feat(ci): auto-sync main → staging to keep staging-as-superset invariant
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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Hongming Wang
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.github/workflows/auto-sync-main-to-staging.yml
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.github/workflows/auto-sync-main-to-staging.yml
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name: Auto-sync main → staging
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# Reflects every push to `main` back onto `staging` so the
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# staging-as-superset-of-main invariant holds.
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#
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# Background:
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#
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# `auto-promote-staging.yml` advances main via `git merge --ff-only`
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# + `git push origin main` — that's a clean fast-forward, no merge
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# commit. But manual merges of `staging → main` PRs through the
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# GitHub UI / API create a merge commit on main that staging
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# doesn't have. The next `staging → main` PR then evaluates as
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# "BEHIND" because staging is missing that merge commit, requiring
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# a manual `gh pr update-branch` round-trip.
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#
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# This happened twice on 2026-04-28 (PRs #2202, #2205, both manual
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# bridges). Each time the bridge needed update-branch + a re-CI
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# round before merging. Operationally annoying and avoidable.
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#
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# This workflow closes the gap automatically:
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#
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# 1. Push to main fires (regardless of source: auto-promote, UI
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# merge, API merge, direct push).
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# 2. Check whether main is already in staging's ancestry — if
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# yes, no-op (auto-promote-staging already kept them in sync
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# via fast-forward).
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# 3. If not, try fast-forward staging to main first (works when
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# staging hasn't diverged with its own commits).
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# 4. If ff fails (staging has commits main doesn't — feature work
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# in flight), do a real merge with a "chore: sync" commit so
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# staging absorbs main's tip while keeping its own history.
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# 5. Push staging.
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#
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# Loop safety:
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#
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# Pushing the synced staging triggers `auto-promote-staging.yml`,
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# which checks gates on staging's new tip and, if green, ff-pushes
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# staging to main. Since staging now == main (ff case) or ⊇ main
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# (merge case where promote then advances), the resulting push to
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# main is either a no-op (no actual ref change → no push event) or
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# advances main further. In the latter case auto-sync fires again,
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# sees main already in staging's ancestry, no-ops. No infinite loop.
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on:
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push:
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branches: [main]
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permissions:
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contents: write
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jobs:
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sync-staging:
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runs-on: ubuntu-latest
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steps:
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- name: Checkout staging
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uses: actions/checkout@v4
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with:
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fetch-depth: 0
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ref: staging
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token: ${{ secrets.GITHUB_TOKEN }}
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- name: Configure git author
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run: |
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git config user.name "github-actions[bot]"
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git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
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- name: Check if staging already contains main
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id: check
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run: |
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set -euo pipefail
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git fetch origin main
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if git merge-base --is-ancestor origin/main HEAD; then
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echo "needs_sync=false" >> "$GITHUB_OUTPUT"
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{
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echo "## ✅ No-op"
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echo
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echo "staging already contains \`origin/main\` ($(git rev-parse --short=8 origin/main))."
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echo "auto-promote-staging or a previous auto-sync run already kept them aligned."
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} >> "$GITHUB_STEP_SUMMARY"
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else
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echo "needs_sync=true" >> "$GITHUB_OUTPUT"
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echo "::notice::staging is missing main's tip — sync needed"
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fi
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- name: Fast-forward staging → main
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if: steps.check.outputs.needs_sync == 'true'
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id: ff
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run: |
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set -euo pipefail
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if git merge --ff-only origin/main; then
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echo "did_ff=true" >> "$GITHUB_OUTPUT"
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echo "::notice::Fast-forwarded staging to origin/main"
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else
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echo "did_ff=false" >> "$GITHUB_OUTPUT"
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echo "::notice::ff failed — staging has its own commits; will create merge"
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fi
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- name: Merge main into staging (when ff fails)
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if: |
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steps.check.outputs.needs_sync == 'true' &&
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steps.ff.outputs.did_ff != 'true'
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run: |
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set -euo pipefail
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# ff failed because staging has commits main doesn't — typical
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# in-flight feature work. Create a merge commit so staging
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# absorbs main's tip while keeping its own history.
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if ! git merge --no-ff origin/main -m "chore: sync main → staging (auto)"; then
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{
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echo "## ❌ Conflict"
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echo
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echo "Auto-merge \`main → staging\` failed with conflicts."
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echo "A human needs to resolve manually:"
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echo
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echo " git checkout staging"
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echo " git merge origin/main"
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echo " # resolve, commit, push"
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} >> "$GITHUB_STEP_SUMMARY"
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exit 1
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fi
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- name: Push staging
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if: steps.check.outputs.needs_sync == 'true'
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run: |
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set -euo pipefail
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git push origin staging
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{
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if [ "${{ steps.ff.outputs.did_ff }}" = "true" ]; then
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echo "## ✅ staging fast-forwarded"
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echo
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echo "staging is now at \`$(git rev-parse --short=8 HEAD)\` (== origin/main)."
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else
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echo "## ✅ staging absorbed main"
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echo
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echo "staging is now at \`$(git rev-parse --short=8 HEAD)\` with a merge commit absorbing main's tip."
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fi
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} >> "$GITHUB_STEP_SUMMARY"
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