Auto-merge package PRs that are trusted to build (#867)

A package PR approved to build, by its author being trusted or by the
build-approved label, sat open after going green until someone merged it by
hand, so nothing it built was published. Enable GitHub's auto-merge on it
with PKGS_BOT_TOKEN, so the merge lands once the required checks pass and
starts publish.yml.

The trust rule is build-pr.yml's. Only PRs changing nothing outside
pkgbuilds/ qualify: a PR's own tooling never runs in its build, and after
merge it runs with the publish secrets. The upstream sync, which labels its
own PRs, stays on the reviewed lane. Removing build-approved withdraws the
auto-merge.

.github/workflows/auto-merge-pr.yml

@@ -0,0 +1,108 @@
+name: Auto-merge approved package PRs
+
+# A package PR trusted to build is trusted to ship: once its builds are
+# green it should merge and publish without a maintainer pressing the
+# button. This enables GitHub's auto-merge on such PRs; branch protection
+# still holds the merge until `result`, `self-tests` and `build-isolation`
+# pass, and a red build stays an open PR. .github/scripts/auto-merge-pr.cjs
+# has the rule.
+#
+# Auto-merge is enabled with the PAT in PKGS_BOT_TOKEN: a merge made with the
+# built-in GITHUB_TOKEN does not start publish.yml.
+#
+# pull_request_target runs this default-branch code with secrets; the PR's
+# code is never checked out here.
+on:
+  pull_request_target:
+    types: [opened, reopened, synchronize, ready_for_review, labeled, unlabeled]
+  workflow_dispatch:
+    inputs:
+      pr:
+        description: 'PR number to evaluate'
+        required: true
+
+permissions:
+  contents: read
+  pull-requests: read
+
+concurrency:
+  group: auto-merge-pr-${{ github.event.pull_request.number || github.event.inputs.pr }}
+  cancel-in-progress: true
+
+jobs:
+  auto-merge:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.repository.default_branch }}
+          persist-credentials: false
+
+      - name: Find the PR's author
+        id: pr
+        uses: actions/github-script@v7
+        env:
+          NUMBER: ${{ github.event.pull_request.number || github.event.inputs.pr }}
+        with:
+          script: |
+            const { data: pr } = await github.rest.pulls.get({
+              ...context.repo, pull_number: Number(process.env.NUMBER),
+            });
+            core.setOutput('number', String(pr.number));
+            core.setOutput('author', pr.user.login);
+
+      - id: vouch
+        uses: mitchellh/vouch/action/check-user@f23dbb5e745334f97414ec70463ce7301071a661 # v1
+        with:
+          user: ${{ steps.pr.outputs.author }}
+          allow-fail: true
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Decide
+        id: decide
+        uses: actions/github-script@v7
+        env:
+          NUMBER: ${{ steps.pr.outputs.number }}
+          VOUCH_STATUS: ${{ steps.vouch.outputs.status }}
+        with:
+          script: |
+            const autoMerge = require('./.github/scripts/auto-merge-pr.cjs');
+            await autoMerge({ github, context, core,
+              number: Number(process.env.NUMBER), vouchStatus: process.env.VOUCH_STATUS });
+
+      - name: Enable auto-merge
+        if: steps.decide.outputs.enable == 'true'
+        env:
+          GH_TOKEN: ${{ secrets.PKGS_BOT_TOKEN }}
+          PR: ${{ steps.pr.outputs.number }}
+          HEAD_SHA: ${{ steps.decide.outputs.head_sha }}
+        run: |
+          if [[ -z "$GH_TOKEN" ]]; then
+            echo "::error::Set PKGS_BOT_TOKEN; a GITHUB_TOKEN merge would not publish."
+            exit 1
+          fi
+          # Idempotent: enabling twice errors. Bot lanes enable their own.
+          if [[ "$(gh pr view "$PR" -R "$GITHUB_REPOSITORY" --json autoMergeRequest --jq '.autoMergeRequest != null')" == true ]]; then
+            echo "auto-merge already enabled on #$PR"
+            exit 0
+          fi
+          # --match-head-commit: never arm a merge for a commit newer than
+          # the one just judged.
+          gh pr merge --auto --squash --match-head-commit "$HEAD_SHA" "$PR" -R "$GITHUB_REPOSITORY"
+
+      # Removing build-approved withdraws the approval, so withdraw the
+      # auto-merge it armed too. Only on that event: auto-merge a maintainer
+      # enabled by hand on any other PR is theirs to keep.
+      - name: Withdraw auto-merge
+        if: >-
+          steps.decide.outputs.enable == 'false' &&
+          github.event.action == 'unlabeled' && github.event.label.name == 'build-approved'
+        env:
+          GH_TOKEN: ${{ secrets.PKGS_BOT_TOKEN }}
+          PR: ${{ steps.pr.outputs.number }}
+        run: |
+          if [[ "$(gh pr view "$PR" -R "$GITHUB_REPOSITORY" --json autoMergeRequest --jq '.autoMergeRequest != null')" == true ]]; then
+            gh pr merge --disable-auto "$PR" -R "$GITHUB_REPOSITORY"
+          fi