protect IPython from bad custom exception handlers

Previously, errors in custom handlers would result in the custom exception
handler's error being printed in lieu of the real exception, and certain cases could cause infinite loops.

Now, if CustomTB fails it is unregistered immediately, and the original TB is also displayed.

IPython's own BdbQuit_IPython_excepthook had an invalid signature, which revealed this issue, and has also been fixed.

test included.

closes  #692

IPython/core/debugger.py

@@ -64,7 +64,7 b' def BdbQuit_excepthook(et,ev,tb):'
     else:
         BdbQuit_excepthook.excepthook_ori(et,ev,tb)
 
-def BdbQuit_IPython_excepthook(self,et,ev,tb):
+def BdbQuit_IPython_excepthook(self,et,ev,tb,tb_offset=None):
     print 'Exiting Debugger.'
 
 

IPython/core/interactiveshell.py

@@ -1478,16 +1478,30 b' class InteractiveShell(SingletonConfigurable, Magic):'
         assert type(exc_tuple)==type(()) , \
                "The custom exceptions must be given AS A TUPLE."
 
-        def dummy_handler(self,etype,value,tb):
+        def dummy_handler(self,etype,value,tb,tb_offset=None):
             print '*** Simple custom exception handler ***'
             print 'Exception type :',etype
             print 'Exception value:',value
             print 'Traceback      :',tb
             #print 'Source code    :','\n'.join(self.buffer)
 
-        if handler is None: handler = dummy_handler
-
-        self.CustomTB = types.MethodType(handler,self)
+        if handler is None:
+            wrapped = dummy_handler
+        else:
+            def wrapped(self,etype,value,tb,tb_offset=None):
+                try:
+                    return handler(self,etype,value,tb,tb_offset=tb_offset)
+                except:
+                    # clear custom handler immediately
+                    self.set_custom_exc((), None)
+                    print >> io.stderr, "Custom TB Handler failed, unregistering"
+                    # show the exception in handler first
+                    stb = self.InteractiveTB.structured_traceback(*sys.exc_info())
+                    print >> io.stdout, self.InteractiveTB.stb2text(stb)
+                    print >> io.stdout, "The original exception:"
+                    self.showtraceback((etype,value,tb), tb_offset=tb_offset)
+
+        self.CustomTB = types.MethodType(wrapped,self)
         self.custom_exceptions = exc_tuple
 
     def excepthook(self, etype, value, tb):

IPython/core/tests/test_interactiveshell.py

@@ -146,3 +146,21 b' class InteractiveShellTestCase(unittest.TestCase):'
         finally:
             # Reset compiler flags so we don't mess up other tests.
             ip.compile.reset_compiler_flags()
+
+    def test_bad_custom_tb(self):
+        """Check that InteractiveShell is protected from bad custom exception handlers"""
+        ip = get_ipython()
+        from IPython.utils import io
+        save_stderr = io.stderr
+        try:
+            # capture stderr
+            io.stderr = StringIO()
+            ip.set_custom_exc((IOError,),lambda etype,value,tb: None)
+            self.assertEquals(ip.custom_exceptions, (IOError,))
+            ip.run_cell(u'raise IOError("foo")')
+            self.assertEquals(ip.custom_exceptions, ())
+            self.assertTrue("Custom TB Handler failed" in io.stderr.getvalue())
+        finally:
+            io.stderr = save_stderr
+
+