> From what I can tell, vfork probably also works on macOS (darwin).
> 
> Lets let this run for a bit on Linux and it can be a separate issue to 
> open vfork usage up to other platforms.

I'd prefer to not use vfork on macOS. For one I don't particularly trust that vfork would work reliably when using higher level APIs, but more importantly posix_spawn on macOS has some options that are hard to achieve otherwise and might be useful to expose in subprocess. An example of this is selecting the CPU architecture to use for the launched process when using fat binaries and a system that supports CPU emulation, such as the intel emulation on the upcoming Apple Silicon systems.