Our APT repositories

We have three kinds of APT repositories:

  • our custom APT repository, that stores our custom Debian packages;
  • (full) time-based snapshots of the upstream APT repositories we use, so that we can use frozen versions thereof when needed;
  • (partial) tagged snapshots of upstream APT repositories we need, so that one can rebuild a released ISO in the future, and we keep the corresponding source code around.

Snapshots and branches

Here we discuss what APT snapshots of upstream repositories are used when building a Tails ISO image. This a is function of the branch we build from, as well as if we are building an ISO that is meant to be released (i.e. whether there is a tag in Git corresponding the last entry in debian/changelog).

Note: when we say "building an ISO from the B branch" below, we only refer to the base branch, i.e. B is encoded in config/base_branch. Hence such statements also refers to topic branches with B as base branch.

Building an ISO from the devel branch always uses the freshest set of APT repository snapshots available. Resolving what's the set of freshest APT repository snapshots is done at the beginning of the build (auto/config, auto/scripts/apt-mirror), so that the entire build uses the exact same state of these repositories. This is needed for reproducible builds, and has a nice side effect: so long, Hashsum mismatch, and thanks for the fish.

When building an ISO from the branch used to prepare the next major release (testing):

  • outside of the freeze period: the testing branch is not used;
  • freeze period: at freeze time, the RM encodes in the Git testing branch the set of APT repository snapshots (via their serial numbers) that shall be used during the freeze; the only exception is security.debian.org, for which we always use our latest snapshot;
  • at release time: when building from a tagged branch, similarly to what we do for our custom APT repository, instead of using time-based APT repository snapshots, we use snapshots labeled with the Git tag (note that this is not needed, strictly speaking, as the APT sources used at Tails runtime will anyway be the official (and not frozen) Debian ones; this is mostly needed for legal purposes (this allows to distribute for a long time the source packages needed to build a given Tails ISO image), and it will be useful when we want to be able to reproduce a given Tails ISO build).

When building an ISO from the branch used to prepare the next point-release (stable) we use the same time-based snapshots that were used to prepare the latest Tails release, except:

  • we use our latest snapshot of security.debian.org;
  • at release time: when building from a tagged branch, similarly to what we do for our custom APT repository, instead of using time-based APT repository snapshots, we use snapshots labeled with the Git tag.

Design notes

Miscellaneous

A given APT repository snapshot is immutable after it's been taken. We deal with freeze exceptions separately.

We want to have reproducible builds some day. Therefore, the APT sources.list shipped in the ISO must be stable across rebuilds from the same release Git tag.

APT sources used inside Tails

A running Tails' APT must be pointed at the official, live Debian archive, and not to a Tails-specific and already obsolete snapshot.

Say kedit is a package shipped in Debian, but not in Tails. Then, when run inside Tails, apt install kedit must fetch kedit from current Debian, as opposed to installing it from a Tails-specific, and generally obsolete, snapshot of the Debian APT repository.

To achieve that we tweak sources.list in config/chroot local-includes/lib/live/config/1500-reconfigure-APT.

Upgrading to a new snapshot

In other words: bumping, in Git, the pointers to the set of snapshots that shall be used by a given branch.

Let's use, as an example of a situation in which we might want to do that, upgrading to a new Debian point-release.

With this design:

  • devel gets them automatically because it closely tracks the Debian archive;
  • for release branches (stable, testing): on a case-by-case basis, depending on the respective Debian/Tails release schedule timing, we can choose whether to switch to using a new snapshot of the Debian archive for the next release. Note that this can be done via a topic-branch since this information is encoded in Git. If we choose not to manually pick the point release, which is the default if we don't act at all, then:
    • testing will start using the new Debian point-release as soon as it is unfrozen, that is as soon as it has been used to release a new major version of Tails;
    • stable will start using the new Debian point-release once a testing branch that uses that point-release is merged into stable.