/relations: GreenletExit for requests with many corpora

[janiemi]

Some /relations requests on many corpora sometimes terminate in a GreenletExit error after processing some of the corpora. For example, this search first terminated after processing 6 of the 20 corpora in 41 seconds, then after 8 corpora in 152 seconds, then after 9 corpora in 74 seconds. The error with traceback was the following (pretty-printed here):

"ERROR": {
  "type": "GreenletExit",
  "value": "",
  "traceback": [
    "Traceback (most recent call last):",
    "  File \"/home/fkkorp/korp-backend/v8/korp.py\", line 146, in incremental_json",
    "    for response in ff:",
    "  File \"/home/fkkorp/korp-backend/v8/korp.py\", line 232, in decorated",
    "    msg = q.get(block=True, timeout=timeout)",
    "  File \"/home/fkkorp/korp-backend/v8/venv/lib64/python3.6/site-packages/gevent/queue.py\", line 283, in get",
    "    return self.__get_or_peek(self._get, block, timeout)",
    "  File \"/home/fkkorp/korp-backend/v8/venv/lib64/python3.6/site-packages/gevent/queue.py\", line 260, in __get_or_peek",
    "    result = waiter.get()",
    "  File \"/home/fkkorp/korp-backend/v8/venv/lib64/python3.6/site-packages/gevent/hub.py\", line 898, in get",
    "    return self.hub.switch()",
    "  File \"/home/fkkorp/korp-backend/v8/venv/lib64/python3.6/site-packages/gevent/hub.py\", line 630, in switch",
    "    return RawGreenlet.switch(self)",
    "greenlet.GreenletExit"
  ]
}

Does the GreenletExit result from a timeout of some kind? Could that perhaps be avoided, or could the error message be somehow more informative? However, sometimes a similar search succeeds even if it takes much longer time, so what causes a GreenletExit? Does the server load perhaps have an effect?

At least in the above case, it seemed that successive searches always got a bit further in the list of corpora before a GreenletExit. Is that effect due to caching the result by corpus?