site: take sitemap dates from git, because mtime is a lie in CI

apps/web/app/sitemap.ts

@@ -1,37 +1,59 @@
+import { execFile } from 'node:child_process'
 import { stat } from 'node:fs/promises'
 import { join } from 'node:path'
+import { promisify } from 'node:util'
 import type { MetadataRoute } from 'next'
 import { listDocs } from '@/lib/docs'
 import { SITE } from '@/lib/site'
 
+const run = promisify(execFile)
+
 /**
  * Every URL used to carry `new Date()`, so all eighteen shared one build
- * timestamp. A lastmod that changes on every deploy and is identical across the
- * site says nothing about what actually changed, and crawlers discount it —
- * which is worse than sending none, because it costs a field and buys nothing.
+ * timestamp. A lastmod that is identical site-wide and changes on every deploy
+ * says nothing about what actually changed, so crawlers discount it — which is
+ * worse than sending none, because it costs a field and buys nothing.
  *
- * The real date is on disk. Doc pages are rendered from the repository's own
- * `docs/*.md`, so that file's mtime *is* when the page last changed; the rest
- * are their own source files. Falls back to the build time only when a stat
- * fails, which should not happen but must not break the sitemap if it does.
+ * The date has to come from git, not from the filesystem. An earlier version of
+ * this read mtimes, which is correct on a working tree and useless in CI: a
+ * clone stamps every file with the checkout time, so in production all eighteen
+ * collapsed back to a single date and the fix silently did nothing. mtime is
+ * kept only as a fallback for a build with no git history, where it is no worse
+ * than the build clock it replaces.
  */
 const ROOT = join(process.cwd(), '..', '..')
 const buildTime = new Date()
 
-async function modified(...relative: string[]): Promise<Date> {
-  const times = await Promise.all(
-    relative.map(async (path) => {
-      try {
-        return (await stat(join(ROOT, path))).mtime
-      } catch {
-        return null
-      }
-    }),
+/** When git last changed this path, or null if git cannot say. */
+async function committed(path: string): Promise<Date | null> {
+  try {
+    const { stdout } = await run('git', ['log', '-1', '--format=%cI', '--', path], { cwd: ROOT })
+    const iso = stdout.trim()
+    if (!iso) return null
+    const date = new Date(iso)
+    return Number.isNaN(date.getTime()) ? null : date
+  } catch {
+    // No git, no history, or a shallow clone that does not reach this commit.
+    return null
+  }
+}
+
+async function touched(path: string): Promise<Date | null> {
+  try {
+    return (await stat(join(ROOT, path))).mtime
+  } catch {
+    return null
+  }
+}
+
+/** The newest real date across the sources a page is built from. */
+async function modified(...paths: string[]): Promise<Date> {
+  const dates = await Promise.all(
+    paths.map(async (path) => (await committed(path)) ?? (await touched(path))),
   )
-  const known = times.filter((time): time is Date => time !== null)
+  const known = dates.filter((date): date is Date => date !== null)
   if (known.length === 0) return buildTime
-  // The newest of the sources a page is built from.
-  return new Date(Math.max(...known.map((time) => time.getTime())))
+  return new Date(Math.max(...known.map((date) => date.getTime())))
 }
 
 export default async function sitemap(): Promise<MetadataRoute.Sitemap> {
@@ -40,9 +62,9 @@ export default async function sitemap(): Promise<MetadataRoute.Sitemap> {
 
   const [home, download, docsIndex, security, privacy] = await Promise.all([
     modified(`${web}/app/page.tsx`, `${web}/app/layout.tsx`),
-    modified(`${web}/app/download/page.tsx`),
+    modified(`${web}/app/download/page.tsx`, `${web}/lib/releases.ts`),
     modified(`${web}/app/docs/page.tsx`),
-    modified(`${web}/app/security/page.tsx`),
+    modified(`${web}/app/security/page.tsx`, 'docs/security.md'),
     modified(`${web}/app/privacy/page.tsx`),
   ])