tree f10cdb8232719d0ee5ff419e6d07db4f2bcd6696
parent 2b814cde29535ab356401e91c3edf7ca3dcddb49
author John Bowler <jbowler@acm.org> 1703976938 -0800
committer Cosmin Truta <ctruta@gmail.com> 1705615026 +0200

Do not build unused Adler32 code

This removes the default build of an undocumented feature to disable
Adler32 checksums on those systems where it was the default.

The PR is motived by github #187 however it fixes a much more general
problem (#187 is limited to an issue where libpng "crashes" on some
manufacturer systems).  The fix is based on a suggestion by @sgowdev who
is the originator of the issue.

When libpng disables the checking of Adler32 checksums it does so by an
undocumented and therefore possibly unsupported call to a zlib function
which does not exist in some versions of zlib.

Fortunately libpng only does this if the caller of libpng explicitly
asks for it to happen.  Unfortunately the call to the undocumented
function is still in the compiled and built libpng and this means that
on some systems (as identified in #187) libpng can fail to load or maybe
even crash.

The libpng authors are currently unaware of any program or system that
uses this feature and none has been identified by the contributors to

In this fix an option is added to *enable* the code so that by default
the code is *disabled* - this is a simple generalization of the
suggestion by @sgowdev.

BENEFITS: the problem is eliminated, users of the functionality, if any,
are idenfified, the functionality can be implemented correctly in the
future or it can be removed.  Hardly anyone complains.

COSTS: someone will complain that they have to enable an option in a
libpng build to use a feature that never worked consistently in the
first place.

This patch has been tested both with the option enabled and with it
disabled via pngusr.dfa.  Tests, checks pass with cmake and configure,
make distcheck passes on configure.

Reported-by: Stephen Gowen <dev.sgowen@gmail.com>
Signed-off-by: John Bowler <jbowler@acm.org>
Signed-off-by: Cosmin Truta <ctruta@gmail.com>