[Bug 1803534] Re: Backport uuid based cache file naming scheme

[Sebastien Bacher]

I installed 'fontconfig' from proposed and the package installation failed on that error
'fc-cache: symbol lookup error: fc-cache: undefined symbol: FcDirCacheCreateUUID'

It looks like the fontconfig binary should depends on the new version of
the lib and doesn't?

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Title:
  Backport uuid based cache file naming scheme

Status in fontconfig package in Ubuntu:
  New
Status in fontconfig source package in Bionic:
  Fix Committed

Bug description:
  Fontconfig 2.13.0 changed the cache file naming scheme to be based on
  the contents of ".uuid" file in the directory rather than a hash of
  the directory name.  On a pure debs system this doesn't really matter
  since everything is using the same libfontconfig.so.

  When snaps are involved, it can lead to some apps not seeing the cache
  files produced by a different fontconfig version.  In particular,
  while libfontconfig 2.13 can reuse 2.12's cache files for read only
  directories, the reverse is not true.  This will be a problem for apps
  built on top of the "core18" base snap when run on later Ubuntu
  releases (cosmic, disco, etc).

  By providing a backport of the UUID cache file feature to bionic for
  use by snap applications, we'd avoid this.  Having it in bionic-
  updates would be ideal so that snapcraft picks it up automatically.

  [Impact]

   * This update changes how fontconfig cache files are named, instead
  of using md5($dir), it instead uses the contents of a $dir/.uuid file.
  No changes are made to the format of the cache file contents.

   * This change is primarily intended for use by snap applications
  built with core18: as the updated libfontconfig will fall back to the
  md5 cache file names for read-only directories without a .uuid file,
  they will be able to reuse cache files from any host system running
  fontconfig >= 2.11.95

  [Test Case]

   * After installing the update, cache files should be generated in
  /var/cache/fontconfig/ with names like "07b67f7a-16ea-4440-9b6c-
  21fc9153568c-le64.cache-7" (note the extra dashes not present in the
  MD5 based cache file names).

  [Regression Potential]

   * Applications using a non-default libfontconfig could end up not
  finding the new cache files and regenerating them in
  ~/.cache/fontconfig.

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