journal-detect.py

#!/usr/bin/env python3
"""
Determine whether stderr is connected to systemd journal, and hence whether
structured events can be written to the journal.

Based on https://systemd.io/JOURNAL_NATIVE_PROTOCOL/#automatic-protocol-upgrading
"""

import os
import sys


def stderr_is_systemd_journal() -> bool:
    try:
        journal_stream = os.environ['JOURNAL_STREAM']
    except KeyError:
        return False

    try:
        part1, part2 = journal_stream.split(':', maxsplit=1)
    except ValueError:
        return False

    try:
        journal_dev = int(part1, 10)
        journal_inode = int(part2, 10)
    except ValueError:
        return False

    stderr_stat = os.fstat(sys.stderr.fileno())
    return (stderr_stat.st_dev, stderr_stat.st_ino) == (journal_dev, journal_inode)


def main():
    stderr_stat = os.fstat(sys.stderr.fileno())
    print(f"{stderr_stat.st_dev=} {stderr_stat.st_ino=}")
    print(f"{os.environ.get('JOURNAL_STREAM')=}")
    print(f"{stderr_is_systemd_journal()=}")

    if stderr_is_systemd_journal():
        print("Hello systemd journal")


if __name__ == '__main__':
    main()

journal_detect.go

package main

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

// StdErrIsJournal returns true if standard error is connected to the systemd
// journal, false otherwise
func StderrIsJournal() (bool, error) {
	// When a processes is started by systemd and standard error is connected
	// to the journal, then $JOURNAL_STREAM contains the device and inode
	// number of the standard error file descriptor (e.g. "123:789").
	// https://www.freedesktop.org/software/systemd/man/systemd.exec.html#%24JOURNAL_STREAM
	js := os.Getenv("JOURNAL_STREAM")
	parts := strings.SplitN(js, ":", 2)
	if len(parts) != 2 {
		return false, errors.New(js)
	}

	journalDev, err := strconv.ParseUint(parts[0], 10, 64)
	if err != nil {
		return false, err
	}

	journalIno, err := strconv.ParseUint(parts[1], 10, 64)
	if err != nil {
		return false, err
	}

	stat, err := os.Stderr.Stat()
	if err != nil {
		return false, err
	}
	statRaw, ok := stat.Sys().(*syscall.Stat_t)
	if !ok {
		return false, errors.New("couldn't convert stat")
	}

	return journalDev == statRaw.Dev && journalIno == statRaw.Ino, nil
}

func main() {
	x, y := StderrIsJournal()
	fmt.Println(x, y)
}

Invoked from a terminal the script correctly detects that stderr is not attached to the journal. The journal is available, but stderr is a different file.

:~/src$ ./journal-detect.py 
stderr_stat.st_dev=24 stderr_stat.st_ino=3
os.environ.get('JOURNAL_STREAM')='8:57469'
stderr_is_systemd_journal()=False

Invoked as a (temporary) systemd unit the script correctly detects that stderr is the journal, and it can adjust behaviour as desired.

:~/src$ systemd-run --user --wait ./journal-detect.py
Running as unit: run-u240.service
Finished with result: success
Main processes terminated with: code=exited/status=0
Service runtime: 27ms
:~/src$ journalctl --no-pager --user -u run-u240.service
-- Logs begin at Fri 2020-05-15 13:14:09 BST, end at Tue 2021-06-08 23:55:16 BST. --
Jun 08 23:55:16 martha systemd[2428]: Started /home/alex/src/./journal-detect.py.
Jun 08 23:55:16 martha journal-detect.py[6989]: stderr_stat.st_dev=8 stderr_stat.st_ino=94208
Jun 08 23:55:16 martha journal-detect.py[6989]: os.environ.get('JOURNAL_STREAM')='8:94208'
Jun 08 23:55:16 martha journal-detect.py[6989]: stderr_is_systemd_journal()=True
Jun 08 23:55:16 martha journal-detect.py[6989]: Hello systemd journal
Jun 08 23:55:16 martha systemd[2428]: run-u240.service: Succeeded.

:~/tmp$ go build journal_detect.go 
:~/tmp$ ./journal_detect
false <nil>

:~/tmp$ systemd-run --user ./journal_detect
Running as unit: run-r4668e6b91b744684a0f6ac2bf75defee.service
:~/tmp$ journalctl --user -u run-r4668e6b91b744684a0f6ac2bf75defee.service
-- Logs begin at Fri 2020-05-15 13:14:09 BST, end at Thu 2021-06-17 22:26:20 BST. --
Jun 17 22:26:20 martha systemd[2428]: Started /home/alex/tmp/./main.
Jun 17 22:26:20 martha journal_detect[293679]: true <nil>
Jun 17 22:26:20 martha systemd[2428]: run-r4668e6b91b744684a0f6ac2bf75defee.service: Succeeded.