[Bug 1957123] Re: livecd-rootfs incorrectly installs core snap as implicit dep of other base snaps

[Steve Langasek]

** Description changed:

- The focal and jammy desktop images include a gtk-common-themes snap
- which uses base: bare.  This is obviously supposed to mean the snap has
- no external dependencies on a particular core snap.  Unfortunately, the
- logic in livecd-rootfs is broken, and when processing the bare snap
- itself, seeing that it has no base: declared in its own metadata,
- interprets this as meaning there is an implicit dependency on the core
- snap and pulls in a whole separate root filesystem pointlessly.
+ [Justification]
+ The focal and jammy desktop images include a gtk-common-themes snap which uses base: bare.  This is obviously supposed to mean the snap has no external dependencies on a particular core snap.  Unfortunately, the logic in livecd-rootfs is broken, and when processing the bare snap itself, seeing that it has no base: declared in its own metadata, interprets this as meaning there is an implicit dependency on the core snap and pulls in a whole separate root filesystem pointlessly.
  
  We should:
  - fix livecd-rootfs to not look for base snaps of snaps which are themselves type: base
  - make it a fatal build error for focal an later to try to build an image that pulls in the core snap, since this is legacy (the Ubuntu 16.04 rootfs should not be used in Ubuntu 20.04 and later images) and it's a bad bug if it does get pulled in.
+ 
+ [Test case]
+ All daily images should build successfully for all flavors with the updated livecd-rootfs.  If this change introduces any build failures, it should be reverted until the underlying problem (seeding a snap that requires 'core' as a base) can be resolved.

** Also affects: livecd-rootfs (Ubuntu Focal)
   Importance: Undecided
       Status: New

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https://bugs.launchpad.net/bugs/1957123

Title:
  livecd-rootfs incorrectly installs core snap as implicit dep of other
  base snaps

Status in livecd-rootfs package in Ubuntu:
  New
Status in livecd-rootfs source package in Focal:
  New

Bug description:
  [Justification]
  The focal and jammy desktop images include a gtk-common-themes snap which uses base: bare.  This is obviously supposed to mean the snap has no external dependencies on a particular core snap.  Unfortunately, the logic in livecd-rootfs is broken, and when processing the bare snap itself, seeing that it has no base: declared in its own metadata, interprets this as meaning there is an implicit dependency on the core snap and pulls in a whole separate root filesystem pointlessly.

  We should:
  - fix livecd-rootfs to not look for base snaps of snaps which are themselves type: base
  - make it a fatal build error for focal an later to try to build an image that pulls in the core snap, since this is legacy (the Ubuntu 16.04 rootfs should not be used in Ubuntu 20.04 and later images) and it's a bad bug if it does get pulled in.

  [Test case]
  All daily images should build successfully for all flavors with the updated livecd-rootfs.  If this change introduces any build failures, it should be reverted until the underlying problem (seeding a snap that requires 'core' as a base) can be resolved.

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