break cycle in chained exceptions

CHANGES.rst

@@ -18,6 +18,8 @@ Unreleased
     float values. (:issue:`1511`)
 -   Update :class:`~middleware.lint.LintMiddleware` to work on Python 3.
     (:issue:`1510`)
+-   The debugger detects cycles in chained exceptions and does not time
+    out in that case. (:issue:`1536`)
 
 
 Version 0.15.2

src/werkzeug/debug/tbtools.py

@@ -245,12 +245,14 @@ def __init__(self, exc_type, exc_value, tb):
         self.exception_type = exception_type
 
         self.groups = []
+        memo = set()
         while True:
             self.groups.append(Group(exc_type, exc_value, tb))
+            memo.add(exc_value)
             if PY2:
                 break
             exc_value = exc_value.__cause__ or exc_value.__context__
-            if exc_value is None:
+            if exc_value is None or exc_value in memo:
                 break
             exc_type = type(exc_value)
             tb = exc_value.__traceback__

tests/test_debug.py

@@ -354,3 +354,20 @@ def app(environ, start_response):
         assert "The debugger caught an exception in your WSGI application" in r.text
     else:
         assert r.text == "hello"
+
+
+@pytest.mark.skipif(PY2, reason="Python 2 doesn't have chained exceptions.")
+@pytest.mark.timeout(2)
+def test_chained_exception_cycle():
+    try:
+        try:
+            raise ValueError()
+        except ValueError:
+            raise TypeError()
+    except TypeError as e:
+        # create a cycle and make it available outside the except block
+        e.__context__.__context__ = error = e
+
+    # if cycles aren't broken, this will time out
+    tb = Traceback(TypeError, error, error.__traceback__)
+    assert len(tb.groups) == 2

tox.ini

@@ -10,6 +10,7 @@ skip_missing_interpreters = true
 deps =
     coverage
     pytest
+    pytest-timeout
     pytest-xprocess
     requests
     requests_unixsocket