Boost: don't cache pages with errors that exit very early (#38054)

* Check for early fatal errors that should stop caching

* changelog

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/9675839062

Upstream-Ref: Automattic/jetpack@ee67524

CHANGELOG.md

@@ -9,6 +9,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 This is an alpha version! The changes listed here are not final.
 
+### Added
+- Jetpack Boost: ensure that "init" fires before allowing a page to be cached to avoid caching error pages
+
 ### Fixed
 - Detect when WP_CACHE is defined with "const" in wp-config.php
 - General: Fix instance where Boost can break caching for other caching plugins when getting deactivated.

app/modules/optimizations/page-cache/pre-wordpress/Boost_Cache.php

@@ -54,6 +54,11 @@ class Boost_Cache {
 	 */
 	private static $cache_engine_loaded = false;
 
+	/**
+	 * @var bool - Indicates whether WordPress initialized correctly and we can cache the page.
+	 */
+	private $do_cache = false;
+
 	/**
 	 * @param ?Storage\Storage $storage - Optionally provide a Storage subclass to handle actually storing and retrieving cached content. Defaults to a new instance of File_Storage.
 	 */
@@ -75,6 +80,7 @@ public function init_actions() {
 		add_action( 'switch_theme', array( $this, 'invalidate_cache' ) );
 		add_action( 'wp_trash_post', array( $this, 'delete_on_post_trash' ), 10, 2 );
 		add_filter( 'wp_php_error_message', array( $this, 'disable_caching_on_error' ) );
+		add_filter( 'init', array( $this, 'init_do_cache' ) );
 	}
 
 	/**
@@ -176,6 +182,11 @@ public function ob_start() {
 	public function ob_callback( $buffer ) {
 		if ( strlen( $buffer ) > 0 && $this->request->is_cacheable() ) {
 
+			// Do not cache the page as WordPress did not initialize correctly.
+			if ( ! $this->do_cache ) {
+				return $buffer;
+			}
+
 			if ( false === stripos( $buffer, '</html>' ) ) {
 				Logger::debug( 'Closing HTML tag not found, not caching' );
 				return $buffer;
@@ -468,4 +479,14 @@ public function disable_caching_on_error( $message ) {
 		Logger::debug( 'Fatal error detected, caching disabled' );
 		return $message;
 	}
+
+	/**
+	 * This function is called after WordPress is loaded, on "init".
+	 * It is used to indicate that it is safe to cache and that no
+	 * fatal errors occurred.
+	 */
+	public function init_do_cache() {
+		Logger::debug( 'postload: init succeeded' );
+		$this->do_cache = true;
+	}
 }