//go:build unix

package hostexec

import (
	"errors"

	"golang.org/x/sys/unix"
)

// Observers, including root and adopt, never create or repair an agent's
// lifetime locks. Missing locks deny admission without changing the namespace.
func probeLifetime(dir, host string) (func(), error) { return probeExisting(dir, host, ".lock") }
func probeLive(dir, host string) (func(), error)     { return probeExisting(dir, host, ".live") }
func probeExisting(dir, host, suffix string) (func(), error) {
	if _, err := Key(host, "check"); err != nil {
		return nil, err
	}
	fd, err := openDir(dir)
	if err != nil {
		return nil, err
	}
	defer unix.Close(fd)
	lock, err := unix.Openat(fd, host+suffix, unix.O_RDONLY|unix.O_NOFOLLOW|unix.O_CLOEXEC|unix.O_NONBLOCK, 0)
	if err != nil {
		return nil, err
	}
	fail := func(err error) (func(), error) { _ = unix.Close(lock); return nil, err }
	var st unix.Stat_t
	if err = unix.Fstat(lock, &st); err != nil {
		return fail(err)
	}
	if st.Mode&unix.S_IFMT != unix.S_IFREG || st.Nlink != 1 {
		return fail(ErrState)
	}
	if err = unix.Flock(lock, unix.LOCK_EX|unix.LOCK_NB); err == nil {
		return func() { _ = unix.Flock(lock, unix.LOCK_UN); _ = unix.Close(lock) }, nil
	}
	if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
		return fail(ErrControllerBusy)
	}
	return fail(err)
}