workflows: close incomplete PRs

- Close PRs that appear to retain most of the template text so
  maintainers do not need to triage unfilled submissions manually.
- Reopen workflow-closed PRs once the template is properly filled in,
  while avoiding manually closed PRs.
- Keep the matching logic in style-checked Ruby and the workflow logic
  in a shell helper.

Author: MikeMcQuaid

.github/workflows/incomplete-prs.rb

@@ -0,0 +1,26 @@
+# typed: strict
+# frozen_string_literal: true
+
+# Checks whether a pull request body preserves enough of the current pull request template.
+
+CHECKBOX_MARKER = /\A- \[[ xX]\] /
+NORMALISED_CHECKBOX_MARKER = "- [ ] "
+REQUIRED_TEMPLATE_PERCENTAGE = 51
+PERCENTAGE_SCALE = 100
+
+pr_body_path = ARGV.fetch(0)
+template_path = ARGV.fetch(1)
+
+pr_lines, template_lines = [pr_body_path, template_path].map do |path|
+  File.foreach(path, chomp: true).filter_map do |line|
+    line = line.strip.sub(CHECKBOX_MARKER, NORMALISED_CHECKBOX_MARKER)
+    line unless line.empty?
+  end
+end
+
+template_lines.uniq!
+matching_template_lines = template_lines.count { |line| pr_lines.include?(line) }
+scaled_matching_percentage = matching_template_lines * PERCENTAGE_SCALE
+scaled_required_percentage = template_lines.count * REQUIRED_TEMPLATE_PERCENTAGE
+
+exit scaled_matching_percentage >= scaled_required_percentage

.github/workflows/incomplete-prs.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+
+# Closes incomplete pull requests and reopens workflow-closed pull requests once the template is filled in.
+
+set -xeuo pipefail
+
+PR_BODY_FILE="$(mktemp)"
+trap 'rm -f "${PR_BODY_FILE}"' EXIT
+printf "%s" "${PR_BODY:-}" >"${PR_BODY_FILE}"
+
+if brew ruby .github/workflows/incomplete-prs.rb "${PR_BODY_FILE}" .github/PULL_REQUEST_TEMPLATE.md
+then
+  complete_template=true
+else
+  complete_template=false
+fi
+
+if [[ "${PR_STATE:?}" == "closed" ]]
+then
+  if [[ "${complete_template}" == "true" ]] &&
+     gh api --paginate "repos/${GITHUB_REPOSITORY:?}/issues/${PR_NUMBER:?}/comments" \
+     --jq '.[] | select(.user.login == "github-actions[bot]" and (.body | contains("<!-- incomplete-pr-template -->"))) | .id' |
+     grep -q .
+  then
+    gh api --method PATCH "repos/${GITHUB_REPOSITORY:?}/pulls/${PR_NUMBER:?}" \
+      -f state=open
+  fi
+
+  exit 0
+fi
+
+if [[ "${complete_template}" == "true" ]]
+then
+  exit 0
+fi
+
+gh api --method POST "repos/${GITHUB_REPOSITORY:?}/issues/${PR_NUMBER:?}/comments" \
+  --raw-field body="$(
+    cat <<COMMENT
+<!-- incomplete-pr-template -->
+Thanks for your pull request. This has been closed because it appears to use an incomplete or outdated pull request template.
+
+This pull request may be reopened on request if a human properly fills in the current [pull request template](${PR_TEMPLATE_URL:?}).
+COMMENT
+  )"
+
+gh api --method PATCH "repos/${GITHUB_REPOSITORY:?}/pulls/${PR_NUMBER:?}" \
+  -f state=closed

.github/workflows/incomplete-prs.yml

@@ -0,0 +1,43 @@
+name: Close incomplete pull requests
+
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - reopened
+
+permissions: {}
+
+defaults:
+  run:
+    shell: bash -xeuo pipefail {0}
+
+concurrency:
+  group: "incomplete-pr-${{ github.event.pull_request.number }}"
+  cancel-in-progress: true
+
+jobs:
+  manage:
+    if: github.repository == 'Homebrew/brew'
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write
+    env:
+      GH_TOKEN: ${{ github.token }}
+      PR_BODY: ${{ github.event.pull_request.body }}
+      PR_NUMBER: ${{ github.event.pull_request.number }}
+      PR_STATE: ${{ github.event.pull_request.state }}
+      PR_TEMPLATE_URL: ${{ github.server_url }}/${{ github.repository }}/blob/HEAD/.github/PULL_REQUEST_TEMPLATE.md
+    steps:
+      - name: Set up Homebrew
+        id: set-up-homebrew
+        uses: Homebrew/actions/setup-homebrew@main
+        with:
+          core: false
+          cask: false
+
+      - name: Manage incomplete pull request
+        working-directory: ${{ steps.set-up-homebrew.outputs.repository-path }}
+        run: bash .github/workflows/incomplete-prs.sh