I wonder if a dedicated datatype should be created for all os-specific parameters like https://golang.org/pkg/syscall/#SysProcAttr. Popen already has way too many parameters. And prctl is a very general interface; probably 98% of prctls would never need to be called pre-exec.

(Separately, os.prctl should be created to expose prctl in all its multiplexed glory?)

(Also, but PDEATHSIG has an infamous footgun where the the signal is sent on exit of the forking thread, which is not necessarily the exit of the invoking process.)