nixos: Split paras by \n\n in option descriptions

What annoyed me for a long time was the fact, that in order to break
into a new paragraph, you need to insert </para><para> in the
description attribute of an option.

Now we will automatically create <para/> elements for every block that
is separated by two consecutive newlines.

I first tried to do this within options-to-docbook.xsl, but it turns
out[1] that this isn't directly possible with XSLT 1.0, so I added
another XSLT file that postprocesses the option descriptions that are
now enclosed in <nixos:option-description/> by options-to-docbook.xsl.

The splitting itself is a bit more involved, because we can't simply
split on every \n\n because we'd also split text nodes of elements, for
example:

  <screen><![CDATA[

    one line

    another one

  ]]></screen>

This would create one <para/> element for "one line" and another for
"another line", which we obviously don't want because <screen/> is used
to display verbatim contents of what a user is seeing on the screen.

So what we do instead is splitting *only* the top-level text nodes
within the outermost <para/> and leave all elements as-is. If there are
more than one <para/> elements at the top-level, we simply don't process
it at all, because the description then already contains </para><para>.

https://www.mhonarc.org/archive/html/xsl-list/2012-09/msg00319.html

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @edolstra, @domenkozar

nixos/doc/manual/default.nix

@@ -90,7 +90,9 @@ let
     fi
     ${buildPackages.libxslt.bin}/bin/xsltproc \
       --stringparam revision '${revision}' \
-      -o $out ${./options-to-docbook.xsl} $optionsXML
+      -o intermediate.xml ${./options-to-docbook.xsl} $optionsXML
+    ${buildPackages.libxslt.bin}/bin/xsltproc \
+      -o "$out" ${./postprocess-option-descriptions.xsl} intermediate.xml
   '';
 
   sources = lib.sourceFilesBySuffices ./. [".xml"];

nixos/doc/manual/options-to-docbook.xsl

@@ -4,6 +4,7 @@
                 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                 xmlns:str="http://exslt.org/strings"
                 xmlns:xlink="http://www.w3.org/1999/xlink"
+                xmlns:nixos="tag:nixos.org"
                 xmlns="http://docbook.org/ns/docbook"
                 extension-element-prefixes="str"
                 >
@@ -30,10 +31,12 @@
 
             <listitem>
 
+              <nixos:option-description>
                 <para>
                   <xsl:value-of disable-output-escaping="yes"
                                 select="attr[@name = 'description']/string/@value" />
                 </para>
+              </nixos:option-description>
 
               <xsl:if test="attr[@name = 'type']">
                 <para>

nixos/doc/manual/postprocess-option-descriptions.xsl

@@ -0,0 +1,115 @@
+<?xml version="1.0"?>
+
+<xsl:stylesheet version="1.0"
+                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
+                xmlns:str="http://exslt.org/strings"
+                xmlns:exsl="http://exslt.org/common"
+                xmlns:db="http://docbook.org/ns/docbook"
+                xmlns:nixos="tag:nixos.org"
+                extension-element-prefixes="str exsl">
+  <xsl:output method='xml' encoding="UTF-8" />
+
+  <xsl:template match="@*|node()">
+    <xsl:copy>
+      <xsl:apply-templates select="@*|node()" />
+    </xsl:copy>
+  </xsl:template>
+
+  <xsl:template name="break-up-description">
+    <xsl:param name="input" />
+    <xsl:param name="buffer" />
+
+    <!-- Every time we have two newlines following each other, we want to
+         break it into </para><para>. -->
+    <xsl:variable name="parbreak" select="'&#xa;&#xa;'" />
+
+    <!-- Similar to "(head:tail) = input" in Haskell. -->
+    <xsl:variable name="head" select="$input[1]" />
+    <xsl:variable name="tail" select="$input[position() &gt; 1]" />
+
+    <xsl:choose>
+      <xsl:when test="$head/self::text() and contains($head, $parbreak)">
+        <!-- If the haystack provided to str:split() directly starts or
+             ends with $parbreak, it doesn't generate a <token/> for that,
+             so we are doing this here. -->
+        <xsl:variable name="splitted-raw">
+          <xsl:if test="starts-with($head, $parbreak)"><token /></xsl:if>
+          <xsl:for-each select="str:split($head, $parbreak)">
+            <token><xsl:value-of select="node()" /></token>
+          </xsl:for-each>
+          <!-- Something like ends-with($head, $parbreak), but there is
+               no ends-with() in XSLT, so we need to use substring(). -->
+          <xsl:if test="
+            substring($head, string-length($head) -
+                             string-length($parbreak) + 1) = $parbreak
+          "><token /></xsl:if>
+        </xsl:variable>
+        <xsl:variable name="splitted"
+                      select="exsl:node-set($splitted-raw)/token" />
+        <!-- The buffer we had so far didn't contain any text nodes that
+             contain a $parbreak, so we can put the buffer along with the
+             first token of $splitted into a para element. -->
+        <para xmlns="http://docbook.org/ns/docbook">
+          <xsl:apply-templates select="exsl:node-set($buffer)" />
+          <xsl:apply-templates select="$splitted[1]/node()" />
+        </para>
+        <!-- We have already emitted the first splitted result, so the
+             last result is going to be set as the new $buffer later
+             because its contents may not be directly followed up by a
+             $parbreak. -->
+        <xsl:for-each select="$splitted[position() &gt; 1
+                              and position() &lt; last()]">
+          <para xmlns="http://docbook.org/ns/docbook">
+            <xsl:apply-templates select="node()" />
+          </para>
+        </xsl:for-each>
+        <xsl:call-template name="break-up-description">
+          <xsl:with-param name="input" select="$tail" />
+          <xsl:with-param name="buffer" select="$splitted[last()]/node()" />
+        </xsl:call-template>
+      </xsl:when>
+      <!-- Either non-text node or one without $parbreak, which we just
+           want to buffer and continue recursing. -->
+      <xsl:when test="$input">
+        <xsl:call-template name="break-up-description">
+          <xsl:with-param name="input" select="$tail" />
+          <!-- This essentially appends $head to $buffer. -->
+          <xsl:with-param name="buffer">
+            <xsl:if test="$buffer">
+              <xsl:for-each select="exsl:node-set($buffer)">
+                <xsl:apply-templates select="." />
+              </xsl:for-each>
+            </xsl:if>
+            <xsl:apply-templates select="$head" />
+          </xsl:with-param>
+        </xsl:call-template>
+      </xsl:when>
+      <!-- No more $input, just put the remaining $buffer in a para. -->
+      <xsl:otherwise>
+        <para xmlns="http://docbook.org/ns/docbook">
+          <xsl:apply-templates select="exsl:node-set($buffer)" />
+        </para>
+      </xsl:otherwise>
+    </xsl:choose>
+  </xsl:template>
+
+  <xsl:template match="nixos:option-description">
+    <xsl:choose>
+      <!--
+        Only process nodes that are comprised of a single <para/> element,
+        because if that's not the case the description already contains
+        </para><para> in between and we need no further processing.
+      -->
+      <xsl:when test="count(db:para) > 1">
+        <xsl:apply-templates select="node()" />
+      </xsl:when>
+      <xsl:otherwise>
+        <xsl:call-template name="break-up-description">
+          <xsl:with-param name="input"
+                          select="exsl:node-set(db:para/node())" />
+        </xsl:call-template>
+      </xsl:otherwise>
+    </xsl:choose>
+  </xsl:template>
+
+</xsl:stylesheet>