Function netns_available 

pub fn netns_available() -> bool

Whether an unprivileged, self-contained network namespace cut-off is available on THIS process’s kernel — a real probe, but one that (unlike landlock_available) genuinely can’t be done risk-free in the calling process itself (unshare(2) acts on the CALLING process/thread, so probing it directly would isolate supercode’s own network, not just check availability). Instead this forks a disposable, single-purpose child that does nothing but attempt the unshare and immediately _exit() with the result — never touches the allocator, locks, or any other state the parent might hold mid-fork (the standard safe shape for a post-fork child that never execs), so it carries none of pre_exec’s usual multi-threaded-fork hazards. Cached for the process lifetime.