Fallback to Compose file in locale directory if no mapping exists in compose.dir

[JSkold]

I've created and distribute a custom locale that a mix between my native language's locale and en_US. There unfortunately seems to be no way for me to extend the compose.dir file to include a mapping for my new locale.

This MR add a new fallback in case a compose.dir entry is missing for the current locale and will cause libxkbcommon to use the compose file at:
/usr/share/X11/locale/en_XX.UTF-8/Compose
In this case en_XX is missing from compose.dir.

[bluetech]

Do you also create a custom Compose file for your locale?

[JSkold]

In my case I just have a simple Compose file with:
include "/usr/share/X11/locale/en_US.UTF-8/Compose"

But with this change anyone shipping a non-standard locale could bring their own Compose file. My use case is primarily to get dead keys to work without having every user put that line in their XCompose in their home directoy.

[bluetech]

While having a custom locale is probably rare, having a custom locale and a custom Compose is surely very rare. Most people will want to fallback to the en_US.UTF-8 Compose.

Current status quo is that if you're using a custom locale, you have no solution.

With this PR, if you're using a custom locale, Compose is still broken, but you can fix with an additional step of adding an /usr/share/X11/locale/en_XX.UTF-8/Compose file with the magic include incantation. This seems not much better to me than adding this include line to your ~/.XCompose or setting XCOMPOSEFILE (both of which take precedence over the locale).

I think the preferable state would be that, if you're using a custom locale, you fall back to en_US.UTF-8, with (optionally) some provision to provide a custom file. However, this will need to be considered carefully, as doing it naively means xkb_compose_table_from_locale will no longer fail on bad/misspelled locales, which is not great.

[JSkold]

In my case fallback to en_US.UTF-8 would work, but I don't like to assume that is what everyone wants. My hope is that any locale should just be able to be installed like any other package.

The benefit of checking for a fallback is that the administrator does not need to make additional configuration and the locale would work just like any other. A package would ship with two files:
/usr/share/i18n/locales/en_XX
/usr/share/X11/locale/en_XX.UTF-8/Compose
and it would "Just Work". In the other case the administrator would have to either configure every user to have a specific ~/.XCompose or put XCOMPOSEFILE in something like /etc/environment. It works but it's more unnecessary steps for the administrator to take.