Multi-dimensional transactions and rollbacks, oh my!

Ludovic Courtès — 24 липня 2018 року

One of the highlights of version 0.15.0 was the overhaul of guix pull, the command that updates Guix and its package collection. In Debian terms, you can think of guix pull as:

apt-get update && apt-get install apt

Let’s be frank, guix pull does not yet run as quickly as this apt-get command—in the “best case”, when pre-built binaries are available, it currently runs in about 1m30s on a recent laptop. More about the performance story in a future post…

One of the key features of the new guix pull is the ability to roll back to previous versions of Guix. That’s a distinguishing feature that opens up new possibilities.

“Profile generations”

Transactional upgrades and rollbacks have been a distinguishing feature of Guix since Day 1. They come for free as a consequence of the functional package management model inherited from the Nix package manager. To many users, this alone is enough to justify using a functional package manager: if an upgrade goes wrong, you can always roll back. Let’s recap how this all works.

As a user, you install packages in your own profile, which defaults to ~/.guix-profile. Then from time to time you update Guix and its package collection:

$ guix pull

This updates ~/.config/guix/current, giving you an updated guix executable along with an updated set of packages. You can now upgrade the packages that are in your profile:

$ guix package -u
The following packages will be upgraded:
   diffoscope   93 → 96     /gnu/store/…-diffoscope-96
   emacs    25.3 → 26.1     /gnu/store/…-emacs-26.1
   gimp     2.8.22 → 2.10.4 /gnu/store/…-gimp-2.10.4
   gnupg    2.2.7 → 2.2.9   /gnu/store/…-gnupg-2.2.9

The upgrade creates a new generation of your profile—the previous generation of your profile, with diffoscope 93, emacs 25.3, and so on is still around. You can list profile generations:

$ guix package --list-generations
Generation 1  Jun 08 2018 20:06:21
   diffoscope   93     out   /gnu/store/…-diffoscope-93
   emacs        25.3   out   /gnu/store/…-emacs-25.3
   gimp         2.8.22 out   /gnu/store/…-gimp-2.8.22
   gnupg        2.2.7  out   /gnu/store/…-gnupg-2.2.7
   python       3.6.5  out   /gnu/store/…-python-3.6.5

Generation 2  Jul 12 2018 12:42:08     (current)
-  diffoscope   93     out   /gnu/store/…-diffoscope-93
-  emacs        25.3   out   /gnu/store/…-emacs-25.3
-  gimp         2.8.22 out   /gnu/store/…-gimp-2.8.22
-  gnupg        2.2.7  out   /gnu/store/…-gnupg-2.2.7
+  diffoscope   96     out   /gnu/store/…-diffoscope-96
+  emacs        26.1   out   /gnu/store/…-emacs-26.1
+  gimp         2.10.4 out   /gnu/store/…-gimp-2.10.4
+  gnupg        2.2.9  out   /gnu/store/…-gnupg-2.2.9

That shows our two generations with the diff between Generation 1 and Generation 2. We can at any time run guix package --roll-back and get our previous versions of gimp, emacs, and so on. Each generation is just a bunch of symlinks to those packages, so what we have looks like this:

Notice that python was not updated, so it’s shared between both generations. And of course, all the dependencies that didn’t change in between—e.g., the C library—are shared among all packages.

guix pull generations

Like I wrote above, guix pull brings the latest set of package definitions from Git master. The Guix package collection usually contains only the latest version of each package; for example, current master only has version 26.1 of Emacs and version 2.10.4 of the GIMP (there are notable exceptions such as GCC or Python.) Thus, guix package -i gimp, from today’s master, can only install gimp 2.10.4. Often, that’s not a problem: you can keep old profile generations around, so if you really need that older version of Emacs, you can run it from your previous generation.

Still, having guix pull keep track of the changes to Guix and its package collection is useful. Starting from 0.15.0, guix pull creates a new generation, just like guix package does. After you’ve run guix pull, you can now list Guix generations as well:

$ guix pull -l
Generation 10   Jul 14 2018 00:02:03
  guix 27f7cbc
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: origin/master
    commit: 27f7cbc91d1963118e44b14d04fcc669c9618176
Generation 11   Jul 20 2018 10:44:46
  guix 82549f2
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: origin/master
    commit: 82549f2328c59525584b92565846217c288d8e85
  14 new packages: bsdiff, electron-cash, emacs-adoc-mode,
    emacs-markup-faces, emacs-rust-mode, inchi, luakit, monero-gui,
    nethack, openbabel, qhull, r-txtplot, stb-image, stb-image-write
  52 packages upgraded: angband@4.1.2, aspell-dict-en@2018.04.16-0,
    assimp@4.1.0, bitcoin-core@0.16.1, botan@2.7.0, busybox@1.29.1,
    …
Generation 12   Jul 23 2018 15:22:52    (current)
  guix fef7bab
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: origin/master
    commit: fef7baba786a96b7a3100c9c7adf8b45782ced37
  20 new packages: ccrypt, demlo, emacs-dired-du,
    emacs-helm-org-contacts, emacs-ztree, ffmpegthumbnailer, 
    go-github-com-aarzilli-golua, go-github-com-kr-text, 
    go-github-com-mattn-go-colorable, go-github-com-mattn-go-isatty, 
    go-github-com-mgutz-ansi, go-github-com-michiwend-golang-pretty, 
    go-github-com-michiwend-gomusicbrainz, go-github-com-stevedonovan-luar, 
    go-github-com-wtolson-go-taglib, go-github-com-yookoala-realpath, 
    go-gitlab-com-ambrevar-damerau, go-gitlab-com-ambrevar-golua-unicode,
    guile-pfds, u-boot-cubietruck
  27 packages upgraded: c-toxcore@0.2.4, calibre@3.28.0,
    emacs-evil-collection@20180721-2.5d739f5, 
    …

The nice thing here is that guix pull provides high-level information about the differences between two subsequent generations of Guix.

In the end, Generation 1 of our profile was presumably built with Guix Generation 11, while Generation 2 of our profile was built with Guix Generation 12. We have a clear mapping between Guix generations as created by guix pull and profile generations as created with guix package:

Each generation created by guix pull corresponds to one commit in the Guix repo. Thus, if I go to another machine and run:

$ guix pull --commit=fef7bab

then I know that I get the exact same Guix instance as my Generation 12 above. From there I can install diffoscope, emacs, etc. and I know I’ll get the exact same binaries as those I have above, thanks to reproducible builds.

These are very strong guarantees in terms of reproducibility and provenance tracking—properties that are typically missing from “applications bundles” à la Docker.

In addition, you can easily run an older Guix. For instance, this is how you would install the version of gimp that was current as of Generation 10:

$ ~/.config/guix/current-10-link/bin/guix package -i gimp

At this point your profile contains gimp coming from an old Guix along with packages installed from the latest Guix. Past and present coexist in the same profile. The historical dimension of the profile no longer matches exactly the history of Guix itself.

Composing Guix revisions

Some people have expressed interest in being able to compose packages coming from different revisions of Guix—say to create a profile containing old versions of Python and NumPy, but also the latest and greatest GCC. It may seem far-fetched but it has very real applications: there are large collections of scientific packages and in particular bioinformatics packages that don’t move as fast as our beloved flagship free software packages, and users may require ancient versions of some of the tools.

We could keep old versions of many packages but maintainability costs would grow exponentially. Instead, Guix users can take advantage of the version control history of Guix itself to mix and match packages coming from different revisions of Guix. As shown above, it’s already possible to achieve this by running the guix program off the generation of interest. It does the job, but can we do better?

In the process of enhancing guix pull we developed a high-level API that allows an instance of Guix to “talk” to a different instance of Guix—an “inferior”. It’s what allows guix pull to display the list of packages that were added or upgraded between two revisions. The next logical step will be to provide seamless integration of packages coming from an inferior. That way, users would be able to refer to “past” package graphs right from a profile manifest or from the command-line. Future work!

On coupling

The time traveler in you might be wondering: Why are package definitions coupled with the package manager, doesn’t it make it harder to compose packages coming from different revisions? Good point!

Tight coupling certainly complicates this kind of composition: we can’t just have any revision of Guix load package definitions from any other revision; this could fail altogether, or it could provide a different build result. Another potential issue is that guix pulling an older revision not only gives you an older set of packages, it also gives you older tools, bug-for-bug.

The reason for this coupling is that a package definition like this one doesn’t exist in a vacuum. Its meaning is defined by the implementation of package objects, by gnu-build-system, by a number of lower-level abstractions that are all defined as extensions of the Scheme language in Guix itself, and ultimately by Guile, which implements the language Guix is written in. Each instance created by guix pull brings all these components. Because Guix is implemented as a set of programming language extensions and libraries, that package definitions depend on all these parts becomes manifest. Instead of being frozen, the APIs and package definitions evolve together, which gives us developers a lot of freedom on the changes we can make.

Nix results from a different design choice. Nix-the-package-manager implements the Nix language, which acts as a “frozen” interface. Package definitions in Nixpkgs are written in that language, and a given version of Nix can possibly interpret both current and past package definitions without further ado. The Nix language does evolve though, so at one point an old Nix inevitably becomes unable to evaluate a new Nixpkgs, and vice versa.

These two approaches make different tradeoffs. Nix’ loose coupling simplifies the implementation and makes it easy to compose old and new package definitions, to some extent; Guix’ tight coupling makes such composition more difficult to implement, but it leaves developers more freedom and, we hope, may support “time travels” over longer period of times. Time will tell!

It’s like driving a DeLorean

Picture of a DeLorean cabin by Oto Godfrey and Justin Morton, under CC-BY-SA 4.0.

That profile generations are kept around already gave users a time machine of sorts—you can always roll back to a previous state of your software environment. With the addition of roll-back support for guix pull, this adds another dimension to the time machine: you can roll-back to a previous state of Guix itself and from there create alternative futures or even mix bits from the past with bits from the present. We hope you’ll enjoy it!