Meta: add action to check commit messages

Author: ljharb

.github/workflows/check-commit.yml

@@ -0,0 +1,17 @@
+name: 262 Commit Checker
+
+on: pull_request
+
+jobs:
+   test:
+    name: Check commit messages
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v1
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
+    - run: git checkout HEAD^2 # to get the actual PR head
+    - uses: actions/setup-node@v1
+      with:
+        node-version: '12.x'
+    - run: node scripts/check-commit

scripts/check-commit.js

@@ -0,0 +1,37 @@
+#!/usr/bin/env node
+
+'use strict';
+
+const exec = require('child_process').execSync;
+
+const oldestAncestor = String(exec(`bash -c 'diff -u <(git rev-list --first-parent "\${1:-origin/master}") <(git rev-list --first-parent "\${2:-HEAD}") | sed -ne "s/^ //p" | head -1' -`)).trim();
+
+console.log(`Oldest ancestor SHA: ${oldestAncestor}`);
+
+const messages = String(exec(`git log --format=%B ${oldestAncestor}..HEAD | sed '/^[[:space:]]*\$/d'`)).trim().split('\n').reverse();
+
+const [first, ...rest] = messages;
+
+const prefixes = ['Normative', 'Editorial', 'Meta', 'Layering'];
+const firstPrefixRE = new RegExp(`^(?:${prefixes.join('|')}): `);
+const prefixRE = new RegExp(`^(?:${prefixes.concat('fixup', 'squash').join('|')}): `);
+
+const errors = [];
+if (!firstPrefixRE.test(first)) {
+	errors.push(`First commit must start with a valid prefix (${prefixes.join(', ')}), followed by “: ”`);
+}
+
+rest.forEach((msg) => {
+	if (!prefixRE.test(msg)) {
+		errors.push(`Commit “${msg}” must start with a valid prefix (${prefixes.join(', ')}) or “fixup” or “squash”, followed by “: ”`);
+	}
+});
+
+if (errors.length === 0) {
+	console.log('Commit messages are valid!');
+} else {
+	console.error(`Errors:
+${errors.map(x => ` - ${x}`).join('\n')}
+`);
+	process.exitCode = errors.length;
+}