Three small fixes from the self-review of #2209:
1. **Required: concurrency group.** Two pushes to main in quick
succession (manual UI merge then auto-promote-staging's ff-push,
or any back-to-back main pushes) would race two auto-sync runs
against the same staging branch — second `git push origin staging`
fails non-fast-forward, surfacing as a red CI alert for what should
be a no-op. Add `concurrency: { group: auto-sync-main-to-staging,
cancel-in-progress: false }` so the second run waits for the first
and sees its result.
2. **Hygiene: `git merge --abort` on conflict.** The conflict-error
path exits 1 with the work tree in a half-merged state. Doesn't
affect future runs (each gets a fresh checkout) but is an
unpleasant artifact for anyone who shells into the runner. Abort
first, then exit.
3. **Doc accuracy: "Loop safety" comment.** The original said the
chain terminates because "main is either a no-op or advances
further." That's true but understates the actual safety: GitHub
Actions explicitly does NOT trigger downstream workflow runs from
`GITHUB_TOKEN`-authored pushes. So the loop is impossible by
construction, not just by happy coincidence of ref state. Updated
the comment to reflect the actual mechanism.
Plus a step-name nit: "Fast-forward staging → main" reads as if main
is the target. Renamed to "Fast-forward staging to main" for
consistency with the workflow's name (main → staging).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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