load all substitutions from etc

On some distributions (like CentOS Stream and Oracle Linux), we need
more substitutions to be able to load repositories properly.

DNF has a helper for that: conf.substitutions.update_from_etc.

On pure DNF distributions, calling this should be sufficient.
On EL7, where the primary tool is YUM, DNF does not load vars from
/etc/yum, only from /etc/dnf, so we have to help it a bit and explicitly
try to load releasever from /etc/yum.
(DNF since 4.2.15 *does* also load substitutions from /etc/yum, but EL7
ships with 4.0.x)

Author: evgeni

repos/system_upgrade/common/libraries/module.py

@@ -23,14 +23,20 @@ def _create_or_get_dnf_base(base=None):
         # have repositories only for the exact system version (including the minor number). In a case when
         # /etc/yum/vars/releasever is present, read its contents so that we can access repositores on such systems.
         conf = dnf.conf.Conf()
-        pkg_manager = 'yum' if get_source_major_version() == '7' else 'dnf'
-        releasever_path = '/etc/{0}/vars/releasever'.format(pkg_manager)
-        if os.path.exists(releasever_path):
-            with open(releasever_path) as releasever_file:
-                releasever = releasever_file.read().strip()
-                conf.substitutions['releasever'] = releasever
-        else:
-            conf.substitutions['releasever'] = get_source_major_version()
+
+        # preload releasever from what we know, this will be our fallback
+        conf.substitutions['releasever'] = get_source_major_version()
+
+        # dnf on EL7 doesn't load vars from /etc/yum, so we need to help it a bit
+        if get_source_major_version() == '7':
+            try:
+                with open('/etc/yum/vars/releasever') as releasever_file:
+                    conf.substitutions['releasever'] = releasever_file.read().strip()
+            except:
+                pass
+
+        # load all substitutions from etc
+        conf.substitutions.update_from_etc('/')
 
         base = dnf.Base(conf=conf)
         base.init_plugins()