Saturday, November 15, 2014

RDepending on Perl itself

Writing correct dependency specifications is an art in itself. So, here's a small guide for Gentoo developers how to specify runtime dependencies on dev-lang/perl. First, the general rule.
Check the following two things: 1) is your package linking anywhere to, 2) is your package installing any Perl modules into Perl's vendor directory (e.g., /usr/lib64/perl5/vendor_perl/5.20.1/)? If at least one of these two questions is answered with yes, you need in your dependency string a slot operator, i.e. "dev-lang/perl:=" Obviously, your ebuild will have to be EAPI=5 for that. If neither 1) nor 2) are the case, "dev-lang/perl" is enough.
Now, with eclasses. If you use perl-module.eclass or perl-app.eclass, two variables control automatic adding of dependencies. GENTOO_DEPEND_ON_PERL sets whether the eclass automatically adds a dependency on Perl, and defaults to yes in both cases. GENTOO_DEPEND_ON_PERL_SUBSLOT controls whether the slot operator ":=" is used. It defaults to yes in perl-module.eclass and to no in perl-app.eclass. (This is actually the only difference between the eclasses.) The idea behind that is that a Perl module package always installs modules into vendor_dir, while an application can have its own separate installation path for its modules or not install any modules at all.
In many cases, if a package installs Perl modules you'll need Perl at build time as well since the module build system is written in Perl. If a package links to Perl, that is obviously needed at build time too.

So, summarizing:
eclass 1) or 2) true 1) false, 2) false
none "dev-lang/perl:=" needed in RDEPEND and most likely also DEPEND "dev-lang/perl" needed in RDEPEND, maybe also in DEPEND
perl-module.eclass no need to do anything GENTOO_DEPEND_ON_PERL_SUBSLOT=no possible before inherit
perl-app.eclass GENTOO_DEPEND_ON_PERL_SUBSLOT=yes needed before inherit no need to do anything

Monday, November 10, 2014

NJP accepted: Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube

Today's good news is that our manuscript "Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube" has been accepted for publication by New Journal of Physics.
In a way, this work is directly building on our previous publication on thermally induced quasiparticles in niobium-carbon nanotube hybrid systes. As a contribution mainly from our theory colleagues, now the modelling of transport processes is enhanced and extended to cotunneling processes within Coulomb blockade. A generalized master equation based on the reduced density matrix approach in the charge conserved regime is derived, applicable to any strength of the intradot interaction and to finite values of the superconducting gap.
We show both theoretically and experimentally that also in cotunneling spectroscopy distinct thermal "replica lines" due to the finite quasiparticle occupation of the superconductor occur at higher temperature T~1K: the now possible transport processes lead to additional conductance both at zero bias and at finite voltage corresponding to an excitation energy; experiment and theoretical result match very well.

"Thermally induced subgap features in the cotunneling spectroscopy of a carbon nanotube"
S. Ratz, A. Donarini, D. Steininger, T. Geiger, A. Kumar, A. K. Hüttel, Ch. Strunk, and M. Grifoni
accepted for publication by New Journal of Physics, arXiv:1408.5000 (PDF)

Sunday, October 19, 2014

How do I run ~arch Perl on a stable Gentoo system?

Here's a small piece of advice for all who want to upgrade their Perl to the very newest available, but still keep running an otherwise stable Gentoo installation: These three lines are exactly what needs to go into /etc/portage/package.keywords:
Of course, as always, bugs may be present; what you get as Perl installation is called unstable or testing for a reason. We're looking forward to your reports on our bugzilla.

Friday, August 22, 2014

EAPI=5 adoption has reached 50% of ebuilds in the portage tree

As of today, more than 50% of the 37527 ebuilds in the Gentoo portage tree use the newest ebuild API (EAPI) version, EAPI=5!
The details of the various EAPIs can be found in the package manager specification (PMS); the most notable new feature of EAPI 5, which has sped up acceptance a lot is the introduction of so-called subslots. A package A can specify a subslot, another package B that depends on it can specify that it needs to be rebuilt when the subslot of A changes. This leads to much more elegant solutions for many of the the link or installation path problems that revdep-rebuild, emerge @preserved-rebuild, or e.g. perl-cleaner try to solve... Another useful new feature in EAPI=5 is the masking of use-flags specifically for stable-marked ebuilds.
You can follow the adoption of EAPIs in the portage tree on an automatically updated graph page.

Sunday, August 3, 2014

Perl in Gentoo: Upgrading pains, perl-cleaner, and EAPI=5

In a previous post, we've already looked at the structure of Perl ebuilds in Gentoo Linux. Now, let's see what happens in the case of a major Perl update.

Does this look familiar?
After updating dev-lang/perl you must reinstall
the installed perl modules.
Use: perl-cleaner --all
Then maybe you have updated your major Perl version recently, since this important message is printed by emerge afterwards. So, what is it about? In short, a certain disconnect between the "Perl way" of doing things and the rest of the world. Both have their merits, they just don't play very well with each other... and the result is that major Perl updates in Gentoo have traditionally also been a major pain. (This will become much better in the future, see below.)

Let's see where a perl package stores its files.
caipi ~ # equery files dev-perl/Email-Address
 * Searching for Email-Address in dev-perl ...
 * Contents of dev-perl/Email-Address-1.898.0:
caipi ~ #
Interesting- the installation path contains the Perl version! The reasons for upstream to do this are pretty much obvious, the application binary interface for compiled modules can change and it's necessary to keep the installed modules for different versions apart. Also, in theory you can keep different Perl versions installed in parallel. Nice idea, however if you have only one "system Perl" installation, and you exchange that for a newer version (say, 5.18.1 instead of 5.16.3), the result is that the new version won't find the installed packages anymore.

The results are rather annoying. Imagine you haven't updated your system for a while, one of the many packages to be updated is dev-lang/perl, and later maybe (just picking an example at random) gnome-base/gsettings-desktop-schemas. Perl is updated fine, but when portage arrives at building the gnome package, the build fails with something like
checking for perl >= 5.8.1... 5.18.2
checking for XML::Parser... configure: error: XML::Parser perl module is required for intltool
Right. Perl is updated, dev-perl/XML-Parser is still installed in the old path, and Perl doesn't find it. Bah.

Enter perl-cleaner, the traditional "solution". This small program checks for files in "outdated" Perl installation paths, finds out which packages they belong to, and makes portage rebuild the corresponding packages. During the rebuild, the installation is run by the updated Perl, which makes the files go into the new, now correct path.

This sounds like a good solution, but there are a lot of details and potential problems hidden. For once, most likely you'll run perl-cleaner after a failed emerge command, and some unrelated packages still need updates. Portage will try to figure out how to do this, but blockers and general weirdness may happen. Then, sometimes a package isn't needed with the new Perl version anymore, but perl-cleaner can't know that. Again the result may be a blocker. We've added the following instructions to the perl-cleaner output, which may help cleaning up the most frequent difficulties:
 * perl-cleaner is stopping here:
 * Fix the problem and start perl-cleaner again.
 * If you encounter blockers involving virtuals and perl-core, here are
 * some things to try:
 *   Remove all perl-core packages from your world file
 *     emerge --deselect --ask $(qlist -IC 'perl-core/*')
 *   Update all the installed Perl virtuals
 *     emerge -uD1a $(qlist -IC 'virtual/perl-*')
 *   Afterwards re-run perl-cleaner
In the end, you may have to try several repeated emerge and perl-cleaner commands until you have an updated and consistent system again. So far, it always worked somehow with fiddling, but the situation was definitely not nice.

So what's the future? Well...

EAPI=5 brings the beautiful new feature of subslots and slot operator dependencies. In short, a package A may declare a subslot, and a package B that depends on A may declare "rebuild me if A changes subslot". This mechanism is now used to automate the Perl rebuilds directly from within emerge: dev-lang/perl declares a subslot corresponding to its major version, say "5.18", and every package that installs Perl modules needs to depend on it with the subslot-rebuild requested, e.g.
The good news about this is that portage now knows the dependency tree and can figure out the correct reinstallation order.

The bad news is, it can only work perfectly after all Perl packages have been converted to EAPI=5 and stabilized. perl-core is done, but with about 2100 ebuilds that use perl-module.eclass in the portage tree still quite some work remains. I've plotted the current EAPI distribution of ebuilds using perl-module.eclass in a pie chart for illustration... Maybe we're done when Perl 5.20 goes stable. Who knows. :)

Sunday, July 27, 2014

Perl in Gentoo: dev-lang/perl, virtuals, and perl-core packages

We've got the stabilization of Perl 5.18 upcoming, so what better chance is there to explain a bit how the Perl-related ebuilds in Gentoo work...

First of all, there is dev-lang/perl. This contains the Perl core distribution, installing the binaries and all the Perl modules that are bundled with Perl itself.

Then, there is the perl-core category. It contains independent ebuilds for Perl modules that are also present in the core Perl distribution. Most Perl modules that are bundled with Perl are also in addition released as independent tarballs. If any of these packages is installed from perl-core, its files are placed such that the perl-core download overrides the bundled copy. This means you can also update part of the bundled Perl modules, e.g. in case of a bug, without updating Perl itself.

Next, there are a lot of virtuals "virtual/perl-..." in the virtual category of the portage tree. What are these good for? Well, imagine you want to depend on a specific version of a module that is usually bundled with Perl. For example, you need Module::CoreList at at least version 3.  This can either be provided by a new enough Perl (for example, now hardmasked Perl 5.20 contains Module::CoreList 3.10), or by a separate package from perl-core (where we have Module::CoreList 5.021001 as perl-core/Module-CoreList-5.21.1).
To make sure that everything works, you should never directly depend on a perl-core package, but always on the corresponding virtual (here virtual/perl-Module-CoreList; any perl-core package must have a corresponding virtual). Then both ways to fulfil the dependency are automatically taken into account. (Future repoman versions will warn if you directly depend on perl-core. Also you should never have anything perl-core in your world file!)

Last, we have lots of lots of modules in the dev-perl category. Most of them are from CPAN, and the only thing they have in common is that they have no copy inside core Perl.

I hope this clarifies things a bit. More Perl posts coming...

Friday, June 27, 2014

Emmy Noether grant extended

Today we've received the good news that our Emmy Noether project on the electronic and nano-electromechanical properties of carbon nanotubes has been given a positive intermediate evaluation from the referees. This means funding for an additional period will be granted. Cheers!