fix(db): Prevent two connections for single node databases

lib/private/DB/Connection.php

@@ -31,6 +31,7 @@
 use OCP\Server;
 use Psr\Clock\ClockInterface;
 use Psr\Log\LoggerInterface;
+use function count;
 use function in_array;
 
 class Connection extends PrimaryReadReplicaConnection {
@@ -73,7 +74,7 @@ class Connection extends PrimaryReadReplicaConnection {
 	 * @throws \Exception
 	 */
 	public function __construct(
-		array $params,
+		private array $params,
 		Driver $driver,
 		?Configuration $config = null,
 		?EventManager $eventManager = null
@@ -138,6 +139,15 @@ public function connect($connectionName = null) {
 		}
 	}
 
+	protected function performConnect(?string $connectionName = null): bool {
+		if (($connectionName ?? 'replica') === 'replica'
+			&& count($this->params['replica']) === 1
+			&& $this->params['primary'] === $this->params['replica'][0]) {
+			return parent::performConnect('primary');
+		}
+		return parent::performConnect($connectionName);
+	}
+
 	public function getStats(): array {
 		return [
 			'built' => $this->queriesBuilt,

tests/lib/DB/ConnectionTest.php

@@ -0,0 +1,101 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * SPDX-FileCopyrightText: 2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\DB;
+
+use Doctrine\DBAL\Configuration;
+use Doctrine\DBAL\Driver;
+use Doctrine\DBAL\Driver\Connection as DriverConnection;
+use Doctrine\DBAL\Platforms\MySQLPlatform;
+use OC\DB\Adapter;
+use OC\DB\Connection;
+use Test\TestCase;
+
+/**
+ * @group DB
+ */
+class ConnectionTest extends TestCase {
+
+	public function testSingleNodeConnectsToPrimaryOnly(): void {
+		$connectionParams = [
+			'user' => 'test',
+			'password' => 'topsecret',
+			'host' => 'test',
+		];
+		$adapter = $this->createMock(Adapter::class);
+		$driver = $this->createMock(Driver::class);
+		$configuration = $this->createMock(Configuration::class);
+		$connection = $this->getMockBuilder(Connection::class)
+			->onlyMethods(['connectTo'])
+			->setConstructorArgs([
+				[
+					'adapter' => $adapter,
+					'platform' => new MySQLPlatform(),
+					'tablePrefix' => 'nctest',
+					'primary' => $connectionParams,
+					'replica' => [
+						$connectionParams,
+					],
+				],
+				$driver,
+				$configuration,
+			])
+			->getMock();
+		$driverConnection = $this->createMock(DriverConnection::class);
+		$connection->expects(self::once())
+			->method('connectTo')
+			->with('primary')
+			->willReturn($driverConnection);
+
+		$connection->ensureConnectedToReplica();
+		$connection->ensureConnectedToPrimary();
+		$connection->ensureConnectedToReplica();
+	}
+
+	public function testClusterConnectsToPrimaryAndReplica(): void {
+		$connectionParamsPrimary = [
+			'user' => 'test',
+			'password' => 'topsecret',
+			'host' => 'testprimary',
+		];
+		$connectionParamsReplica = [
+			'user' => 'test',
+			'password' => 'topsecret',
+			'host' => 'testreplica',
+		];
+		$adapter = $this->createMock(Adapter::class);
+		$driver = $this->createMock(Driver::class);
+		$configuration = $this->createMock(Configuration::class);
+		$connection = $this->getMockBuilder(Connection::class)
+			->onlyMethods(['connectTo'])
+			->setConstructorArgs([
+				[
+					'adapter' => $adapter,
+					'platform' => new MySQLPlatform(),
+					'tablePrefix' => 'nctest',
+					'primary' => $connectionParamsPrimary,
+					'replica' => [
+						$connectionParamsReplica,
+					],
+				],
+				$driver,
+				$configuration,
+			])
+			->getMock();
+		$driverConnection = $this->createMock(DriverConnection::class);
+		$connection->expects(self::exactly(2))
+			->method('connectTo')
+			->willReturn($driverConnection);
+
+		$connection->ensureConnectedToReplica();
+		$connection->ensureConnectedToPrimary();
+		$connection->ensureConnectedToReplica();
+	}
+
+}