Comment 22 for bug 1858636

I tired to figure out what exactly happens when I reinstall fonts-noto-color-emoji. According to dpkg's output, fontconfig's triggers run. fontconfig's postinst runs

  # Force regeneration of all fontconfig cache files.
  mkdir -p /var/cache/fontconfig
  fc-cache -s -v 1>/var/log/fontconfig.log 2>&1 || printf "fc-cache failed.\nSee /var/log/fontconfig.log for more information.\n"

when triggered. I looked at /var/log/fontconfig.log, and I see a lot of "skipped" messages for various directories (because "existing cache is valid" or "looped directory detected"), and then I see

  /var/cache/fontconfig: cleaning cache directory
  /var/cache/fontconfig: invalid cache file: 0bd3dc0958fa2205aaaa8ebb13e2872b-le64.cache-7
  fc-cache: succeeded

Now I'm wondering how a fontconfig cache file becomes invalid, and how do other applications cope.