CRE-1827: detect potential merge conflicts

.github/actions/prconflicts/action.yaml

@@ -0,0 +1,120 @@
+name: Detect conflicting PRs
+description: Detects PRs that would conflict with the current PR and posts a comment
+
+inputs:
+  github-token:
+    description: GitHub token with pull-requests write and contents read permissions
+    required: true
+  repository:
+    description: Repository in owner/repo format
+    required: true
+  pr-number:
+    description: Current PR number
+    required: true
+  head-ref:
+    description: Head branch of the current PR
+    required: true
+  base-ref:
+    description: Base branch of the current PR
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - name: Fetch base and current branch
+      shell: bash
+      run: |
+        git fetch origin ${{ inputs.base-ref }} ${{ inputs.head-ref }}
+
+    - name: Detect conflicts and update comment
+      shell: bash
+      env:
+        GH_TOKEN: ${{ inputs.github-token }}
+        REPO: ${{ inputs.repository }}
+        CURRENT_PR: ${{ inputs.pr-number }}
+        CURRENT_BRANCH: ${{ inputs.head-ref }}
+        BASE_BRANCH: ${{ inputs.base-ref }}
+      run: |
+        set -e
+
+        MARKER="<!-- conflict-check -->"
+
+        # Get files changed in current PR
+        current_files=$(gh api --method GET --paginate "repos/${REPO}/pulls/${CURRENT_PR}/files" --jq '.[].filename' | sort -u)
+        current_files_count=$(echo "$current_files" | wc -l)
+        echo "Current PR touches $current_files_count files"
+
+        # Get all open PRs targeting same base (paginated)
+        prs=$(gh api --method GET --paginate "repos/${REPO}/pulls?state=open&base=${BASE_BRANCH}" --jq '.[].number')
+        total_prs=$(echo "$prs" | wc -w)
+        echo "Found $total_prs open PRs targeting ${BASE_BRANCH}"
+
+        # Find PRs with overlapping files (fast API check)
+        candidates=()
+        for pr in $prs; do
+          [ "$pr" = "$CURRENT_PR" ] && continue
+
+          other_files=$(gh api --method GET --paginate "repos/${REPO}/pulls/${pr}/files" --jq '.[].filename' | sort -u)
+
+          # Check for file intersection
+          if comm -12 <(echo "$current_files") <(echo "$other_files") | grep -q .; then
+            candidates+=("$pr")
+          fi
+        done
+
+        echo "Found ${#candidates[@]} PRs with overlapping files, checking for conflicts..."
+
+        if [ ${#candidates[@]} -eq 0 ]; then
+          conflicts=()
+        else
+          # Fetch only the PR refs we need
+          fetch_refs=""
+          for pr in "${candidates[@]}"; do
+            fetch_refs="$fetch_refs +refs/pull/${pr}/head:refs/remotes/origin/pr/${pr}"
+          done
+          git fetch origin $fetch_refs
+
+          mkdir -p worktrees
+          conflicts=()
+
+          for pr in "${candidates[@]}"; do
+            other_branch="origin/pr/$pr"
+            dir="worktrees/$pr"
+
+            git worktree add -q --detach "$dir" "origin/${BASE_BRANCH}"
+
+            if git -C "$dir" merge --no-commit --no-ff "$other_branch" >/dev/null 2>&1; then
+              git -C "$dir" -c user.email="ci@local" -c user.name="CI" commit --no-edit -m "temp" >/dev/null 2>&1
+
+              if ! git -C "$dir" merge --no-commit --no-ff "origin/${CURRENT_BRANCH}" >/dev/null 2>&1; then
+                conflicts+=("#$pr")
+              fi
+            fi
+
+            git worktree remove -f "$dir"
+          done
+        fi
+
+        echo "Found ${#conflicts[@]} conflicting PRs"
+
+        if [ ${#conflicts[@]} -eq 0 ]; then
+          BODY="${MARKER}
+        ✅ No conflicts with other open PRs targeting \`${BASE_BRANCH}\`"
+        else
+          BODY="${MARKER}
+        ❌ Conflicts with:
+
+        $(printf '%s\n' "${conflicts[@]}")"
+        fi
+
+        COMMENT_ID=$(gh api --method GET "repos/${REPO}/issues/${CURRENT_PR}/comments" \
+          --jq ".[] | select(.body | contains(\"${MARKER}\")) | .id" \
+          | head -n1)
+
+        if [ -n "$COMMENT_ID" ]; then
+          gh api --method PATCH "repos/${REPO}/issues/comments/${COMMENT_ID}" \
+            -f body="$BODY"
+        else
+          gh api --method POST "repos/${REPO}/issues/${CURRENT_PR}/comments" \
+            -f body="$BODY"
+        fi

.github/workflows/pr-conflicts.yaml

@@ -0,0 +1,29 @@
+name: Detect PR Conflicts
+
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+jobs:
+  detect-conflicts:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+      contents: read
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - uses: ./.github/actions/prconflicts
+        with:
+          github-token: ${{ github.token }}
+          repository: ${{ github.repository }}
+          pr-number: ${{ github.event.pull_request.number }}
+          head-ref: ${{ github.head_ref }}
+          base-ref: ${{ github.base_ref }}