Merge branch 'master' into python3

diff --cc tornado/iostream.py
Simple merge

diff --cc tornado/stack_context.py
index b2e41815806c225897e958013805836f1f7cf0f9,d12c6bc4483d7383344af1c1537ad5eb9efb1c36..fd3ac50e1c43ce0f4748e2de448a9a5282738538
--- 1/tornado/stack_context.py
--- 2/tornado/stack_context.py
+++ b/tornado/stack_context.py
@@@ -172,45 -179,7 +180,42 @@@ def wrap(fn)
                  callback(*args, **kwargs)
          else:
              callback(*args, **kwargs)
-     if getattr(fn, 'stack_context_wrapped', False):
+     if isinstance(fn, (_StackContextWrapper, NoneType)):
          return fn
-     contexts = _state.contexts
-     result = functools.partial(wrapped, fn, contexts)
-     result.stack_context_wrapped = True
-     return result
+     return _StackContextWrapper(wrapped, fn, _state.contexts)
  
 +@contextlib.contextmanager
 +def _nested(*managers):
 +    """Support multiple context managers in a single with-statement.
 +
 +    Copied from the python 2.6 standard library.  It's no longer present
 +    in python 3 because the with statement natively supports multiple
 +    context managers, but that doesn't help if the list of context
 +    managers is not known until runtime.
 +    """
 +    exits = []
 +    vars = []
 +    exc = (None, None, None)
 +    try:
 +        for mgr in managers:
 +            exit = mgr.__exit__
 +            enter = mgr.__enter__
 +            vars.append(enter())
 +            exits.append(exit)
 +        yield vars
 +    except:
 +        exc = sys.exc_info()
 +    finally:
 +        while exits:
 +            exit = exits.pop()
 +            try:
 +                if exit(*exc):
 +                    exc = (None, None, None)
 +            except:
 +                exc = sys.exc_info()
 +        if exc != (None, None, None):
 +            # Don't rely on sys.exc_info() still containing
 +            # the right information. Another exception may
 +            # have been raised and caught by an exit method
 +            raise exc[0], exc[1], exc[2]
 +