During a package upgrade done in a graphical terminal, the update of the fonts-cantarell package can lead to a desktop showing squares instead of the expected text labels. The reason is that dpkg unpacks files with .dpkg-tmp extensions and when it has unpacked all the files, it renames them back into their definitive name. Unfortunately, it seems that the gnome-settings-daemon monitoring of the directory can trigger in during this process and fontconfig will happily register the .dpkg-tmp files which are then removed shortly afterwards.

For some reason (and that's what I call the race condition), the removal of those .dpkg-tmp files is not caught by gnome-settings-daemon and the cache is not updated a second time. In parallel, the Debian packaging will run "fc-cache -s" after the unpack has been finished.

Somehow I have the feeling that the removal of the timeout in https://bugzilla.gnome.org/show_bug.cgi?id=748776 makes this race condition much more likely to happen...

Related links/bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=96896 (asking fontconfig to ignore .dpkg-tmp files)
https://bugs.debian.org/828037