> It seems like Connection.close() was called while Connection._send() was called. I added debug logs:

The connection was closed by terminate_broken() called by _ExecutorManagerThread.run() thread:

test_killed_child (test.test_concurrent_futures.ProcessPoolSpawnProcessPoolExecutorTest) ... close handle 4
  File "/home/vstinner/python/master/Lib/threading.py", line 908, in _bootstrap
    self._bootstrap_inner()
  File "/home/vstinner/python/master/Lib/threading.py", line 950, in _bootstrap_inner
    self.run()
  File "/home/vstinner/python/master/Lib/concurrent/futures/process.py", line 313, in run
    self.terminate_broken(cause)
  File "/home/vstinner/python/master/Lib/concurrent/futures/process.py", line 456, in terminate_broken
    self.join_executor_internals()
  File "/home/vstinner/python/master/Lib/concurrent/futures/process.py", line 503, in join_executor_internals
    self.thread_wakeup.close()
  File "/home/vstinner/python/master/Lib/concurrent/futures/process.py", line 75, in close
    self._writer.close()