libpng compatibility issue with zTXt text containing non-latin characters

[jcupitt]

Hello @randy408, this image reads fine with libpng, but produces non-recoverable state with git master libspng.

some.png.zip

Original bug report:

libvips/libvips#1778

[randy408]

That PNG has a zTXt chunk where the text field contains UTF-8 characters, it should be in a iTXt chunk instead, according to the standard:

Decompression of this datastream yields Latin-1 text that is identical to the text that would be stored in an equivalent tEXt chunk.

Looks like libpng does not check the text field at all.

[jcupitt]

Thanks!

[randy408]

libpng itself can create these malformed files: https://github.com/glennrp/libpng/blob/e19a576157911315c1e045fcd0e4dec3e80fba81/pngwutil.c#L1556