[danudey]

> I think there's a narrow window, at least in some programming languages, when environment variables can be set at the start of a process.

I mean, based on this issue I would say the only safe time is "at the start of the program, before any new threads may have been created".

But again, as others have said, there's no good reason I'm aware of to set environment variables in your own process, and when you spawn a new process you can give it its own environment with any changes you want.