[Bug 2078720] Re: Upgrading from jammy to noble results in a linux-headers package being in a broken state

[Julian Andres Klode]

ubuntu-release-upgrader SRU uploaded to noble and also uploaded to
oracular.

These reintroduce the slow path as a fallback, specifically for APT
versions prior to this SRU (and hence mantic); they also fix that code
to correctly consider running kernels (and KeepInstalledSection) in the
removal of reverse dependencies of obsolete packages.

** Changed in: ubuntu-release-upgrader (Ubuntu)
       Status: Triaged => Fix Committed

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Title:
  Upgrading from jammy to noble results in a linux-headers package being
  in a broken state

Status in apt package in Ubuntu:
  New
Status in linux-aws package in Ubuntu:
  Confirmed
Status in linux-gcp package in Ubuntu:
  Confirmed
Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Committed
Status in apt source package in Jammy:
  In Progress
Status in linux-aws source package in Jammy:
  New
Status in linux-gcp source package in Jammy:
  New
Status in apt source package in Noble:
  New
Status in linux-aws source package in Noble:
  New
Status in linux-gcp source package in Noble:
  New
Status in ubuntu-release-upgrader source package in Noble:
  In Progress

Bug description:
  [Impact]
  Obsolete packages can be removed despite still having reverse dependencies installed, for example:

  
  Now that 24.04.1 has been released, 22.04 users are encouraged to upgrade to 24.04 via the `do-release-upgrade` command. This issue was seen whilst testing this upgrade path.
  Upgrading and later rebooting a jammy GCP instance results in `linux-headers-6.5.0-1025-gcp` being a broken state.

  ```
  $ sudo apt install
  Reading package lists... Done
  Building dependency tree... Done
  Reading state information... Done
  You might want to run 'apt --fix-broken install' to correct these.
  The following packages have unmet dependencies:
   linux-headers-6.5.0-1025-gcp : Depends: linux-gcp-6.5-headers-6.5.0-1025 but it is not installable
  E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).
  ```
  `linux-gcp-6.5-headers-6.5.0-1025` is a dependency of `linux-image-6.5.0-1025-gcp` which is also not removed during the upgrade.

  ```
  $ apt-cache rdepends linux-headers-6.5.0-1025-gcp
  linux-headers-6.5.0-1025-gcp
  Reverse Depends:
    linux-image-6.5.0-1025-gcp
  $ apt-cache rdepends linux-image-6.5.0-1025-gcp
  linux-image-6.5.0-1025-gcp
  Reverse Depends:
  ```

  Running `apt --fix-broken install` resolves the error.
  ```
  $ sudo apt --fix-broken install
  Reading package lists... Done
  Building dependency tree... Done
  Reading state information... Done
  Correcting dependencies... Done
  The following packages will be REMOVED:
    linux-headers-6.5.0-1025-gcp
  0 upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
  After this operation, 27.9 MB disk space will be freed.
  Do you want to continue? [Y/n] y
  (Reading database ... 83770 files and directories currently installed.)
  Removing linux-headers-6.5.0-1025-gcp (6.5.0-1025.27~22.04.1) ...
  $ sudo apt install
  Reading package lists... Done
  Building dependency tree... Done
  Reading state information... Done
  0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
  ```

  This issue was also observed after upgrading a jammy AWS instance to
  noble.

  ```
  $ sudo apt install
  Reading package lists... Done
  Building dependency tree... Done
  Reading state information... Done
  You might want to run 'apt --fix-broken install' to correct these.
  The following packages have unmet dependencies:
   linux-headers-6.5.0-1024-aws : Depends: linux-aws-6.5-headers-6.5.0-1024 but it is not installable
  E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).
  ```

  [Test plan]

  To prepare the VMs, follow the following scheme:

  $ lxc launch ubuntu:jammy jammy-to-noble
  $ lxc exec jammy-to-noble apt-mark lxd-agent-launcher # otherwise it resets

  To validate the apt change on jammy,

  ubuntu at jammy-to-noble$ add-apt-repository -p proposed
  ubuntu at jammy-to-noble$ apt install libapt-pkg6.0 # to upgrade libapt-pkg6.0
  ubuntu at jammy-to-noble$ do-release-upgrade

  To validate the APT change on noble, we must rely on the test suite as
  we won't have a noble->oracular upgrade bug causing it, most likely.

  To validate an ubuntu-release-upgrader change, instead run:

  ubuntu at jammy-to-noble$ do-release-upgrade -p
  ubuntu at mantic-to-noble$ do-release-upgrade -p

  [Where problems could occur: APT SRU]

  For the APT change, the function in question is used in a small number
  of places:

  In APT library:
  - In the `upgrade` command and library function. Removals are already undone there before calling it so we never reach the new code path.
  - In the phased update implementation, during dist-upgrade. I expect a failure is lurking there that is fixed by this, but I haven't been able to write a reproducer to trigger it just yet.

  In aptitude:
  - The function is wrapped, but the wrapper never called

  In QApt:
  - QApt seems to rely on it for upgrading and doesn't seem to have any error checking of the return value. I expect it will read the error 

  [Where problems could occur: u-r-u SRU]
  Upgrades will now result in a consistent state, but may spend hours searching for obsolete software again.

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