[Image-SIG] Image crop race condition
rhinchliff at adgholdings.co.uk
Wed Dec 1 14:35:44 CET 2010
I am writing a python application that crops an image and processes it in a number of threads.
The lazy loading of the cropped image is causing me exceptions, I think because of this race condition in Image.py v1.1.7 starting at line 1682:
# Lazy operations
def __init__(self, im, box):
x0, y0, x1, y1 = box
if x1 < x0:
x1 = x0
if y1 < y0:
y1 = y0
self.mode = im.mode
self.size = x1-x0, y1-y0
self.__crop = x0, y0, x1, y1
self.im = im.im
# lazy evaluation!
self.im = self.im.crop(self.__crop)
#self.__crop = None
# FIXME: future versions should optimize crop/paste
The problem is in 'load' at line 1707.
'self.__crop' is checked for 'None', and enters the block.
Another thread that has already entered the block sets self.__crop to None on line 1709.
The original thread then calls crop with a None object, causing an exception.
I've fixed the problem in my local copy by commenting out line 1709.
I don't know the design reasons for setting '__crop' to None (free the memory?), but I think you should consider using double-checked locking pattern here if you wish to keep it.
Otherwise, thankyou for making this excellent library available for public consumption!
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