From 9333feae7610427832e15cb1d40b948a4a43aada Mon Sep 17 00:00:00 2001
From: Alex Kavanagh <alex.kavanagh@canonical.com>
Date: Mon, 23 Oct 2023 12:41:31 +0100
Subject: [PATCH] Mock get_platform / add CHARMHELPERS_IN_UNITTEST

Patch osplatform.get_platform() to return "ubuntu" if the environment
variable CHARMHELPERS_IN_UNITTEST is set to something truthy. This is to
enable unit testing in classic charms on NON ubuntu systems (e.g. Debian
bookworm) where the function is often called via a decorator that is
evaluated at module load time and thus is very, very tricky to mock out,
as it is often 'hit' during the test discovery phase.
---
 README.rst                 | 13 +++++++++++++
 charmhelpers/osplatform.py |  9 ++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/README.rst b/README.rst
index 35c168ccb..8967d675b 100644
--- a/README.rst
+++ b/README.rst
@@ -36,6 +36,19 @@ Dev/Test
 
 See the HACKING.md file for information about testing and development.
 
+Unit Test Mode
+==============
+
+If the environment variable `CHARMHELPERS_IN_UNITTEST` is set to a truthy value
+(e.g. 'on', 'true', 'anything') then then `osplatform.get_platform()` will
+*always* return `ubuntu`. This prevents it from attempting to call into
+platform libraries. This is needed as many charms try to call this function
+during module loading (via function decorators) and this makes it very
+difficult to mock out.
+
+Set this variable in the `tox.ini` of the charm using the `setenv` parameter in
+a testenv.
+
 License
 =======
 
diff --git a/charmhelpers/osplatform.py b/charmhelpers/osplatform.py
index 1ace468f7..69cd3bd9a 100644
--- a/charmhelpers/osplatform.py
+++ b/charmhelpers/osplatform.py
@@ -2,7 +2,7 @@
 import os
 
 
-def get_platform():
+def _get_platform():
     """Return the current OS platform.
 
     For example: if current os platform is Ubuntu then a string "ubuntu"
@@ -47,3 +47,10 @@ def _get_platform_from_fs():
     for k, v in content.items():
         content[k] = v.strip('"')
     return content["NAME"]
+
+
+## If the unit-test mode is set, the platform is always "ubuntu"
+if not os.environ.get('CHARMHELPERS_IN_UNITTEST', False):
+    get_platform = _get_platform
+else:
+    get_platform = lambda: "ubuntu"