Fixed #29393 -- Prevented infinite loop in ExceptionReporter.get_trac…

…eback_frames().

django/views/debug.py

@@ -397,6 +397,9 @@ def explicit_or_implicit_cause(exc_value):
         while exc_value:
             exceptions.append(exc_value)
             exc_value = explicit_or_implicit_cause(exc_value)
+            if exc_value in exceptions:
+                # Avoid infinite loop if there's a cyclic reference (#29393).
+                break
 
         frames = []
         # No exceptions were supplied to ExceptionReporter

tests/view_tests/tests/test_debug.py

@@ -4,6 +4,7 @@
 import re
 import sys
 import tempfile
+import threading
 from io import StringIO
 from pathlib import Path
 
@@ -403,6 +404,44 @@ def test_reporting_frames_without_source(self):
         text = reporter.get_traceback_text()
         self.assertIn('"generated" in funcName', text)
 
+    def test_reporting_frames_for_cyclic_reference(self):
+        try:
+            def test_func():
+                try:
+                    raise RuntimeError('outer') from RuntimeError('inner')
+                except RuntimeError as exc:
+                    raise exc.__cause__
+            test_func()
+        except Exception:
+            exc_type, exc_value, tb = sys.exc_info()
+        request = self.rf.get('/test_view/')
+        reporter = ExceptionReporter(request, exc_type, exc_value, tb)
+
+        def generate_traceback_frames(*args, **kwargs):
+            nonlocal tb_frames
+            tb_frames = reporter.get_traceback_frames()
+
+        tb_frames = None
+        tb_generator = threading.Thread(target=generate_traceback_frames, daemon=True)
+        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
+            # exits. This is resource heavy when running the full test suite.
+            # Setting the following values to None makes
+            # reporter.get_traceback_frames() exit early.
+            exc_value.__traceback__ = exc_value.__context__ = exc_value.__cause__ = None
+            tb_generator.join()
+            self.fail('Cyclic reference in Exception Reporter.get_traceback_frames()')
+        if tb_frames is None:
+            # can happen if the thread generating traceback got killed
+            # or exception while generating the traceback
+            self.fail('Traceback generation failed')
+        last_frame = tb_frames[-1]
+        self.assertIn('raise exc.__cause__', last_frame['context_line'])
+        self.assertEqual(last_frame['filename'], __file__)
+        self.assertEqual(last_frame['function'], 'test_func')
+
     def test_request_and_message(self):
         "A message can be provided in addition to a request"
         request = self.rf.get('/test_view/')