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Desktop Extensions Font Hook

Scope

Setup the general font cache in the desktop extensions using an extension provided configure hook to cache the system fonts and let the launcher only process user fonts (e.g.; .fonts) to decouple font caching issues found on many systems and offload first run times to the system (e.g.; snapd install and refreshes).

The cross distro story shall benefit from decoupling font processing from the system as there will be no more font cache misses due to cache ABI changes between the cache generation logic found within the snap and that found on the system.

General mechanism

The fonts.conf file shall be modified to achieve fallback mechanisms.

When the configure hook runs it must run fc-cache to update the fonts cache with from fonts that are generally available on the system using a privately generated fonts.conf. This cache must reside in a location that is generally available to the snap when run by a desktop user, to achieve such result, the cache shall reside in $SNAP_DATA.

When the desktop application launches, the launcher generated by the extension shall contain a reference to this cache and only process fonts that are installed on the user’s private font instance, ~/.fonts.

Extension changes

The desktop extensions require changes in two of its components, the desktop parts that generate the required command-chain shall generate a new command-chain proper for the configure hook that will generate the system fonts cache.

The fonts.conf that shall be written on hook run (to allow for future modification) and referenced by fc-cache shall look as follows:

<fontconfig>
  <cachedir>$SNAP_DATA/fontconfig</cachedir>
  <include ignore_missing="yes">/etc/fonts/fonts.conf</include>
</fontconfig>

The YAML generated by the desktop extensions shall be extended to add a hook entry that references this new command-chain, such that the following is generated:

hooks:
  configure:
    command-chain:
    - snap/command-chain/snapcraft-runner
    - snap/command-chain/hooks-configure-desktop
    plugs:
    - desktop

Meta Generation changes

Logic must be added to support running a command-chain for a hook when no hook was provided by the snap developer, Snapcraft must support the scenario where a command-chain for a hook was defined in snapcraft.yaml but no hook was provided.

To achieve this result and considering the current logic existing in Snapcraft’s meta package is that Snapcraft shall generate a stub for a hook for when it finds command-chains in hooks which shall be overwritten by actual hooks.