Future exception was never retrieved; Set changed size during iteration

[lukehutch]

I got this exception multiple times while trying to download with -a 99 -d 1

Future exception was never retrieved
future: <Future finished exception=RuntimeError('Set changed size during iteration',)>
Traceback (most recent call last):
  File "/usr/lib64/python3.6/concurrent/futures/thread.py", line 56, in run
    result = self.fn(*self.args, **self.kwargs)
  File "getmyancestors.py", line 765, in add_parents
    self.add_trio(father, mother, fid)
  File "getmyancestors.py", line 750, in add_trio
    self.add_indi(father)
  File "getmyancestors.py", line 741, in add_indi
    self.indi[fid] = Indi(fid, self)
  File "getmyancestors.py", line 442, in __init__
    self.sources.add((self.tree.add_source(json),))
  File "getmyancestors.py", line 804, in add_source
    for s in self.sources:
RuntimeError: Set changed size during iteration