//go:build linux

package logger

import (
	"os"
	"strconv"
	"strings"
	"syscall"
)

// stderrIsJournal reports whether os.Stderr is currently connected to
// the systemd journal. systemd.exec(5) sets JOURNAL_STREAM to
// "<dev>:<inode>" of the journal stream at service start. Detection
// must fstat(2) the actual stderr and compare — a child process may
// redirect stderr while inheriting the env var, so presence of the
// variable alone is not sufficient.
func stderrIsJournal() bool {
	dev, ino, ok := parseJournalStream(os.Getenv("JOURNAL_STREAM"))
	if !ok {
		return false
	}
	var st syscall.Stat_t
	if err := syscall.Fstat(int(os.Stderr.Fd()), &st); err != nil {
		return false
	}
	return uint64(st.Dev) == dev && uint64(st.Ino) == ino
}

func parseJournalStream(v string) (dev, ino uint64, ok bool) {
	sep := strings.IndexByte(v, ':')
	if sep <= 0 || sep == len(v)-1 {
		return 0, 0, false
	}
	d, err1 := strconv.ParseUint(v[:sep], 10, 64)
	n, err2 := strconv.ParseUint(v[sep+1:], 10, 64)
	if err1 != nil || err2 != nil {
		return 0, 0, false
	}
	return d, n, true
}