In multiprocessing/reduction.py, there is a hack workaround in the sendfds() and recvfds() methods for darwin, as determined by the "ACKNOWLEDGE" constant. There is a reference to issue #14669 in the code related to why this was added in the first place. This bug exists in both 3.6.3 and the latest 3.7.0a2.

When a file descriptor is received, this workaround/hack sends an acknowledgement message to the sender. The problem is that this completely breaks Duplex pipes depending on the timing of the acknowledgement messages, as your "sock.send(b'A')" and "sock.recv(1) != b'A'" calls are being interwoven with my own messages.

Specifically, I have a parent process with child processes. I send socket file descriptors from the parent to the children, and am also duplexing messages from the child processes to the parent. If I am in the process of sending/receiving a message around the same time as your workaround is performing this acknowledge step, then your workaround corrupts the pipe. 

In a multi-process program, each end of a pipe must only be read or written to by a single process, but this workaround breaks this requirement. A different workaround must be found for the original bug that prompted this "acknowledge" step to be added, because library code must not be interfering with the duplex pipe.