AsciiDoctor: How can I add custom xmlns' - asciidoctor

Question

How can I add a custom xmlns in the output when I convert an asciidoc file with AsciiDoctor?
I'd like to add xmlns:xi="http://www.w3.org/2001/XInclude" in the top book tag.
The current implementation seems to generate:
<?xml version="1.0" encoding="UTF-8"?>
<?asciidoc-toc?>
<?asciidoc-numbered?>
<book xmlns="http://docbook.org/ns/docbook" xmlns:xl="http://www.w3.org/1999/xlink" version="5.0" xml:lang="en">
<info>
<title>title</title>
</info>
</book>
from this:
= title
:lang: en
When I run:
$ asciidoctor -b docbook5 -d book -o out.xml source.txt
There is a built-in attribute xmlns, but it seems to be for docbook 4.5.
The reason I want to use XInclude is to include some xml files from Docinfo files and Passthrough Blocks

Answer 1

With a bit of research inside the asciidoctor code it quickly became clear that the part you'd like to modify is fairly static.
See asciidoctor/converter/docbook5.rb Line 44 for more info.
The best approach is to create a postprocessor extension which modifies the output. The example below is just to show a possible implementation.
Create a file with the following content and call it docbook_postprocessor.rb.
class Docbook5XiPostprocessor < Asciidoctor::Extensions::Postprocessor
def process document, output
if document.basebackend? 'docbook'
input_regex = %r{^(<.*xmlns:xl="http://www.w3.org/1999/xlink") (version="5.0".*>)}
replacement = %(\\1 xmlns:xi="http://www.w3.org/2001/XInclude" \\2)
output = output.sub(input_regex, replacement)
end
output
end
end
Asciidoctor::Extensions.register do
postprocessor Docbook5XiPostprocessor
end
Note: The above extension is for the sake of brevity placed in the same directory as the asciidoctor source file called source.adoc.
The run the asciidoctor command with the -r ./docbook_postprocessor.rb parameters.
$ asciidoctor -r ./docbook_postprocessor.rb -b docbook5 -d book -o - source.adoc
<?xml version="1.0" encoding="UTF-8"?>
<?asciidoc-toc?>
<?asciidoc-numbered?>
<book
xmlns="http://docbook.org/ns/docbook"
xmlns:xl="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
version="5.0"
xml:lang="en">
<info>
<title>test</title>
<date>2020-12-19</date>
</info>
</book>
* Above output has been slightly reformatted to eliminate the scrollbar
Creating ruby gem with the above code for easier distribution is a task left to the reader.