Fix a crash in undefined-variable caused by chained attributes in metaclass

Close #3742

Author: PCManticore

ChangeLog

@@ -28,6 +28,10 @@ What's New in Pylint 2.6.1?
 ===========================
 Release date: TBA
 
+* Fix a crash in `undefined-variable` caused by chained attributes in metaclass
+
+  Close #3742
+
 * Fix false positive for `not-async-context-manager` when `contextlib.asynccontextmanager` is used
 
   Close #3862

pylint/checkers/variables.py

@@ -2037,7 +2037,10 @@ def _check_classdef_metaclasses(self, klass, parent_node):
         if isinstance(klass._metaclass, astroid.Name):
             name = klass._metaclass.name
         elif isinstance(klass._metaclass, astroid.Attribute) and klass._metaclass.expr:
-            name = klass._metaclass.expr.name
+            attr = klass._metaclass.expr
+            while not isinstance(attr, astroid.Name):
+                attr = attr.expr
+            name = attr.name
         elif metaclass:
             name = metaclass.root().name
 

tests/functional/u/undefined_variable_crash_on_attribute.py

@@ -0,0 +1,6 @@
+# pylint: disable=import-error,missing-module-docstring,missing-class-docstring,too-few-public-methods
+import unknown
+
+
+class Cls(metaclass=unknown.path.MetaFoo):
+    pass