[Bug 1926819] Autopkgtest regression report (update-notifier/3.192.40.3)

[Ubuntu SRU Bot]

All autopkgtests for the newly accepted update-notifier (3.192.40.3) for hirsute have finished running.
The following regressions have been reported in tests triggered by the package:

update-manager/1:21.04.10 (amd64, s390x, armhf, arm64, ppc64el)


Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-
migration/hirsute/update_excuses.html#update-notifier

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

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https://bugs.launchpad.net/bugs/1926819

Title:
  Empty MOTD on up-to-date systems (includes fix)

Status in update-notifier package in Ubuntu:
  Fix Released
Status in update-notifier source package in Xenial:
  Fix Committed
Status in update-notifier source package in Bionic:
  Fix Committed
Status in update-notifier source package in Focal:
  Fix Committed
Status in update-notifier source package in Hirsute:
  Fix Committed

Bug description:
  [Impact]
  On a system without updates to be installed, not showing any message in motd feels that something went wrong in the backend. Also, currently we are printing unwanted white spaces in that scenario, which is not a good user experience here.

  [Test case]

  To reproduce the issue, you can:

  1. Launch a xenial container
  2. Remove ubuntu-advantage-tools from the system
     This package will make update-notifier show more messages which are not needed in that test
  3. Run an apt update
  4. Install update-notifier
  5. Run apt upgrade
  6. Run /usr/lib/update-notifier/apt_check.py --human-readable
  7. Verify we deliver an empty message

  To verify that the error is fixed:

  1. Run the past scenario until step 4
  2. Install the new update-notifier from this ppa:
  https://launchpad.net/~lamoura/+archive/ubuntu/update-notifier-test-ppa/
  3. Run `/usr/lib/update-notifier/apt-check --human-readable`
  4. Verify that the command print the following message:

     0 updates can be applied immediately.

  [Where problems could occur]

  We are bringing back a behavior that was already present in the
  package, but removed in the last version of it. We will now be
  impacting users that may not be relying in that message anymore. But
  we believe this should not be a huge issue if that happens, since
  users were already relying on the old behavior.

  [Discussion]
  Currently, motd will not output a message if the system does not have any updates to apply. However, this is not ideal since this may cause the impression that there is something wrong with motd or the system. Also, the empty message we produce in that scenario is not completely empty, it does contain some white spaces on it, which is completely unnecessary.

  Because of that, we are bringing back the behavior of printing the
  message:

  0 updates can be applied immediately.

  If no updates need to be applied in the system.

  [Original description]

  
  Starting with update-notifier-common 3.192.30.7 (focal), my up-to-date systems are showing this MOTD on login:

  Welcome to Ubuntu 20.04.2 LTS (GNU/Linux 5.4.0-72-generic x86_64)

  user at host:~$

  I've attached a patch that fixes this issue and adds more test cases.

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