exif: implement reading user-defined EXIF tags from files

[horazont]

This is a follow-up from #19622, where @zisoft pointed out that there's a mechanism to read custom EXIF tags into darktable's database for filtering and such. That didn't work, so I cobbled together this patch.

This does not fully work yet. For some reason, on first import of an image, the metadata is not set. I have to reread the EXIF data manually via the UI in order for it to appear. Any ideas why that might be welcome.

[zisoft]

I am currently busy with the gtk4 migration, so great you have a look here.

[horazont]

Figured it out! The xmp read was called with exif_read == FALSE, leading to a dt_metadata_clear call. I fixed that in a manner which hopefully makes sense.

EDIT: I'm not exactly sure in which case the exif_read == FALSE code path should be triggered except on image import, so I don't know how to test that I didn't break anything... I tried to look through the usages of the various functions, but I seem to circle back to imports always.

[wpferguson]

I'm not exactly sure in which case the exif_read == FALSE code path should be triggered

Let's say I modify the exif information using a script (which I do sometimes) and I get it all screwed up. I can force the metadata to be reread, which cleans up my mess and sets the metadata in the database to agree with the metadata in the image.

[horazont]

@wpferguson As far as I can tell, that doesn't trigger an xmp read with exif_read == FALSE, but calls dt_exif_read which calls _exif_decode_xmp_data with exif_read set to TRUE.

(assuming you mean the "refresh EXIF" button)

[wpferguson]

Wont build

In file included from /usr/include/c++/12/string:40,
                 from /usr/include/c++/12/bits/locale_classes.h:40,
                 from /usr/include/c++/12/bits/ios_base.h:41,
                 from /usr/include/c++/12/streambuf:41,
                 from /usr/include/c++/12/bits/streambuf_iterator.h:35,
                 from /usr/include/c++/12/iterator:66,
                 from /usr/include/exiv2/slice.hpp:26,
                 from /usr/include/exiv2/types.hpp:27,
                 from /home/bill/src/darktable/test/19656_user_exif_tags/src/common/exif.cc:23:
In static member function ‘static constexpr std::size_t std::char_traits<char>::length(const char_type*)’,
    inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::assign(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12/bits/basic_string.h:1655:19,
    inlined from ‘std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator=(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]’ at /usr/include/c++/12/bits/basic_string.h:823:28,
    inlined from ‘bool _exif_decode_exif_data(dt_image_t*, Exiv2::ExifData&)’ at /home/bill/src/darktable/test/19656_user_exif_tags/src/common/exif.cc:2385:15:
/usr/include/c++/12/bits/char_traits.h:399:32: error: argument 1 null where non-null expected [-Werror=nonnull]
  399 |         return __builtin_strlen(__s);
      |                ~~~~~~~~~~~~~~~~^~~~~
compilation terminated due to -Wfatal-errors.
cc1plus: all warnings being treated as errors
gmake[2]: *** [bin/CMakeFiles/lib_darktable.dir/build.make:488: bin/CMakeFiles/lib_darktable.dir/common/exif.cc.o] Error 1
gmake[2]: *** Waiting for unfinished jobs....
gmake[1]: *** [CMakeFiles/Makefile2:2575: bin/CMakeFiles/lib_darktable.dir/all] Error 2
gmake: *** [Makefile:156: all] Error 2

gcc 12.3.0

[horazont]

Hmmm… I can't see from the code what's wrong and it builds for me …

Unfortunately, I'm mostly AFK until Monday, so I probably won't be able to look into it before that.

[horazont]

@wpferguson found it, fixed!