self.esema.release()
self.mutex.release()
return item
self.not_empty.release()
def get_nowait(self):
"""Remove and return an item from the queue without blocking.
@ -176,7 +144,16 @@ class Queue:
Only get an item if one is immediately available. Otherwise
raise the Empty exception.
"""
return self.get(False)
self.not_empty.acquire()
try:
if self._empty():
raise Empty
else:
item = self._get()
self.not_full.notify()
return item
finally:
self.not_empty.release()
# Override these methods to implement other queue organizations
# (e.g. stack or priority queue).

11
Misc/NEWS
View File

@ -29,6 +29,17 @@ Extension modules
Library
-------
- Bug #788520. Queue.{get, get_nowait, put, put_nowait} have new
implementations, exploiting Conditions (which didn't exist at the time
Queue was introduced). A minor semantic change is that the Full and
Empty exceptions raised by non-blocking calls now occur only if the
queue truly was full or empty at the instant the queue was checked (of
course the Queue may no longer be full or empty by the time a calling
thread sees those exceptions, though). Before, the exceptions could
also be raised if it was "merely inconvenient" for the implementation
to determine the true state of the Queue (because the Queue was locked
by some other method in progress).
- Bugs #979794 and #980117: difflib.get_grouped_opcodes() now handles the
case of comparing two empty lists. This affected both context_diff() and
unified_diff(),