Avoid allocating in pre_exec closure

[purplesyringa]

Error::new allocates memory (see rust-lang/rust#148971). This is bad in multi-threaded programs, which microsandbox AFAIK is. If the fork occurs while the allocator lock is held by another thread, deadlocks can occur, since there's no one left in the new process to unlock the mutex. I do not believe this is UB, and modern libc offer protections against this issue, but this isn't POSIX-compliant and should preferably be avoided.

I've found two other places where setsid are invoked, and there it's called through libc, ignoring the resulting error (which should be impossible in a new process anyway). I adjusted the third place to use the same pattern.

[appcypher]

@purplesyringa Can you extend ur fix to these lines as well:

microsandbox/microsandbox-utils/lib/runtime/supervisor.rs

Lines 129 to 131 in 0d99770

	command.pre_exec(|| {
	nix::unistd::setsid()
	.map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, e))?;

[purplesyringa]

Isn't that the exact same location?

[appcypher]

you are right.