Improve Query Monitor setup to better support multisite installs (#222)

This PR improves the Query Monitor setup to better support multisite
installs.

The Query Monitor integration logic tries to eagerly check whether the
Query Monitor plugin is active. This is non-trivial for multisites (at
that stage, we don't know yet which site is active, etc.).

I created a fix that doesn’t do the eager load on multisites. All is set
up correctly a bit later, when the QM plugin loads. Not loading it
eagerly means we won't log a couple of queries that WP core does before
it loads the QM plugin.

For single sites, the eager load is still in place.

Author: JanJakes

integrations/query-monitor/boot.php

@@ -21,7 +21,7 @@
 	define( 'QM_DB_SYMLINK', false );
 }
 
-// 1. Check if we should load Query Monitor (as per the original "db.php" file).
+// Check if we should load Query Monitor (as per the original "db.php" file).
 if ( ! defined( 'ABSPATH' ) ) {
 	exit;
 }
@@ -56,29 +56,77 @@
 	}
 }
 
+/*
+ * Register SQLite enhancements for Query Monitor when plugins are loaded.
+ *
+ * This will also ensure that the plugin Query Monitor is fully initialized even
+ * when we can't load it eagerly, e.g. on a multisite install.
+ */
+function register_sqlite_enhancements_for_query_monitor() {
+	if ( ! class_exists( 'QM_Backtrace' ) ) {
+		return;
+	}
+
+	require_once __DIR__ . '/plugin.php';
+
+	if ( ! defined( 'SQLITE_QUERY_MONITOR_LOADED' ) ) {
+		define( 'SQLITE_QUERY_MONITOR_LOADED', true );
+	}
+}
+
+if ( function_exists( 'add_action' ) ) {
+	add_action( 'plugins_loaded', 'register_sqlite_enhancements_for_query_monitor' );
+}
+
+/*
+ * On a multisite install, we can't easily determine the current site eagerly.
+ * Therefore, let's bail out and let Query Monitor activate later as a plugin.
+ */
+if ( is_multisite() ) {
+	return;
+}
+
+/*
+ * Now, let's try to load Query Monitor eagerly, to start logging queries early.
+ * When the plugin is installed, we will check the database if it is also active.
+ */
 global $wpdb;
 if ( ! isset( $wpdb ) ) {
 	return;
 }
 
-// 2. Check if Query Monitor is active.
+// Check if Query Monitor is installed.
+if ( defined( 'WP_PLUGIN_DIR' ) ) {
+	$plugins_dir = WP_PLUGIN_DIR;
+} else {
+	$plugins_dir = WP_CONTENT_DIR . '/plugins';
+}
+
+$qm_dir = "{$plugins_dir}/query-monitor";
+$qm_php = "{$qm_dir}/classes/PHP.php";
+
+if ( ! is_readable( $qm_php ) ) {
+	return;
+}
+
+// Check if Query Monitor is active.
 if ( null === $wpdb->options ) {
 	global $table_prefix;
 	$wpdb->set_prefix( $table_prefix ?? '' );
 }
 
 $query_monitor_active = false;
 try {
-	$value = $wpdb->get_row(
+	// Make sure no errors are displayed when the query fails.
+	$show_errors = $wpdb->hide_errors();
+	$value       = $wpdb->get_row(
 		$wpdb->prepare(
 			"SELECT option_value FROM $wpdb->options WHERE option_name = %s LIMIT 1",
 			'active_plugins'
 		)
 	);
-	/**
-	 * $value may be null during WordPress Playground multisite setup.
-	 * @see https://github.com/WordPress/sqlite-database-integration/pull/219.
-	 */
+	$wpdb->show_errors( $show_errors );
+
 	if ( null !== $value ) {
 		$query_monitor_active = in_array(
 			'query-monitor/query-monitor.php',
@@ -94,20 +142,7 @@
 	return;
 }
 
-// 3. Determine the plugins directory.
-if ( defined( 'WP_PLUGIN_DIR' ) ) {
-	$plugins_dir = WP_PLUGIN_DIR;
-} else {
-	$plugins_dir = WP_CONTENT_DIR . '/plugins';
-}
-
-// 4. Load Query Monitor (as per the original "db.php" file).
-$qm_dir = "{$plugins_dir}/query-monitor";
-$qm_php = "{$qm_dir}/classes/PHP.php";
-
-if ( ! is_readable( $qm_php ) ) {
-	return;
-}
+// Load Query Monitor eagerly (as per the original "db.php" file).
 require_once $qm_php;
 
 if ( ! QM_PHP::version_met() ) {
@@ -129,14 +164,5 @@
 	define( 'SAVEQUERIES', true );
 }
 
-// 5. Mark the Query Monitor integration as loaded.
+// Mark the Query Monitor integration as loaded.
 define( 'SQLITE_QUERY_MONITOR_LOADED', true );
-
-// 6. Register the SQLite enhancements for Query Monitor.
-function register_sqlite_enhancements_for_query_monitor() {
-	require_once __DIR__ . '/plugin.php';
-}
-
-if ( function_exists( 'add_action' ) ) {
-	add_action( 'plugins_loaded', 'register_sqlite_enhancements_for_query_monitor' );
-}