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

[Robert C Jennings]

Public bug reported:

Building an ubuntu-cpc minimal image by default would have no snaps
installed.  When I 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.

** Affects: livecd-rootfs (Ubuntu)
     Importance: Undecided
     Assignee: Robert C Jennings (rcj)
         Status: Triaged

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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:
  Triaged

Bug description:
  Building an ubuntu-cpc minimal image by default would have no snaps
  installed.  When I 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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