add synchronous value validation

system/Database/SQLite3/Connection.php

@@ -17,6 +17,7 @@
 use CodeIgniter\Database\Exceptions\DatabaseException;
 use CodeIgniter\Database\TableName;
 use Exception;
+use InvalidArgumentException;
 use SQLite3;
 use SQLite3Result;
 use stdClass;
@@ -81,6 +82,9 @@ public function initialize()
         }
 
         if (is_int($this->synchronous)) {
+            if (! in_array($this->synchronous, [0, 1, 2, 3], true)) {
+                throw new InvalidArgumentException('Invalid synchronous value.');
+            }
             $this->connID->exec('PRAGMA synchronous = ' . $this->synchronous);
         }
     }

tests/system/Database/Live/SQLite3/ConnectTest.php

@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * This file is part of CodeIgniter 4 framework.
+ *
+ * (c) CodeIgniter Foundation <[email protected]>
+ *
+ * For the full copyright and license information, please view
+ * the LICENSE file that was distributed with this source code.
+ */
+
+namespace CodeIgniter\Database\Live\SQLite3;
+
+use CodeIgniter\Test\CIUnitTestCase;
+use Config\Database;
+use InvalidArgumentException;
+use PHPUnit\Framework\Attributes\Group;
+
+/**
+ * @internal
+ */
+#[Group('DatabaseLive')]
+final class ConnectTest extends CIUnitTestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        $this->db = Database::connect($this->DBGroup);
+
+        if ($this->db->DBDriver !== 'SQLite3') {
+            $this->markTestSkipped('This test is only for SQLite3.');
+        }
+    }
+
+    public function testShowErrorMessageWhenSettingInvalidSynchronous(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Invalid synchronous value.');
+
+        $config = config('Database');
+        $group  = $config->tests;
+        // Sets invalid synchronous.
+        $group['synchronous'] = 123;
+        $db                   = Database::connect($group);
+
+        // Actually connect to DB.
+        $db->initialize();
+    }
+}