Setting an XMP seq to the empty list segfaults

[Olivier Tilloy]

This happens with pyexiv2 0.2 (development branch) compiled against libexiv2 0.19.

After setting the value of an XMP tag of type 'seq ProperName' to the empty list [], writing back the metadata raises a segmentation fault. Test script to reproduce:

import pyexiv2
m = pyexiv2.ImageMetadata('image.jpg')
m.read()
m['Xmp.dc.creator'] = pyexiv2.XmpTag('Xmp.dc.creator', [])
m.write()

I am not sure whether emptying a seq is valid anyway, this needs to be investigated.

[Olivier Tilloy (osomon)]

The following minimal C++ program reproduces the issue:

#include "exiv2/image.hpp"
int main(int argc, char** argv)
{
    Exiv2::Image::AutoPtr image = Exiv2::ImageFactory::open(argv[1]);
    image->readMetadata();
    image->xmpData()["Xmp.dc.creator"].setValue(0);
    image->writeMetadata();
    return 0;
}

(to be compiled with `g++ -lexiv2`)
When run with the path to an image file as parameter, here is the output:

terminate called after throwing an instance of 'Exiv2::BasicError<char>'
  what(): Value not set

libexiv2 doesn't accept empty values, which makes sense. To prevent this from happening, pyexiv2 could raise an exception when setting an array (seq, bag, alt) value to the empty list.

[Olivier Tilloy (osomon)]

Note that the same applies to setting the value of a tag of type LangAlt to the empty dictionary. This should be prevented by raising an exception.