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Looks like - another - OS-X bug.
What happens if you reduce the size argument when reading from the error pipe? Try setting it to a value like 512 (minimum guaranteed PIPE_BUF):
"""
newData = os.read(errpipe_read, min(512, rSize))
"""
You could also try to put a short sleep before exiting the child process (after pickling the exception) to see what happens if the reader reads before the pipe is closed by the other end (that would be a huge bug, but hey, who knows...).