sync_milestones.yml

name: sync milestones

on:
  milestone:
    types: [created]

jobs:
  synchronize:
    name: synchronize milestones
    runs-on: ubuntu-latest

    steps:
      # Rather than use a personal access token to interact with the project, we
      # can use this GitHub App. There's an API for exchanging app credentials
      # for a short-term token, and we use that API here.
      - name: get token
        uses: tibdex/github-app-token@v1
        id: app_token
        with:
          app_id: ${{ secrets.PROJECT_APP_ID }}
          installation_id: ${{ secrets.PROJECT_INSTALLATION_ID }}
          private_key: ${{ secrets.PROJECT_PRIVATE_KEY }}
      
      - name: sync milestones
        uses: actions/github-script@v6
        with:
          github-token: ${{ steps.app_token.outputs.token }}
          script: |
            // Get the metadata about this repo from the workflow context
            const { owner, repo } = context.repo;
            // And remove this repo from the list of repos to sync to. No need
            // to sync myself with myself.
            const syncTargets = ["methods", "ux-guide"].filter((r) => r !== repo);

            // Get a list of this repo's milestones
            const myMilestones = await github
              .paginate(`GET /repos/${owner}/${repo}/milestones`)
              .then((milestones) =>
                milestones.map(({ title, description, due_on, state }) => ({
                  title,
                  description,
                  due_on,
                  state,
                }))
              );

            // Now for each sync target repo...
            for await (const target of syncTargets) {
              // Get the titles of their milestones, which we'll use to filter
              // down to which milestones to synchronize
              const theirMilestones = new Set(
                await github
                  .paginate(`GET /repos/${owner}/${target}/milestones`)
                  .then((milestones) => milestones.map(({ title }) => title))
              );

              for await (const milestone of myMilestones) {
                // If a milestone with this title does NOT exist in the target
                // repo, then we should create one.
                if (!theirMilestones.has(milestone.title)) {
                  // Expand the milestone so we push the title, description, due
                  // date, and status (open/closed) all at once.
                  await github.rest.issues.createMilestone({ owner, repo: target, ...milestone });
                }
              }
            }