To my knowledge, a pipe is the *only* way to reliably wakeup the main thread from a signal handler in another thread.  It's not necessary here simply because this bug only names a subset of the signal problems, whereas #1564547 attempts to fix all of them.  Dropping it would be silly unless it were officially declared that the signal module and the threading module were incompatible.

You're right about the .tripped/Py_AddPendingCall order.  I got myself confused as to what Py_AddPendingCall did.