interface: make interface-to-template conversion a config option (defaulting to enabled)

dist/ifupdown-ng.conf.example

@@ -15,3 +15,11 @@ allow_addon_scripts = 1
 # to disable this setting in order to require inheritance from specified
 # templates.
 allow_any_iface_as_template = 1
+
+# implicit_template_conversion:
+# In some legacy configs, a template may be declared as an iface, and
+# ifupdown-ng automatically converts those declarations to a proper
+# template.  If this setting is disabled, inheritance will continue to
+# work against non-template interfaces without converting them to a
+# template.
+implicit_template_conversion = 1

libifupdown/config-file.c

@@ -45,6 +45,7 @@ set_bool_value(const char *key, const char *value, void *opaque)
 static struct lif_config_handler handlers[] = {
 	{"allow_addon_scripts", set_bool_value, &lif_config.allow_addon_scripts},
 	{"allow_any_iface_as_template", set_bool_value, &lif_config.allow_any_iface_as_template},
+	{"implicit_template_conversion", set_bool_value, &lif_config.implicit_template_conversion},
 };
 
 bool

libifupdown/config-file.h

@@ -21,6 +21,7 @@
 struct lif_config_file {
 	bool allow_addon_scripts;
 	bool allow_any_iface_as_template;
+	bool implicit_template_conversion;
 };
 
 extern struct lif_config_file lif_config;

libifupdown/interface.c

@@ -243,7 +243,8 @@ lif_interface_collection_inherit(struct lif_interface *interface, struct lif_dic
 	if (!lif_config.allow_any_iface_as_template && !parent->is_template)
 		return false;
 
-	/* explicitly convert any interface we are inheriting from into a template */
+	/* maybe convert any interface we are inheriting from into a template */
+	if (lif_config.implicit_template_conversion)
 		parent->is_template = true;
 
 	lif_dict_add(&interface->vars, "inherit", strdup(ifname));