tests: move traceback cleanup to test method wrappers

Python 3.11 changed internal details that cleanup_traceback() relied on.
Move the functionality to wrappers around actual test functions. This
way, it's contained within test class, and doesn't depend on any
specific implementation of test runner.
Wrap those methods dynamically in QubesTestCase.__init__(), to handle
any overriden methods in subclasses too. This avoids needing to use
explicit decorators for every single test method.

(cherry picked from commit 5548010)

qubes/tests/__init__.py

@@ -32,6 +32,7 @@
 
 import asyncio
 import collections
+import contextlib
 import functools
 import logging
 import os
@@ -384,6 +385,24 @@ def __exit__(self, exc_type, exc_value, tb):
         self._orig_iter_entry_points = None
 
 
+class _clear_ex_info(contextlib.ContextDecorator):
+    """Remove local variables reference from tracebacks to allow garbage
+    collector to clean all Qubes*() objects, otherwise file descriptors
+    held by them will leak"""
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if exc_val is None:
+            return
+        ex = exc_val
+        while ex is not None:
+            if isinstance(ex, qubes.exc.QubesVMError):
+                ex.vm = None
+            traceback.clear_frames(ex.__traceback__)
+            ex = ex.__context__
+
+
 class QubesTestCase(unittest.TestCase):
     """Base class for Qubes unit tests.
     """
@@ -393,8 +412,13 @@ class QubesTestCase(unittest.TestCase):
     _callTearDown   = never_awaited.detect()(unittest.TestCase._callTearDown)
     _callCleanup    = never_awaited.detect()(unittest.TestCase._callCleanup)
 
-    def __init__(self, *args, **kwargs):
-        super(QubesTestCase, self).__init__(*args, **kwargs)
+    def __init__(self, methodName='runTest'):
+        try:
+            test_method = getattr(self, methodName)
+            setattr(self, methodName, _clear_ex_info()(test_method))
+        except AttributeError:
+            pass
+        super(QubesTestCase, self).__init__(methodName)
         self.longMessage = True
         self.log = logging.getLogger('{}.{}.{}'.format(
             self.__class__.__module__,
@@ -403,6 +427,10 @@ def __init__(self, *args, **kwargs):
         self.addTypeEqualityFunc(qubes.devices.DeviceManager,
                                  self.assertDevicesEqual)
 
+        # decorate methods here, to catch also any overriden methods in subclasses
+        self.setUp = _clear_ex_info()(self.setUp)
+        self.tearDown = _clear_ex_info()(self.tearDown)
+
         self.loop = None
 
         global libvirt_event_impl
@@ -422,25 +450,6 @@ def setUp(self):
 
         self.loop = asyncio.get_event_loop()
         self.addCleanup(self.cleanup_loop)
-        self.addCleanup(self.cleanup_traceback)
-
-    def cleanup_traceback(self):
-        """Remove local variables reference from tracebacks to allow garbage
-        collector to clean all Qubes*() objects, otherwise file descriptors
-        held by them will leak"""
-        exc_infos = [e for test_case, e in self._outcome.errors
-                     if test_case is self]
-        if self._outcome.expectedFailure:
-            exc_infos.append(self._outcome.expectedFailure)
-        for exc_info in exc_infos:
-            if exc_info is None:
-                continue
-            ex = exc_info[1]
-            while ex is not None:
-                if isinstance(ex, qubes.exc.QubesVMError):
-                    ex.vm = None
-                traceback.clear_frames(ex.__traceback__)
-                ex = ex.__context__
 
     def cleanup_gc(self):
         gc.collect()