fix: sync-main-to-experimental action correct base branch

.github/workflows/merge-sync-to-experimental.yml

@@ -0,0 +1,67 @@
+name: Merge sync PR into experimental
+
+# The repo allows only squash merges, and squashing the sync PR would drop main's
+# commits and freeze the merge base. This lands it as a real merge commit instead;
+# GitHub closes the sync PR as merged once its commits are reachable from experimental.
+on:
+  workflow_dispatch:
+
+concurrency:
+  group: merge-sync-to-experimental
+  cancel-in-progress: false
+
+permissions:
+  contents: write
+  pull-requests: read
+
+jobs:
+  merge:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out the code
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Configure git identity
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
+
+      - name: Merge chore/sync into experimental
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          set -euo pipefail
+
+          git config --replace-all remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
+          git fetch --no-tags --force --prune origin
+
+          if ! git rev-parse --verify --quiet refs/remotes/origin/chore/sync >/dev/null; then
+            echo "::error::chore/sync does not exist on the remote; there is no sync to land."
+            exit 1
+          fi
+
+          if git merge-base --is-ancestor origin/chore/sync origin/experimental; then
+            echo "experimental already contains chore/sync; nothing to merge."
+            exit 0
+          fi
+
+          PR_NUMBER="$(gh pr list --base experimental --head chore/sync --state open --json number --jq '.[0].number // empty')"
+
+          git checkout -B experimental origin/experimental
+          # --no-ff so the sync always shows up as one labelled merge commit, the way the
+          # PR button would have recorded it.
+          if ! git merge --no-ff -m "chore: sync main to experimental${PR_NUMBER:+ (#$PR_NUMBER)}" origin/chore/sync; then
+            git merge --abort || true
+            echo "::error::Merging chore/sync into experimental failed. Resolve it on chore/sync, push, and re-run this workflow."
+            exit 1
+          fi
+
+          git push origin experimental
+
+          # A PR closed by a push is not covered by delete_branch_on_merge, so remove the
+          # now fully-merged branch here: the sync job treats an existing chore/sync as a
+          # sync still in flight.
+          git push origin --delete chore/sync

.github/workflows/sync-main-to-experimental.yml

@@ -5,35 +5,112 @@ on:
     branches: [ main ]
   workflow_dispatch:
 
+# Two pushes to main must not race on pushing chore/sync.
+concurrency:
+  group: sync-main-to-experimental
+  cancel-in-progress: false
+
+permissions:
+  contents: write
+  pull-requests: write
+
 jobs:
   create-pr:
     runs-on: ubuntu-latest
 
     steps:
       - name: Check out the code
         uses: actions/checkout@v4
+        with:
+          # full history, otherwise the merges below have no common ancestor
+          fetch-depth: 0
+
+      - name: Configure git identity
+        run: |
+          git config user.name "github-actions[bot]"
+          git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
 
       - name: Create or update sync branch
         run: |
-          git fetch origin experimental
-          git checkout -B chore/sync
-          git push --force --set-upstream origin chore/sync
+          set -euo pipefail
+
+          # The wildcard is what the bare fetch below and --force-with-lease resolve
+          # branch names through.
+          git config --replace-all remote.origin.fetch "+refs/heads/*:refs/remotes/origin/*"
+          git fetch --no-tags --force --prune origin
+
+          # An existing chore/sync is a sync still in flight: keep it, so commits
+          # arriving on main accumulate on it and a manual conflict resolution pushed
+          # to it survives. The branch is deleted when its PR merges, so the next sync
+          # starts from experimental again.
+          BASE=origin/experimental
+          if git rev-parse --verify --quiet refs/remotes/origin/chore/sync >/dev/null; then
+            BASE=origin/chore/sync
+          fi
+          git checkout -B chore/sync "$BASE"
+
+          # chore/sync must be experimental + main: anything that only exists on
+          # experimental has to survive the sync, or the package built from the branch
+          # ships main's protos alone.
+          for REF in origin/experimental origin/main; do
+            if ! git merge -m "chore: merge $REF into chore/sync" "$REF"; then
+              git merge --abort || true
+              echo "::error::Merging $REF into chore/sync failed. To recover: branch chore/sync off experimental (or check out the existing one), merge main into it, resolve, push it, and open a PR against experimental. Later runs merge on top of that resolution."
+              exit 1
+            fi
+          done
+
+          # Push whenever the recomputed branch differs from the remote, so a stale
+          # chore/sync can never stay the head of an open PR.
+          REMOTE_HEAD="$(git rev-parse --verify --quiet refs/remotes/origin/chore/sync || echo absent)"
+          if [ "$(git rev-parse HEAD)" = "$REMOTE_HEAD" ]; then
+            echo "chore/sync on the remote is already up to date."
+          else
+            git push --force-with-lease origin chore/sync
+          fi
 
       - name: Create or update PR
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          # Backticks here are data, not shell source: they reach gh through the
+          # variable and are never re-evaluated.
+          PR_BODY: |
+            :crown: *An automated PR to keep experimental in sync with main*
+
+            > [!IMPORTANT]
+            > **Do not merge this PR with the Squash button.**
+            >
+            > Squashing replaces `main`'s commits with a single new one, so `main` stops
+            > being an ancestor of `experimental` and the merge base freezes. Every later
+            > sync then re-applies changes `experimental` already has, and starts
+            > conflicting within a couple of runs.
+            >
+            > Land it by running the **Merge sync PR into experimental** workflow
+            > (Actions -> Merge sync PR into experimental -> Run workflow). It pushes a
+            > real merge commit, and this PR closes as merged.
+            >
+            > If it does get squashed, restore the ancestry with
+            > `git merge -s ours origin/main` on `experimental`.
         run: |
-          PR_NUMBER=$(gh pr list --base experimental --head chore/sync --state open --json number --jq '.[0].number')
+          set -euo pipefail
+
+          # gh pr create fails with "No commits between ..." when nothing is ahead.
+          if git merge-base --is-ancestor HEAD origin/experimental; then
+            echo "experimental already contains chore/sync; nothing to propose."
+            exit 0
+          fi
+
+          PR_NUMBER="$(gh pr list --base experimental --head chore/sync --state open --json number --jq '.[0].number // empty')"
 
           if [ -n "$PR_NUMBER" ]; then
             echo "PR #$PR_NUMBER already exists, adding a comment..."
-            gh pr comment $PR_NUMBER --body "🔄 Updated with latest changes from **main** on $(date -u '+%Y-%m-%d %H:%M UTC')"
+            gh pr comment "$PR_NUMBER" --body "🔄 Updated with latest changes from **main** on $(date -u '+%Y-%m-%d %H:%M UTC')"
           else
             echo "Creating new PR..."
             gh pr create \
               --base experimental \
               --head chore/sync \
               --title "chore: sync main to experimental" \
-              --body ":crown: *An automated PR to keep experimental in sync with main*" \
+              --body "$PR_BODY" \
               --label "auto-pr"
           fi