WP Super Cache: if there is an output buffer active do not create ano…

…ther one (#36124)

* Check if an output buffer is active before setting one up

* changelog

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/8137108292

CHANGELOG.md

@@ -31,6 +31,7 @@ This is an alpha version! The changes listed here are not final.
 - Super Cache: with rebuild enabled, apply that to subdirectories instead of deleting them.
 - WP Super Cache - fix the wp_super_cache_clear_post_cache so the homepage cache isn't deleted too.
 - WP Super Cache: bail if the request uri isn't set. It means the plugin isn't configured yet.
+- WP Super Cache: don't create an output buffer if there's already one active
 
 ## [1.11.0] - 2023-11-08
 ### Added

wp-cache-phase2.php

@@ -1512,6 +1512,16 @@ function wp_cache_phase2() {
 		return false;
 	}
 
+	if ( ob_get_level() > 0 ) {
+		global $wp_super_cache_late_init;
+		$ob_warning = 'Already in an output buffer. Check your plugins, themes, mu-plugins, and other custom code. Exit.';
+		if ( isset( $wp_super_cache_late_init ) && $wp_super_cache_late_init ) {
+			$ob_warning = 'Late init enabled. Disable it. ' . $ob_warning;
+		}
+		wp_cache_debug( 'wp_cache_phase2: ' . $ob_warning );
+		return;
+	}
+
 	wp_cache_debug( 'In WP Cache Phase 2', 5 );
 
 	$wp_cache_gmt_offset   = get_option( 'gmt_offset' ); // caching for later use when wpdb is gone. https://wordpress.org/support/topic/224349