DocBook/XML: add support for docbook external references

Documents external references
-----------------------------
A new feature with version 1.7 is references to documents in other
packages. For example a protocol specifications may refer to another
one or to a library implementing it.

This feature works with PDF, HTML, PS (and epub?) generated documents
and will generate links to the other versions of the same type, i.e. html
docs will link to other html docs,  and pdf docs will link to other pdf
docs.

New: generating target databases
--------------------------------
All documents will generate a target db, analogous to a symbol table
which will be consulted by other documents to find linking information.
This is accomplished with xsltproc.

New: consulting target databases
--------------------------------
While documents are being converted from xml to html/pdf/ps,
they will consult the target dbs of each document they which to refer to.
This is accomplished with xmlto.

New: master database
--------------------
A hand written database has been created which provides an association
between the target dbs and the document themselves. It also reflects
the hierarchical structure of the documents relative to the installation dir.

New: xorg-xhtml.xsl and xorg-fo.xsl
-----------------------------------
A number of motivations are behind the new (and split) versions of xorg.xsl.
This module must remain backward compatible to build previous versions
of documentation. It was originally designed a while ago for sgml.
Given that all documents makefile had to change, it seemed a good opportunity
to break from the past. Among the differences:

The location of xorg.css is no longer set in the stylesheet.
The chunked html is no longer supported.
The html and fo parameters are kept separate.
Additional parameters for external references.
Now allows different values for html/fo xsl for the same parameter
should that be required.
The xorg-xhtml.xsl imports html/docbook.xsl
The xorg-fo.xsl imports fo/docbook.xsl

Co-authored-by: Matt Dew <marcoz@osource.org>

Acked-by: Dan Nicholson <dbn.lists@gmail.com>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>

1 files changed, 133 insertions, 0 deletions

diff --git a/README b/README
index 5d60900..07e760b 100644
--- a/README
+++ b/README
@@ -5,6 +5,139 @@ source who want to produce formatted documentation from their builds,
 or those who have installed the HTML version of the documentation,
 which refers to the included common xorg.css stylesheet.
 
+Documents external references
+-----------------------------
+A new feature with version 1.7 is references to documents in other
+packages. For example a protocol specifications may refer to another
+one or to a library implementing it.
+
+This feature works with PDF, HTML, PS (and epub?) generated documents
+and will generate links to the other versions of the same type, i.e. html
+docs will link to other html docs,  and pdf docs will link to other pdf
+docs.
+
+Due to the modular nature of the X window System and the flexibility
+it provides to O/S builders, one should not expect all links to
+be resolved all the time. A more recent version of a package may be
+installed or an older version may be retained for a long period of time.
+Not all packages are installed either.
+
+Build challenges
+----------------
+The X Window System is composed of over 200 packages. The documentation is
+included with the package it documents so the information matches the code.
+The build is organized to satisfy C code building requirements, but this
+paradigm does not fit very well for building documentation.
+
+For a document to build a reference to another document, it needs to have
+information about this document which may or may not be there, or may be
+at an unexpected version. The document it refers to may also wish to refer
+to the former document.
+
+Local X builds
+--------------
+When building X as a sandbox on a workstation, using a simple build script
+to configure and build all packages in the right order is all you need
+to get a working system, including documentation. You will not get all
+the links in the documentation resolved correctly due to build order
+and/or bi-directional references.
+
+Unless you are updating or testing the documentation, you do not need to
+worry about this glitch. A simple workaround is to build each affected
+package using "make clean install && make clean install" in any order.
+This will give a chance to each package to get information about the other one
+through the installed files.
+
+O/S X builds
+------------
+O/S builders often repackage X code and documentation, however they face the
+same challenges as described above. Users can update packages through
+the package management system or install additional packages. To use the
+earlier example, if a protocol is installed without its implementing
+library there will be a broken reference.
+
+Build assumptions
+-----------------
+Packages provide great flexibility, including the one to install each package
+documentation in a separate, totally unrelated directory using the --docdir
+configure option. For external references to work, it is assumed that
+the each package installs the documentation using the relative
+doc/${PACKAGE_TARNAME} location.
+
+The Docbook stylesheet technology will create references with paths relative
+to this location. It will navigate up to "doc" using ../ and then navigate
+down to the document it refers to.
+
+Diagnosing a broken reference
+-----------------------------
+A document reference is created at build time while the document is being
+transformed from Docbook/XML to HTML or another format. If this reference
+is missing, the document must be rebuilt.
+
+To build the reference, the docbook stylesheet consults a database stored
+in this package which is called masterdb.xml. This database supplies the
+relative directory structure of where the documents are located as well
+as including target document references in the various *.db files.
+These files are generated and installed while the documents are being build.
+
+This is what a Docbook/XML reference from docA to docB looks like (docA.xml):
+<olink targetdoc='docB' targetptr='docBfragment'>Title</olink>
+
+This is what an HTML reference from docA to docB looks like (docA.html):
+<a href="docB.html#docBfragment" class="olink">Title</a>
+
+This is what an HTML targetdoc for docB looks like (docB.html.db):
+<div element="article" href="#docBfragment" number="" targetptr="docBfragment">
+
+This is what an HTML reference from docA to docB looks like (docA.html)
+when the reference cannot be resolved:
+<span class="olink">Title</span>
+
+This can be caused by one or more of the following:
+
+   - docB may not longer contain the text (targetptr) docA is referring to.
+   - docB.html.db file may not have been installed by docB package
+   - masterdb.html.xml is missing a <document/> entry for docB
+   - masterdb.html.xml cannot be loaded so all refs are broken.
+   - an XSLT processor which does not support xinclude is being used
+   - docB.html.db was generated without --xinclude option for the processor
+   - xsltproc is not at version 1.1.26 or later
+   - for PDF/PS formats, the fragment is ignored, always points to top of docB
+
+Some pitfalls
+-------------------------
+<book id="docA"> in docA.xml: the id must match the file basename.
+In xorg-*.xsl, <xsl:param name="current.docid" select="/*/@id"/>
+supplies docid for *.db. The document id must be unique across
+all documents.
+
+The baseuri attribute in masterdb.xml is required for pdf/ps references
+to work. It cannot be replaced with olink.base.uri in the makefile.
+
+The pdf/ps "inside the document" references only started working with
+docbook-xsl v 1.76.1 which is not yet available to your favorite O/S.
+In xorg-fo.xsl, insert.olink.pdf.frag must be set to zero which allows
+the reference to at least point to the top of the document.
+
+XSL toolchain
+-------------
+The following minimum versions are needed for external references to work:
+XSLT processor:	1.1.26
+libxml2:	2.7.6
+libxslt1.1:	1.1.26
+The exact triplet may vary by distribution, sometimes a version is skipped.
+If you are close enough, try it.
+
+References
+----------
+http://docbook.org/tdg/en/html/docbook.html
+http://docbook.sourceforge.net/release/xsl/current/doc/html/
+http://docbook.sourceforge.net/release/xsl/current/doc/fo/
+http://www.sagehill.net/docbookxsl/OlinkPrintOutput.html
+
+
+-------------------------------------------------------------------------------
+
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