New feature flag functionality and fix includes failing silently (#367)

Build time feature flags are now defined in cloudinit/features.py.
Feature flags can be added to toggle configuration options or
deprecated features. Feature flag overrides can be placed in
cloudinit/feature_overrides.py. Further documentation can be found in
HACKING.rst.

Additionally, updated default behavior to exit with an exception
if #include can't retrieve resources as expected. This behavior
can be toggled with a feature flag.

LP: #1734939

1 files changed, 60 insertions, 0 deletions

diff --git a/cloudinit/tests/test_features.py b/cloudinit/tests/test_features.py
new file mode 100644
index 00000000..d7a7226d
--- /dev/null
+++ b/cloudinit/tests/test_features.py
@@ -0,0 +1,60 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+# pylint: disable=no-member,no-name-in-module
+"""
+This file is for testing the feature flag functionality itself,
+NOT for testing any individual feature flag
+"""
+import pytest
+import sys
+from pathlib import Path
+
+import cloudinit
+
+
+@pytest.yield_fixture()
+def create_override(request):
+    """
+    Create a feature overrides file and do some module wizardry to make
+    it seem like we're importing the features file for the first time.
+
+    After creating the override file with the values passed by the test,
+    we need to reload cloudinit.features
+    to get all of the current features (including the overridden ones).
+    Once the test is complete, we remove the file we created and set
+    features and feature_overrides modules to how they were before
+    the test started
+    """
+    override_path = Path(cloudinit.__file__).parent / 'feature_overrides.py'
+    if override_path.exists():
+        raise Exception("feature_overrides.py unexpectedly exists! "
+                        "Remove it to run this test.")
+    with override_path.open('w') as f:
+        for key, value in request.param.items():
+            f.write('{} = {}\n'.format(key, value))
+
+    sys.modules.pop('cloudinit.features', None)
+
+    yield
+
+    override_path.unlink()
+    sys.modules.pop('cloudinit.feature_overrides', None)
+
+
+class TestFeatures:
+    def test_feature_without_override(self):
+        from cloudinit.features import ERROR_ON_USER_DATA_FAILURE
+        assert ERROR_ON_USER_DATA_FAILURE is True
+
+    @pytest.mark.parametrize('create_override',
+                             [{'ERROR_ON_USER_DATA_FAILURE': False}],
+                             indirect=True)
+    def test_feature_with_override(self, create_override):
+        from cloudinit.features import ERROR_ON_USER_DATA_FAILURE
+        assert ERROR_ON_USER_DATA_FAILURE is False
+
+    @pytest.mark.parametrize('create_override',
+                             [{'SPAM': True}],
+                             indirect=True)
+    def test_feature_only_in_override(self, create_override):
+        from cloudinit.features import SPAM
+        assert SPAM is True