SpiderImagePlugin: raise an error when seeking in a non-stack file

[radarhere]

Copy of PR #1741, with a test added.

Using ImageSequence.Iterator on a non-stack SPIDER image leads to infinite loop.
EOFError (which stops the iteration) is never raised because when the image isn't
a stack, seek() returns gently without error.

[coveralls]

Coverage remained the same at 76.92% when pulling fb86d69 on radarhere:spiderimageplugin into fd7fa4e on python-pillow:master.

[hugovk]

Get an infinite loop as expected on latest Pillow 3.2.0 (Python 2.7 / OS X).

Check completes with this patch.

[hugovk]

Please can you update this to "Tests/images/" so it can be run from the project root? That'll make it consistent with other checks/tests.

[radarhere]

Okay, done.

[hugovk]

Thanks. I forgot earlier: is it possible to unit test this raise too?

https://coveralls.io/builds/5638613/source?filename=PIL%2FSpiderImagePlugin.py#L176

[radarhere]

I thought about that. Then I thought that if such a test failed, it would go into DOS, and that would be bad. I would have thought that was the idea behind separating the test into it's own file.

[wiredfool]

If we're checking a non-stack file, we can just bail at 2 or more iterations of the same item. We could then incorporate that test into the normal test routine, since it wouldn't be a DOS.

(though, I do appreciate not having a DOS automatically run in the testing suite either way)

[radarhere]

I realised that it is possible to write the additional test without running the risk of DOS. So I've updated the commit.

[wiredfool]

@radarhere:

We could fold thedos check into the main spider tests like this:

im = Image.open("Tests/images/hopper.spider")
for i, frame in enumerate(ImageSequence.Iterator(im)):
    if i > 1:
        print ("DOS Check Failed")
        # or self.fail("Dos Check failed")
        break

[radarhere]

Thanks for the code. Done. I've removed the original separate dos file, let me know if you think it would be better to have both.

[coveralls]

Changes Unknown when pulling dabb68a on radarhere:spiderimageplugin into * on python-pillow:master*.