core: when a unit template is specified in SYSTEMD_WANTS=, instantiate it with sysfs path

This should make cases like the user's setup in #7109 a lot easier to
handle, as in that case we'll do the right escaping automatically.

1 files changed, 33 insertions, 38 deletions

diff --git a/man/systemd.device.xml b/man/systemd.device.xml
index 6edf1090d0..e121e151bc 100644
--- a/man/systemd.device.xml
+++ b/man/systemd.device.xml
@@ -112,35 +112,36 @@
   <refsect1>
     <title>The udev Database</title>
 
-    <para>The settings of device units may either be configured via
-    unit files, or directly from the udev database (which is
-    recommended). The following udev device properties are understood
-    by systemd:</para>
+    <para>Unit settings of device units may either be configured via unit files, or directly from the udev
+    database. The following udev device properties are understood by the service manager:</para>
 
     <variablelist class='udev-directives'>
       <varlistentry>
         <term><varname>SYSTEMD_WANTS=</varname></term>
         <term><varname>SYSTEMD_USER_WANTS=</varname></term>
-        <listitem><para>Adds dependencies of type
-        <varname>Wants</varname> from the device unit to all listed
-        units. The first form is used by the system systemd instance,
-        the second by user systemd instances. Those settings may be
-        used to activate arbitrary units when a specific device
-        becomes available.</para>
-
-        <para>Note that this and the other tags are not taken into
-        account unless the device is tagged with the
-        <literal>systemd</literal> string in the udev database,
-        because otherwise the device is not exposed as a systemd unit
-        (see above).</para>
-
-        <para>Note that systemd will only act on
-        <varname>Wants</varname> dependencies when a device first
-        becomes active. It will not act on them if they are added to
-        devices that are already active. Use
-        <varname>SYSTEMD_READY=</varname> (see below) to influence on
-        which udev event to trigger the dependencies.
-        </para></listitem>
+        <listitem><para>Adds dependencies of type <varname>Wants=</varname> from the device unit to the specified
+        units. <varname>SYSTEMD_WANTS=</varname> is read by the system service manager,
+        <varname>SYSTEMD_USER_WANTS=</varname> by user service manager instances. These properties may be used to
+        activate arbitrary units when a specific device becomes available.</para>
+
+        <para>Note that this and the other udev device properties are not taken into account unless the device is
+        tagged with the <literal>systemd</literal> tag in the udev database, because otherwise the device is not
+        exposed as a systemd unit (see above).</para>
+
+        <para>Note that systemd will only act on <varname>Wants=</varname> dependencies when a device first becomes
+        active. It will not act on them if they are added to devices that are already active. Use
+        <varname>SYSTEMD_READY=</varname> (see below) to configure when a udev device shall be considered active, and
+        thus when to trigger the dependencies.</para>
+
+        <!-- Note that we don't document here that we actually apply unit_name_mangle() to all specified names, since
+             that's kinda ugly, and people should instead specify correctly escaped names -->
+
+        <para>The specified property value should be a space-separated list of valid unit names. If a unit template
+        name is specified (that is, a unit name containing an <literal>@</literal> character indicating a unit name to
+        use for multiple instantiation, but with an empty instance name following the <literal>@</literal>), it will be
+        automatically instantiated by the device's <literal>sysfs</literal> path (that is: the path is escaped and
+        inserted as instance name into the template unit name). This is useful in order to instantiate a specific
+        template unit once for each device that appears and matches specific properties.</para></listitem>
       </varlistentry>
 
       <varlistentry>
@@ -152,20 +153,14 @@
 
       <varlistentry>
         <term><varname>SYSTEMD_READY=</varname></term>
-        <listitem><para>If set to 0, systemd will consider this device
-        unplugged even if it shows up in the udev tree. If this
-        property is unset or set to 1, the device will be considered
-        plugged if it is visible in the udev tree. This property has
-        no influence on the behavior when a device disappears from the
-        udev tree.</para>
-
-        <para>This option is useful to support devices that initially
-        show up in an uninitialized state in the tree, and for which a
-        <literal>changed</literal> event is generated the moment they
-        are fully set up. Note that <varname>SYSTEMD_WANTS=</varname>
-        (see above) is not acted on as long as
-        <varname>SYSTEMD_READY=0</varname> is set for a
-        device.</para></listitem>
+        <listitem><para>If set to 0, systemd will consider this device unplugged even if it shows up in the udev
+        tree. If this property is unset or set to 1, the device will be considered plugged if it is visible in the udev
+        tree.</para>
+
+        <para>This option is useful for devices that initially show up in an uninitialized state in the tree, and for
+        which a <literal>changed</literal> event is generated the moment they are fully set up. Note that
+        <varname>SYSTEMD_WANTS=</varname> (see above) is not acted on as long as <varname>SYSTEMD_READY=0</varname> is
+        set for a device.</para></listitem>
       </varlistentry>
 
       <varlistentry>