Background
`auto-promote-staging.yml` advances main via `git merge --ff-only`
+ `git push origin main` — clean fast-forward, no merge commit. But
manual `staging → main` merges via the GitHub UI / API create a merge
commit on main that staging doesn't have. The next `staging → main`
PR then evaluates as "BEHIND" because staging is missing that merge
commit, requiring a manual `gh pr update-branch` round-trip.
This pattern bit twice on 2026-04-28 (PRs #2202 and #2205, both
manual bridges to land pipeline fixes themselves). Each needed
update-branch + re-CI before they could merge. Annoying and
avoidable.
What this workflow does
Triggered on every push to main (regardless of source: auto-promote,
UI merge, API merge, direct push):
1. Check whether main is already in staging's ancestry. If yes,
no-op — auto-promote-staging keeps them aligned via ff push,
and the no-op case is the steady state.
2. If not (manual merge commit on main, or direct main hotfix):
try `git merge --ff-only origin/main` first. Works when staging
hasn't diverged with its own commits.
3. If ff fails (staging has its own in-flight feature work):
`git merge --no-ff origin/main -m "chore: sync main → staging"`.
Absorbs main's tip while keeping staging's own history.
4. Push staging.
Loop safety
Pushing the synced staging triggers auto-promote-staging.yml, which
checks gates on staging's new tip and, if green, ff-pushes staging
to main. Since staging now ⊇ main, the resulting push to main is
either a no-op (no ref change → no push event fires → auto-sync
doesn't re-trigger) or advances main further. In the latter case
auto-sync fires once more, sees main already in staging's ancestry,
no-ops. Bounded.
Conflict handling
If the merge step hits conflicts (staging and main diverged with
incompatible changes), the workflow fails with a clear summary
pointing to manual resolution. This shouldn't happen in practice —
staging is the integration branch; conflicts indicate a direct main
hotfix touching the same code as in-flight staging work.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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