name: Retarget and Merge Dependabot PRs

on:
  schedule:
    - cron: "0 6 * * *"
  workflow_dispatch:

permissions:
  contents: write
  pull-requests: write

jobs:
  retarget-and-merge:
    runs-on: blacksmith-2vcpu-ubuntu-2404
    steps:
      - name: Checkout repository
        uses: actions/checkout@v6
        with:
          fetch-depth: 1

      - name: Resolve newest dev branch
        id: dev
        env:
          GH_TOKEN: ${{ secrets.GHCR_TOKEN }}
        run: |
          REFS=$(gh api "repos/${{ github.repository }}/branches" --paginate -q '.[].name')
          # The helper exits non-zero when no dev-X.Y.Z branch exists; treat that
          # as "nothing to do" rather than a workflow failure.
          if DEV_BRANCH=$(printf '%s\n' "$REFS" | node scripts/latest-dev-branch.cjs 2>/dev/null); then
            echo "Newest dev branch: $DEV_BRANCH"
            echo "branch=$DEV_BRANCH" >> "$GITHUB_OUTPUT"
          else
            echo "No dev-X.Y.Z branch open; nothing to retarget."
            echo "branch=" >> "$GITHUB_OUTPUT"
          fi

      - name: Retarget and merge Dependabot PRs
        if: ${{ steps.dev.outputs.branch != '' }}
        env:
          GH_TOKEN: ${{ secrets.GHCR_TOKEN }}
          DEV_BRANCH: ${{ steps.dev.outputs.branch }}
          REPO: ${{ github.repository }}
        run: |
          set -uo pipefail

          CONFLICT_LABEL="dependabot-rebase-requested"

          # Ensure the bookkeeping label exists (no-op if it already does).
          gh label create "$CONFLICT_LABEL" --repo "$REPO" \
            --color "D93F0B" --description "Retarget workflow asked Dependabot to rebase a conflicting PR" \
            2>/dev/null || true

          # True if the PR already carries the conflict label.
          has_conflict_label() {
            gh pr view "$1" --repo "$REPO" --json labels \
              -q '.labels[].name' | grep -qx "$CONFLICT_LABEL"
          }

          # Wait until GitHub has a definite mergeable verdict for a PR (it
          # returns UNKNOWN while recomputing after a base change or a push).
          # Echoes "<mergeable> <mergeStateStatus>".
          wait_for_verdict() {
            local pr="$1" mergeable state
            for _ in $(seq 1 30); do
              read -r mergeable state < <(gh pr view "$pr" --repo "$REPO" \
                --json mergeable,mergeStateStatus \
                -q '.mergeable + " " + .mergeStateStatus')
              if [ "$mergeable" != "UNKNOWN" ] && [ "$state" != "UNKNOWN" ]; then
                echo "$mergeable $state"
                return 0
              fi
              sleep 20
            done
            echo "$mergeable $state"
          }

          # Phase 1: retarget every open Dependabot PR from main onto the dev
          # branch. This kicks off a Dependabot rebase for each.
          PR_NUMBERS=$(gh pr list --repo "$REPO" \
            --author "app/dependabot" \
            --base main \
            --state open \
            --json number -q '.[].number')

          # Pick up PRs already sitting on the dev branch from a previous run too.
          PR_NUMBERS="$PR_NUMBERS $(gh pr list --repo "$REPO" \
            --author "app/dependabot" \
            --base "$DEV_BRANCH" \
            --state open \
            --json number -q '.[].number')"

          PR_NUMBERS=$(printf '%s\n' $PR_NUMBERS | sort -un)

          if [ -z "$PR_NUMBERS" ]; then
            echo "No open Dependabot PRs to process."
            exit 0
          fi

          for PR in $PR_NUMBERS; do
            BASE=$(gh pr view "$PR" --repo "$REPO" --json baseRefName -q .baseRefName)
            if [ "$BASE" != "$DEV_BRANCH" ]; then
              echo "Retargeting PR #$PR ($BASE -> $DEV_BRANCH)"
              gh pr edit "$PR" --repo "$REPO" --base "$DEV_BRANCH"
            fi
          done

          # Phase 2: merge one at a time. Each merge can make the remaining npm
          # PRs stale, so re-check immediately before merging and rebase stragglers.
          for PR in $PR_NUMBERS; do
            echo "::group::PR #$PR"

            read -r MERGEABLE STATE < <(wait_for_verdict "$PR")
            echo "  mergeable=$MERGEABLE mergeStateStatus=$STATE"

            # BEHIND = clean but needs the latest base; ask Dependabot to rebase
            # and skip for now (next run merges it once it is up to date).
            if [ "$STATE" = "BEHIND" ]; then
              echo "  PR #$PR is behind $DEV_BRANCH; asking Dependabot to rebase."
              gh pr comment "$PR" --repo "$REPO" --body "@dependabot rebase"
              echo "::endgroup::"
              continue
            fi

            # DIRTY / CONFLICTING = a real conflict. Try a rebase once (label it so
            # we can tell next time); if it is STILL conflicting on a later run
            # despite already being labelled, the rebase failed for good - close it
            # so Dependabot reopens a fresh PR against the current dev branch.
            if [ "$MERGEABLE" = "CONFLICTING" ] || [ "$STATE" = "DIRTY" ]; then
              if has_conflict_label "$PR"; then
                echo "  PR #$PR still conflicts after a prior rebase request; closing so Dependabot reopens it fresh."
                gh pr close "$PR" --repo "$REPO" --delete-branch \
                  --comment "Closing: this PR still conflicts with $DEV_BRANCH after a rebase attempt (its changes are likely already merged). Dependabot will reopen a fresh PR computed against the current $DEV_BRANCH."
              else
                echo "  PR #$PR conflicts with $DEV_BRANCH; requesting a rebase and labelling it."
                gh pr edit "$PR" --repo "$REPO" --add-label "$CONFLICT_LABEL"
                gh pr comment "$PR" --repo "$REPO" --body "@dependabot rebase"
              fi
              echo "::endgroup::"
              continue
            fi

            # A clean PR that was previously flagged has recovered - drop the label.
            if has_conflict_label "$PR"; then
              gh pr edit "$PR" --repo "$REPO" --remove-label "$CONFLICT_LABEL" || true
            fi

            echo "  Squash-merging PR #$PR"
            if gh pr merge "$PR" --repo "$REPO" --squash --admin; then
              echo "  Merged PR #$PR"
              # Give GitHub a moment to mark the now-stale siblings BEHIND.
              sleep 15
            else
              echo "  Could not merge PR #$PR now; it will be retried next run."
            fi

            echo "::endgroup::"
          done