[Issue 14770] std.process should use lightweight forks where available

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=14770

--- Comment #7 from Hiroki Noda <kubo39 at gmail.com> ---
(In reply to anonymous4 from comment #6)
> Maybe have a branch for linux and call clone there, call posix_spawn or fork
> for other systems. vfork looks scary and posix deprecated it.

If we use clone(2), the stack of the child process is newly allocated and the
stack of the parent process is not shared. It's preferred.

Maybe I should add clone(2) to druntime before...

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=9ff72da471a509a8c19791efe469f47fa6977410

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