Fix Last-Modified header on archive pages (#2042)

Author: mattstauffer

include/layout.inc

@@ -367,6 +367,21 @@ function news_archive_sidebar(): void
     }
 }
 
+// Get Last-Modified timestamp for a year's archive page:
+// mtime of file or latest article, whichever is newer
+/** @param list<array<array-key, mixed>> $news */
+function news_archive_last_modified(array $news, string $pageFile): int
+{
+    $timestamps = array_map(static fn (array $entry): int => strtotime($entry['published']), $news);
+
+    $mtime = filemtime($pageFile);
+    if ($mtime !== false) {
+        $timestamps[] = $mtime;
+    }
+
+    return $timestamps === [] ? time() : max($timestamps);
+}
+
 // Print news
 function print_news($news, $dog, $max = 5, $return = false) {
     $retval = [];

public/archive/2026.php

@@ -5,7 +5,12 @@
 $_SERVER['BASE_PAGE'] = 'archive/2026.php';
 require_once __DIR__ . '/../../include/prepend.inc';
 news_archive_sidebar();
-site_header("News Archive - 2026", ['cache' => true, 'cache_control' => 15 * 60]);
+$news = (new NewsHandler())->getNewsByYear(2026);
+
+site_header("News Archive - 2026", [
+    'cache' => news_archive_last_modified($news, __FILE__),
+    'cache_control' => 15 * 60,
+]);
 ?>
 
 <h1>News Archive - 2026</h1>
@@ -18,5 +23,5 @@
 
 <?php
 
-print_news((new NewsHandler())->getNewsByYear(2026), null, 500);
+print_news($news, null, 500);
 site_footer(['elephpants' => true, 'sidebar' => $SIDEBAR_DATA]);