GitHub Actions: auto-delete a PR's stranded caches when the PR closes

cleanup-pr-caches.yml

# .github/workflows/cleanup-pr-caches.yml
#
# Deletes all GitHub Actions caches scoped to a PR's merge ref as soon as the
# PR closes (merged or not). Those caches live on refs/pull/<N>/merge and can
# NEVER be restored by any other branch, so after the PR closes they are pure
# dead weight that otherwise lingers until 7-day-inactivity or LRU eviction.
#
# Drop this file into each repo at: .github/workflows/cleanup-pr-caches.yml
name: Cleanup PR caches

on:
  pull_request:
    types: [closed]

permissions:
  actions: write   # required to delete caches
  contents: read

jobs:
  cleanup:
    runs-on: ubuntu-latest
    steps:
      - name: Delete caches for this PR
        env:
          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          REPO: ${{ github.repository }}
          BRANCH: refs/pull/${{ github.event.pull_request.number }}/merge
        run: |
          echo "Clearing caches on $BRANCH for $REPO"
          ids=$(gh api --paginate \
            "/repos/$REPO/actions/caches?per_page=100&ref=$BRANCH" \
            --jq '.actions_caches[].id')
          if [ -z "$ids" ]; then
            echo "No caches to delete."
            exit 0
          fi
          for id in $ids; do
            echo "Deleting cache $id"
            gh api -X DELETE "/repos/$REPO/actions/caches/$id" || true
          done