fix: environment leak on Windows

[awson]

This fixes an old todo, where spawning a child process on Windows modified the environment variables of the parent.

Fixes #1282

[gebner]

Thanks for fixing this really old todo!

Please squash the commits!

Please remove the (#1282) from the commit message and PR title. Instead, please write Fixes #1282 in the PR message!

[gebner]

@awson Is there a reason you decided to stick with the statically-sized buffer instead of the dynamically-growing std::vector<char> version?

Is 0x8000 some magic limit on Windows? If so, we should document that!

[awson]

@awson Is there a reason you decided to stick with the statically-sized buffer instead of the dynamically-growing std::vector<char> version?

Is 0x8000 some magic limit on Windows? If so, we should document that!

Exactly. MS documentation says "The total size of the environment block for a process may not exceed 32,767 characters".

[gebner]

Exactly. MS documentation says "The total size of the environment block for a process may not exceed 32,767 characters".

Good to know! Can you put that in a comment? Then the PR is good to go!

[awson]

That said, my interpretation has always been that 32767 is the size of the block without the extra null char, but after reading MS docs here and there I see them using "total size" wording, hence we, perhaps, need to use 0x7fff here.