gdatetime: Fix incorrect alt-digits being used after changing locale

The alt-digits are loaded from nl_langinfo() in a GOnce section, which means nl_langinfo() is not re-queried after the process changes locale (if that happens).

So, change the GOnce to a mutex and store the locale of the alt-digits alongside them. This will introduce contention when calling format_number() is called, but how often are multiple threads trying to format dates at the same time?

If this does get highlighted as a performance problem, the other approach I considered was a GPrivate struct containing all the locale-specific cached data. That comes at the cost of using a GPrivate slot (although that’s only particularly expensive on Windows, and the locale code is quite different for Windows, so perhaps that could be avoided entirely). It does mean that all locale printing could be lock-free and still safely update cached data on a locale change.

Signed-off-by: Philip Withnall pwithnall@endlessos.org