cc-ci-orchestrator/cc-ci-plan/plan-phase-samever-older-base-fallback.md

# Phase `samever` — step back to an older base when last-green == head (no same-version upgrade)

**Mission (operator-specified 2026-06-17):** close a gap in the `prevb` dynamic upgrade-base resolver.
When the resolved **last-green (warm-canonical) base version equals the PR head version**, the upgrade
tier currently deploys the **same version** as base and head — a vacuous, non-upgrade "upgrade." Instead
of skipping the tier, **step back to a genuinely older base**: the **newest published version strictly
older than the head version**. The upgrade must always cross a real version delta when an older version
exists. (This is design **A**; design B "canonical history" is deferred — see `cc-ci-plan/IDEAS.md`.)

State files: `STATUS-samever.md`, `BACKLOG-samever.md`, `REVIEW-samever.md`, `JOURNAL-samever.md`. DECISIONS.md shared.

## 1. Background / root cause

`resolve_upgrade_base` (`runner/run_recipe_ci.py:111`) resolves the upgrade base. Its two paths are
guarded **unequally**:
- **ref (main-tip) path — guarded:** `if main_tip == head_ref → skip "head == main tip (no predecessor
  delta)"`.
- **version (last-green canonical) path — NOT guarded:** it returns `BasePlan("version", rec["version"],
  …)` without checking that the canonical version differs from the head. The resolver isn't even given
  the head's *version* (only `head_ref`, a commit), so it currently can't compare.

When does the canonical equal the head version? The canonical advances **only** on a GREEN + COLD +
LATEST run of a `WARM_CANONICAL`-enrolled recipe (`should_promote_canonical` = `is_enrolled and
overall==0 and not quick and not ref`; a PR `!testme` carries `ref` so it NEVER promotes). So the
canonical is always the latest-published version that last passed a cold sweep. The gap therefore bites
when a PR **does not bump the version** (head version == latest published == canonical), or on a re-test
after the canonical advanced to the head's version — base and head become the same version.

## 2. Design (A)

Give the resolver the **head version** (read the `coop-cloud.*.version` label from the head's
`compose.yml` — the head checkout already exists), and extend the chain:

1. explicit `UPGRADE_BASE_VERSION` override → use it (unchanged).
2. **last-green canonical, IF its version ≠ head version** → use it (`kind="version"`; the green-verified
   primary).
3. **last-green canonical version == head version** → **do NOT skip.** Step back: from the recipe's
   published version tags (`warm_reconcile.recipe_tags` + the existing version-ordering used by
   `latest_version`), pick the **newest published version STRICTLY OLDER than the head version** and use
   it (`kind="version"`). `previous/` still applies version-guarded against whatever base version is
   chosen.
4. no canonical at all → existing **main-tip ref** path (use if `main_tip ≠ head_ref`, else skip) —
   unchanged.
5. **only if no older published version exists** (genuinely the first version / no predecessor) → skip
   with a declared reason (`"base == head and no older published predecessor"`).

Constraints:
- "Strictly older" — exclude any tag equal to the head version; reuse the existing coop-cloud version
  ordering, do not hand-roll semver. If the head version isn't in the published tag list (a brand-new
  version above all tags), the canonical-≠-head branch already handles it — the step-back only triggers
  when canonical == head.
- Preserve the **F1d-2** protections: the chosen older base must actually deploy that *pinned* version
  (checkout the tag so the on-disk tree matches), never LATEST.
- Pure resolver change where possible; keep the `ref` and `skip` paths' behavior identical for all other
  cases (don't perturb discourse #4 or any version-bump PR).

## 3. Gates

**M1 — implemented + unit-tested.** Resolver reads the head version and implements the chain above.
Unit tests (extend `tests/unit/test_upgrade_base.py`): canonical==head → resolves to the newest-older
published version (assert it's strictly older); canonical≠head → uses canonical (unchanged); no
older-published → declared skip with the new reason; head-version parsed from compose; version ordering
picks the correct strictly-older tag; override + ref + existing-skip paths unchanged. Adversary
cold-verifies from a clean checkout: a same-version PR now upgrades from a **real older version** (base
version < head version, evidenced), not a no-op and not a skip; teeth (a broken head still RED); the
version-bump path (canonical→head) is untouched.

**M2 — proven in real CI.** Demonstrate on a **same-version scenario**: a recipe+PR where head version ==
canonical version (a non-version-bump PR, or construct it by pointing the canonical at the head's
version), and show the upgrade tier deploys an **older published base → head** with evidence
`base_version < head_version` (a genuine delta, HC1 confirms head is head). Confirm a normal version-bump
PR — re-run **discourse #4** or an equivalent — is **UNAFFECTED** (still canonical→head). Spot-check ≥1
other enrolled recipe. Fresh Adversary PASS on both milestones → `## DONE`.

## 4. Guardrails

- **Never a same-version no-op, and never a needless skip when an older base exists.** Skip only when
  there is genuinely no older published predecessor.
- **The base must be strictly older than the head version.**
- **Don't regress the version-bump path** — the common upgrade-PR case (canonical → head) must behave
  exactly as before; discourse #4 must still test the official-image migration.
- Never weaken a test; minimal, well-scoped resolver change; `previous/` stays the last resort.
- Commit author `autonomic-bot <autonomic-bot@noreply.git.autonomic.zone>`; push every commit; abra over a
  pseudo-TTY. Recipe mirrors PR-only; never merge.

## 5. Definition of Done

The resolver steps back to the newest-published-version-older-than-head whenever the last-green canonical
equals the head version — never a same-version no-op, never a needless skip when an older base exists;
unit-tested; proven in real CI on a same-version scenario with evidence of a real base<head delta, and the
version-bump path (discourse #4) confirmed unaffected; M1 + M2 fresh Adversary PASSes in REVIEW-samever.md.
Design B (canonical history) recorded in `cc-ci-plan/IDEAS.md`, out of scope here.