Merge BlockstreamResearch/secp256k1-zkp#360: sync-upstream: Simplify

c5be752e2e gha: Simplify sync.yml according to sync-upstream.sh changes (Tim Ruffing)
260fad30aa sync-upstream: Optionally switch to sync branch (Tim Ruffing)
f0ff45f3ab sync-upstream: Simplify (Tim Ruffing)

Pull request description:

  This is a half-rewrite of the sync-upstream script because I still wasn't happy with it. Looking for early feedback on this.

  Advantages:
   - The script does not hardcode any repo URLs, branches, etc.
   - Nicer separation of responsibilities between the script and the GHA workflow. This brings back the usefulness of the script when used locally outside GHA.
   - Much simpler code, and we could simplify also the GHA workflow.

  Still to do:
   - Rewrite the "Tips" section in the generated PR description, e.g., add how to resolve merge conflicts.
   - Change the GHA workflow accordingly.

ACKs for top commit:
  mllwchrry:
    ACK c5be752

Tree-SHA512: 7335b4690642c07965366da952a74065ec404f750e0c6c7e735a9a48285cbe6b9c7db27ff4bdeba1d5ce59717d77be1ecb2334200b56a1a0a2c0fcddc7530d0b

.github/workflows/sync.yml

@@ -9,11 +9,10 @@ jobs:
   sync-upstream:
     runs-on: ubuntu-latest
     env:
-      UPSTREAM: "https://github.com/bitcoin-core/secp256k1.git" 
-      UPSTREAM_BRANCH: "master"
-      ORIGIN_BRANCH: "master"
-      MIN_UPSTREAM_MERGES: 1
-    
+      UPSTREAM: "https://github.com/bitcoin-core/secp256k1.git"
+      BASE_BRANCH: "master"
+      UPSTREAM_REF: "upstream/master"
+
     steps:
       - name: Checkout repository
         uses: actions/checkout@v6
@@ -26,66 +25,37 @@ jobs:
           git config user.name "github-actions[bot]"
           git config user.email "github-actions[bot]@users.noreply.github.com"
 
-      - name: Fetch upstream & generate branch name & check for new commits
-        id: check_commits
-        env:
-          GH_TOKEN: ${{ secrets.SYNC_PAT }}
+      - name: Fetch upstream
         run: |
-          gh repo set-default ${{ github.repository }}  # Set the default repo to the origin repository
           git remote add upstream ${{ env.UPSTREAM }}
           git fetch upstream
-          
-          MERGES=$(git rev-list --count --merges HEAD..upstream/${{ env.UPSTREAM_BRANCH }})
-          echo "Found $MERGES new merge commits in upstream."
-          
-          if [ "$MERGES" -lt ${{ env.MIN_UPSTREAM_MERGES }} ]; then
-            echo "Exiting."
-            echo "skip=true" >> "$GITHUB_OUTPUT"
-          else
-            echo "skip=false" >> "$GITHUB_OUTPUT"
-          fi
-          
-          # Generate a sync branch name "sync-UPSTREAM_HEAD", where UPSTREAM_HEAD is a commit ID.
-          UPSTREAM_HEAD=$(git rev-parse --short upstream/${{ env.UPSTREAM_BRANCH }})
-          SYNC_BRANCH="sync-$UPSTREAM_HEAD"
-          echo "Sync branch name: $SYNC_BRANCH"
-          echo "SYNC_BRANCH=$SYNC_BRANCH" >> "$GITHUB_ENV"
-
-          # Check if the sync branch already exists in the origin repository
-          if git ls-remote --heads origin "$SYNC_BRANCH" | grep -q "$SYNC_BRANCH"; then
-            echo "Branch $SYNC_BRANCH already exists. Skipping the sync."
-            echo "skip=true" >> "$GITHUB_OUTPUT"
-          else
-            echo "skip=false" >> "$GITHUB_OUTPUT"
-          fi
-
-      - name: Generate PR metadata
-        id: branch_pr_metadata
-        if: steps.check_commits.outputs.skip == 'false'
-        run: |
-          # Call the sync script to generate PR title and body
-          ./contrib/sync-upstream.sh -b ${{ env.ORIGIN_BRANCH }} "$SYNC_BRANCH"
-
-      - name: Push a sync branch
-        id: push_branch
-        if: steps.check_commits.outputs.skip == 'false'
-        run: |
-          echo "Creating sync branch: $SYNC_BRANCH"
-          git checkout upstream/${{ env.UPSTREAM_BRANCH }}
-          git checkout -b "$SYNC_BRANCH"
-          git push -u origin "$SYNC_BRANCH"
-
-      - name: Create pull request
-        id: create_pr
-        if: steps.check_commits.outputs.skip == 'false'
+          gh repo set-default ${{ github.repository }}
         env:
           GH_TOKEN: ${{ secrets.SYNC_PAT }}
-        run: |
-          # Execute the generated PR creation script
-          ./contrib/gh-pr-create.sh 
 
-      - name: Cleanup sync branch on failure
-        if: steps.push_branch.outcome == 'success' && steps.create_pr.outcome == 'failure'
+      - name: Run sync-upstream.sh
+        id: sync
         run: |
-          echo "PR creation failed but branch was pushed. Deleting: $SYNC_BRANCH"
-          git push origin --delete "$SYNC_BRANCH"
+          OUTPUT=$(./contrib/sync-upstream.sh --switch "${{ env.BASE_BRANCH }}" "${{ env.UPSTREAM_REF }}" 2>&1) || { echo "$OUTPUT"; exit 1; }
+          echo "$OUTPUT"
+          if echo "$OUTPUT" | grep -qv "^No merge commits"; then
+            echo "newcommits=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "Skipping further workflow steps."
+          fi
+
+      - name: Push sync branch
+        id: push
+        if: steps.sync.outputs.newcommits == 'true'
+        run: |
+          if git push -u origin HEAD; then
+            echo "pushed=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "Skipping further workflow steps."
+          fi
+
+      - name: Create pull request
+        if: steps.sync.outputs.newcommits == 'true' && steps.push.outputs.pushed == 'true'
+        env:
+          GH_TOKEN: ${{ secrets.SYNC_PAT }}
+        run: ./gh-pr-create.sh