//go:build linux

package emission

import (
	"errors"
	"os"
	"syscall"
)

func acquireWriterLock(path string) (*os.File, error) {
	file, err := openOwnedFile(path, os.O_CREATE|os.O_RDWR, 0600)
	if err != nil {
		return nil, err
	}
	if err := syscall.Flock(int(file.Fd()), syscall.LOCK_EX|syscall.LOCK_NB); err != nil {
		file.Close()
		return nil, errors.New("outbox already has a writer or filesystem lock unavailable")
	}
	return file, nil
}
func infoSys(info os.FileInfo) (*syscall.Stat_t, bool) {
	if info == nil {
		return nil, false
	}
	stat, ok := info.Sys().(*syscall.Stat_t)
	return stat, ok
}

// Shared journal/cursor aliases must not evade directory-based exclusion.
// Nonblocking open also rejects unexpected FIFOs/devices without waiting.
func openOwnedFile(path string, flags int, mode os.FileMode) (*os.File, error) {
	file, err := os.OpenFile(path, flags|syscall.O_NOFOLLOW|syscall.O_NONBLOCK, mode)
	if err != nil {
		return nil, err
	}
	info, err := file.Stat()
	stat, ok := infoSys(info)
	if err != nil || !ok || !info.Mode().IsRegular() || info.Mode().Perm()&0007 != 0 || stat.Nlink != 1 || stat.Uid != uint32(os.Geteuid()) || (info.Mode().Perm()&0070 != 0 && stat.Gid != uint32(os.Getegid())) {
		file.Close()
		return nil, errors.New("outbox file must be a single-link owner/current-group regular file")
	}
	return file, nil
}