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The following hangs at 100% CPU on Python 3.5, though not on Python 3.4:
1) Start an asyncio coroutine with run_until_complete().
2) Inside the coroutine, enter an ExitStack using a with-statement.
3) Inside the with-statement, call ExitStack.enter_context() with a generator context manager. It doesn't matter what the generator yields.
4) After the enter_context() call, raise an exception.
Here's an example script that does all of this and repros the hang: https://gist.github.com/oconnor663/483db2820bb5f877c9ed 

Interestingly, it doesn't hang when you raise a different error. There's some new code dealing with the RuntimeError coming out of a generator if it raises StopIteration (instead of returning) introduced by issue #22906. Yury, it looks like you introduced that? (The diff is this:
changeset: 95932:36a8d935c322
user: Yury Selivanov <yselivanov@sprymix.com>
date: Sat May 09 11:44:30 2015 -0400
summary: PEP 479: Change StopIteration handling inside generators.

Trying to reproduce without contextstack.

Here's a minimal test to reproduce:
 import reprlib
 def main():
 if 0:
 yield
 raise RuntimeError
 m = main()
 try:
 m.send(None)
 except RuntimeError as ex:
 ex.__context__ = ex
 reprlib.repr(ex)
 
Looks like it's a bug in reprlib. It's not related to PEP 492/479. 
It's also reproducible in Python 3.4 and 3.3.
Nick, ExitStack does this (indirectly) 'ex.__context__ = ex' thing -- I think that's a bug of contextlib.

Created another issue for the reprlib bug: issue 25781. It appears we don't even need a generator:
 import reprlib
 try:
 raise RuntimeError
 except RuntimeError as ex:
 ex.__context__ = ex
 reprlib.repr(ex)
Closing this one with "not a bug".

It's not even a reprlib bug:
try:
 raise Exception
except Exception as ex:
 ex.__context__ = ex
 hasattr(1, 'aa')

Thanks for chasing this down. Yury, can you suggest a workaround?

> Thanks for chasing this down. Yury, can you suggest a workaround?
I'm not sure how to workaround this :( Hopefully we can fix this in 3.5.1.

FWIW the bug was identified in issue 25782. I've drafted a patch to fix it, please review.

The question is whether we should raise an exception or not:
 ex.__context__ = ex

Another issue for contextlib: http://bugs.python.org/issue25786 

Yury, can you help me understand why `hasattr("foo", "bar")` triggers the infinite loop there, but not `print("foo")`?

> Yury, can you help me understand why `hasattr("foo", "bar")` triggers the infinite loop there, but not `print("foo")`?
hasattr uses getattr under the hood. getattr raises an AttributeError, and that triggers PyErr_SetError, which has an infinite "while" loop. Instead of "hasattr" you can use anything that raises an error.