Fallback to traceback.format_tb when the formatter is called recursively.

i.e. Don't let errors in the formatter pass silently.

This patch saves lots of hair pulling with doctest encoding issues under
zope.testrunner.

2 files changed, 25 insertions, 1 deletions

diff --git a/src/zope/exceptions/exceptionformatter.py b/src/zope/exceptions/exceptionformatter.py
index 7fdbd69..407032c 100644
--- a/src/zope/exceptions/exceptionformatter.py
+++ b/src/zope/exceptions/exceptionformatter.py
@@ -171,7 +171,8 @@ class TextExceptionFormatter(object):
         while tb is not None and (limit is None or n < limit):
             if tb.tb_frame.f_locals.get('__exception_formatter__'):
                 # Stop recursion.
-                result.append('(Recursive formatException() stopped)\n')
+                result.append('(Recursive formatException() stopped, trying traceback.format_tb)\n')
+                result.extend(traceback.format_tb(tb))
                 break
             line = self.formatLine(tb)
             result.append(line + '\n')
diff --git a/src/zope/exceptions/tests/test_exceptionformatter.py b/src/zope/exceptions/tests/test_exceptionformatter.py
index 1243141..c4d139c 100644
--- a/src/zope/exceptions/tests/test_exceptionformatter.py
+++ b/src/zope/exceptions/tests/test_exceptionformatter.py
@@ -152,6 +152,29 @@ class Test(TestCase):
                            '               ^',
                            'SyntaxError: invalid syntax'])
 
+    def testRecursionFailure(self):
+        from zope.exceptions.exceptionformatter import TextExceptionFormatter
+
+        class FormatterException(Exception):
+            pass
+            
+        class FailingFormatter(TextExceptionFormatter):
+            def formatLine(self, tb):
+                raise FormatterException("Formatter failed")
+
+        fmt = FailingFormatter()
+        try:
+            raise ExceptionForTesting
+        except ExceptionForTesting:
+            try:
+                fmt.formatException(*sys.exc_info())
+            except FormatterException:
+                s = tb()
+        # Recursion was detected
+        self.assertTrue('(Recursive formatException() stopped, trying traceback.format_tb)' in s, s)
+        # and we fellback to the stdlib rather than hid the real error
+        self.assertEquals(s.splitlines()[-2], '    raise FormatterException("Formatter failed")')
+        self.assertTrue('FormatterException: Formatter failed' in s.splitlines()[-1])
 
 def test_suite():
     return makeSuite(Test)