From 3634560fa9ddb342c3a823b78cc63e7000ccabd8 Mon Sep 17 00:00:00 2001
From: Vinay Karanam <vinayinvicible@gmail.com>
Date: Thu, 6 Dec 2018 18:28:29 +0530
Subject: [PATCH] Fixed #29393 -- Prevented infinite loop in
 ExceptionReporter.get_traceback_frames().

---
 django/views/debug.py                |  3 +++
 tests/view_tests/tests/test_debug.py | 39 ++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)

diff --git a/django/views/debug.py b/django/views/debug.py
index 1128301c57..0efaf0c6af 100644
--- a/django/views/debug.py
+++ b/django/views/debug.py
@@ -397,6 +397,9 @@ class ExceptionReporter:
         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
diff --git a/tests/view_tests/tests/test_debug.py b/tests/view_tests/tests/test_debug.py
index 1ff1220803..a6e5275c9f 100644
--- a/tests/view_tests/tests/test_debug.py
+++ b/tests/view_tests/tests/test_debug.py
@@ -4,6 +4,7 @@ import os
 import re
 import sys
 import tempfile
+import threading
 from io import StringIO
 from pathlib import Path
 
@@ -403,6 +404,44 @@ class ExceptionReporterTests(SimpleTestCase):
         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/')