Fix root_cache invalidation triggered by config changes

The root_cache plugin assures that no new configuration file in the
`config_opts['config_paths']` list is newer than the root_cache cache
tarball, otherwise the caches are invalidated.

The bug is caused by use of improper set operation (union() doesn't
modify the calling object).

mock/py/mockbuild/config.py

@@ -685,8 +685,7 @@ def update_config_from_file(config_opts, config_file):
         raise exception.ConfigError("Config error: {}: {}".format(config_file, str(exc)))
 
     # this actually allows multiple inclusion of one file, but not in a loop
-    new_paths = set(config_opts["config_paths"])
-    new_paths.union(included_files)
+    new_paths = set(config_opts["config_paths"]) | included_files
     config_opts["config_paths"] = list(new_paths)
 
 

releng/release-notes-next/root_cache_invalidation.bugfix

@@ -0,0 +1,6 @@
+The `root_cache` plugin is designed to invalidate the cache tarball anytime the
+corresponding Mock configuration changes (any file in the list
+`config_opts['config_paths']` changes).  This cache invalidation mechanism had
+been broken since Mock v3.2 when we rewrote the configuration file loader, and
+broke the `config_opts['config_paths']`.  The config loader has been fixed now,
+and the cache invalidation works again as expected.