You can use snap to safely run python3 incompatible python2 programs on Ubuntu 20.04 (Focal Fossa).

Why is bup missing on Ubuntu 20.04?

I use bup as one way to backup my systems. It’s exactly what you should expect of a good backup system. I love it!

Some of my hosts run on Ubuntu. I was shocked when I upgraded one of my hosts to the new Ubuntu LTS 20.04 (Focal Fossa): The bup package is gone!

I thought “WTF? Really?” and started to google. I found a discourse thread on the issue: On 2020-01-01 Python 2 reached its end-of-life. Consequently, Ubuntu 20.04 (Focal Fossa) does not include Python 2 any more. Debian dropped bup, as well.

The next logical step was to google about bup’s roadmap towards python 3. The bup mailing list thread about the Python 3 migration status turned out to be awesome reading. Rob Browning shares his insights

  • about the troubles behind the migration, including design flaws in python 3, and
  • about creative approaches to overcome the situation, including some interesting ideas about switching base technology altogether.

In the meantime, Rob Browning and Johannes Berg seem to have gone on a tour de force and created a set of patches on bup’s master branch, which will be released as 0.40 soon after 0.30.1. The team is also active on keeping bup in Debian-based distros.

So long story short: It’s worth waiting. To bridge the time until the new bup is available, I offer a little workaround here. It’s inspired by merlijn-sebrechts discourse post.

Forward-porting packages from Ubuntu 18.04 LTS using snap core18

In order to port python2 packages forward to Ubuntu 20.04, you can use core18 on snap.

Snap is a packaging system for creating portable, batteries-included and immutable snapshot images of applications. Actually, I avoided Snap until now for 3 main reasons:

  1. Snap falls behind in terms of some fundamental freedom aspects.
  2. Snap bloats my mount list with its proliferating use of squashfs mounts.
  3. Snap keeps a snapd process running on my system all the time. Why should a package manager need this?

But for the very use case at hand, it just seems perfect.

core18 is a snap image based on Ubuntu 18.04, so it still supports python2. Even better: bup is available as a native apt package on Ubuntu 18.04.

Let’s dive into the creation of our bup snap.

Install snapcraft

First, I install snapcraft. Snapcraft is a tool for creating your own snap packages.

sudo snap install snapcraft --classic

Create a snapcraft.yaml

snapcraft uses a declarative file snapcraft.yaml to describe its packages. Mine is really quick and dirty, but serves its purpose. It’s inspired by the tutorial about snap for python apps.

name: bup
version: "0.29"
summary: "It backs things up"
description: |
  bup is a program that backs things up. It's short for "backup."
base: core18
confinement: classic

    plugin: nil
      - bup

    command: bup

The snapcraft nil plugin does not actually do anything, but enables you to use snapcraft base features without performing a real plugin execution. For example, I will use stage-packages in order to install Ubuntu 18.04 apt packages.

If you want to package other python2 tools, which are available on PyPI, but not on Ubuntu 18.04, you will need the snapcraft python plugin.

Build the snap

By default, snapcraft uses multipass to build on a real VM. I use the snapcraft lxd provider, because I run these experiments jailed in a Virtual Machine and can not spawn new VMs from inside it.

snapcraft --use-lxd

snap will prompt you before setting up the necessary dependencies.

Install the snap

Next, I install the freshly created snap package.

sudo snap install bup_0.29_amd64.snap --classic --dangerous

This command line sounds creepy, right?

Thou shalt not fear: The classic option is snap’s confinement mode that resembles classic local packages. The dangerous option overrides the signature checks, which is perfectly fine for a package you created yourself.

Have fun with bup

Afterwards, use bup as usual!

BUP_DIR=/path/to/my-bup-backup/ bup index --one-file-system /home
BUP_DIR=/path/to/my-bup-backup/ bup save -n my-backup /home

Post header background image by Markus Distelrath from Pixabay.

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