[Image-SIG] Image crop race condition

Rob Hinchliff 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

class _ImageCrop(Image):

    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

    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,


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