A pipe was was left over from a development version of pdeathsig.c and is not needed.
Process C waits for a signal that'll be generated when process B exists. Process B waits for process D to send it a byte via pipe_db before it exits. Process D sends the byte after it has started ptrace()ing process C. The point of the test is to show that process C receives the signal because process B exited, even though C has been reparented to process D. The pipe pipe_cd isn't doing anything useful (though in an earlier version of the patch it did). Clean that up by removing the useless pipe.