[Bug 1851858] Re: adds ESM to sources.list.d unconditionally, despite it being x86-only

[Andreas Hasenack]

** Description changed:

  [Impact]
  The ubuntu-advantage-tools package ("uat", for short) installs by default a sources.list snippet so that the machine can become aware of available ESM updates. They cannot be downloaded without authentication, so to prevent them from being considered in updates, an apt preferences file is also installed pinning the esm repository down.
  
  Turns out that ESM is only available for the x86 architecture, and
  installing that sources.list snippet for other architectures leads to
  apt-get update failures.
  
  A mitigation was put in place on the ESM repository to publish an empty
  archive for these unsupported architectures, so apt-get update won't
  fail.
  
  The change in this SRU adds another case for when postinst configures
  and unconfigures ESM, and that is an architecture check via `dpkg
  --print-architecture`.
  
  This by itself is not enough to prevent users from trying to enable esm-
  infra on non-x86 architectures, as the contract server is still
  incorrectly advertising that support. This has been fixed in the staging
  deployment and a production deployment with this change should happen
  soon as well, completing the fix for this issue.
  
  [Test Case]
  # Install the current trusty-updates ubuntu-advantage-tools on a non-x86 system (armhf, arm64, ppc64el, or s390x).
  $ sudo apt install ubuntu-advantage-tools
  
  # verify that apt-get update is hitting the esm repository:
  $ sudo apt-get update | grep esm
  Get:1 https://esm.ubuntu.com trusty-infra-security InRelease
  Get:2 https://esm.ubuntu.com trusty-infra-updates InRelease
  ...
  
  # verify that an esm pinning file was installed. Check that esm.ubuntu.com shows up in the apt-cache policy output, and that its pinning is negative:
  $ apt-cache policy | grep esm
  -32768 https://esm.ubuntu.com/ubuntu/ trusty-infra-updates/main ppc64el Packages
       origin esm.ubuntu.com
  -32768 https://esm.ubuntu.com/ubuntu/ trusty-infra-security/main ppc64el Packages
       origin esm.ubuntu.com
  
  # upgrade to the ubuntu-advantage-tools package from proposed and repeat the test. apt-get update shouldn't be hitting the ESm repository anymore, and the policy output should be empty as well:
  $ sudo apt install ubuntu-advantage-tools # from trusty-proposed
  $ apt-get update
  <empty>
  $ apt-cache policy | grep esm
  <empty>
  
  # conversely, on a x86 system, the output should remain the same, i.e.,
  esm should be listed and again with a negative pinning
  
  [Regression Potential]
  The logic relies on architecture names returned by `dpkg --print-architecture`. It there is a change in its output, or some other bug, we could be disabling (or enabling) the esm repository where we shouldn't.
  
  [Other Info]
  The knowledge about which architectures are supported is now statically stored in the package, which is a bit unfortunate. The final authority is the contract server, and the actual esm repository. This information is sent to the client, but we are not making a network call in postinst to verify that. One reason being that the launchpad builders and DEP8 runners block such egress traffic.
  
+ If esm-infra was already enabled before applying the update, it will
+ become disabled on non-x86 architectures. Since there are no non-x86 ESM
+ updates available, this is just reflecting the truth about the support.
+ 
  [Original Description]
  The shiny new ubuntu-advantage-tools client adds ESM to sources.list.d unconditionally on all architectures, but the ESM archive itself currently only publishes for x86.  One of those two things is a bug.
  
  Personally, I think it's a bug we don't publish ESM for all the same
  arches as we released for (even if we don't update all the packages for
  all arches, people would at least get things like tzdata updates), but
  if the intent is to be strictly x86-only, then ubuntu-advantage-tools is
  very much in the wrong here, as it creates a situation where apt-get
  update fails on all !x86.

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https://bugs.launchpad.net/bugs/1851858

Title:
  adds ESM to sources.list.d unconditionally, despite it being x86-only

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