Fixed #31675 -- Added warning to ExceptionReporter when exception chain has a cycle.

django/views/debug.py

@@ -2,6 +2,7 @@ import functools
 import re
 import sys
 import types
+import warnings
 from pathlib import Path
 
 from django.conf import settings
@@ -28,6 +29,10 @@ DEBUG_ENGINE = Engine(
 CURRENT_DIR = Path(__file__).parent
 
 
+class ExceptionCycleWarning(UserWarning):
+    pass
+
+
 class CallableSettingWrapper:
     """
     Object to wrap callable appearing in settings.
@@ -401,6 +406,11 @@ class ExceptionReporter:
             exceptions.append(exc_value)
             exc_value = explicit_or_implicit_cause(exc_value)
             if exc_value in exceptions:
+                warnings.warn(
+                    "Cycle in the exception chain detected: exception '%s' "
+                    "encountered again." % exc_value,
+                    ExceptionCycleWarning,
+                )
                 # Avoid infinite loop if there's a cyclic reference (#29393).
                 break
 

tests/view_tests/tests/test_debug.py

@@ -23,8 +23,8 @@ from django.utils.functional import SimpleLazyObject
 from django.utils.regex_helper import _lazy_re_compile
 from django.utils.safestring import mark_safe
 from django.views.debug import (
-    CallableSettingWrapper, ExceptionReporter, Path as DebugPath,
-    SafeExceptionReporterFilter, default_urlconf,
+    CallableSettingWrapper, ExceptionCycleWarning, ExceptionReporter,
+    Path as DebugPath, SafeExceptionReporterFilter, default_urlconf,
     get_default_exception_reporter_filter, technical_404_response,
     technical_500_response,
 )
@@ -518,7 +518,12 @@ class ExceptionReporterTests(SimpleTestCase):
 
         tb_frames = None
         tb_generator = threading.Thread(target=generate_traceback_frames, daemon=True)
-        tb_generator.start()
+        msg = (
+            "Cycle in the exception chain detected: exception 'inner' "
+            "encountered again."
+        )
+        with self.assertWarnsMessage(ExceptionCycleWarning, msg):
+            tb_generator.start()
         tb_generator.join(timeout=5)
         if tb_generator.is_alive():
             # tb_generator is a daemon that runs until the main thread/process