Cache version bump breaks Flatpak apps
Fontconfig bumped its font cache version in 5d84745e. This was the first cache version bump since 2015, which means it was also the first version bump since widespread adoption of Flatpak. Unfortunately exposed some serious problems when the cache version on the host does not match the cache version of Fontconfig in the Flatpak runtime, e.g.:
https://github.com/fedora-silverblue/issue-tracker/issues/305
In this particular case, Fedora can fix the problem because Fedora controls the Fontconfig version in its runtime, but most people use upstream runtimes where we cannot control the version of Fontconfig in the runtime. So I don't know what to do about this. It will require changes in either Fontconfig or Flatpak. One suggestion is to build multiple host caches in both older and newer formats, but I'm not sure how practical that would be.
Note: Flatpak mounts the host Fontconfig cache in the sandbox because it exposes host fonts there too, and without the host cache, graphical apps take a long time to start as the cache is rebuilt. So not mounting the host cache is not a good solution.