Race-free process creation in the GNU C Library

But my point is as long is we're able to guarantee that child's pid is not reused, there is no race if/when parent calls whatever set_xyz on child's pid (it may find child dead, but it'll never confuse this child with another process). And classic mechanism of zombies gives us exactly that. Child's pid won't get reused until parent collects child's status.

P.S. Also I was under the impression that lot/most of those "many things" (setsid, unshare etc) are typically what child does for itself (after clone_vfork but before exec, for which posix_spawn has numerous attributes).