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, 33 insertions, 0 deletions

diff --git a/cloudinit/features.py b/cloudinit/features.py
new file mode 100644
index 00000000..e455213d
--- /dev/null
+++ b/cloudinit/features.py
@@ -0,0 +1,33 @@
+# This file is part of cloud-init. See LICENSE file for license information.
+"""
+Feature flags are used as a way to easily toggle configuration
+**at build time**. They are provided to accommodate feature deprecation and
+downstream configuration changes.
+
+Currently used upstream values for feature flags are set in
+``cloudinit/features.py``. Overrides to these values (typically via quilt
+patch) can be placed
+in a file called ``feature_overrides.py`` in the same directory. Any value
+set in ``feature_overrides.py`` will override the original value set
+in ``features.py``.
+
+Each flag should include a short comment regarding the reason for
+the flag and intended lifetime.
+
+Tests are required for new feature flags, and tests must verify
+all valid states of a flag, not just the default state.
+"""
+
+ERROR_ON_USER_DATA_FAILURE = True
+"""
+If there is a failure in obtaining user data (i.e., #include or
+decompress fails), old behavior is to log a warning and proceed.
+After the 20.2 release, we instead raise an exception.
+This flag can be removed after Focal is no longer supported
+"""
+
+try:
+    # pylint: disable=wildcard-import
+    from cloudinit.feature_overrides import *  # noqa
+except ImportError:
+    pass