Ability to skip crc checks for ancillary chunks

[kkopachev]

I encountered that some PNG files could not be open with Pillow since they have crc mismatch for iTXt chunks.
Browsers and imagemagick can open them just fine (though imagemagick shows a warning)
libpng defines a way to set it's behavior to throw an error or warning for ancillary chunks.

So I am proposing use of LOAD_TRUNCATED_IMAGES to skip crc checks for ancillary chunks. No need to calculate if we're going to ignore an error.

[homm]

I'd prefer to ignore the error after it occurs. Of course, it is meaningless to do the check, if we are going to accept the result in any case. But in the future, we can add some warnings or other decoder info.

[kkopachev]

Hm, why bother doing this now, if in both cases code has to be changed to throw a warning?
But anyway, I can keep crc check if you'd like. Would you like to ignore checksum mismatch or incomplete checksum as well?

[homm]

Does libpng and friends open images with errors in data chunks?

[kkopachev]

From what I can tell, in libpng user of it can ask to throw errors for any crc corruptions, or ignore ancillary, or ignore all. Not sure what happens if it encounters errors in data chunks, though. My thought here was that since Pillow does not expose any methods to change decoder behavior, but only LOAD_TRUNCATED_IMAGES, using that flag would be pretty safe bet to let read files at least with ancillary chunks broken.
Not sure where my original image came from, but looks like there was an issue with libpng where it sometimes could produce invalid CRC for iTXt chunk.

[homm]

self.assertRaises(SyntaxError, PngImagePlugin.PngImageFile, BytesIO(image_data))

[kkopachev]

Fixed. FYI there are few more lambda usages in that file in same context.

[homm]

I haven't met such images in a wild. But I'm always for decoding more with LOAD_TRUNCATED_IMAGES.