// Copyright (c) 2014-2020 Canonical Ltd
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License version 3 as
// published by the Free Software Foundation.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <http://www.gnu.org/licenses/>.

package systemd

import (
	"bytes"
	"fmt"
	"log/syslog"
	"net"
	"os"
)

var journalStdoutPath = "/run/systemd/journal/stdout"

// NewJournalStreamFile creates log stream file descriptor to the journal. The
// semantics is identical to that of sd_journal_stream_fd(3) call.
func NewJournalStreamFile(identifier string, priority syslog.Priority, levelPrefix bool) (*os.File, error) {
	conn, err := net.DialUnix("unix", nil, &net.UnixAddr{Name: journalStdoutPath})
	if err != nil {
		return nil, err
	}
	// does not affect *os.File created through conn.File() later on
	defer conn.Close()

	if err := conn.CloseRead(); err != nil {
		return nil, err
	}

	// header contents taken from the original systemd code:
	// https://github.com/systemd/systemd/blob/97a33b126c845327a3a19d6e66f05684823868fb/src/journal/journal-send.c#L395
	header := bytes.Buffer{}
	header.WriteString(identifier)
	header.WriteByte('\n')
	header.WriteByte('\n')
	header.WriteByte(byte('0') + byte(priority))
	header.WriteByte('\n')
	var prefix int
	if levelPrefix {
		prefix = 1
	}
	header.WriteByte(byte('0') + byte(prefix))
	header.WriteByte('\n')
	header.WriteByte('0')
	header.WriteByte('\n')
	header.WriteByte('0')
	header.WriteByte('\n')
	header.WriteByte('0')
	header.WriteByte('\n')

	if _, err := conn.Write(header.Bytes()); err != nil {
		return nil, fmt.Errorf("failed to write header: %v", err)
	}

	return conn.File()
}
