Add cascading config finder

Boxed plugin can only get a static hard-coded configuration, delivered as a resource inside
a Python package. It cannot be overridden without explicit change of a package-delivered data.
This is also makes it impossible to programmatically integrate it with Kiwi as a library. Under
these circumstances, a cascading mechanism is required, which is searching for the configuration
file in the following order:

* $KIWI_BOXED_PLUGIN_CFG full path, the name can be anything
* Current directory, from where a software was called
* $HOME/.config/kiwi/kiwi_boxed_plugin.yml
* /etc/kiwi_boxed_plugin.yml
* Resource (default config, coming with the package)

.gitignore

@@ -77,3 +77,6 @@ doc/xml/
 
 # man pages
 *.1.gz
+
+# VSC
+.vscode/

kiwi_boxed_plugin/defaults.py

@@ -16,6 +16,7 @@
 # along with kiwi-boxed-build.  If not, see <http://www.gnu.org/licenses/>
 #
 import os
+import pathlib
 from typing import List
 from kiwi.path import Path
 from pkg_resources import resource_filename
@@ -37,8 +38,38 @@ class Defaults:
 
     @staticmethod
     def get_plugin_config_file() -> str:
+        """
+        Config file name: `kiwi_boxed_plugin.yml`
+        Locations are searched in this order:
+            1. Environment variable "KIWI_BOXED_PLUGIN_CFG" (full path, name can be different)
+            2. Current directory
+            3. $HOME/.config/kiwi/kiwi_boxed_plugin.yml
+            4. /etc/kiwi_boxed_plugin.yml
+            5. Resource (default config, coming with the package)
+        """
+        cfg_n = "kiwi_boxed_plugin.yml"
+
+        # Path via env. Here is no limit what name/path of the cfg
+        cfg_p: str | None = os.environ.get("KIWI_BOXED_PLUGIN_CFG")
+        if cfg_p is not None and os.path.exists(cfg_p):
+            return cfg_p
+
+        # Curr dir
+        cfg_p = os.path.abspath(cfg_n)
+        if os.path.exists(cfg_p):
+            return cfg_p
+
+        # Local Kiwi config
+        cfg_pth: pathlib.Path = pathlib.Path.home().joinpath(f".config/kiwi/{cfg_n}")
+        if cfg_pth.exists():
+            return cfg_pth.as_posix()
+
+        cfg_p = f"/etc/{cfg_n}"
+        if os.path.exists(cfg_p):
+            return cfg_p
+
         return resource_filename(
-            'kiwi_boxed_plugin', 'config/kiwi_boxed_plugin.yml'
+            'kiwi_boxed_plugin', f'config/{cfg_n}'
         )
 
     @staticmethod

test/unit/defaults_test.py

@@ -1,10 +1,52 @@
 from pkg_resources import resource_filename
-
 from kiwi_boxed_plugin.defaults import Defaults
+from mock import patch
+import os
+
+
+class MockedPath:
+    def __init__(self):
+        self.p: str | None = None
+
+    def home(self):
+        return self
+
+    @staticmethod
+    def exists():
+        return True
+
+    def joinpath(self, f: str):
+        self.p = os.path.join("/home/zoidberg", f)
+        return self
+
+    def as_posix(self):
+        return self.p
 
 
 class TestDefaults:
     def test_get_plugin_config_file(self):
         assert Defaults.get_plugin_config_file() == resource_filename(
             'kiwi_boxed_plugin', 'config/kiwi_boxed_plugin.yml'
         )
+
+    @patch("os.path.exists", lambda f: True)
+    @patch.dict(os.environ, KIWI_BOXED_PLUGIN_CFG="aarchderwelt.conf")
+    def test_get_plugin_config_file_env(self):
+        assert Defaults.get_plugin_config_file() == "aarchderwelt.conf", \
+            "aarch64 aonfiguration from the environment variable do not match"
+
+    @patch("os.path.abspath", lambda f: "/highway/to/hell.conf")
+    @patch("os.path.exists", lambda f: True)
+    def test_get_plugin_config_file_currdir(self):
+        assert Defaults.get_plugin_config_file() == "/highway/to/hell.conf", \
+            "Should contain absolute path to the config"
+
+    @patch("pathlib.Path", MockedPath())
+    def test_get_plugin_config_file_local_kiwi(self):
+        assert Defaults.get_plugin_config_file() == "/home/zoidberg/.config/kiwi/kiwi_boxed_plugin.yml", \
+            "Should contain local Kiwi Box config"
+
+    @patch("os.path.exists", lambda f: f == "/etc/kiwi_boxed_plugin.yml")
+    def test_get_plugin_config_file_etc(self):
+        assert Defaults.get_plugin_config_file() == "/etc/kiwi_boxed_plugin.yml", \
+            "Should contain Kiwi Box config in /etc dir"