Timeout for pending jobs raises InvalidStateError

[Zeniuus]

Environment:

• macOS HighSierra 10.13.6
• Python v3.6.6 with pyenv
• aiojobs v0.2.2

How to reproduce:

import asyncio

import aiojobs


async def some_unexpectedly_long_async_job():
    await asyncio.sleep(10000)


async def reproduce_bug():
    scheduler = await aiojobs.create_scheduler(limit=1)

    jobs = await asyncio.gather(*[scheduler.spawn(some_unexpectedly_long_async_job())
                                  for _ in range(2)])
    try:
        waited_jobs = await asyncio.gather(*[job.wait(timeout=3) for job in jobs])
    except:
        # Deal with async jobs timeout
        pass

    await scheduler.close()

loop = asyncio.get_event_loop()
loop.run_until_complete(reproduce_bug())

bug:

Exception in callback Job._done_callback(<Task cancell...at test.py:6>>)
handle: <Handle Job._done_callback(<Task cancell...at test.py:6>>)>
Traceback (most recent call last):
  File "/Users/zeniuus/.pyenv/versions/3.6.6/lib/python3.6/asyncio/events.py", line 145, in _run
    self._callback(*self._args)
  File "/Users/zeniuus/.pyenv/versions/aiojobs-bug-test/lib/python3.6/site-packages/aiojobs/_job.py", line 138, in _done_callback
    scheduler._done(self)
  File "/Users/zeniuus/.pyenv/versions/aiojobs-bug-test/lib/python3.6/site-packages/aiojobs/_scheduler.py", line 141, in _done
    new_job._start()
  File "/Users/zeniuus/.pyenv/versions/aiojobs-bug-test/lib/python3.6/site-packages/aiojobs/_job.py", line 134, in _start
    self._started.set_result(None)
asyncio.base_futures.InvalidStateError: invalid state

[Zeniuus]

I tested with various cases and I found that this error occurs when

1. # of jobs is bigger than # of job limit of the scheduler so that there exists a pending job, and
2. the pending job's coroutine does not start until its timeout given by param.

I was able to fix this error by fixing Job::_do_wait as below.
Original:

    async def _do_wait(self, timeout):
        with async_timeout.timeout(timeout=timeout, loop=self._loop):
            # TODO: add a test for waiting for a pending coro
            await self._started
            result = await self._task
            return result

Fixed:

    async def _do_wait(self, timeout):
        with async_timeout.timeout(timeout=timeout, loop=self._loop):
            # TODO: add a test for waiting for a pending coro
            try:
                await self._started
            except:
                self._closed = True
                raise
            result = await self._task
            return result

[Dreamsorcerer]

It would be great if you could verify the problem still exists on master, and create a test in a PR that reproduces the issue if still present.