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<p class=MsoNormal>I am writing a python application that crops an image and
processes it in a number of threads.<o:p></o:p></p>
<p class=MsoNormal>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:<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>#
--------------------------------------------------------------------<o:p></o:p></p>
<p class=MsoNormal># Lazy operations<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>class _ImageCrop(Image):<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> def __init__(self, im, box):<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>
Image.__init__(self)<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> x0, y0, x1, y1 =
box<o:p></o:p></p>
<p class=MsoNormal> if x1 < x0:<o:p></o:p></p>
<p class=MsoNormal>
x1 = x0<o:p></o:p></p>
<p class=MsoNormal> if y1 < y0:<o:p></o:p></p>
<p class=MsoNormal>
y1 = y0<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> self.mode =
im.mode<o:p></o:p></p>
<p class=MsoNormal> self.size =
x1-x0, y1-y0<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> self.__crop = x0,
y0, x1, y1<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> self.im = im.im<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> def load(self):<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> # lazy
evaluation!<o:p></o:p></p>
<p class=MsoNormal> if self.__crop:<o:p></o:p></p>
<p class=MsoNormal>
self.im = self.im.crop(self.__crop)<o:p></o:p></p>
<p class=MsoNormal>
#self.__crop = None<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> if self.im:<o:p></o:p></p>
<p class=MsoNormal>
return self.im.pixel_access(self.readonly)<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal> # FIXME: future
versions should optimize crop/paste<o:p></o:p></p>
<p class=MsoNormal> # sequences!<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>The problem is in ‘load’ at line 1707.<o:p></o:p></p>
<p class=MsoNormal>‘self.__crop’ is checked for ‘None’,
and enters the block.<o:p></o:p></p>
<p class=MsoNormal>Another thread that has already entered the block sets self.__crop
to None on line 1709.<o:p></o:p></p>
<p class=MsoNormal>The original thread then calls crop with a None
object, causing an exception.<o:p></o:p></p>
<p class=MsoNormal>I’ve fixed the problem in my local copy by commenting
out line 1709.<o:p></o:p></p>
<p class=MsoNormal>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.<o:p></o:p></p>
<p class=MsoNormal>Otherwise, thankyou for making this excellent library
available for public consumption!<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Best Regards,<o:p></o:p></p>
<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Rob<o:p></o:p></p>
</div>
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