ci: add deterministic pull request size labels (#42615)

Co-authored-by: surbadest <surbadest@users.noreply.github.com>

.github/scripts/size-labeler.test.cjs

@@ -0,0 +1,94 @@
+const fs = require('node:fs')
+const assert = require('node:assert/strict')
+const test = require('node:test')
+const source = fs.readFileSync(
+  require('node:path').join(__dirname, '../workflows/size-labeler.yml'),
+  'utf8',
+)
+const script = source
+  .replace(/\r\n/g, '\n')
+  .split('          script: |\n')[1]
+  .split('\n')
+  .map((l) => l.replace(/^            /, ''))
+  .join('\n')
+const run = new (Object.getPrototypeOf(async function () {}).constructor)(
+  'github',
+  'context',
+  'core',
+  script,
+)
+async function scenario(files, existing = [], changed = files.length) {
+  const calls = []
+  const warnings = []
+  const api = {
+    pulls: {
+      get: async () => ({ data: { changed_files: changed } }),
+      listFiles: 'files',
+    },
+    issues: {
+      listLabelsOnIssue: 'labels',
+      addLabels: async (p) => calls.push(['add', p.labels[0]]),
+      removeLabel: async (p) => calls.push(['remove', p.name]),
+    },
+  }
+  await run(
+    {
+      rest: api,
+      paginate: async (endpoint) =>
+        endpoint === 'files' ? files : existing.map((name) => ({ name })),
+    },
+    {
+      repo: { owner: 'test', repo: 'repo' },
+      payload: { pull_request: { number: 1 } },
+    },
+    { warning: (s) => warnings.push(s) },
+  )
+  return { calls, warnings }
+}
+for (const [n, label] of [
+  [0, 'XS'],
+  [9, 'XS'],
+  [10, 'S'],
+  [29, 'S'],
+  [30, 'M'],
+  [99, 'M'],
+  [100, 'L'],
+  [499, 'L'],
+  [500, 'XL'],
+  [999, 'XL'],
+  [1000, 'XXL'],
+]) {
+  test(`threshold ${n}`, async () =>
+    assert.deepEqual((await scenario([{ filename: 'a.py', additions: n, deletions: 0 }])).calls, [
+      ['add', `size:${label}`],
+    ]))
+}
+test('count deletions', async () =>
+  assert.deepEqual((await scenario([{ filename: 'a.py', additions: 0, deletions: 30 }])).calls, [
+    ['add', 'size:M'],
+  ]))
+test('ignore nested lockfiles', async () =>
+  assert.deepEqual(
+    (
+      await scenario([
+        { filename: 'api/uv.lock', additions: 9000, deletions: 5 },
+        { filename: 'web/pnpm-lock.yaml', additions: 5000, deletions: 0 },
+      ])
+    ).calls,
+    [['add', 'size:XS']],
+  ))
+test('replace stale size only', async () =>
+  assert.deepEqual(
+    (await scenario([{ filename: 'a.py', additions: 10, deletions: 0 }], ['size:L', 'bug', 'web']))
+      .calls,
+    [
+      ['add', 'size:S'],
+      ['remove', 'size:L'],
+    ],
+  ))
+test('idempotent', async () => assert.deepEqual((await scenario([], ['size:XS', 'bug'])).calls, []))
+test('incomplete file list', async () => {
+  const r = await scenario([], ['size:L'], 3001)
+  assert.deepEqual(r.calls, [])
+  assert.equal(r.warnings.length, 1)
+})