man: add specifiers section to directives index

The hack with getparent().txt is not very pretty, but the whole
thing seems to work well enough. It is useful to figure out whihc
specifiers are supported where.

man/directives-template.xml

@@ -135,6 +135,14 @@
     <variablelist id='miscellaneous' />
   </refsect1>
 
+  <refsect1>
+    <title>Specifiers</title>
+
+    <para>Short strings which are substituted in configuration directives.</para>
+
+    <variablelist id='specifiers' />
+  </refsect1>
+
   <refsect1>
     <title>Files and directories</title>
 

man/systemd.dnssd.xml

@@ -75,7 +75,7 @@
             <para>An instance name of the network service as defined in the section 4.1.1 of <ulink
             url="https://tools.ietf.org/html/rfc6763">RFC 6763</ulink>, e.g. <literal>webserver</literal>.</para>
             <para>The option supports simple specifier expansion. The following expansions are understood:</para>
-            <table>
+            <table class='specifiers'>
               <title>Specifiers available</title>
               <tgroup cols='3' align='left' colsep='1' rowsep='1'>
                 <colspec colname="spec" />

man/systemd.unit.xml

@@ -1695,7 +1695,7 @@
     and resolvable for the setting to be valid. The following
     specifiers are understood:</para>
 
-    <table>
+    <table class='specifiers'>
       <title>Specifiers available in unit files</title>
       <tgroup cols='3' align='left' colsep='1' rowsep='1'>
         <colspec colname="spec" />
@@ -1710,7 +1710,8 @@
         </thead>
         <tbody>
           <row>
-            <!-- We do not use the common definition from standard-specifiers.xml here since it includes a reference onto our own man page, which would make the rendered version self-referential. -->
+            <!-- We do not use the common definition from standard-specifiers.xml here since it includes a
+                 reference onto our own man page, which would make the rendered version self-referential. -->
             <entry><literal>%a</literal></entry>
             <entry>Architecture</entry>
             <entry>A short string identifying the architecture of the local system. A string such as <constant>x86</constant>, <constant>x86-64</constant> or <constant>arm64</constant>. See the architectures defined for <varname>ConditionArchitecture=</varname> above for a full list.</entry>
@@ -1740,7 +1741,8 @@
 Note that this setting is <emphasis>not</emphasis> influenced by the <varname>User=</varname> setting configurable in the [Service] section of the service unit.</entry>
           </row>
           <row>
-            <!-- We do not use the common definition from standard-specifiers.xml here since we want a slightly more verbose explanation here, referring to the reload cycle. -->
+            <!-- We do not use the common definition from standard-specifiers.xml here since we want a
+                 slightly more verbose explanation here, referring to the reload cycle. -->
             <entry><literal>%H</literal></entry>
             <entry>Host name</entry>
             <entry>The hostname of the running system at the point in time the unit configuration is loaded.</entry>

man/sysusers.d.xml

@@ -237,7 +237,7 @@ r     -        500-900
     <para>Specifiers can be used in the "Name", "ID", "GECOS", "Home directory", and "Shell" fields.
     An unknown or unresolvable specifier is treated as invalid configuration.
     The following expansions are understood:</para>
-      <table>
+      <table class='specifiers'>
         <title>Specifiers available</title>
         <tgroup cols='3' align='left' colsep='1' rowsep='1'>
           <colspec colname="spec" />

man/tmpfiles.d.xml

@@ -618,7 +618,7 @@ w- /proc/sys/vm/swappiness - - - - 10</programlisting></para>
     <para>Specifiers can be used in the "path" and "argument" fields.
     An unknown or unresolvable specifier is treated as invalid configuration.
     The following expansions are understood:</para>
-      <table>
+      <table class='specifiers'>
         <title>Specifiers available</title>
         <tgroup cols='3' align='left' colsep='1' rowsep='1'>
           <colspec colname="spec" />

tools/make-directive-index.py

@@ -87,6 +87,15 @@ def _extract_directives(directive_groups, formatting, page):
         storfile[name.text].append((pagename, section))
         formatting[name.text] = name
 
+    storfile = directive_groups['specifiers']
+    for name in t.iterfind(".//table[@class='specifiers']//entry/literal"):
+        if name.text[0] != '%' or name.getparent().text is not None:
+            continue
+        if name.attrib.get('index') == 'false':
+            continue
+        storfile[name.text].append((pagename, section))
+        formatting[name.text] = name
+
 def _make_section(template, name, directives, formatting):
     varlist = template.find(".//*[@id='{}']".format(name))
     for varname, manpages in sorted(directives.items()):