Fix PermissionError in _clear_all_cache_files on Windows (separate_files=True) (#348)

* Fix PermissionError in _clear_all_cache_files on Windows by adding retry loop

Co-authored-by: shaypal5 <917954+shaypal5@users.noreply.github.com>

* Add tests for _clear_all_cache_files retry logic to reach 100% patch coverage

Co-authored-by: shaypal5 <917954+shaypal5@users.noreply.github.com>

* Fix ruff-format pre-commit.ci failure in test_pickle_core.py

Co-authored-by: shaypal5 <917954+shaypal5@users.noreply.github.com>

* Add pragma no branch to inner for loop to reach 100% patch coverage

Co-authored-by: shaypal5 <917954+shaypal5@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: shaypal5 <917954+shaypal5@users.noreply.github.com>

Author: Copilot

src/cachier/cores/pickle.py

@@ -145,7 +145,19 @@ def _clear_all_cache_files(self) -> None:
         path, name = os.path.split(self.cache_fpath)
         for subpath in os.listdir(path):
             if subpath.startswith(f"{name}_"):
-                os.remove(os.path.join(path, subpath))
+                fpath = os.path.join(path, subpath)
+                # Retry loop to handle Windows mandatory file-locking (WinError 32):
+                # portalocker holds an exclusive lock while a thread is computing,
+                # so os.remove() may fail transiently until the lock is released.
+                for attempt in range(3):  # pragma: no branch
+                    try:
+                        os.remove(fpath)
+                        break
+                    except PermissionError:
+                        if attempt < 2:
+                            time.sleep(0.1 * (attempt + 1))
+                        else:
+                            raise
 
     def _clear_being_calculated_all_cache_files(self) -> None:
         path, name = os.path.split(self.cache_fpath)

tests/pickle_tests/test_pickle_core.py

@@ -1212,6 +1212,81 @@ def mock_func():
         core.delete_stale_entries(timedelta(hours=1))
 
 
+@pytest.mark.pickle
+def test_clear_all_cache_files_retries_on_permission_error(tmp_path):
+    """Test _clear_all_cache_files retries on PermissionError then succeeds."""
+    core = _PickleCore(
+        hash_func=None,
+        cache_dir=tmp_path,
+        pickle_reload=False,
+        wait_for_calc_timeout=10,
+        separate_files=True,
+    )
+
+    def mock_func():
+        pass
+
+    core.set_func(mock_func)
+
+    # Create a cache file that matches the name pattern
+    cache_fpath = core.cache_fpath
+    dummy_file = cache_fpath + "_dummykey"
+    with open(dummy_file, "wb") as f:
+        f.write(b"")
+
+    # os.remove fails twice then succeeds on the third call
+    real_remove = os.remove
+    call_count = 0
+
+    def flaky_remove(path):
+        nonlocal call_count
+        call_count += 1
+        if call_count < 3:
+            raise PermissionError("locked")
+        real_remove(path)
+
+    with (
+        patch("cachier.cores.pickle.os.remove", side_effect=flaky_remove),
+        patch("cachier.cores.pickle.time.sleep") as mock_sleep,
+    ):
+        core._clear_all_cache_files()
+        assert mock_sleep.call_count == 2
+        mock_sleep.assert_any_call(0.1)
+        mock_sleep.assert_any_call(0.2)
+
+    assert not os.path.exists(dummy_file)
+
+
+@pytest.mark.pickle
+def test_clear_all_cache_files_raises_on_persistent_permission_error(tmp_path):
+    """Test _clear_all_cache_files re-raises PermissionError after all retries."""
+    core = _PickleCore(
+        hash_func=None,
+        cache_dir=tmp_path,
+        pickle_reload=False,
+        wait_for_calc_timeout=10,
+        separate_files=True,
+    )
+
+    def mock_func():
+        pass
+
+    core.set_func(mock_func)
+
+    # Create a cache file that matches the name pattern
+    cache_fpath = core.cache_fpath
+    dummy_file = cache_fpath + "_dummykey"
+    with open(dummy_file, "wb") as f:
+        f.write(b"")
+
+    with (
+        patch("cachier.cores.pickle.os.remove", side_effect=PermissionError("locked")),
+        patch("cachier.cores.pickle.time.sleep"),
+        pytest.raises(PermissionError),
+    ):
+        core._clear_all_cache_files()
+
+
 # Redis core static method tests
 @pytest.mark.parametrize(
     ("test_input", "expected"),