Add FileWatcher unit tests

The existing tests are really integration tests, as they are not only
testing `FileWatcher`, but also `Select` and `PeriodicTimer`, which is
confusing when there is a bug in any of the other classes as we might
get test failures here.

This commit adds a a proper unit tests mocking `awatch`.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

tests/utils/test_file_watcher.py

@@ -3,13 +3,77 @@
 
 """Tests for `channel.FileWatcher`."""
 
+from __future__ import annotations
+
 import os
 import pathlib
+from collections.abc import AsyncGenerator, Iterator, Sequence
 from datetime import timedelta
+from typing import Any
+from unittest import mock
+
+import pytest
+from watchfiles import Change
+from watchfiles.main import FileChange
 
 from frequenz.channels.util import FileWatcher, Select, Timer
 
 
+class _FakeAwatch:
+    """Fake awatch class to mock the awatch function."""
+
+    def __init__(self, changes: Sequence[FileChange] = ()) -> None:
+        """Create a `_FakeAwatch` instance.
+
+        Args:
+            changes: A sequence of file changes to be returned by the fake awatch
+                function.
+        """
+        self.changes: Sequence[FileChange] = changes
+
+    async def fake_awatch(
+        self, *paths: str, **kwargs: Any  # pylint: disable=unused-argument
+    ) -> AsyncGenerator[set[FileChange], None]:
+        """Fake awatch function.
+
+        Args:
+            paths: Paths to watch.
+            kwargs: Keyword arguments to pass to the awatch function.
+        """
+        for change in self.changes:
+            yield {change}
+
+
+@pytest.fixture
+def fake_awatch() -> Iterator[_FakeAwatch]:
+    """Fixture to mock the awatch function."""
+    fake = _FakeAwatch()
+    with mock.patch(
+        "frequenz.channels.util._file_watcher.awatch",
+        autospec=True,
+        side_effect=fake.fake_awatch,
+    ):
+        yield fake
+
+
+async def test_file_watcher_receive_updates(
+    fake_awatch: _FakeAwatch,  # pylint: disable=redefined-outer-name
+) -> None:
+    """Test the file watcher receive the expected events."""
+    filename = "test-file"
+    changes = (
+        (Change.added, filename),
+        (Change.deleted, filename),
+        (Change.modified, filename),
+    )
+    fake_awatch.changes = changes
+    file_watcher = FileWatcher(paths=[filename])
+
+    for change in changes:
+        recv_changes = await file_watcher.receive()
+        assert recv_changes == pathlib.Path(change[1])
+
+
 async def test_file_watcher(tmp_path: pathlib.Path) -> None:
     """Ensure file watcher is returning paths on file events.