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
class _ImageCrop(Image):
def __init__(self, im, box):
Image.__init__(self)
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
def load(self):
# lazy evaluation!
if self.__crop:
self.im = self.im.crop(self.__crop)
#self.__crop = None
if self.im:
return self.im.pixel_access(self.readonly)
# FIXME: future versions should optimize crop/paste
# sequences!
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!
Best Regards,
Rob
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