From 9f8b1ebaf80710e5ab817fcc0622ed5f14dcf3a8 Mon Sep 17 00:00:00 2001
From: Ryan McCue <me@ryanmccue.info>
Date: Tue, 18 Feb 2025 16:08:57 +0000
Subject: [PATCH] Add SWR threshold capability

This will work automatically with Batcache, as it only instructs
CloudFront to change its behaviour, rather than everything.
---
 README.md         |  1 +
 inc/namespace.php | 25 +++++++++++++++++++++++--
 2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 1677e49..e3535bb 100644
--- a/README.md
+++ b/README.md
@@ -35,6 +35,7 @@ Smartcache has a variety of filters available. These include:
 * `smartcache.is_old_post` - Filter whether a specific post is considered old (infrequently updated). (Default true for posts older than the old threshold.)
 * `smartcache.is_new_post` - Filter whether a specific post is considered new (frequently updated). (Default true if published in previous 24 hours.)
 * `smartcache.max-age` - Filter the maximum lifetime for the current page directly.
+* `smartcache.swr_threshold` - Filter the stale-while-revalidate timeframe. (Default 30s for frequently updated content, 60s for infrequently updated content.)
 * `smartcache.should_cache` - Filter whether a page should be cached at all. (Only affects whether Smartcache generates a header, but may be overridden by other behaviour or by the cache itself.)
 * `smartcache.urls_to_invalidate_for_post` - Filter which URLs to invalidate for a given post.
 * `smartcache.should_invalidate` - Should we invalidate URLs for this post? (Default true, false for new content as it will expire naturally quickly.)
diff --git a/inc/namespace.php b/inc/namespace.php
index 7aed448..5b9b5ce 100644
--- a/inc/namespace.php
+++ b/inc/namespace.php
@@ -28,6 +28,16 @@
  */
 const LIFETIME_LONG = 1209600;
 
+/**
+ * Threshold for the SWR (Stale-While-Revalidate) cache.
+ */
+const SWR_THRESHOLD_LONG = 60;
+
+/**
+ * Threshold for short SWR (Stale-While-Revalidate) cache.
+ */
+const SWR_THRESHOLD_SHORT = 30;
+
 /**
  * Bootstrap function to set up the plugin.
  *
@@ -155,11 +165,22 @@ function set_cache_ttl() : void {
 
 	global $batcache;
 	$max_age = absint( apply_filters( 'smartcache.max-age', get_default_lifetime() ) );
+	$swr_time = apply_filters( 'smartcache.swr_threshold', $max_age <= 300 ? SWR_THRESHOLD_SHORT : SWR_THRESHOLD_LONG );
 	if ( ! $batcache || ! is_object( $batcache ) ) {
-		header( 'Cache-Control: s-maxage=' . $max_age . ', must-revalidate' );
+		$value = sprintf(
+			's-maxage=%d, stale-while-revalidate=%d, must-revalidate',
+			$max_age,
+			$swr_time
+		);
 	} else {
-		header( 'Cache-Control: s-maxage=' . $max_age . ', max-age=' . $batcache->max_age . ', must-revalidate' );
+		$value = sprintf(
+			's-maxage=%d, max-age=%d, stale-while-revalidate=%d, must-revalidate',
+			$max_age,
+			$batcache->max_age,
+			$swr_time
+		);
 	}
+	header( $value );
 }
 
 /**