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"Set changed size during iteration" Error in Worker.put_key_in_memory()

See original GitHub issue

I am getting an error in this line: https://github.com/dask/distributed/blob/d54388c92f5cb6643180749f7d30f580609379a2/distributed/worker.py#L1970

Maybe a temporary copy of ts.dependents helps here?

Exception in callback functools.partial(<bound method IOLoop._discard_future_result of <tornado.platform.asyncio.AsyncIOMainLoop object at 0x7fa72878fb90>>, <Task finished coro=<Worker.gather_dep() done, defined at [...]/python3.7/site-packages/distributed/worker.py:2000> exception=RuntimeError('Set changed size during iteration')>)
Traceback (most recent call last):
File "[...]/python3.7/site-packages/tornado/ioloop.py", line 743, in _run_callback
  ret = callback()
File "[...]/python3.7/site-packages/tornado/ioloop.py", line 767, in _discard_future_result
  future.result()
File "[...]/python3.7/site-packages/distributed/worker.py", line 2119, in gather_dep
  self.transition(ts, "memory", value=data[d])
File "[...]/python3.7/site-packages/distributed/worker.py", line 1539, in transition
  state = func(ts, **kwargs)
File "[...]/python3.7/site-packages/distributed/worker.py", line 1605, in transition_flight_memory
  self.put_key_in_memory(ts, value)
File "[...]/python3.7/site-packages/distributed/worker.py", line 1970, in put_key_in_memory
  for dep in ts.dependents:
RuntimeError: Set changed size during iteration