feat(test): Add unit tests for MOTD

* Card ID: CCT-414

src/insights_client/tests/test_motd.py

@@ -0,0 +1,116 @@
+import os
+import tempfile
+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.
+    """
+
+    def __init__(self):
+        self.dir = tempfile.TemporaryDirectory()
+
+        # /etc/insights-client/insights-client.motd
+        self.dir_src = unittest.mock.patch(
+            "insights_client.MOTD_SRC", f"{self.dir.name}/src"
+        )
+        self.dir_src.start()
+
+        # /etc/motd.d/insights-client
+        self.dir_motd = unittest.mock.patch(
+            "insights_client.MOTD_FILE", f"{self.dir.name}/motd"
+        )
+        self.dir_motd.start()
+
+        # /etc/insights-client/.registered
+        self.dir_reg = unittest.mock.patch(
+            "insights_client.REGISTERED_FILE", f"{self.dir.name}/reg"
+        )
+        self.dir_reg.start()
+
+        # /etc/insights-client/.unregistered
+        self.dir_unreg = unittest.mock.patch(
+            "insights_client.UNREGISTERED_FILE", f"{self.dir.name}/unreg"
+        )
+        self.dir_unreg.start()
+
+        with open(f"{self.dir.name}/src", "w") as f:
+            f.write("This is the content of the MOTD file.")
+
+    def __del__(self) -> None:
+        self.dir_unreg.stop()
+        self.dir_reg.stop()
+        self.dir_motd.stop()
+        self.dir_src.stop()
+        self.dir = None
+
+
+@pytest.fixture
+def mock_fs():
+    fs = MockFS()
+    yield fs
+    del fs
+
+
+def test_present(mock_fs):
+    assert not os.path.exists(insights_client.MOTD_FILE)
+
+    # The file gets created/symlinked when .registered & .unregistered do not exist
+    insights_client.update_motd_message()
+    assert os.path.exists(insights_client.MOTD_FILE)
+    assert os.path.samefile(insights_client.MOTD_FILE, insights_client.MOTD_SRC)
+
+
+def test_absent_on_dot_registered(mock_fs):
+    # The MOTD file exists by default
+    insights_client.update_motd_message()
+    assert os.path.exists(insights_client.MOTD_FILE)
+
+    # It gets removed when .registered exists
+    with open(f"{mock_fs.dir.name}/reg", "w") as f:
+        f.write("")
+    insights_client.update_motd_message()
+    assert not os.path.exists(insights_client.MOTD_FILE)
+
+    # It stays absent when run multiple times
+    insights_client.update_motd_message()
+    assert not os.path.exists(insights_client.MOTD_FILE)
+
+
+def test_absent_on_dot_unregistered(mock_fs):
+    # The MOTD file exists by default
+    insights_client.update_motd_message()
+    assert os.path.exists(insights_client.MOTD_FILE)
+
+    # It gets removed when .unregistered exists
+    with open(f"{mock_fs.dir.name}/unreg", "w") as f:
+        f.write("")
+    insights_client.update_motd_message()
+    assert not os.path.exists(insights_client.MOTD_FILE)
+
+    # It stays absent when run multiple times
+    insights_client.update_motd_message()
+    assert not os.path.exists(insights_client.MOTD_FILE)
+
+
+def test_ignored_on_dev_null(mock_fs):
+    # When the /etc/motd.d/insights-client is a symbolic link to /dev/null...
+    os.symlink(os.devnull, f"{mock_fs.dir.name}/motd")
+
+    # ...it should not be overwritten...
+    insights_client.update_motd_message()
+    assert os.path.samefile(os.devnull, f"{mock_fs.dir.name}/motd")
+
+    # ...whether the MOTD file should be present or not.
+    with open(f"{mock_fs.dir.name}/reg", "w") as f:
+        f.write("")
+    insights_client.update_motd_message()
+    assert os.path.samefile(os.devnull, f"{mock_fs.dir.name}/motd")