add-milestone.yml

name: Add Milestone on a Merged PR

on:
  pull_request:
    types:
      - closed
    branches:
      - main
      - "v[0-9]+.[0-9]+"

permissions: {}

jobs:
  add-milestone-pr:
    name: Add Milestone on PR
    if: github.event.pull_request.merged == true
    runs-on: ubuntu-latest
    permissions:
      pull-requests: write
    env:
      GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
      GH_REPO: ${{ github.repository }}
    steps:
      - name: Get current milestone from branch
        id: current-milestone
        run: |
          BASE_BRANCH="${{ github.base_ref }}"

          if [[ "$BASE_BRANCH" == "main" ]]; then
            # For main, use the open milestone with the highest creation order (latest upcoming release).
            MILESTONE=$(gh api repos/${{ github.repository }}/milestones \
              --jq '[.[] | select(.state == "open")] | sort_by(.number) | last | .title')
          elif [[ "$BASE_BRANCH" =~ ^v([0-9]+)\.([0-9]+)$ ]]; then
            # For release branches like v1.25, derive the milestone as v1.25.0.
            MILESTONE="v${BASH_REMATCH[1]}.${BASH_REMATCH[2]}.0"
          else
            echo "Error: Unexpected branch '$BASE_BRANCH'."
            exit 1
          fi

          if [ -z "$MILESTONE" ]; then
            echo "Error: Couldn't determine a current milestone for branch '$BASE_BRANCH'."
            exit 1
          fi

          if [[ ! $MILESTONE =~ ^v[0-9]+\.[0-9]+\.[0-9]+$ ]]; then
            echo "Error: Malformed milestone '$MILESTONE'. It should be of the form 'vX.Y.Z'."
            exit 1
          fi

          echo "MILESTONE=$MILESTONE" >> "$GITHUB_OUTPUT"

      - name: Set the merged PR milestone to current milestone
        run: |
          echo "Setting milestone $MILESTONE to PR $NUMBER."
          gh issue edit "$NUMBER" --milestone "$MILESTONE"
        env:
          NUMBER: ${{ github.event.number }}
          MILESTONE: ${{ steps.current-milestone.outputs.MILESTONE }}