Download not available

<?php
/**
 * Gallery archive download: /gallery/download.php?g=<slug>
 *
 * Hands the visitor a short-lived presigned URL for the gallery's prebuilt ZIP
 * and gets out of the way. The transfer runs browser ↔ S3, so a 3 GB archive is
 * unaffected by the host's 60 s execution limit and stays resumable — this
 * script only ever sends a redirect.
 *
 * An out-of-date archive is refused rather than served, mirroring the disabled
 * button on the gallery page: a client must not be handed a ZIP that silently
 * omits the photos uploaded since it was built.
 *
 * Every failure looks the same as an unknown gallery, so the endpoint reveals
 * nothing the visitor does not already know from the link.
 */
define('SITE_BASE', '../');

require dirname(__DIR__) . '/app/bootstrap.php';

session_boot();

$slug = (string)($_GET['g'] ?? '');
$gallery = $slug !== '' ? gallery_load($slug) : null;

$allowed = $gallery !== null
    && !gallery_is_expired($gallery)
    && !empty($gallery['downloads_enabled'])
    && !empty($gallery['archive']['key'])
    && !archive_is_stale($gallery)
    // The same per-gallery session unlock the viewer sets.
    && (empty($gallery['password_hash']) || !empty($_SESSION['gallery_unlocked'][$slug]));

if (!$allowed) {
    http_response_code(404);
    public_header('Download not available');
    echo '<div class="gate"><div class="gate-card"><h1>Download not available</h1>'
       . '<p class="page-sub">This download does not exist or is not ready yet.</p></div></div>';
    public_footer();
    exit;
}

// Short TTL on purpose: the browser only needs the URL long enough to start the
// transfer. A GET already in flight is not cut off when the signature expires.
redirect(s3_presign_get((string)$gallery['archive']['key'], 900));