Merge branch 'master' into log_future

diff --cc tornado/concurrent.py
index b03e9d2549a267dce060704af963b3aae8925e62,6bab5d2e0add53c17682cd3a1058829218823ad0..d32f2c6ecdea708cd11472ba01ff5589a6759543
--- 1/tornado/concurrent.py
--- 2/tornado/concurrent.py
+++ b/tornado/concurrent.py
@@@ -25,12 -25,11 +25,13 @@@ module
  from __future__ import absolute_import, division, print_function, with_statement
  
  import functools
 +import traceback
  import sys
  
 +from tornado.log import app_log
  from tornado.stack_context import ExceptionStackContext, wrap
  from tornado.util import raise_exc_info, ArgReplacer
+ from tornado.log import app_log
  
  try:
      from concurrent import futures
@@@ -288,30 -174,12 +289,33 @@@ class Future(object)
      def _set_done(self):
          self._done = True
          for cb in self._callbacks:
-             # TODO: error handling
-             cb(self)
+             try:
+                 cb(self)
+             except Exception:
+                 app_log.exception('exception calling callback %r for %r',
+                                   cb, self)
          self._callbacks = None
 +        
 +    # On Python 3.3 or older, objects with a destructor part of a reference
 +    # cycle are never destroyed. It's not more the case on Python 3.4 thanks to
 +    # the PEP 442.
 +    if _PY34:
 +        def __del__(self):
 +            if not self._log_traceback:
 +                # set_exception() was not called, or result() or exception()
 +                # has consumed the exception
 +                return
 +            
 +            exc = self._exception
 +            tb = traceback.format_exception(exc.__class__, exc,
 +                                                 exc.__traceback__)
 +            
 +            msg = '%s exception was never retrieved: %s' % \
 +                    (self.__class__.__name__, ''.join(tb).rstrip())
 +            
 +            # HACK: should probably call something
 +            app_log.error(msg)
 +            
  
  TracebackFuture = Future