Controllers report Ubuntu Core version in the Snap

Adding snapd as it appears LXD can get this information through a special permission to get host information. MAAS needs a simple interface which just gives read-only access to /etc/os-release on the host. This will help bring feature parity between the MAAS Debian package and Snap package.

Can MAAS just use /etc/lsb-release instead? That correctly identifies the host system with the hello-world snap:

$ snap run --shell hello-world
To run a command as administrator (user "root"), use "sudo <command>".
See "man sudo_root" for details.

$ cat /etc/os-release
NAME="Ubuntu Core"
VERSION="16"
ID=ubuntu-core
PRETTY_NAME="Ubuntu Core 16"
VERSION_ID="16"
HOME_URL="http://www.snapcraft.io/"
BUG_REPORT_URL="http://bugs.launchpad.net/snappy/"
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04 LTS"

MAAS is actually using LXD's library to get this information. I asked stgraber how this works on LXD and why LXD uses /etc/os-release.

<stgraber> lsb-release isn't available on all platforms IIRC
<stgraber> we get os-release just fine because the LXD snap has an alternative view of /etc where we have os-release point to /var/lib/snapd/host/etc/os-release
<ltrager> stgraber: ah so its some of the special permissions LXD has :)
<stgraber> yeah
<ltrager> stgraber: I supposed /etc/lsb-release is available where MAAS is running so it should be safe to use
<stgraber> of all the distros we test, only Ubuntu and Manjaro seem to have lsb-release
<stgraber> all others do not
<stgraber> https://paste.ubuntu.com/p/cP5rycYdq6/

I can use /etc/lsb-release for now but I'm wondering if snapd should pass /etc/os-release in the same way it does /etc/lsb-release as lsb-release is only available on Ubuntu and Manjaro.

[1] https://github.com/lxc/lxd/blob/master/shared/osarch/release.go

If lsb-release is not available on all distros, perhaps maas could ask for system-files interface access to /var/lib/snapd/hostfs/etc/os-release ?

@Ian how do you do that? I tried the following in a test snap and it didn't work (after connecting the interface):

name: testsnap
version: "1.0"
summary: testsnap
description: testsnap
architectures:
  - amd64
base: core18
confinement: strict
grade: stable

plugs:
  read-system-os-release:
    interface: system-files
    read:
      - /var/lib/snapd/hostfs/etc/os-release

apps:
  testsnap:
    command: script
    plugs:
      - read-system-os-release

where "script" does the following:

#!/bin/bash

for file in /etc/os-release /var/lib/snapd/hostfs/etc/os-release; do
    echo "------- $file"
    cat "$file"
    echo "-------"
done

After connecting "testsnap:read-system-os-release" I still get:

$ testsnap
------- /etc/os-release
NAME="Ubuntu Core"
VERSION="18"
ID=ubuntu-core
PRETTY_NAME="Ubuntu Core 18"
VERSION_ID="18"
HOME_URL="https://snapcraft.io/"
BUG_REPORT_URL="http://bugs.launchpad.net/snappy/"
-------
------- /var/lib/snapd/hostfs/etc/os-release
cat: /var/lib/snapd/hostfs/etc/os-release: Permission denied
-------

So, the reason this doesn't work is that in ubuntu /etc/os-release is a symlink to /usr/lib/os-release.

So even using the hostfs trick won't really work across distributions in a reliable way.

LXD actually handles that case

https://github.com/lxc/lxd/blob/master/shared/osarch/release.go#L12

The system-observer interface in snapd should allow reading /var/lib/snapd/hostfs/etc/os-release

@mvo it does, but unfortunately this still yields a permission denied since /etc/os-release on Ubuntu is a symlink and the target is not allowed:

root@maas:~# cat /var/lib/snapd/hostfs/etc/os-release
cat: /var/lib/snapd/hostfs/etc/os-release: Permission denied

root@maas:~# ls -la /var/lib/snapd/hostfs/etc/os-release
lrwxrwxrwx 1 root root 21 Jan 30 12:28 /var/lib/snapd/hostfs/etc/os-release -> ../usr/lib/os-release

root@maas:~# cat /var/lib/snapd/hostfs/usr/lib/os-release
cat: /var/lib/snapd/hostfs/usr/lib/os-release: Permission denied