[Mesa-dev] shader cache backward compatibility

[Alexander Larsson]

Hi, I'm the developer behind flatpak (https://flatpak.org/) and we've
recently run into some issues with the mesa shader cache. Flatpak has
a app/runtime split where the runtime is shared between app and
provides /usr. The runtime contains a version of mesa, but this can be
overridden by runtime extensions to add other OpenGL drivers.

Each app has a separate $XDG_CACHE_HOME, pointing into the per-app
writable storage. For example, gedit has
XDG_CACHE_HOME="/home/alex/.var/app/org.gnome.gedit/cache". This
causes mesa to store the shader cache per-app in
$XDG_CACHE_HOME/mesa_shader_cache.

In the regular case this works fine, but sometimes the version of mesa
is changed, with the shader cache being left in place. For example,
sometimes we update the mesa version in the runtime, and sometimes the
app switches to a new runtime which has a different mesa version.

Such updates have caused a lot of issues for us, ranging from direct
crashes at startup as in
https://github.com/flatpak/flatpak/issues/2052 and sometimes just
super-slow performance. In all cases, blowing away the shader cache
directory fixed all issues.

The steam flatpak has a bunch of workaround for the cache:
  https://github.com/flathub/com.valvesoftware.Steam/blob/master/steam_wrapper/steam_wrapper.py#L35
But we can't expect every app to do this.

So, my question is, is the cache supposed to be backward compatible,
or at least versioned? Are we missing something in our mesa builds to
make that work? Is this fixed somewhere with a patch i can backport?
And if not, do we need to add some magic to flatpak to automatically
clean up the shader cache after an update?

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 Alexander Larsson                                Red Hat, Inc
       alexl at redhat.com         alexander.larsson at gmail.com