[Bug 1968154] Re: Only keep 2 kernels

[Julian Andres Klode]

bionic: apt 1.6.17, Started with linux-image-4.15.0-208-generic

Log:
1. (in 208) installed 206
2. (in 208) no kernels to autoremove
3. (in 208) installed 204
4. (in 208) SUCCESS: autoremove would remove 204
5. (in 204) SUCCESS: After reboot into 204, 206 is autoremovable and 204, 208 are kept
6. (in 208 again) 204 is offered for removal, but I remove 206 per instructions
7. (in 208) SUCCESS: Both the remaining 204 and the 208 are kept.


** Tags removed: verification-needed verification-needed-bionic
** Tags added: verification-done verification-done-bionic

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https://bugs.launchpad.net/bugs/1968154

Title:
  Only keep 2 kernels

Status in apt package in Ubuntu:
  Fix Released
Status in apt source package in Bionic:
  Fix Committed
Status in apt source package in Focal:
  Fix Released
Status in apt source package in Impish:
  Fix Released

Bug description:
  [Impact]
  APT currently keeps 3 kernels or even 4 in some releases. Our boot partition is sized for a steady state of 2 kernels + 1 new one being unpacked, hence users run out of space and new kernels fail to install, upgrade runs might abort in the middle. It's not nice.

  [Test plan]
  1. Have two kernels installed (let's call them version 3, 2)
  2. Check that both kernels are not autoremovable
  3. Install an old kernel (let's call it 1), and mark it automatic
  4. Check that 1 will be autoremovable (apt autoremove -s)
  5. Reboot into 1, check that 2 is autoremovable (apt autoremove -s)
  6. Actually remove 2
  7. Reboot into 3 and check that both 1 and 3 are now not autoremovable

  [Where problems could occur]
  We could keep the wrong kernels installed that the user did not expect.

  We remove the requirement to keep the most recently installed version,
  previously recorded in APT::LastInstalledKernel, to achieve this, as
  we had 3 hard requirements so far:

  1. keep booted kernel
  2. keep highest version
  3. keep most recently installed

  1 can't be removed as it would break running systems, 2 is what you
  definitely want to keep.

  During normal system lifetime, the most recently installed kernel is
  the same as the highest version, so 2==3, and there are no changes to
  behavior.

  Likewise, if you most recently installed an older kernel manually for
  debugging, it would be manually installed and not subject to removal,
  even if the rule is dropped.

  The behavior really only changes if you install an older kernel, and
  then mark it auto - that older kernel becomes automatically removable
  immediately after it is marked as auto.

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