[Buildroot] State of Perl: remaining questions

[Thomas Petazzoni]

Hello Peter,

As we discussed on IRC, here is my opinion on the state of the Perl
patch set from François and the remaining points of discussion.

 * I am fine with patches 1/10, 2/10, 3/10, 4/10, 5/10, 6/10. I think
   those patches could be committed now.

 * I have asked François to refactor a bit the removal of microperl
   (patches 7/10, 8/10 and 10/10). In the end, it will make no
   difference, it's just the order in which are done that will be
   different. But this also requests for your approval to remove the
   microperl package without having a deprecation period. I believe
   that since we're adding a full Perl installation instead, there
   should be no problem in doing this.

 * I am still not fully convinced by 9/10, the integration of
   cpanminus, and I will give some details below.

François will correct me if I'm wrong, but cpanminus is a tool that
easily allows to download and build Perl modules, taking care of the
necessary dependencies. It is a bit like easy_install for Python, if
I'm correct.

So the idea is that instead of having Buildroot packages for each and
every Perl module, you only have cpanminus, which provides two important
Kconfig knobs:

 * BR2_PACKAGE_CPANMINUS_MODULES, thanks to which you provide a
   space-separated list of Perl modules you want.

 * BR2_PACKAGE_CPANMINUS_NATIVE_DEPENDENCIES, thanks to which you
   provide a space-separated list of Buildroot packages that are native
   dependencies needed to build the Perl modules listed in
   BR2_PACKAGE_CPANMINUS_MODULES.

For example, BR2_PACKAGE_CPANMINUS_MODULES can be "Curses::UI" to get
the Perl module of this name, in which case
BR2_PACKAGE_CPANMINUS_NATIVE_DEPENDENCIES has to be "curses" so that
our "curses" package gets built as a dependency of cpanminus.

So, cpanminus is doing its own package management, download process,
dependency tracking, which to me creates a number of problems:

 *) No integration with the licensing infrastructure. The individual
    Perl modules are not visible in terms of licensing report. This is
    maybe not a big issue since most of them are released under the
    Artistic license.

 *) No integration with the download mechanisms of Buildroot. So it
    doesn't put the downloaded tarballs in $(DL_DIR), doesn't take care
    of $(BR2_PRIMARY_SITE), breaks "make source", "make external-deps",
    prevents people from having guarantees to make offline builds
    (unless they create a specific cpan mirror, of course).

 *) The dependency tracking seems a bit fragile, i.e the user needs to
    know on which native package a given Perl module depend. This is
    maybe not that problematic, we can assume that the developer crazy
    enough to write Perl code will know his dependencies :-)

On the other end, as Arnout and Francois have noted, create Buildroot
packages for each and every Perl module around is going to bring us a
crazy number of packages. Perl modules are apparently even more split
in fine-grained components than Python packages are, so the number of
dependencies and components if even larger.

Note that I am not carrying a strong NAK on cpanminus, I am just
pointing out how it moves away from the normal Buildroot
infrastructure, and which problems it generates. I will not oppose to
see it merged, I don't plan to be using it myself.

So to summarize, the only part of this series that to me remains a bit
controversial is this cpanminus thing, patch 9/10.

I would like to thank again François for his excellent persistence and
perseverance in his work on the Lua and Perl stuff in Buildroot. This
is _highly_ appreciated. Thanks!

Thomas
-- 
Thomas Petazzoni, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com