Tolerate selecting MySQL system variables (#109)

The purpose of this PR is to avoid errors when selecting MySQL system variables. Fixes #104.

Related to:
WordPress/wordpress-playground#1272 -
"UpdraftPlus plugins doesn't fully work in Playground."

tests/WP_SQLite_Translator_Tests.php

@@ -2247,4 +2247,33 @@ public function testDefaultNullValue() {
 			$result
 		);
 	}
+
+	/**
+	 * @dataProvider mysqlVariablesToTest
+	 */
+	public function testSelectVariable( $variable_name ) {
+		// Make sure the query does not error
+		$this->assertQuery( "SELECT $variable_name;" );
+	}
+
+	public static function mysqlVariablesToTest() {
+		return array(
+			// NOTE: This list was derived from the variables used by the UpdraftPlus plugin.
+			// We will start here and plan to expand supported variables over time.
+			array( '@@character_set_client' ),
+			array( '@@character_set_results' ),
+			array( '@@collation_connection' ),
+			array( '@@GLOBAL.gtid_purged' ),
+			array( '@@GLOBAL.log_bin' ),
+			array( '@@GLOBAL.log_bin_trust_function_creators' ),
+			array( '@@GLOBAL.sql_mode' ),
+			array( '@@SESSION.max_allowed_packet' ),
+			array( '@@SESSION.sql_mode' ),
+
+			// Intentionally mix letter casing to help demonstrate case-insensitivity
+			array( '@@cHarActer_Set_cLient' ),
+			array( '@@gLoBAL.gTiD_purGed' ),
+			array( '@@sEssIOn.sqL_moDe' ),
+		);
+	}
 }

wp-includes/sqlite/class-wp-sqlite-translator.php

@@ -1453,8 +1453,9 @@ private function execute_select() {
 			$updated_query                     = $this->get_information_schema_query( $updated_query );
 			$params                            = array();
 		} elseif (
-			strpos( $updated_query, '@@SESSION.sql_mode' ) !== false
-			|| strpos( $updated_query, 'CONVERT( ' ) !== false
+			// Examples: @@SESSION.sql_mode, @@GLOBAL.max_allowed_packet, @@character_set_client
+			preg_match( '/@@((SESSION|GLOBAL)\s*\.\s*)?\w+\b/i', $updated_query ) === 1 ||
+			strpos( $updated_query, 'CONVERT( ' ) !== false
 		) {
 			/*
 			 * If the query contains a function that is not supported by SQLite,