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man: try to make clearer that /var/ is generally not available in /usr/lib/systemd/system-shutdown/ callouts

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I made the mistake to look into what is installed into
/usr/lib/systemd/system-shutdown/ on Fedora. fwdupd among other things
assumes /var/ is available from these callouts, though it is not in the
general case.

Hence, let's emphasize this in the documentation a bit more.
This commit is contained in:
Lennart Poettering 2023-04-20 10:03:18 +02:00
parent 4d49f44f0f
commit e002b8a28a
1 changed files with 25 additions and 32 deletions
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57
man/systemd-poweroff.service.xml
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@ -36,41 +36,34 @@
<refsect1>
<title>Description</title>
<para><filename>systemd-poweroff.service</filename> is a system
service that is pulled in by <filename>poweroff.target</filename> and
is responsible for the actual system power-off operation. Similarly,
<filename>systemd-halt.service</filename> is pulled in by
<filename>halt.target</filename>,
<filename>systemd-reboot.service</filename> by
<filename>reboot.target</filename> and
<filename>systemd-kexec.service</filename> by
<filename>kexec.target</filename> to execute the respective
actions.</para>
<para><filename>systemd-poweroff.service</filename> is a system service that is pulled in by
<filename>poweroff.target</filename> and is responsible for the actual system power-off
operation. Similarly, <filename>systemd-halt.service</filename> is pulled in by
<filename>halt.target</filename>, <filename>systemd-reboot.service</filename> by
<filename>reboot.target</filename> and <filename>systemd-kexec.service</filename> by
<filename>kexec.target</filename> to execute the respective actions.</para>
<para>When these services are run, they ensure that PID 1 is
replaced by the
<filename>/usr/lib/systemd/systemd-shutdown</filename> tool which
is then responsible for the actual shutdown. Before shutting down,
this binary will try to unmount all remaining file systems,
disable all remaining swap devices, detach all remaining storage
devices and kill all remaining processes.</para>
<para>When these services are run, they ensure that PID 1 is replaced by the
<filename>/usr/lib/systemd/systemd-shutdown</filename> tool which is then responsible for the actual
shutdown. Before shutting down, this binary will try to unmount all remaining file systems (or at least
remount them read-only), disable all remaining swap devices, detach all remaining storage devices and
kill all remaining processes.</para>
<para>It is necessary to have this code in a separate binary
because otherwise rebooting after an upgrade might be broken — the
running PID 1 could still depend on libraries which are not
available any more, thus keeping the file system busy, which then
cannot be re-mounted read-only.</para>
<para>It is necessary to have this code in a separate binary because otherwise rebooting after an upgrade
might be broken — the running PID 1 could still depend on libraries which are not available any more,
thus keeping the file system busy, which then cannot be re-mounted read-only.</para>
<para>Immediately before executing the actual system
power-off/halt/reboot/kexec <filename>systemd-shutdown</filename>
will run all executables in
<filename>/usr/lib/systemd/system-shutdown/</filename> and pass
one arguments to them: either <literal>poweroff</literal>,
<literal>halt</literal>, <literal>reboot</literal>, or
<literal>kexec</literal>, depending on the chosen action. All
executables in this directory are executed in parallel, and
execution of the action is not continued before all executables
finished.</para>
<para>Shortly before executing the actual system power-off/halt/reboot/kexec
<filename>systemd-shutdown</filename> will run all executables in
<filename>/usr/lib/systemd/system-shutdown/</filename> and pass one arguments to them: either
<literal>poweroff</literal>, <literal>halt</literal>, <literal>reboot</literal>, or
<literal>kexec</literal>, depending on the chosen action. All executables in this directory are executed
in parallel, and execution of the action is not continued before all executables finished. Note that
these executables are run <emphasis>after</emphasis> all services have been shut down, and after most
mounts have been detached (the root file system as well as <filename>/run/</filename> and various API
file systems are still around though). This means any programs dropped into this directory must be
prepared to run in such a limited execution environment and not rely on external services or hierarchies
such as <filename>/var/</filename> to be around (or writable).</para>
<para>Note that <filename>systemd-poweroff.service</filename> (and the related units) should never be
executed directly. Instead, trigger system shutdown with a command such as <literal>systemctl