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Calling Pool.map (and friends) on empty lists [] causes Pool._cache to hang on to the MapResults forever:
tp = ThreadPool(5)
xs = tp.map(lambda x: 'a' * int(10e6), range(100))
# now using 1GB mem = 100 * 10MB strs
del xs
gc.collect()
# still using 1GB mem
tp.close(); tp.join()
# now using ~0GB mem

Oops, sorry - pasted a wrong example.
In [38]: tp = ThreadPool(5)
In [39]: xs = tp.map(lambda x: x, [])
In [40]: xs
Out[40]: []
In [41]: tp._cache
Out[41]: {3: <multiprocessing.pool.MapResult at 0x244ab50>}

It's a duplicate of #12157.