fail works as expected but success hangs

a.py

# -*- coding: utf-8 -*-

import asyncio, functools
from nose.tools import raises

class ExitFailure(RuntimeError):
  def __init__(self, exit_code):
    self.exit_code = exit_code

class ExitSuccess:
  pass

class ZeroExitCodeSubprocess(asyncio.SubprocessProtocol):
  def __init__(self, future):
    self.future = future

  def connection_made(self, transport):
    self.transport = transport

  def process_exited(self):
    ec = self.transport.get_returncode()
    print(ec)
    if ec:
      print("fail")
      self.future.set_exception(ExitFailure(ec))
    else:
      print("succ")
      self.future.set_result(ExitSucess())

@asyncio.coroutine
def _call(loop, *args, **kw):
    f = asyncio.Future(loop = loop)
    t, p = yield from loop.subprocess_exec(
      lambda: ZeroExitCodeSubprocess(f)
    , *args
    , **kw
    )
    yield from p.future


def test_call_success():
  loop = asyncio.get_event_loop()
  x = _call(loop, "true")
  loop.run_until_complete(x)

@raises(ExitFailure)
def test_call_fail():
  loop = asyncio.get_event_loop()
  x = _call(loop, "false")
  loop.run_until_complete(x)