snap prepare-image gets confused when the default track is not "latest"

[Alfonso Sanchez-Beato]

I have a model assertion (https://paste.ubuntu.com/p/vsNjth2WwF/) for UC18 that includes modem-manager as a required snap:

type: model
authority-id: VvBqrmj0g4M1DDyvelq2ktWQbP6SXXKT
series: 16
brand-id: VvBqrmj0g4M1DDyvelq2ktWQbP6SXXKT
model: pc
architecture: amd64
base: core18
gadget: pc=18
kernel: pc-kernel=18
required-snaps:
  - modem-manager
...

An important detail is that modem-manager has the 1.10 track as the default track now. When I run "snap prepare-image <this_assertion> <dir>", it downloads the snap in the 1.10 (default) track, as I see "./image/var/lib/snapd/seed/snaps/modem-manager_414.snap" (revision 414 is in 1.10/stable at the moment) in the seed. But, in seed.yaml the channel is set to "stable", which is actually "latest/stable", not "1.10/stable":

- name: modem-manager
  snap-id: KtwxgRlwCAVKFw92BUdt1WloH1Va3QPo
  channel: stable
  contact: mailto:<email address hidden>
  file: modem-manager_414.snap

So, when I start an image that uses this seed, the modem-manager that has been installed is from the 1.10 track, but it is tracking "latest/stable", so when a refresh happens it installs the snap in latest.

To be coherent, the tracked channel should be 1.10/stable, or use latest/stable when downloading the snap too.

In any case, required-snaps should allow setting the track as it happens for the gadget and kernel snaps, to remove the ambiguities here. It should be possible to control the tracked channel from the model.

[Ian Johnson (anonymouse67)]

I confirmed this is happening with snapd edge version 2.44.3+git1161.g0ed5ef1

[Samuele Pedroni (pedronis)]

Proposed https://github.com/snapcore/snapd/pull/8523 for 2.45.

Also a reminder that one can override channels with prepare-image (for UC16/18 images and dangerous UC20 model ones) with --snap <SNAP>=<CHANNEL> .