test: workaroud flaky TEST-53-TIMER.restart-trigger against journald cgroup attributi...

test: workaroud flaky TEST-53-TIMER.restart-trigger against journald cgroup attribution race

The restart-trigger subtest occasionally fails on CI with:

    + assert_eq 0 1
    FAIL: expected: '1' actual: '0'

even though the timer fires correctly and the echo message is in fact
written to the journal. The failure happens because the test relies on
`journalctl --unit=$UNIT_NAME` to find the message, and that filter is
based on the cgroup journald looks up for the writer PID at the time
the stdout message is received.

For very short-lived processes spawned via systemd-executor (like
`echo`), that lookup is racy: the writer's `/proc/$PID/cgroup` can
still resolve to `/init.scope` (systemd-executor's own cgroup) rather
than the service's cgroup, so the message ends up attributed to
`init.scope` and `--unit=` filtering misses it.

    __CURSOR=s=6f90ff5b6a0e47c3a527a9b4892af965;i=f8ed;b=3dad0cc689a04781879e4dd846d24432;m=17703dc;t=6513be1be2506;x=8d3009a687724b5e
    __REALTIME_TIMESTAMP=1778167492519174
    __MONOTONIC_TIMESTAMP=24576988
    __SEQNUM=63725
    __SEQNUM_ID=6f90ff5b6a0e47c3a527a9b4892af965
    _BOOT_ID=3dad0cc689a04781879e4dd846d24432
    _HOSTNAME=H
    PRIORITY=6
    SYSLOG_FACILITY=3
    _UID=0
    _GID=0
    _CAP_EFFECTIVE=1ffffffffff
    _SYSTEMD_CGROUP=/init.scope
    _SYSTEMD_UNIT=init.scope
    _SYSTEMD_SLICE=-.slice
    _EXE=/usr/lib/systemd/systemd-executor
    _TRANSPORT=stdout
    _COMM=18
    _MACHINE_ID=89ef83adc0bc4a33a83a227201b57203
    _RUNTIME_SCOPE=system
    _PID=816
    _CMDLINE=/usr/lib/systemd/systemd-executor --deserialize 50 --log-level debug,console:info --log-target journal-or-kmsg
    _STREAM_ID=8e8e4166c99e40afaa58bcd04a50a7f4
    SYSLOG_IDENTIFIER=echo
    MESSAGE=Hello from timer 29581

Note _SYSTEMD_UNIT=init.scope / _SYSTEMD_CGROUP=/init.scope on the
echo output: this is what causes `--unit=timer-restart-14362` to
return 0 hits. The test failure logs from the same run confirm this:

    + JOURNAL_TS=1778160292
    + journalctl -p info --since=@1778160292 --unit=timer-restart-14362 '--grep=Hello from timer 29581'
    -- No entries --
    + systemctl restart timer-restart-14362.timer
    ...
    + date '--set=+2 hours'
    Thu May  7 15:24:52 UTC 2026
    + sleep 1
    ...
    echo[816]: Hello from timer 29581
    ...
    ++ journalctl -q -p info --since=@1778160292 --unit=timer-restart-14362 '--grep=Hello from timer 29581'
    ++ wc -l
    + assert_eq 0 1
    FAIL: expected: '1' actual: '0'

For comparison, in a passing local run the same message is attributed
correctly to the service unit (_SYSTEMD_UNIT=timer-restart-24147.service),
so `--unit=` matches.

Work around the underlying journald race in the test by setting an
explicit `SyslogIdentifier=` on the service and matching with `-t` plus
the unique grep pattern: `SyslogIdentifier` is carried over the stdout
stream protocol and is not affected by the cgroup lookup race.

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>