stages/yum.config: add an option to configure langpacks plugin

The new stage enables users to configure the langpacks plugin of YUM.
Currently, only locales option is supported.

stages/org.osbuild.yum.config

@@ -1,10 +1,12 @@
 #!/usr/bin/python3
 """
-Change YUM configuration.
+Configure yellowdog updater modified (YUM)
 
-The stage changes persistent YUM configuration on the filesystem.
+The stage currently supports configuring http_caching in YUM config and
+locales in langpacks plugin. If the config files don't exist, they
+are created.
 """
-
+import os
 import sys
 import iniparse
 
@@ -26,13 +28,71 @@ SCHEMA = r"""
         "description": "Determines how upstream HTTP caches are instructed to handle any HTTP downloads that YUM does."
       }
     }
+  },
+  "plugins": {
+    "additionalProperties": false,
+    "type": "object",
+    "description": "YUM plugins configuration",
+    "properties": {
+      "langpacks": {
+        "additionalProperties": false,
+        "type": "object",
+        "description": "'langpacks' YUM plugin configuration",
+        "minProperties": 1,
+        "properties": {
+          "locales": {
+            "type": "array",
+            "minItems": 1,
+            "description": "list of locales for YUM",
+            "items": {
+              "type": "string"
+            }
+          }
+        }
+      }
+    }
   }
 }
 """
 
 
+def configure_plugins(tree, plugins_options):
+    for plugin, plugin_options in plugins_options.items():
+        plugin_conf_path = f"{tree}/etc/yum/pluginconf.d/{plugin}.conf"
+        plugin_conf = iniparse.SafeConfigParser()
+
+        try:
+            with open(plugin_conf_path, "r") as f:
+                plugin_conf.readfp(f)
+        except FileNotFoundError:
+            print(f"Warning: {plugin} configuration file '{plugin_conf_path}' does not exist, will create it.")
+
+        # ensure that the pluginconf directory exists
+        os.makedirs(f"{tree}/etc/yum/pluginconf.d/", exist_ok=True)
+
+        for option, value in plugin_options.items():
+            if option == "locales":
+                if not plugin_conf.has_section("main"):
+                    plugin_conf.add_section("main")
+
+                locales = ", ".join(value)
+
+                plugin_conf.set("main", "langpack_locales", locales)
+            else:
+                # schema does not allow any additional properties, but keeping this for completeness
+                print(f"Error: unknown property {option} specified for {plugin} plugin.")
+                return 1
+
+        with open(plugin_conf_path, "w") as f:
+            plugin_conf.write(f)
+
+    return 0
+
+
 def main(tree, options):
     config_options = options.get("config")
+    plugins_options = options.get("plugins", {})
+
     yum_config_path = f"{tree}/etc/yum.conf"
     yum_config = iniparse.SafeConfigParser()
 
@@ -53,6 +113,9 @@ def main(tree, options):
         with open(yum_config_path, "w") as f:
             yum_config.write(f)
 
+    if configure_plugins(tree, plugins_options):
+        return 1
+
     return 0