Bug#759405: perl: make pod2man timestamps reproducible

[Niko Tyni]

Package: perl
Version: 5.20.0-4
Severity: wishlist
User: reproducible-builds at lists.alioth.debian.org
Usertags: toolchain, timestamps
X-Debbugs-Cc: rra at debian.org

Manual pages generated with Pod::Man have a date header based on the
timestamp of the source file, or the current time when reading STDIN.
For the benefit of reproducible builds [1], it would be nice if this
date did not vary across rebuilds.

I doubt any normal build system feeds STDIN to Pod::Man, but another
case where the current time is used is when there are Debian patches to
the source file. Quoting the dpkg-source(1) manual page:

  The timestamp of all patched files is reset to the extraction time of
  the source package (this avoids timestamp skews  leading  to  problems
  when  autogenerated  files  are patched)

I've filed #759404 against dpkg-dev about changing this, but in the
meantime it would be nice to fix/work around this somehow. The best thing
I've come up with is an environment variable (maybe POD2MAN_DATE?) that
would override the date; changing build systems to pass the '--date'
option to pod2man or its equivalents would be much more intrusive.

AFAICS this could be done by changing Pod::Man::devise_date() to fall
back to this variable if it is set. Changing /usr/bin/pod2man isn't
enough: at least ExtUtils::MakeMaker build systems use their own pod2man
implementation in ExtUtils::Command::MM, and so does the perl build
itself in the 'installman' script.

I'm not sure who should export this environment variable, but debhelper
springs to mind. It should probably be done for all build systems rather
than just for the perl ones, as there may well be non-perl packages in
the archive that use POD for documentation.

[1] https://wiki.debian.org/ReproducibleBuilds
-- 
Niko Tyni   ntyni at debian.org