fix(log): rotate the log file that is actually written

lib/private/Log/Rotate.php

@@ -11,6 +11,8 @@
 use OCP\AppFramework\Utility\ITimeFactory;
 use OCP\BackgroundJob\TimedJob;
 use OCP\IConfig;
+use OCP\Log\IFileBased;
+use OCP\Log\ILogFactory;
 use OCP\Log\RotationTrait;
 use OCP\Server;
 use Psr\Log\LoggerInterface;
@@ -33,7 +35,11 @@ public function __construct(ITimeFactory $time) {
 	#[\Override]
 	public function run($argument): void {
 		$config = Server::get(IConfig::class);
-		$this->filePath = $config->getSystemValueString('logfile', $config->getSystemValueString('datadirectory', \OC::$SERVERROOT . '/data') . '/nextcloud.log');
+		$logFile = Server::get(ILogFactory::class)->get('file');
+		if (!$logFile instanceof IFileBased) {
+			return;
+		}
+		$this->filePath = $logFile->getLogFilePath();
 
 		$this->maxSize = $config->getSystemValueInt('log_rotate_size', 100 * 1024 * 1024);
 		if ($this->shouldRotateBySize()) {

tests/lib/Log/RotateTest.php

@@ -0,0 +1,56 @@
+<?php
+
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+namespace Test\Log;
+
+use OC\Log\File;
+use OC\Log\Rotate;
+use OC\SystemConfig;
+use OCP\AppFramework\Utility\ITimeFactory;
+use OCP\IConfig;
+use OCP\Log\ILogFactory;
+use Test\TestCase;
+
+class RotateTest extends TestCase {
+	private string $dir;
+
+	#[\Override]
+	protected function setUp(): void {
+		parent::setUp();
+		$this->dir = sys_get_temp_dir() . '/' . self::getUniqueID('rotate');
+		mkdir($this->dir);
+	}
+
+	#[\Override]
+	protected function tearDown(): void {
+		@unlink($this->dir . '/nextcloud.log');
+		@unlink($this->dir . '/nextcloud.log.1');
+		@rmdir($this->dir);
+		parent::tearDown();
+	}
+
+	public function testRotatesFallbackLogFile(): void {
+		$systemConfig = $this->createMock(SystemConfig::class);
+		$systemConfig->method('getValue')->willReturnArgument(1);
+		$logFile = new File($this->dir . '/missing/nextcloud.log', $this->dir . '/nextcloud.log', $systemConfig);
+		file_put_contents($this->dir . '/nextcloud.log', 'log entry');
+
+		$logFactory = $this->createMock(ILogFactory::class);
+		$logFactory->method('get')->with('file')->willReturn($logFile);
+		$this->overwriteService(ILogFactory::class, $logFactory);
+
+		$config = $this->createMock(IConfig::class);
+		$config->method('getSystemValueInt')->with('log_rotate_size')->willReturn(1);
+		$this->overwriteService(IConfig::class, $config);
+
+		$job = new Rotate($this->createMock(ITimeFactory::class));
+		$job->run(null);
+
+		$this->assertFileDoesNotExist($this->dir . '/nextcloud.log');
+		$this->assertFileExists($this->dir . '/nextcloud.log.1');
+	}
+}