Ensure we don't get stuck in formatting loops

Change-Id: I516dc2aca05823add9daf8c6d4c4af7ef7d2a313

taskflow/exceptions.py

@@ -98,7 +98,9 @@ class TaskFlowException(Exception):
         buf.write(self._get_message())
         active_indent = indent
         next_up = self.cause
-        while next_up is not None:
+        seen = []
+        while next_up is not None and next_up not in seen:
+            seen.append(next_up)
             buf.write(os.linesep)
             if isinstance(next_up, TaskFlowException):
                 buf.write(indent_text * active_indent)

taskflow/tests/unit/test_exceptions.py

@@ -14,6 +14,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import string
+
 import six
 import testtools
 
@@ -56,6 +58,19 @@ class TestExceptions(test.TestCase):
         self.assertIsNotNone(capture.cause)
         self.assertIsInstance(capture.cause, IOError)
 
+    def test_no_looping(self):
+        causes = []
+        for a in string.ascii_lowercase:
+            try:
+                cause = causes[-1]
+            except IndexError:
+                cause = None
+            causes.append(exc.TaskFlowException('%s broken' % a, cause=cause))
+        e = causes[0]
+        last_e = causes[-1]
+        e._cause = last_e
+        self.assertIsNotNone(e.pformat())
+
     def test_pformat_str(self):
         ex = None
         try: