Upgrading libpng: shall we move to 1.4.x or 1.5.x?

[Tom Lane]

I have been looking into replacing Fedora's obsolete version of libpng
(1.2.x release series) with something more modern.  The possible choices
are the 1.4.x and 1.5.x release series.  The 1.5.x series adds some more
features that 1.4.x did not have, but it also poses significantly more
migration problems.  The reason is that 1.5.x no longer exposes the
contents of the library's internal "png_info" struct.  Direct access
to fields of png_info has been deprecated for a long time, in favor of
using accessor functions, but up through 1.4 you can get away with it.

I did test rebuilds in mock of all rawhide packages that are reported to
be dependent on libpng.  Out of 964 packages with dependencies on libpng,
we have:

Packages that rebuilt successfully with 1.5	658
Packages that FTBFS for non-libpng reasons	186
Packages that rebuilt with 1.4, but not 1.5	74
Packages that need help even with 1.4		46

(The non-libpng failures seem to mostly be due to the recent upgrade to
glib 2.0.  Some of those probably have libpng issues as well, but didn't
get far enough in their builds to expose them.)

About half of the "need help anyway" group may just need their
BuildRequires to be rebuilt first --- it looked like they had no
source-code dependency, but were just absorbing "-lpng12" from library
link flags reported by other packages.  I built each package independently
rather than trying to chain the builds, so this wasn't catered for.

The vast majority of the 74 packages that would need extra work if we move
to libpng 1.5 are trying to access the png_info struct directly, and so
would need patches to use the accessor functions instead.  This is work
that should be done and upstreamed anyway, but if we move to 1.4 we would
not have to do it Right Now.  It looks like it would be a fairly large
amount of work, too --- I count 1164 "incomplete type" failures in the
build logs for those packages, and it's quite likely there are more in
source files that the build runs didn't get to.  On the other hand, if we
move to 1.4 there will not be any pressing reason for these fixes to get
made, and so I'm not sure that we'll be any better off when we try to move
to 1.5 later.  Another point is that we'd have to build these same 964
packages over again if we plan a two-stage upgrade.

Any opinions on which way to jump?

In either case, as per the discussion at
http://lists.fedoraproject.org/pipermail/devel/2011-October/157712.html
I plan to provide the 1.2.x libpng shared library (and only the library,
not its devel support files) in a libpng-compat subpackage for the time
being.  So existing programs that depend on 1.2.x will continue to work,
but it won't be possible to rebuild a package that has source-level
dependencies on 1.2.x until those are fixed.  I think this is enough
to avoid needing a special build tag for staging the rebuilds.

			regards, tom lane