Draft/POC for SQLite support in Core

.gitignore

@@ -102,3 +102,7 @@ wp-tests-config.php
 
 # Visual regression test diffs
 tests/visual-regression/specs/__snapshots__
+
+src/wp-content/database/.ht.sqlite
+
+src/wp-content/database/

phpcs.xml.dist

@@ -229,7 +229,7 @@
 	#############################################################################
 	SELECTIVE EXCLUSIONS
 	Exclude specific files for specific sniffs and/or exclude sub-groups in sniffs.
-	
+
 	These exclusions are listed ordered by alphabetic sniff name.
 	#############################################################################
 	-->
@@ -295,6 +295,10 @@
 		<exclude-pattern>/tests/phpunit/tests/db/charset\.php</exclude-pattern>
 	</rule>
 
+	<rule ref="WordPress.DB.RestrictedClasses">
+		<exclude-pattern>/src/wp-includes/sqlite/*\.php</exclude-pattern>
+	</rule>
+
 	<!-- Allow the WP DB related tests for issues with prepared SQL placeholders
 		 (as the handling of those are being tested). -->
 	<rule ref="WordPress.DB.PreparedSQLPlaceholders.UnfinishedPrepare">

src/wp-admin/includes/class-wp-debug-data.php

@@ -279,6 +279,11 @@ public static function debug_data() {
 					'label' => 'WP_MAX_MEMORY_LIMIT',
 					'value' => WP_MAX_MEMORY_LIMIT,
 				),
+				'DB_ENGINE'           => array(
+					'label' => 'DB_ENGINE',
+					'value' => ( defined( 'DB_ENGINE' ) ? DB_ENGINE : __( 'Undefined' ) ),
+					'debug' => ( defined( 'DB_ENGINE' ) ? DB_ENGINE : 'undefined' ),
+				),
 				'WP_DEBUG'            => array(
 					'label' => 'WP_DEBUG',
 					'value' => WP_DEBUG ? __( 'Enabled' ) : __( 'Disabled' ),
@@ -879,79 +884,105 @@ public static function debug_data() {
 			'value' => wp_date( 'c', $_SERVER['REQUEST_TIME'] ),
 		);
 
-		// Populate the database debug fields.
-		if ( is_object( $wpdb->dbh ) ) {
-			// mysqli or PDO.
-			$extension = get_class( $wpdb->dbh );
-		} else {
-			// Unknown sql extension.
-			$extension = null;
+		$db_engine = defined( 'DB_ENGINE' ) && 'sqlite' === DB_ENGINE ? 'sqlite' : 'mysql';
+		$extension = null;
+		if ( 'mysql' === $db_engine ) {
+			// Populate the database debug fields.
+			if ( is_object( $wpdb->dbh ) ) {
+				// mysqli or PDO.
+				$extension = get_class( $wpdb->dbh );
+			}
 		}
 
 		$server = $wpdb->get_var( 'SELECT VERSION()' );
 
 		$client_version = $wpdb->dbh->client_info;
 
-		$info['wp-database']['fields']['extension'] = array(
-			'label' => __( 'Extension' ),
-			'value' => $extension,
+		$info['wp-database']['fields']['db_engine'] = array(
+			'label' => __( 'Database engine' ),
+			'value' => 'sqlite' === $db_engine ? 'SQLite' : 'MySQL/MariaDB',
 		);
 
-		$info['wp-database']['fields']['server_version'] = array(
-			'label' => __( 'Server version' ),
-			'value' => $server,
-		);
+		if ( 'mysql' === $db_engine ) {
+			$info['wp-database']['fields']['extension'] = array(
+				'label' => __( 'Extension' ),
+				'value' => $extension,
+			);
 
-		$info['wp-database']['fields']['client_version'] = array(
-			'label' => __( 'Client version' ),
-			'value' => $client_version,
-		);
+			$info['wp-database']['fields']['server_version'] = array(
+				'label' => __( 'Server version' ),
+				'value' => $server,
+			);
 
-		$info['wp-database']['fields']['database_user'] = array(
-			'label'   => __( 'Database username' ),
-			'value'   => $wpdb->dbuser,
-			'private' => true,
-		);
+			$info['wp-database']['fields']['client_version'] = array(
+				'label' => __( 'Client version' ),
+				'value' => $client_version,
+			);
 
-		$info['wp-database']['fields']['database_host'] = array(
-			'label'   => __( 'Database host' ),
-			'value'   => $wpdb->dbhost,
-			'private' => true,
-		);
+			$info['wp-database']['fields']['database_user'] = array(
+				'label'   => __( 'Database username' ),
+				'value'   => $wpdb->dbuser,
+				'private' => true,
+			);
 
-		$info['wp-database']['fields']['database_name'] = array(
-			'label'   => __( 'Database name' ),
-			'value'   => $wpdb->dbname,
-			'private' => true,
-		);
+			$info['wp-database']['fields']['database_host'] = array(
+				'label'   => __( 'Database host' ),
+				'value'   => $wpdb->dbhost,
+				'private' => true,
+			);
+
+			$info['wp-database']['fields']['database_name'] = array(
+				'label'   => __( 'Database name' ),
+				'value'   => $wpdb->dbname,
+				'private' => true,
+			);
+		} elseif ( 'sqlite' === $db_engine ) {
+			$info['wp-database']['fields']['database_version'] = array(
+				'label' => __( 'SQLite version' ),
+				'value' => class_exists( 'SQLite3' ) ? SQLite3::version()['versionString'] : null,
+			);
+
+			$info['wp-database']['fields']['database_file'] = array(
+				'label'   => __( 'Database file' ),
+				'value'   => FQDB,
+				'private' => true,
+			);
+
+			$info['wp-database']['fields']['database_size'] = array(
+				'label' => __( 'Database size' ),
+				'value' => size_format( filesize( FQDB ) ),
+			);
+		}
 
 		$info['wp-database']['fields']['database_prefix'] = array(
 			'label'   => __( 'Table prefix' ),
 			'value'   => $wpdb->prefix,
 			'private' => true,
 		);
 
-		$info['wp-database']['fields']['database_charset'] = array(
-			'label'   => __( 'Database charset' ),
-			'value'   => $wpdb->charset,
-			'private' => true,
-		);
+		if ( 'mysql' === $db_engine ) {
+			$info['wp-database']['fields']['database_charset'] = array(
+				'label'   => __( 'Database charset' ),
+				'value'   => $wpdb->charset,
+				'private' => true,
+			);
 
-		$info['wp-database']['fields']['database_collate'] = array(
-			'label'   => __( 'Database collation' ),
-			'value'   => $wpdb->collate,
-			'private' => true,
-		);
+			$info['wp-database']['fields']['database_collate'] = array(
+				'label'   => __( 'Database collation' ),
+				'value'   => $wpdb->collate,
+				'private' => true,
+			);
 
-		$info['wp-database']['fields']['max_allowed_packet'] = array(
-			'label' => __( 'Max allowed packet size' ),
-			'value' => self::get_mysql_var( 'max_allowed_packet' ),
-		);
+			$info['wp-database']['fields']['max_allowed_packet'] = array(
+				'label' => __( 'Max allowed packet size' ),
+				'value' => self::get_mysql_var( 'max_allowed_packet' ),
+			);
 
-		$info['wp-database']['fields']['max_connections'] = array(
-			'label' => __( 'Max connections number' ),
-			'value' => self::get_mysql_var( 'max_connections' ),
-		);
+			$info['wp-database']['fields']['max_connections'] = array(
+				'label' => __( 'Max connections number' ),
+				'value' => self::get_mysql_var( 'max_connections' ),
+			);
+		}
 
 		// List must use plugins if there are any.
 		$mu_plugins = get_mu_plugins();

src/wp-admin/includes/class-wp-site-health.php

@@ -15,6 +15,7 @@ class WP_Site_Health {
 	private $is_recommended_mysql_version;
 
 	public $is_mariadb                   = false;
+	public $is_sqlite                    = false;
 	private $mysql_server_version        = '';
 	private $mysql_required_version      = '5.5';
 	private $mysql_recommended_version   = '8.0';
@@ -208,6 +209,7 @@ private function prepare_sql_data() {
 		global $wpdb;
 
 		$mysql_server_type = $wpdb->db_server_info();
+		$this->is_sqlite   = defined( 'DB_ENGINE' ) && 'sqlite' === DB_ENGINE ? 'sqlite' : 'mysql';
 
 		$this->mysql_server_version = $wpdb->get_var( 'SELECT VERSION()' );
 
@@ -1230,7 +1232,7 @@ public function get_test_sql_server() {
 
 		$db_dropin = file_exists( WP_CONTENT_DIR . '/db.php' );
 
-		if ( ! $this->is_recommended_mysql_version ) {
+		if ( ! $this->is_sqlite && ! $this->is_recommended_mysql_version ) {
 			$result['status'] = 'recommended';
 
 			$result['label'] = __( 'Outdated SQL server' );
@@ -1246,7 +1248,7 @@ public function get_test_sql_server() {
 			);
 		}
 
-		if ( ! $this->is_acceptable_mysql_version ) {
+		if ( ! $this->is_sqlite && ! $this->is_acceptable_mysql_version ) {
 			$result['status'] = 'critical';
 
 			$result['label']          = __( 'Severely outdated SQL server' );

src/wp-admin/includes/upgrade.php

@@ -51,7 +51,12 @@ function wp_install( $blog_title, $user_name, $user_email, $is_public, $deprecat
 
 		wp_check_mysql_version();
 		wp_cache_flush();
-		make_db_current_silent();
+
+		if ( defined( 'DB_ENGINE' ) && 'sqlite' === DB_ENGINE ) {
+			sqlite_make_db_sqlite(); // phpcs:ignore PHPCompatibility.Extensions.RemovedExtensions.sqliteRemoved
+		} else {
+			make_db_current_silent();
+		}
 
 		/*
 		 * Ensure update checks are delayed after installation.
@@ -3279,6 +3284,57 @@ function make_db_current_silent( $tables = 'all' ) {
 	dbDelta( $tables );
 }
 
+
+/**
+ * Function to create tables according to the schemas of WordPress.
+ *
+ * This is executed only once while installation.
+ *
+ * @since 1.0.0
+ *
+ * @return boolean
+ */
+function sqlite_make_db_sqlite() {
+	include_once ABSPATH . 'wp-admin/includes/schema.php';
+
+	$table_schemas = wp_get_db_schema();
+	$queries       = explode( ';', $table_schemas );
+	try {
+		$pdo = new PDO( 'sqlite:' . FQDB, null, null, array( PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION ) ); // phpcs:ignore WordPress.DB.RestrictedClasses
+	} catch ( PDOException $err ) {
+		$err_data = $err->errorInfo; // phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
+		wp_die( $err_data[2], 'Database Error!' );
+	}
+
+	$translator = new WP_SQLite_Translator( $pdo, $GLOBALS['table_prefix'] );
+	$query      = null;
+
+	try {
+		$translator->begin_transaction();
+		foreach ( $queries as $query ) {
+			$query = trim( $query );
+			if ( empty( $query ) ) {
+				continue;
+			}
+
+			$result = $translator->query( $query );
+			if ( false === $result ) {
+				throw new PDOException( $translator->get_error_message() );
+			}
+		}
+		$translator->commit();
+	} catch ( PDOException $err ) {
+		$err_data = $err->errorInfo; // phpcs:ignore WordPress.NamingConventions.ValidVariableName.UsedPropertyNotSnakeCase
+		$err_code = $err_data[1];
+		$translator->rollback();
+		wp_die( $err_data[2], 'Database Error!' );
+	}
+
+	$pdo = null;
+
+	return true;
+}
+
 /**
  * Creates a site theme from an existing theme.
  *