allow more 'backwards compatible' vars_plugins (#25451)

* allow more 'backwards compatible' vars_plugins

* better version relying on exceptions

lib/ansible/vars/manager.py

@@ -229,6 +229,24 @@ class VariableManager:
             all_group = self._inventory.groups.get('all')
             host_groups = sort_groups([g for g in host.get_groups() if g.name not in ['all']])
 
+            def _get_plugin_vars(plugin, loader, path, entities):
+                data = {}
+                try:
+                    data = plugin.get_vars(self._loader, path, entities)
+                except AttributeError:
+                    try:
+                        for entity in entities:
+                            if isinstance(entity, Host):
+                                data.update(plugin.get_host_vars(entity.name))
+                            else:
+                                data.update(plugin.get_group_vars(entity.name))
+                    except AttributeError:
+                        if hasattr(plugin, 'run'):
+                            raise AnsibleError("Cannot use v1 type vars plugin %s from %s" % (plugin._load_name, plugin._original_path))
+                        else:
+                            raise AnsibleError("Invalid vars plugin %s from %s" % (plugin._load_name, plugin._original_path))
+                return data
+
             # internal fuctions that actually do the work
             def _plugins_inventory(entities):
                 ''' merges all entities by inventory source '''
@@ -240,15 +258,14 @@ class VariableManager:
                         inventory_dir = os.path.dirname(inventory_dir)
 
                     for plugin in vars_loader.all():
-                        data = combine_vars(data, plugin.get_vars(self._loader, inventory_dir, entities))
-
+                        data = combine_vars(data, _get_plugin_vars(plugin, self._loader, inventory_dir, entities))
                 return data
 
             def _plugins_play(entities):
                 ''' merges all entities adjacent to play '''
                 data = {}
                 for plugin in vars_loader.all():
-                    data = combine_vars(data, plugin.get_vars(self._loader, basedir, entities))
+                    data = combine_vars(data, _get_plugin_vars(plugin, self._loader, basedir, entities))
                 return data
 
             # configurable functions that are sortable via config