systemd 257 will now set `DEBUG_INVOCATION=1` in the execution context
of services which need to be debugged — for example if they’ve failed
and been restarted (and `RestartMode=debug`).
If we handle `DEBUG_INVOCATION=1` as being equivalent to
`G_MESSAGES_DEBUG=all`, then any processes using GLib’s default log
writer function (or `g_log_writer_default_would_drop()`) automatically
gains support for this feature.
See https://mastodon.social/@pid_eins/113548825942383777
Signed-off-by: Philip Withnall <pwithnall@gnome.org>
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