Probe whether Charmcraft 4 packs a bases:-only charm

[github-actions]

Claims

From the Charmcraft 4 charmcraft.yaml reference (the bases section):

• A: bases is deprecated but still accepted by Charmcraft 4 — it has not been removed entirely.
• B: bases is "only accepted for bases supported before 2024-01-01", i.e. a newer base such as ubuntu@24.04 is refused when declared via bases:.
• C: bases conflicts with base/build-base/platforms and cannot be mixed with them.

Issue #68 asks the direct question: does Charmcraft 4 (the version used in this repo's workflows) refuse to pack a charm that uses bases: only?

What I tested and why

I tested claim B (which also bears on A). I switched kepler/charmcraft.yaml from the preferred base: ubuntu@24.04 + platforms: form to the deprecated bases:-only form, keeping the same base (ubuntu@24.04, amd64 + arm64) and removing base/platforms so there is no conflict (claim C respected). kosmos/charmcraft.yaml is left untouched as a control — it still uses base + platforms and should pack and deploy as usual.

The test execution here is the CI charmcraft pack step itself, not a pytest test. Charmcraft packing cannot be exercised by run_tox (it only runs format/lint/unit), and the integration tests run after packing and consume the already-packed .charm file — so if packing fails, the integration tests never run. That makes the pack step the natural and only place to observe Charmcraft's behaviour. The charmcraft.yaml change is the test input; the pack step is the test execution.

What I believe is true

I believe Charmcraft 4 refuses to pack kepler with bases: using ubuntu@24.04. The docs state bases is "only accepted for bases supported before 2024-01-01", and ubuntu@24.04 was released in April 2024 — after that cutoff. So I expect kepler's charmcraft pack step to fail, while kosmos (the control) packs and deploys normally.

What the CI result means

• kepler CI fails at the charmcraft pack step (and kosmos CI passes): Charmcraft 4 refuses bases: for ubuntu@24.04. This confirms my belief and the doc's "only old bases" constraint (claim B holds; bases is not removed entirely, claim A holds for old bases).
• kepler CI passes (pack succeeds and integration tests run): Charmcraft 4 accepts bases: even with ubuntu@24.04. This refutes my belief and the doc's "only accepted for bases supported before 2024-01-01" note — bases is more permissive than documented.

Either way, kosmos is the control: if it also fails, the result is about something other than the bases: change and the comparison is inconclusive.