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man: explain ConditionNeedsUpdate a bit more

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We were effectively doing all post-upgrade scripts twice in Fedora. We got this
wrong, so it's likely other people will get it wrong too. So let's explain
what is actually needed to make this work, but also when it's not useful.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2021-06-07 12:55:55 +02:00 committed by Lennart Poettering
parent 16e09d51a7
commit f75420a43a
1 changed files with 14 additions and 2 deletions
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16
man/systemd.unit.xml
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@ -1374,11 +1374,23 @@
<para>If the <varname>systemd.condition-needs-update=</varname> option is specified on the kernel
command line (taking a boolean), it will override the result of this condition check, taking
precedence over any file modification time checks. If it is used
precedence over any file modification time checks. If the kernel command line option is used,
<filename>systemd-update-done.service</filename> will not have immediate effect on any following
<varname>ConditionNeedsUpdate=</varname> checks, until the system is rebooted where the kernel
command line option is not specified anymore.</para>
</listitem>
<para>Note that to make this scheme effective, the timestamp of <filename>/usr/</filename> should
be explicitly updated after its contents are modified. The kernel will automatically update
modification timestamp on a directory only when immediate children of a directory are modified; an
modification of nested files will not automatically result in mtime of <filename>/usr/</filename>
being updated.</para>
<para>Also note that if the update method includes a call to execute appropriate post-update steps
itself, it should not touch the timestamp of <filename>/usr/</filename>. In a typical distribution
packaging scheme, packages will do any required update steps as part of the installation or
upgrade, to make package contents immediately usable. <varname>ConditionNeedsUpdate=</varname>
should be used with other update mechanisms where such an immediate update does not
happen.</para></listitem>
</varlistentry>
<varlistentry>