feat(test): Add unit tests for MOTD

* Card ID: CCT-414

src/insights_client/tests/test_motd.py

@@ -0,0 +1,113 @@
+import os
+import pathlib
+import tempfile
+import typing
+import unittest
+import unittest.mock
+
+import pytest
+
+import insights_client
+
+
+class MockFS:
+    """Mock filesystem.
+
+    Ensures that the files required for the MOTD manipulation are created
+    in a temporary directory instead of working with actual system files.
+    """
+
+    _dir: tempfile.TemporaryDirectory
+
+    @property
+    def _chroot(self) -> pathlib.Path:
+        return pathlib.Path(self._dir.name)
+
+    def touch(self, path: str):
+        (self._chroot / path).open("w").close()
+
+    def exists(self, path: str) -> bool:
+        return (self._chroot / path).exists()
+
+    def symlink_to(self, path: str, point_to: str):
+        (self._chroot / path).symlink_to(point_to)
+
+    def samefile(self, first: str, second: str) -> bool:
+        return (self._chroot / first).samefile(self._chroot / second)
+
+    def __init__(self):
+        self._dir = tempfile.TemporaryDirectory(prefix="test-chroot-")
+
+        paths: typing.Dict[str, str] = {
+            "insights_client.MOTD_SRC": "etc/insights-client/insights-client.motd",
+            "insights_client.MOTD_FILE": "etc/motd.d/insights-client",
+            "insights_client.REGISTERED_FILE": "etc/insights-client/.registered",
+            "insights_client.UNREGISTERED_FILE": "etc/insights-client/.unregistered",
+        }
+
+        self.patches: typing.List[unittest.mock.patch] = []
+        for target, path in paths.items():
+            mocked_path = self._chroot / path
+            mocked_path.parent.mkdir(parents=True, exist_ok=True)
+            patch = unittest.mock.patch(target, f"{mocked_path!s}")
+            self.patches.append(patch)
+            patch.start()
+
+        self.touch("etc/insights-client/insights-client.motd")
+
+    def __del__(self):
+        for patch in self.patches:
+            patch.stop()
+        self._dir.cleanup()
+
+
+@pytest.fixture(scope="function")
+def mock_fs():
+    fs = MockFS()
+    yield fs
+    del fs
+
+
+def test_present(mock_fs):
+    assert not mock_fs.exists("etc/motd.d/insights-client")
+
+    # The file gets created/symlinked when .registered & .unregistered do not exist
+    insights_client.update_motd_message()
+    assert mock_fs.exists("etc/motd.d/insights-client")
+    assert mock_fs.samefile(
+        "etc/motd.d/insights-client", "etc/insights-client/insights-client.motd"
+    )
+
+
+@pytest.mark.parametrize(
+    "filename",
+    [".registered", ".unregistered"],
+)
+def test_absent_on_dot(mock_fs, filename):
+    # The MOTD gets created by default...
+    insights_client.update_motd_message()
+    assert mock_fs.exists("etc/motd.d/insights-client")
+
+    # ...and gets removed when .registered/.unregistered exists.
+    mock_fs.touch(f"etc/insights-client/{filename}")
+
+    insights_client.update_motd_message()
+    assert not mock_fs.exists("etc/motd.d/insights-client")
+
+    # ...and it stays absent when run multiple times.
+    insights_client.update_motd_message()
+    assert not mock_fs.exists("etc/motd.d/insights-client")
+
+
+def test_ignored_on_dev_null(mock_fs):
+    # When the /etc/motd.d/insights-client is a symbolic link to /dev/null...
+    mock_fs.symlink_to("etc/motd.d/insights-client", os.devnull)
+
+    # ...it should not be overwritten...
+    insights_client.update_motd_message()
+    assert mock_fs.samefile("etc/motd.d/insights-client", os.devnull)
+
+    # ...whether the MOTD file should be present or not.
+    mock_fs.touch("etc/insights-client/.registered")
+    insights_client.update_motd_message()
+    assert mock_fs.samefile("etc/motd.d/insights-client", os.devnull)