Python-based harvesters sometimes deadlock

Sometimes, the Python-based harvesters somethimes but not always deadlock while waiting for the parent process to write data to their standard input, i.e. sys.stdin.readline().

This suggest that their standard output is not properly flushed by the print(json.dumps(..), flush=True) sequence. This is further corroborated by the problem apparently going away when Python's -u command line flag or PYTHONUNBUFFERED environment variable is used. However, since we always write a long JSON string followed by a single newline, buffering is highly beneficial to efficiency and should stay enabled either automatically or managed explicitly. It actually happens even with -u set...