Comment 20 for bug 1749546

Aren't this:

(In reply to Alistair Buxton from comment #6)
> 5. The timestamp of /usr/share/fonts inside the squashfs becomes
> 1522818676.0.
> 6. Installer boots and fontconfig sees the mismatched time stamps so
> rebuilds the cache in the live user's home directory - which is wiped at
> reboot.
> 7. During install the squashfs is copied back to an ext4 FS.

and this:

> 8. User reboots and logs in.
> 9. /usr/share/fonts is now on a filesystem which supports nanos, but the
> original time stamp nanos has been lost.
> 10. Fontconfig decides the cache is out of date and rebuilds it again, in
> the new user's home directory. This happens each time a newly created user
> logs in, until such time as a postinst script rebuilds the system font cache.

actually the same thing happening twice? The nanosecond portion of the timestamp is lost when the squashfs is built, and it's still not there when it's copied to the target system, but the fontconfig cache has nanos in it.

In that case, my proposed fix fixes this. I tried with a modified Xubuntu ISO, and after installation:

laney@bionic:~$ ls -l .cache
total 24
-rw-rw-r-- 1 laney laney 5 Apr 5 12:52 blueman-applet-1000
drwxr-xr-x 2 laney laney 4096 Apr 5 12:52 gstreamer-1.0
-rw-r--r-- 1 laney laney 0 Apr 5 13:05 motd.legal-displayed
drwx------ 2 laney laney 4096 Apr 5 12:53 obexd
drwx------ 2 laney laney 4096 Apr 5 12:52 sessions
drwxrwxr-x 2 laney laney 4096 Apr 5 13:01 update-manager-core
-rw-rw-r-- 1 laney laney 381 Apr 5 12:52 xfce4-indicator-plugin.log

(laney@bionic:~$ stat /usr/share/fonts
...
Modify: 2018-04-04 06:44:44.000000000 +0100
laney@bionic:~$ FC_DEBUG=16 fc-cache
FC_DEBUG=16
FcCacheTimeValid dir "/usr/share/fonts" cache checksum 1522820684.549609025 dir checksum 1522820684.0
tv_nsec == 0, ignoring nanoseconds)