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[[!meta title="Time-based snapshots of upstream APT repositories"]]

[[!toc levels=2]]

Overview
========

Our time-based snapshots of upstream APT repositories are published on
<http://time-based.snapshots.deb.tails.boum.org/>.

These are _full_ snapshots of the upstream APT repositories we use for
building Tails ISO images. They contain exactly the same set of
packages as the mirrored repository. This has the advantage that some
workflows are trivially handled, e.g. working on a topic branch that
installs additional Debian packages; if such snapshots were not full
ones, then to work on one such branch, one would need either that to
have the credentials to import new packages from Debian into our own
mirror or repositories (which raises the barrier for contributing), or
that during some phases of Tails development the regular Debian
archive is used instead of our own mirror, which feels prone to "time
to QA vs. time to release" issues.

We snapshot each upstream APT repository N times a day, and without
further action, each snapshot is kept for D days.

The main goal here is to be able to freeze the APT repositories used
by a branch, whenever we freeze it.

A time-based snapshot's name contains:

 * an identifier of the APT repository this snapshot is about, e.g.
   `debian`, `debian-security`, `torproject`;
 * a `YYYYMMDD$ID` serial, `$ID` being an incremental decimal number
   formatted on two digits (`01`, `02`, etc.).

The APT repository mirroring infrastructure publishes the name of the
latest snapshot for each mirrored repository over HTTP, in the
`project/trace/$archive` file
([example](http://time-based.snapshots.deb.tails.boum.org/debian-security/project/trace/debian-security)).
Similarly, every ISO
build exports the names of the APT repository snapshots it uses
([example](http://nightly.tails.boum.org/build_Tails_ISO_devel/lastSuccessful/archive/latest.iso.apt-sources)).

The corresponding data is not critical: we can restart the whole thing
from scratch if needed, without too much pain ⇒ no need to synchronize
this content to the failover server; no need to back it up.

We don't bother merging mirrored APT repositories / suites into
aggregated ones. It loses information, gives us more work, and brings
little value.

# Source code

* `tails::reprepro::snapshots::time_based` class in
  [[!tails_gitweb_repo puppet-tails]]
* bits scattered in the main Tails Git repository (details below)

SSH access
==========

One must configure their SSH client to connect to the APT server:

	Host incoming.deb.tails.boum.org
		Port 3003

Workflow
========

<a id="freeze"></a>

Freeze snapshots
----------------

For example, to encode in the `$RELEASE_BRANCH` branch the set of
[[time-based APT repository snapshots|APT_repository/time-based snapshots]]
that shall be used during the freeze:

        git checkout "$RELEASE_BRANCH" && \
        ./auto/scripts/apt-snapshots-serials freeze && \
        git commit \
            -m "Freeze APT snapshots for ${VERSION}." \
            config/APT_snapshots.d/*/serial

<a id="thaw"></a>

Thaw snapshots
--------------

For example, to encode in the `devel` Git branch the fact
that it is not frozen anymore, that is remove the indication that
a specific set of APT repository snapshots must be used:

        git checkout devel && \
        ./auto/scripts/apt-snapshots-serials thaw && \
        git commit \
            -m "Thaw APT snapshots after Tails $VERSION was released." \
            config/APT_snapshots.d/*/serial

<a id="bump-expiration-date"></a>

Bump expiration date
--------------------

We set `Valid-Until` of time-based snapshots 10 days after they are
generated. In some cases, this can be too short, and we need to
manually bump `Valid-Until` for a given time-based snapshot.

Only release managers and sysadmins can do such operations.

### Bump one specific snapshot's expiration date

To bump `Valid-Until`, for a given snapshot (`$SERIAL`) of a given
archive (`$ARCHIVE`), so that they are valid for `$DAYS_FROM_NOW` days
from now:

    ssh reprepro-time-based-snapshots@incoming.deb.tails.boum.org \
       tails-bump-apt-snapshot-valid-until \
           "$ARCHIVE" "$SERIAL" "$DAYS_FROM_NOW"

<a id="bump-expiration-date-for-all-snapshots"></a>

### Bump all snapshots' expiration date

To bump `Valid-Until`, for every snapshot used by the current frozen
`$RELEASE_BRANCH` branch, so that they are valid for `$DAYS_FROM_NOW`
days from now:

	git checkout "$RELEASE_BRANCH" && \
	(
	   cd config/APT_snapshots.d && \
	   for ARCHIVE in * ; do
           if ! grep -qs '^latest$' "$ARCHIVE"/serial; then
               ssh reprepro-time-based-snapshots@incoming.deb.tails.boum.org \
                   tails-bump-apt-snapshot-valid-until \
                       "$ARCHIVE" "$(cat "$ARCHIVE"/serial)" \
                       "$DAYS_FROM_NOW"
           fi
	   done
	)

Stop tracking a distribution
----------------------------

After we stop tracking a distribution, e.g. after we release Tails
based on a new Debian, we need to manually remove all corresponding
time-based snapshots, and the packages that are not referenced
anymore.

For example, when we stopped tracking Wheezy, we did:

	reprepro dumpreferences \
       | grep -E '^s=wheezy' \
       | awk '{print $1}' \
       | sort -u \
       | xargs -n 1 reprepro _removereferences \
    && reprepro deleteunreferenced

<a id="freeze-exception"></a>

Freeze exception
----------------

### Grant a freeze exception

1. Import the package you want to upgrade into our own
   [[custom APT repository|contribute/APT repository/custom]], in the
   suite corresponding to the branch that we want to see this
   package in.

   For example:

           PBUILDER_OPTIONS='--basetgz /var/cache/pbuilder/base-jessie-i386.tgz' \
           TARGET_DIST='testing' \
           ./bin/import-package libgsecuredelete

   See [[!tails_gitweb bin/import-package]] for more detailed usage information.

2. If the imported package comes from a Debian distribution whose
   pinning value is at least 990 in `config/chroot_apt/preferences`:
   you can stop right here. Otherwise, read on.

3. Add a pinning entry in `config/chroot_apt/preferences` for the
   package you imported:

        Explanation: freeze exception
        Package: XYZ
        Pin: origin deb.tails.boum.org
        Pin-Priority: 999

4. Commit:

        git commit config/chroot_apt/preferences \
            -m "Add freeze exceptions for $(dpkg-parsechangelog -SVersion)"

5. Push to Git.

<a id="freeze-exceptions-post-release"></a>

### Post-release

Thaw the packages that were granted freeze exceptions, now that they
can be fetched from a newer time-based snapshot of the repository
we've initially pulled it from.

1. For each entry in `config/chroot_apt/preferences` that has
   `Explanation: freeze exception`: set `Pin-Priority` to `-1`.

2. Commit:

        git commit config/chroot_apt/preferences \
            -m "Remove freeze exceptions added for $(dpkg-parsechangelog -SVersion)"

3. Push to Git.

# Design notes

## gensnapshot

We use reprepro's `gensnapshot` command, that basically copies
a distribution, keeping references to the packages it contains.

Compared to the "snapshots as full-blown distributions + `reprepro
pull`" option we
[used in our initial experiments](https://labs.riseup.net/code/issues/6295#note-14),
we are saving _a lot_ on database size, and thus in performance,
because reprepro does less tracking on snapshots, than what it does
for real distributions.

The counterpart of using snapshots created with `gensnapshot` is that:

 * garbage collecting expired snapshots is a bit more involved, i.e.
   we have to
   [do it ourselves](https://git-tails.immerda.ch/puppet-tails/tree/files/reprepro/snapshots/time_based/tails-delete-expired-apt-snapshots);
 * bumping `Valid-Until` for a given time-based snapshot has to be
   done directly in `dist`, without any help from reprepro; so here
   again, we
   [do it ourselves](https://git-tails.immerda.ch/puppet-tails/tree/files/reprepro/snapshots/time_based/tails-bump-apt-snapshot-valid-until).

None of these problems warrant going back to the other option... and
having to deal with 80GB+ Berkeley DB databases.

## Garbage collection and Valid-Until

We expire snapshots older than 10 days in order to save disk space,
and to avoid the reprepro database to grow too much.

To ensure that garbage collection doesn't delete a snapshot we still
need, e.g. the one currently referenced in the frozen `testing`
branch, we rely on the `Valid-Until` field found in `Release` files:
the way to express "I want to keep a given snapshot around" is to
postpone its expiration date; i.e. we don't differentiate "keep
a given snapshot around" from "keep a given snapshot usable", which
seems to make sense.

See [[above|time-based_snapshots#bump-expiration-date]] for how we
can manage `Valid-Until` manually, whenever needed.

One advantage of this design is that we don't have to regularly update
`Valid-Until` fields, and the corresponding signatures: we only do
that on a case-by-case basis, when needed. And thus, we can actually
benefit from the protections offered by APT when `Valid-Until` fields
are present, as any snapshot will expire unless we do something
about it.

In practice, the main use case for keeping a given time-based APT
repository snapshot around and valid is when it's being used by
a release branch:

 - `testing`: while it's frozen, that is during 5-10 days most of the
   time;
 - `stable`: that's a corner case, since `stable` generally uses the
   set of tagged snapshots of the latest Tails release; if and when we
   decide to manually point `stable` to a different set of snapshots,
   then we can as well deal with `Valid-Until` manually.

In passing, note that we ship an empty `/var/cache/apt/lists/` in the
ISO ⇒ modifying `Release` and `Release.gpg` files on our APT
repository won't prevent the ISO build from being deterministic.

## APT vs. reprepro: dist names

We need to encode in the APT sources' base URL the exact snapshot we
want to use, in order to be able to pass it to `lb config --mirror-*`.
But this doesn't match reprepro's directory structure as-is.

Thankfully this problem can be workaround'ed with some symlinks or
HTTP rewrite rules. Here's how.

Let's assume:

    lb config --distribution jessie
    lb config --mirror-chroot \
       http://time-based.snapshots.deb.tails.boum.org/debian/2016031101/
    lb config --mirror-chroot-security \
       http://time-based.snapshots.deb.tails.boum.org/debian-security/2016031102/
    [...]

Which generates this APT `sources.list`:

    deb http://time-based.snapshots.deb.tails.boum.org/debian/2016031101/ jessie main
    deb http://time-based.snapshots.deb.tails.boum.org/debian-security/2016031102/ jessie/updates main
    [...]

As a result APT sends HTTP requests with URLs such as:

 * <http://time-based.snapshots.deb.tails.boum.org/debian/2016032401/dists/jessie/Release>
 * <http://time-based.snapshots.deb.tails.boum.org/debian/2016032401/pool/XYZ>
 * <http://time-based.snapshots.deb.tails.boum.org/debian-security/2016032402/dists/jessie/updates/Release>
 * <http://time-based.snapshots.deb.tails.boum.org/debian-security/2016032402/pool/XYZ>

The corresponding files in reprepro's filesystem (given that we have
one reprepro instance per mirrored archive) are:

 * in Debian archive's reprepro:
   - `/srv/apt-snapshots/time-based/repositories/debian/dists/jessie/snapshots/2016032401/Release`,
     that contains `Suite: jessie/snapshots/2016032401` and `Codename: jessie`
   - `/srv/apt-snapshots/time-based/repositories/debian/pool/XYZ`

 * in Debian security archive's reprepro:
   - `/srv/apt-snapshots/time-based/repositories/debian-security/dists/jessie/updates/snapshots/2016031102/Release`,
     that contains `Suite: jessie/updates/snapshots/2016031102` and
     `Codename: jessie/updates`
   - `/srv/apt-snapshots/time-based/repositories/debian-security/pool/XYZ`

To have the above HTTP requests translate to access to these files,
we use
[a set of HTTP rewrite rules](https://git-tails.immerda.ch/puppet-tails/tree/templates/reprepro/snapshots/time_based/nginx_site.erb).

Note: this works because APT only warns when the codename in the
`Release` file doesn't match the one requested in `sources.list`.
There's a code comment around this check, dating back from 2004, that
says something like "This might become fatal in the future". We bet that if it
becomes fatal some day, it will be possible to turn it back into
a warning via configuration. This affects only development builds
since we're not going to configure APT _in the Tails ISO_ to point to
our own snapshots of the Debian archive anyway.

<a id="design-freeze-exceptions"></a>

## Freeze exceptions

This is a new problem brought by using "frozen" snapshot of APT
repositories during a Tails code freeze: some bug, that we want to see
fixed in the release we are preparing, would be resolved if we pulled
an upgraded package as-is from a freshest Debian APT repository.
Before we could freeze APT repositories, we would have got this bugfix
for free. Now we need to grant freeze exceptions.

This is similar to "Upgrading to a new snapshot", except that we want
to upgrade one package only. By definition, this only affects *frozen*
release branches (`stable`, `testing`), and topic branches based on
them: all other branches use the freshest set of APT repository
snapshots available.

Most of the time, a bugfix branch we want to merge into a frozen
release branch doesn't need to upgrade packages from Debian, so this
is a corner case for the time being. Moreover, so far we have always
dealt with this problem entirely by hand, so it's not critical to
provide much improved tools. What makes it tempting to improve the
situation here is mostly:

 * even though freeze exceptions will remain exceptions, frozen will
   add one use case:
 * this will become a relatively common operation if we are based on
   Debian testing some day, so let's check that it's not only
   possible, but also reasonably easy to handle with this design
   (otherwise we may have to switch to more powerful tools, such as
   dak + britney).

To grant a freeze exception to a given package, we simply import it
into our own
[[custom APT repository|contribute/APT repository/custom]], in the
suite corresponding to the branch that we want to see this package in
⇒ in the general case, the upgraded package will be installed in the
next Tails release.

This works because our APT pinning ranks Tails custom APT suites at
the same level as the other APT sources corresponding to the current
version of Debian Tails is based on, and higher than other Debian
distribution (which, in passing, implies that we have to manually pin,
in Git, the packages from our custom APT suites, that we want to
override the ones found in other repositories regardless of version
numbers):

 * if the imported package comes from Debian stable: it will be
   installed simply because its version is greater than the version of
   the same package from Debian stable; and once we have thawed the
   corresponding snapshot, the package can be pulled equally from any
   of these two sources (Debian, and our custom APT repository), until
   a newer version of this package is uploaded to Debian, and then the
   newer one will supersede the package we have in our custom APT
   repository;

 * if the imported package comes from another Debian distribution,
   that has a pinning value strictly lower than 990, such as Debian
   unstable: if we did nothing more, the package would be installed
   because its pinning (`origin deb.tails.boum.org`) is higher than
   the one from the Debian distribution we're importing it from;
   *however*, in this case we need to track this package, and to
   remove it from our custom repository after we have thawed the
   corresponding snapshot — otherwise, due to this pinning
   configuration, we would stick to the version of the package we have
   one day imported, while in most cases we want to resume tracking
   the version from Debian; so, we do this that way:

   1. Import the package we want to upgrade into our own
      [[custom APT repository|contribute/APT repository/custom]], in
      the suite corresponding to the branch that we want to see this
      package in.

   2. Explicitly pin, in `config/chroot_apt/preferences`, the upgraded
      package we have just imported to a value higher than 990, with
      a proper `Explanation:` field; this pinning is not required at
      this stage, but it is one way to encode in Git the packages we
      have imported, which simplifies the following (clean up) step.

   3. Once the corresponding APT snapshot has been thawed, that is
      once the upgraded package can be fetched from a newer time-based
      snapshot of the repository we've initially pulled it from: make
      it so branches stop using the upgraded package, and resume
      tracking the one available in Debian. To do that, we modify the
      pinning entry added at the previous step, and give it a value of
      `-1`. This should be done by the release manager, immediately
      after a release, when they thaw the APT snapshots used for the
      release, and merge it into other release branches.

      Ideas for future improvements:

      * At some point a helper tool can do this automatically,
        assuming we always use the same `Explanation:` field to mark
        these pinning entries. (Ideally we would simply use
        a dedicated file under `apt/preferences.d/` for freeze
        exceptions, but `live-build` 2.x doesn't support that.)

      * Ideally we would remove these imported packages from our
        custom APT repository at post-release time as well, so we can
        get rid of the `-1` pinning entries, but it really needs to be
        done in a 100% correct order, to ensure that after all the
        merges we do post-release (and sometimes at other times)
        between release branches, the imported packages are _not_
        present anymore in any of the corresponding APT suites.