When system have been installed, and we adjust the system to past, the fc-cahce will not cache new fontdir. It is because that the cache file's mtime is newer than the fontdirs. It causes fontconfig applications can not use new fonts.
Could we save fontdir's mtime in the cache file, and decide the cache's availability by checking whether the mtime in cache is equal to fontdirs instead of newer than fontdirs?
Font caches now contain the directory mtime; the cache is invalid when that time is not the same as the directory mtime, which eliminates random time mis-orderings between cache and font directory.
*** Bug 10900 has been marked as a duplicate of this bug. ***