PICARD-3290: Add tests for _retry_on_permission_error

Tests mock time.sleep to avoid actual delays and mock IS_WIN to run
the Windows-specific retry logic on any platform.

Author: zas

test/test_file.py

@@ -28,6 +28,7 @@
 from unittest.mock import (
     MagicMock,
     Mock,
+    patch,
 )
 
 from test.picardtestcase import PicardTestCase
@@ -847,3 +848,57 @@ class MyFormat(File):
 
         score = MyFormat.score('/somepath/somefile.ogg', Mock(), b"abc")
         self.assertEqual(score, 0)
+
+
+class RetryOnPermissionErrorTest(PicardTestCase):
+    def setUp(self):
+        super().setUp()
+        self.patch_tagger_instance('picard.item')
+        self.file = File('somepath/somefile.mp3')
+
+    @patch('picard.file.IS_WIN', False)
+    def test_no_retry_on_non_windows(self):
+        """On non-Windows, PermissionError propagates immediately."""
+        func = Mock(side_effect=PermissionError("locked"))
+        with self.assertRaises(PermissionError):
+            self.file._retry_on_permission_error(func)
+        func.assert_called_once()
+
+    @patch('picard.file.IS_WIN', True)
+    @patch('picard.file.time.sleep')
+    def test_succeeds_first_try(self, mock_sleep):
+        func = Mock(return_value='result')
+        result = self.file._retry_on_permission_error(func)
+        self.assertEqual('result', result)
+        func.assert_called_once()
+        mock_sleep.assert_not_called()
+
+    @patch('picard.file.IS_WIN', True)
+    @patch('picard.file.time.sleep')
+    def test_succeeds_after_retry(self, mock_sleep):
+        """Succeeds on second attempt after one PermissionError."""
+        func = Mock(side_effect=[PermissionError("locked"), 'result'])
+        result = self.file._retry_on_permission_error(func)
+        self.assertEqual('result', result)
+        self.assertEqual(2, func.call_count)
+        mock_sleep.assert_called_once_with(self.file._PERMISSION_ERROR_RETRY_DELAY)
+
+    @patch('picard.file.IS_WIN', True)
+    @patch('picard.file.time.sleep')
+    def test_raises_after_max_retries(self, mock_sleep):
+        """Raises PermissionError after exhausting all retries."""
+        func = Mock(side_effect=PermissionError("locked"))
+        with self.assertRaises(PermissionError):
+            self.file._retry_on_permission_error(func)
+        self.assertEqual(self.file._PERMISSION_ERROR_RETRIES, func.call_count)
+        self.assertEqual(self.file._PERMISSION_ERROR_RETRIES - 1, mock_sleep.call_count)
+
+    @patch('picard.file.IS_WIN', True)
+    @patch('picard.file.time.sleep')
+    def test_non_permission_error_not_retried(self, mock_sleep):
+        """Other exceptions propagate immediately without retry."""
+        func = Mock(side_effect=OSError("other error"))
+        with self.assertRaises(OSError):
+            self.file._retry_on_permission_error(func)
+        func.assert_called_once()
+        mock_sleep.assert_not_called()