[Bug 1871919] Re: images with only core18-based snaps fail to build

[Robert C Jennings]

I have used livecd-rootfs 2.620.2 from proposed to build ec2 and gce
images.  EC2 currently has a mix of core and core18 based snaps while
GCE has only core18 based snaps.  I confirmed that snapd was seeded when
core18 was the only core snap present.  Boot testing each of these
images confirmed that snap preseeding is working and snaps are usable.
I'm going to mark this as verification-done

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

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https://bugs.launchpad.net/bugs/1871919

Title:
  images with only core18-based snaps fail to build

Status in livecd-rootfs package in Ubuntu:
  Fix Released
Status in livecd-rootfs source package in Eoan:
  Fix Committed

Bug description:
  [Impact]

   * Building an ubuntu-cpc minimal image by default would have no snaps
  installed.  When a binary hook seeds a snap that has a core18 base
  the build will fail when calling 'snap debug validate-seed' because
  the snapd snap is missing.

   * Additionally, an image with snaps that only depend on core18 (which
  isn't in the ubuntu-cpc:minimized project) would still ship 'core'.
  This was found by code inspection and reported in bug #1871912.

  [Test Case]

   * Build with ubuntu-cpc as that seeds lxd (now that it is based on
  core18).  With the proposed fix the image build should succeed with
  lxd, core18, and snapd preseeded.  The build should not fail and
  should not include core.

  [Regression Potential]

   * Image builds fail in some fashion related to snap preseeding.  It
  is unlikely to build an image that fails to pre-seed when booted as we
  call 'snap debug validate-seed' in the build, which is catching the
  current bug of omitting snapd with core18.

  [Other Info]

   * None

  [Original description]

  Building an ubuntu-cpc minimal image by default would have no snaps
  installed.  When a binary hook seeds a snap that has a core18 base
  the build will fail when calling 'snap debug validate-seed' because
  the snapd snap is missing.

  Additionally, an image with snaps that only depend on core18 (which
  isn't in the ubuntu-cpc:minimized project) would still ship 'core'.
  This was found by code inspection and reported in bug #1871912.

  We want to build images that only contain the appropriate core image
  and when that is not core it should have snapd preseeded.

  The _snap_post_process function is meant to install snapd if core18 is the
  only core snap installed or remove snapd if core is installed and snapd
  was not explicitly installed. But the current logic in _snap_preseed
  will never call _snap_post_process. $core_name will never be empty
  with the existing logic, and even if it were that would only be for the
  'core' snap and we'd miss using the 'core18' logic that pulls in snapd.
  Given the case statement in _snap_post_process can handle doing the
  right thing given any snap we can just call it unconditionally.

  Seeding any snap via snap_preseed will evaluate the base for each snap
  and seed the appropriate base. There should be no reason to explicitly
  seed the 'core' snap and with snaps moving to 'core18' this will add
  'core' without need.

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