man: fix invalid description of template handling in WantedBy=

We don't need to talk about Alias=. The approach of using Alias= to enable
units is still supported, but hasn't been advertised as the way to do thing
for many years. Using it as an explanation is just confusing.

Also, the description of templated units did not take DefaultInstance=
into account. It is updated and extended.

man/systemd.unit.xml

@@ -1904,34 +1904,31 @@
         <term><varname>WantedBy=</varname></term>
         <term><varname>RequiredBy=</varname></term>
 
-        <listitem><para>This option may be used more than once, or a
-        space-separated list of unit names may be given. A symbolic
-        link is created in the <filename>.wants/</filename> or
-        <filename>.requires/</filename> directory of each of the
-        listed units when this unit is installed by <command>systemctl
-        enable</command>. This has the effect that a dependency of
-        type <varname>Wants=</varname> or <varname>Requires=</varname>
-        is added from the listed unit to the current unit. The primary
-        result is that the current unit will be started when the
-        listed unit is started. See the description of
-        <varname>Wants=</varname> and <varname>Requires=</varname> in
-        the [Unit] section for details.</para>
+        <listitem><para>This option may be used more than once, or a space-separated list of unit names may
+        be given. A symbolic link is created in the <filename>.wants/</filename> or
+        <filename>.requires/</filename> directory of each of the listed units when this unit is installed by
+        <command>systemctl enable</command>. This has the effect of a dependency of type
+        <varname>Wants=</varname> or <varname>Requires=</varname> being added from the listed unit to the
+        current unit. The primary result is that the current unit will be started when the listed unit is
+        started, see the description of <varname>Wants=</varname> and <varname>Requires=</varname> in the
+        [Unit] section for details.</para>
 
-        <para><command>WantedBy=foo.service</command> in a service
-        <filename>bar.service</filename> is mostly equivalent to
-        <command>Alias=foo.service.wants/bar.service</command> in the
-        same file. In case of template units listing non template units,
-        <command>systemctl enable</command> must be called with an
-        instance name, and this instance will be added to the
-        <filename>.wants/</filename> or
-        <filename>.requires/</filename> list of the listed unit. E.g.
-        <command>WantedBy=getty.target</command> in a service
-        <filename>getty@.service</filename> will result in
-        <command>systemctl enable getty@tty2.service</command>
-        creating a
-        <filename>getty.target.wants/getty@tty2.service</filename>
-        link to <filename>getty@.service</filename>.
-        </para></listitem>
+        <para>In case of template units listing non template units, the listing unit must have
+        <varname>DefaultInstance=</varname> set, or <command>systemctl enable</command> must be called with
+        an instance name. The instance (default or specified) will be added to the
+        <filename>.wants/</filename> or <filename>.requires/</filename> list of the listed unit. For example,
+        <command>WantedBy=getty.target</command> in a service <filename>getty@.service</filename> will result
+        in <command>systemctl enable getty@tty2.service</command> creating a
+        <filename>getty.target.wants/getty@tty2.service</filename> link to
+        <filename>getty@.service</filename>. This also applies to listing specific instances of templated
+        units: this specific instance will gain the dependency. A template unit may also list a template
+        unit, in which case a generic dependency will be added where each instance of the listing unit will
+        have a dependency on an instance of the listed template with the same instance value. For example,
+        <command>WantedBy=container@.target</command> in a service <filename>monitor@.service</filename> will
+        result in <command>systemctl enable monitor@.service</command> creating a
+        <filename>container@.target.wants/monitor@.service</filename> link to
+        <filename>monitor@.service</filename>, which applies to all instances of
+        <filename>container@.target</filename>.</para></listitem>
       </varlistentry>
 
       <varlistentry>

test/test-systemctl-enable.sh

@@ -324,6 +324,31 @@ test ! -h "$root/etc/systemd/system/services.target.wants/templ1@333.service"
 test ! -h "$root/etc/systemd/system/services.target.wants/templ1@one.service"
 test ! -h "$root/etc/systemd/system/services.target.wants/templ1@two.service"
 
+: -------template enablement for another template-------------
+cat >"$root/etc/systemd/system/templ2@.service" <<EOF
+[Install]
+RequiredBy=another-template@.target
+EOF
+
+"$systemctl" --root="$root" enable 'templ2@.service'
+islink "$root/etc/systemd/system/another-template@.target.requires/templ2@.service" "/etc/systemd/system/templ2@.service"
+
+"$systemctl" --root="$root" enable 'templ2@two.service'
+islink "$root/etc/systemd/system/another-template@.target.requires/templ2@.service" "/etc/systemd/system/templ2@.service"
+islink "$root/etc/systemd/system/another-template@.target.requires/templ2@two.service" "/etc/systemd/system/templ2@.service"
+
+"$systemctl" --root="$root" disable 'templ2@other.service'
+islink "$root/etc/systemd/system/another-template@.target.requires/templ2@.service" "/etc/systemd/system/templ2@.service"
+islink "$root/etc/systemd/system/another-template@.target.requires/templ2@two.service" "/etc/systemd/system/templ2@.service"
+
+"$systemctl" --root="$root" disable 'templ2@two.service'
+islink "$root/etc/systemd/system/another-template@.target.requires/templ2@.service" "/etc/systemd/system/templ2@.service"
+test ! -h "$root/etc/systemd/system/another-template@.target.requires/templ2@two.service"
+
+"$systemctl" --root="$root" disable 'templ2@.service'
+test ! -h "$root/etc/systemd/system/another-template@.target.requires/templ2@.service"
+test ! -h "$root/etc/systemd/system/another-template@.target.requires/templ2@two.service"
+
 : -------aliases w/ and w/o instance--------------------------
 test ! -e "$root/etc/systemd/system/link4.service"
 cat >"$root/etc/systemd/system/link4.service" <<EOF
@@ -529,6 +554,30 @@ check_alias % '%' && { echo "Expected failure because % is not legal in unit nam
 
 check_alias z 'z' && { echo "Expected failure because %z is not known" >&2; exit 1; }
 
+: -------specifiers in WantedBy-------------------------------
+# We don't need to repeat all the tests. Let's do a basic check that specifier
+# expansion is performed.
+
+cat >"$root/etc/systemd/system/some-some-link7.socket" <<EOF
+[Install]
+WantedBy=target@%p.target
+WantedBy=another-target@.target
+RequiredBy=target2@%p.target
+RequiredBy=another-target2@.target
+EOF
+
+"$systemctl" --root="$root" enable 'some-some-link7.socket'
+islink "$root/etc/systemd/system/target@some-some-link7.target.wants/some-some-link7.socket" "/etc/systemd/system/some-some-link7.socket"
+islink "$root/etc/systemd/system/another-target@.target.wants/some-some-link7.socket" "/etc/systemd/system/some-some-link7.socket"
+islink "$root/etc/systemd/system/target2@some-some-link7.target.requires/some-some-link7.socket" "/etc/systemd/system/some-some-link7.socket"
+islink "$root/etc/systemd/system/another-target2@.target.requires/some-some-link7.socket" "/etc/systemd/system/some-some-link7.socket"
+
+"$systemctl" --root="$root" disable 'some-some-link7.socket'
+test ! -h "$root/etc/systemd/system/target@some-some-link7.target.wants/some-some-link7.socket"
+test ! -h "$root/etc/systemd/system/another-target@.target.wants/some-some-link7.socket"
+test ! -h "$root/etc/systemd/system/target2@some-some-link7.target.requires/some-some-link7.socket"
+test ! -h "$root/etc/systemd/system/another-target2@.target.requires/some-some-link7.socket"
+
 # TODO: repeat the tests above for presets
 
 : -------SYSTEMD_OS_RELEASE relative to root------------------