Remote.html: Preload the SQLite database plugin, but only execute it …

…if there's no custom db.php inside wp-content (#1424)

This PR always preloads the SQLite plugin in remote.html, even if the
provided WordPress already ships one. The preloaded plugin will only be
used if no `wp-content/db.php` drop-in plugin is found in the
filesystem. This change sets the stage for:

* #1426, which ships a minified WordPress build without the SQLite
plugin baked-in.
* #1425, which runs the WordPress installation wizard if the site isn't
installed yet

Related to [Boot
Protocol](#1398)

## Problem solved

We want the official WordPress release `.zip` to just work in
Playground. Currently it doesn't because remote.html expects a
pre-processed build with the SQLite plugin already baked-in. This PR, in
conjunction with #1425 and #1426, remove that requirement.

## Testing Instructions

Confirm the CI checks pass.

packages/playground/remote/src/lib/worker-thread.ts

@@ -6,8 +6,12 @@ import {
 	getWordPressModuleDetails,
 	LatestSupportedWordPressVersion,
 	SupportedWordPressVersions,
+	sqliteDatabaseIntegration,
 } from '@wp-playground/wordpress-builds';
-import { wordPressRewriteRules } from '@wp-playground/wordpress';
+import {
+	preloadSqliteIntegration,
+	wordPressRewriteRules,
+} from '@wp-playground/wordpress';
 import { PHPRequestHandler } from '@php-wasm/universal';
 import {
 	SyncProgressCallback,
@@ -57,6 +61,14 @@ if (
 	wordPressAvailableInOPFS = await playgroundAvailableInOpfs(virtualOpfsDir!);
 }
 
+// The SQLite integration must always be downloaded, even when using OPFS or Native FS,
+// because it can't be assumed to exist in WordPress document root. Instead, it's installed
+// in the /internal directory to avoid polluting the mounted directory structure.
+downloadMonitor.expectAssets({
+	[sqliteDatabaseIntegration.url]: sqliteDatabaseIntegration.size,
+});
+const sqliteIntegrationRequest = monitoredFetch(sqliteDatabaseIntegration.url);
+
 // Start downloading WordPress if needed
 let wordPressRequest = null;
 if (!wordPressAvailableInOPFS) {
@@ -210,6 +222,12 @@ try {
 		});
 	}
 
+	const sqliteIntegrationZip = await (await sqliteIntegrationRequest).blob();
+	await preloadSqliteIntegration(
+		primaryPhp,
+		new File([sqliteIntegrationZip], 'sqlite.zip')
+	);
+
 	// Always setup the current site URL.
 	await defineSiteUrl(primaryPhp, {
 		siteUrl: scopedSiteUrl,

packages/playground/wordpress/src/index.ts

@@ -198,12 +198,21 @@ export async function preloadSqliteIntegration(
 			"'{SQLITE_PLUGIN}'",
 			phpVar(joinPaths(SQLITE_PLUGIN_FOLDER, 'load.php'))
 		);
+	const dbPhpPath = joinPaths(await php.documentRoot, 'wp-content/db.php');
+	const stopIfDbPhpExists = `<?php
+	// Do not preload this if WordPress comes with a custom db.php file.
+	if(file_exists(${phpVar(dbPhpPath)})) {
+		return;
+	}
+	?>`;
 	const SQLITE_MUPLUGIN_PATH =
 		'/internal/shared/mu-plugins/sqlite-database-integration.php';
-	await php.writeFile(SQLITE_MUPLUGIN_PATH, dbPhp);
+	await php.writeFile(SQLITE_MUPLUGIN_PATH, stopIfDbPhpExists + dbPhp);
 	await php.writeFile(
 		`/internal/shared/preload/0-sqlite.php`,
-		`<?php
+		stopIfDbPhpExists +
+			`<?php
+
 /**
  * Loads the SQLite integration plugin before WordPress is loaded
  * and without creating a drop-in "db.php" file. 
@@ -256,6 +265,18 @@ class Playground_SQLite_Integration_Loader {
     }
 }
 $wpdb = $GLOBALS['wpdb'] = new Playground_SQLite_Integration_Loader();
+
+/**
+ * WordPress is capable of using a preloaded global $wpdb. However, if
+ * it cannot find the drop-in db.php plugin it still checks whether
+ * the mysqli_connect() function exists even though it's not used.
+ *
+ * What WordPress demands, Playground shall provide.
+ */
+if(!function_exists('mysqli_connect')) {
+	function mysqli_connect() {}
+}
+
 		`
 	);
 	/**