I think I figured out the problem. The signal module has a “main_pid” variable which it checks inside the low-level handler: <https://hg.python.org/cpython/file/v3.5.1/Modules/signalmodule.c#l300>. It looks like main_pid gets updated inside PyOS_AfterFork(), but I guess there is a race with a signal being received before main_pid is updated.

Maybe there should be some signal masking/blocking or something in the critical section between fork() and PyOS_AfterFork()?