making port:xz optionally independent of other ports

[RJVB (René Bertin)]

There is an option in etc/macports/macports.conf to create xz'ipped software archives for ports built locally. That gives a substantial reduction in the disk footprint of those archives, but also introduces a dependency issue.

Xz depends on port:libiconv and port:gettext, and esp. the latter is updated rather frequently. The update sequence (when building from source) is as follows, taking gettext as an example (using xz compression and with the +universal variant which is the reason many of us have to build each upgrade from source):

1. port destroot gettext +universal
2. create the new archive in ${prefix}/var/macports/software/gettext/gettext???.txz
3. port deactivate gettext@current-version+universal
4. unpack the archive created in (2) in a temporary directory in ${prefix}/var/macports/software/gettext

And that's where things go wrong: since port:gettext has been deactivated, port:xz can no longer function.

The proposed enhancement allows to work around this by adding a default variant for xz, +nls. This default variant corresponds to the current situation, in which xz depends on libiconv and gettext to be "internationalised". Users who want to have xz compressed software archives can port install xz -nls, in which case said dependencies are dropped, and xz still functions when gettext or libiconv have been deactivated.

Upgrading xz itself will remain a problem.

The correct way to address this issue is of course to change the order of steps outline above:

1. port destroot xz +universal
2. create the new archive in ${prefix}/var/macports/software/xz/xz???.txz
3. unpack the archive created in (2) in a temporary directory in ${prefix}/var/macports/software/xz
4. port deactivate xz@current-version+universal
5. move the new files in place from the temporary directory into ${prefix}

This has 2 advantages: 1) the to-be-updated functionality remains available right up to the moment where standard system commands will be used to update it, so that tools used in the procedure can update themselves and 2) the "gap" during which the to-be-updated functionality is unavailable is reduced to the strict minimum, which means users are unlikely to notice it even when they're running the update in the background. This "gap" can be quite lengthy for large ports like Qt.

[larryv (Lawrence Velázquez)]

For what it’s worth, I don’t like this idea.

1. Variants are generally undesirable.
2. Frankly, we should not do anything to encourage people to use .txz archives until we make it reasonable to do so without the use of the xz port. As you already noted, systems that use .txz archives via the xz port would continue to break when xz itself is updated. The only stable way to address this would be to bundle xz with base.