René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Graceful loop shutdown with running executor tasks","articleBody":"When I send a long task to tread-pool executor and immediately close the loop without waiting future returned by `.run_in_executor()` call I get exception report in logs:\n\n```\nERROR:concurrent.futures:exception calling callback for \u003cFuture at 0x7ff1ae393630 state=finished returned NoneType\u003e\nTraceback (most recent call last):\n  File \"/usr/lib/python3.4/concurrent/futures/_base.py\", line 297, in _invoke_callbacks\n    callback(self)\n  File \"/home/andrew/projects/asyncio/asyncio/futures.py\", line 410, in \u003clambda\u003e\n    new_future._copy_state, future))\n  File \"/home/andrew/projects/asyncio/asyncio/base_events.py\", line 487, in call_soon_threadsafe\n    handle = self._call_soon(callback, args)\n  File \"/home/andrew/projects/asyncio/asyncio/base_events.py\", line 461, in _call_soon\n    self._check_closed()\n  File \"/home/andrew/projects/asyncio/asyncio/base_events.py\", line 288, in _check_closed\n    raise RuntimeError('Event loop is closed')\nRuntimeError: Event loop is closed\n```\n\nThat's because task executed in thread has finished after loop closing. The task calls `call_soon_threadsafe()` (see `futures.wrap_future()` for details) and it fails on check for loop health.\n\nDue threaded nature it's hard to avoid situations like this: threads cannot be cancelled, we need cope with these even after loop closing. That's pretty common situation, especially for client-side jobs like web page fetching: DNS resolver is executed in thread pool also.\n\nI see two options:\n1.  Ignore loop closing in `wrap_future` call, skip `call_soon_threadsafe()` for closed loops\n2. Do the same but only if special boolean flag passed into `run_in_executor()` (while I don't see situations when I don't like the behavior, maybe flag should be `True` by default).\n","author":{"url":"https://github.com/asvetlov","@type":"Person","name":"asvetlov"},"datePublished":"2015-07-26T07:45:38.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":12},"url":"https://github.com/258/asyncio/issues/258"}