man: Add some notes about variable $prefix for StateDirectory=

tl;dr: It isn’t supported.

Wording by Zbigniew Jędrzejewski-Szmek.

See https://twitter.com/pid_eins/status/1102639279614906369 and
https://gitlab.freedesktop.org/libfprint/fprintd/merge_requests/5#note_125536
onwards.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

diff --git a/man/systemd.exec.xml b/man/systemd.exec.xml
index bacd539b152aa3b4edadcc2d96b46351b4fb9fc4..56a029a82eeb95e63d0786b501369b29013de28d 100644 (file)
--- a/man/systemd.exec.xml
+++ b/man/systemd.exec.xml
@@ -972,6 +972,15 @@ CapabilityBoundingSet=~CAP_B CAP_C</programlisting>
         configuration or lifetime guarantees, please consider using
         <citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
 
+        <para>The directories defined by these options are always created under the standard paths used by systemd
+        (<filename>/var</filename>, <filename>/run</filename>, <filename>/etc</filename>, …). If the service needs
+        directories in a different location, a different mechanism has to be used to create them.</para>
+
+        <para><citerefentry><refentrytitle>tmpfiles.d</refentrytitle><manvolnum>5</manvolnum></citerefentry> provides
+        functionality that overlaps with these options. Using these options is recommended, because the lifetime of
+        the directories is tied directly to the lifetime of the unit, and it is not necessary to ensure that the
+        <filename>tmpfiles.d</filename> configuration is executed before the unit is started.</para>
+
         <para>Example: if a system service unit has the following,
         <programlisting>RuntimeDirectory=foo/bar baz</programlisting>
         the service manager creates <filename>/run/foo</filename> (if it does not exist),