GNU Guix can be used as a “package manager” to install and upgrade software packages as is familiar to GNU/Linux users, or as an environment manager, but it can also provision containers or virtual machines, and manage the operating system running on your machine.

One foundation that sets it apart from other tools in these areas is reproducibility. From a high-level view, Guix allows users to declare complete software environments and instantiate them. They can share those environments with others, who can replicate them or adapt them to their needs. This aspect is key to reproducible computational experiments: scientists need to reproduce software environments before they can reproduce experimental results, and this is one of the things we are focusing on in the context of the Guix-HPC effort. At a lower level, the project, along with others in the Reproducible Builds community, is working to ensure that software build outputs are reproducible, bit for bit.

Work on reproducibility at all levels has been making great progress. Guix, for instance, allows you to travel back in time. That Guix can travel back in time and build software reproducibly is a great step forward. But there’s still an important piece that’s missing to make this viable: a stable source code archive. This is where Software Heritage (SWH for short) comes in.

When source code vanishes

Guix contains thousands of package definitions. Each package definition specifies the package’s source code URL and hash, the package’s dependencies, and its build procedure. Most of the time, the package’s source code is an archive (a “tarball”) fetched from a web site, but more and more frequently the source code is a specific revision checked out directly from a version control system.

The obvious question, then, is: what happens if the source code URL becomes unreachable? The whole reproducibility endeavor collapses when source code disappears. And source code does disappear, or, even worse, it can be modified in place. At GNU we’re doing a good job of having stable hosting that keeps releases around “forever”, unchanged (modulo rare exceptions). But a lot of free software out there is hosted on personal web pages with a short lifetime and on commercial hosting services that come and go.

By default Guix would look up source code by hash in the caches of our build farms. This comes for free: the “substitute” mechanism extends to all “build artifacts”, including downloads. However, with limited capacity, our build farms do not keep all the source code of all the packages for a long time. Thus, one could very well find oneself unable to rebuild a package months or years later, simply because its source code disappeared or moved to a different location.

Connecting to the archive

It quickly became clear that reproducible builds had “reproducible source code downloads”, so to speak, as a prerequisite. The Software Heritage archive is the missing piece that would finally allow us to reproduce software environments years later in spite of the volatility of code hosting sites. Software Heritage’s mission is to archive essentially “all” the source code ever published, including version control history. Its archive already periodically ingests release tarballs from the GNU servers, repositories from GitHub, packages from PyPI, and much more.

We quickly settled on a scheme where Guix would fall back to the Software Heritage archive whenever it fails to download source code from its original location. That way, package definitions don’t need to be modified: they still refer to the original source code URL, but the downloading machinery transparently goes to Software Heritage when needed.

There are two types of source code downloads in Guix: tarball downloads, and version control checkouts. In the former case, resorting to Software Heritage is easy: Guix knows the SHA256 hash of the tarball so it can look it up by hash using the /content endpoint of the archive’s interface.

Fetching version control checkouts is more involved. In this case, the downloader would first resolve the commit identifier to obtain a Software Heritage revision. The actual code for that revision is then fetched through the vault.

The vault conveniently allows users to fetch the tarball corresponding to a revision. However, not all revisions are readily available as tarballs (understandably), so the vault has an interface that allows you to request the “cooking” of a specific revision. Cooking is asynchronous and can take some time. Currently, if a revision hasn’t been cooked yet, the Guix download machinery will request it and wait until it is available. The process can take some time but will eventually succeed.

Success! At this point, we have essentially bridged the gap between the stable archive that Software Heritage provides and the reproducible software deployment pipeline of Guix. This code was integrated in November 2018, making it the first free software distribution backed by a stable archive.

The challenges ahead

This milestone was encouraging: we had seemingly achieved our goal, but we also knew of several shortcomings. First, even though the software we package is still primarily distributed as tarballs, Software Heritage keeps relatively few of these tarballs. Software Heritage does ingest tarballs, notably those found on the GNU servers, but the primary focus is on preserving complete version control repositories rather than release tarballs.

It is not yet clear to us what to do with plain old tarballs. On one hand, they are here and cannot be ignored. Furthermore, some provide artifacts that are not in version control, such as configure scripts, and often enough they are accompanied by a cryptographic signature from the developers that allows recipients to authenticate the code—an important piece of information that’s often missing from version control history. On the other hand, version control tags are increasingly becoming the mechanism of choice to distribute software releases. It may be that tags will become the primary mechanism for software release distribution soon enough.

Version control tags turn out not to be ideal either, because they’re mutable and per-repository. Conversely, Git commit identifiers are unambiguous and repository-independent because they’re essentially content-addressed, but our package definitions often refer to tags, not commits, because that makes it clearer that we’re providing an actual release and not an arbitrary revision (this is another illustration of Zooko’s triangle).

This leads to another limitation that stems from the mismatch between the way Guix and Software Heritage compute hashes over version control checkouts: both compute a hash over a serialized representation of the directory, but they serialize the directory in a different way (SWH serializes directories as Git trees, while Guix uses “normalized archives” or Nars, the format the build daemon manipulates, which is inherited from Nix.) That prevents Guix from looking up revisions by content hash. The solution will probably involve changing Guix to support the same method as Software Heritage, and/or adding Guix’s method to Software Heritage.

Having to wait for “cooking” completion can also be problematic. The Software Heritage team is investigating the possibility to automatically cook all version control tags. That way, relevant revisions would almost always be readily available through the vault.

Similarly, we have no guarantee that software provided by Guix is available in the archive. Our plan is to extend Software Heritage such that it would periodically archive the source code of software packaged by Guix.

Going further

In the process of adding support for Software Heritage, Guix gained Guile bindings to the Software Heritage HTTP interface. Here’s a couple of things we can do:

(use-modules (guix swh))

;; Check whether SWH has ever crawled our repository.
(define o (lookup-origin "https://git.savannah.gnu.org/git/guix.git"))
⇒ #<<origin> id: 86312956 …>

;; It did! When was its last visit?
(define last-visit
  (first (origin-visits o)))

(date->string (visit-date last-visit))
⇒ "Fri Mar 29 10:07:45Z 2019"

;; Does it have our “v0.15.0” Git tag?
(lookup-origin-revision "https://git.savannah.gnu.org/git/guix.git" "v0.15.0")
⇒ #<<revision> id: "359fdda40f754bbf1b5dc261e7427b75463b59be" …>

Guix itself is a Guile library so when we combine the two, there are interesting things we can do:

(use-modules (guix) (guix swh)
             (gnu packages base)
             (gnu packages golang))

;; This is our GNU Coreutils package.
coreutils
⇒ #<package coreutils@8.30 gnu/packages/base.scm:342 1c67b40>

;; Does SWH have its tarball?
(lookup-content (origin-sha256 (package-source coreutils))
                "sha256")
⇒ #<<content> checksums: (("sha1" …)) data-url: …>

;; Our package for HashiCorp’s Configuration Language (HCL) is
;; built from a Git commit.
(define commit
  (git-reference-commit
    (origin-uri (package-source go-github-com-hashicorp-hcl))))

;; Is this particular commit available in the archive?
(lookup-revision commit)
⇒ #<<revision> id: "23c074d0eceb2b8a5bfdbb271ab780cde70f05a8" …>

We’re currently using a subset of this interface, but there’s certainly more we could do. For example, we can compute archive coverage of the Guix packages; we can also request the archival of each package’s source code via the “save code” interface—though all this is subject to rate limiting.

Wrap-up

Software Heritage support in Guix creates a bridge from the stable source code archive to reproducible software deployment with complete provenance tracking. For the first time it gives us a software package distribution that can be rebuilt months or years later. This is particularly beneficial in the context of reproducible science: finally we can describe reproducible software environments, a prerequisite for reproducible computational experiments.

Going further, we can provide a complete software supply tool chain with provenance tracking that links revisions in the archive to bit-reproducible build artifacts produced by Guix. Oh and Guix itself is archived, so we have this meta-level where we could link Guix revisions to the revisions of packages it provides… There are still technical challenges to overcome, but that vision is shaping up.

Originally published on the Guix blog.