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.

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
+    <para>Unit settings of device units may either be configured via unit files, or directly from the udev
-    unit files, or directly from the udev database (which is
+    database. The following udev device properties are understood by the service manager:</para>
-    recommended). The following udev device properties are understood
-    by systemd:</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
+        <listitem><para>Adds dependencies of type <varname>Wants=</varname> from the device unit to the specified
-        <varname>Wants</varname> from the device unit to all listed
+        units. <varname>SYSTEMD_WANTS=</varname> is read by the system service manager,
-        units. The first form is used by the system systemd instance,
+        <varname>SYSTEMD_USER_WANTS=</varname> by user service manager instances. These properties may be used to
-        the second by user systemd instances. Those settings may be
+        activate arbitrary units when a specific device becomes available.</para>
-        used to activate arbitrary units when a specific device
-        becomes available.</para>
 
-        <para>Note that this and the other tags are not taken into
+        <para>Note that this and the other udev device properties are not taken into account unless the device is
-        account unless the device is tagged with the
+        tagged with the <literal>systemd</literal> tag in the udev database, because otherwise the device is not
-        <literal>systemd</literal> string in the udev database,
+        exposed as a systemd unit (see above).</para>
-        because otherwise the device is not exposed as a systemd unit
-        (see above).</para>
 
-        <para>Note that systemd will only act on
+        <para>Note that systemd will only act on <varname>Wants=</varname> dependencies when a device first becomes
-        <varname>Wants</varname> dependencies when a device first
+        active. It will not act on them if they are added to devices that are already active. Use
-        becomes active. It will not act on them if they are added to
+        <varname>SYSTEMD_READY=</varname> (see below) to configure when a udev device shall be considered active, and
-        devices that are already active. Use
+        thus when to trigger the dependencies.</para>
-        <varname>SYSTEMD_READY=</varname> (see below) to influence on
+
-        which udev event to trigger the dependencies.
+        <!-- Note that we don't document here that we actually apply unit_name_mangle() to all specified names, since
-        </para></listitem>
+             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
+        <listitem><para>If set to 0, systemd will consider this device unplugged even if it shows up in the udev
-        unplugged even if it shows up in the udev tree. If this
+        tree. If this property is unset or set to 1, the device will be considered plugged if it is visible in the udev
-        property is unset or set to 1, the device will be considered
+        tree.</para>
-        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
+        <para>This option is useful for devices that initially show up in an uninitialized state in the tree, and for
-        show up in an uninitialized state in the tree, and for which a
+        which a <literal>changed</literal> event is generated the moment they are fully set up. Note that
-        <literal>changed</literal> event is generated the moment they
+        <varname>SYSTEMD_WANTS=</varname> (see above) is not acted on as long as <varname>SYSTEMD_READY=0</varname> is
-        are fully set up. Note that <varname>SYSTEMD_WANTS=</varname>
+        set for a device.</para></listitem>
-        (see above) is not acted on as long as
-        <varname>SYSTEMD_READY=0</varname> is set for a
-        device.</para></listitem>
       </varlistentry>
 
       <varlistentry>

src/core/device.c

@@ -277,11 +277,28 @@ static int device_add_udev_wants(Unit *u, struct udev_device *dev) {
                 if (r == -ENOMEM)
                         return log_oom();
                 if (r < 0)
-                        return log_unit_error_errno(u, r, "Failed to add parse %s: %m", property);
+                        return log_unit_error_errno(u, r, "Failed to parse property %s with value %s: %m", property, wants);
 
-                r = unit_name_mangle(word, UNIT_NAME_NOGLOB, &k);
+                if (unit_name_is_valid(word, UNIT_NAME_TEMPLATE) && DEVICE(u)->sysfs) {
-                if (r < 0)
+                        _cleanup_free_ char *escaped = NULL;
-                        return log_unit_error_errno(u, r, "Failed to mangle unit name \"%s\": %m", word);
+
+                        /* If the unit name is specified as template, then automatically fill in the sysfs path of the
+                         * device as instance name, properly escaped. */
+
+                        r = unit_name_path_escape(DEVICE(u)->sysfs, &escaped);
+                        if (r < 0)
+                                return log_unit_error_errno(u, r, "Failed to escape %s: %m", DEVICE(u)->sysfs);
+
+                        r = unit_name_replace_instance(word, escaped, &k);
+                        if (r < 0)
+                                return log_unit_error_errno(u, r, "Failed to build %s instance of template %s: %m", escaped, word);
+                } else {
+                        /* If this is not a template, then let's mangle it so, that it becomes a valid unit name. */
+
+                        r = unit_name_mangle(word, UNIT_NAME_NOGLOB, &k);
+                        if (r < 0)
+                                return log_unit_error_errno(u, r, "Failed to mangle unit name \"%s\": %m", word);
+                }
 
                 r = unit_add_dependency_by_name(u, UNIT_WANTS, k, NULL, true, UNIT_DEPENDENCY_UDEV);
                 if (r < 0)