message: Kill all processes when finished
  Only really relevant on a timeout, I think: we give up on the last
  process, but we manually need to kill the rest of the pipeline.
diff: @@ -37,10 +37,14 @@ def run_pipe(cmds, input):
  )
  procs.append(proc)
  + try:
  # Send stdin and collect stdout.
  procs[0].stdin.write(input)
  procs[0].stdin.close()
  return procs[-1].communicate(timeout=TIMEOUT)
  + finally:
  + for proc in procs:
  + proc.kill()
  def run_bench(pipeline, fn):
  