[Fontconfig] Next steps for a reproducible Fontconfig?

[Chris Lamb]

[Adding 864082 at bugs.debian.org to CC]

Dear fontconfig maintainers,

I've just spent a coffee-or-two unpicking this to get the latest status
and to load the history back into my brain.

As a bit of background, I'm working on the Reproducible Builds
effort and fontconfig — in its usual usage, or at least in Debian
at the time — generated unreproducible cache files.

This was due to it using the timestamps of each directory in the
`checksum` member of the `_FcCache` struct. This is so that it can
identify which cache files remain valid and/or require regeneration
(or similar logic).

So therefore in June 2017 I sent an initial patch:

  https://lists.freedesktop.org/archives/fontconfig/2017-June/005948.html

… which, after some (private?) discussion regarding the implementation, resulted in:

  https://lists.freedesktop.org/archives/fontconfig/2018-May/006285.html

… and that was merged after some further round-trips in f098adac54:

   https://lists.freedesktop.org/archives/fontconfig/2018-May/006289.html

… which was released as part of:

   $ git tag --contains f098adac54 | head -n1
   2.13.1

So far, so good. However, Johannes Schauer then reported that
fontconfig "still" installs unreproduciby:

  https://bugs.debian.org/864082#101

… so I prepared a new patch:

  https://lists.freedesktop.org/archives/fontconfig/2018-October/006374.html

… and that was "soft NACK'd" in the sense that Keith mentions:

    > I've dug into this a bit more and I think an architectural change in the
    > cache files made last year is probably not what we want.

       — https://lists.freedesktop.org/archives/fontconfig/2018-October/006376.html

(I am now inferring that it was this "architectural change"
resulted in the regression Johannes reported, rather than the bug
being incomplete from the beginning.)

Anyway, the upshot from my proposal was that some larger/different
changes are/were "requested" instead.

Behdad Esfahbod also chimed in with:

    > I don't like the new mechanism either, but I think it was added to resolve
    > bind-mounted font dirs

       — https://lists.freedesktop.org/archives/fontconfig/2018-October/006381.html

… in the context of Flatpak apps. Keith then addressed all this
with a branch which he published here:

  https://gitlab.freedesktop.org/keithp/fontconfig

… the most salient commit being (I think?):

  https://gitlab.freedesktop.org/keithp/fontconfig/commit/a04751b2e624d034becec7588159ef2f9a8dfc1b

Since then, I don't believe there has been any review of this
branch both in the sense of the code itself but also in terms of
the architectural changes that it implies. I might be able to help
on the former front but without knowing the "lore" of Fontconfig I
simply cannot comment on the latter parts.

Anyway, I'd love to get this resolved once and for all ideally get
it into Debian buster which is about to start "freezing" very
soon.

What would be the best way for me to help here? Can I entreat Keith
to merge his branch? I can put some cycles onto this issue if that is
of some assistance.


Best wishes,

-- 
Chris Lamb
chris-lamb.co.uk / @lolamby