doc: Add helper for converting DocBook files to Markdown

maintainers/scripts/doc/escape-code-markup.py

@@ -0,0 +1,97 @@
+#! /usr/bin/env nix-shell
+#! nix-shell -I nixpkgs=channel:nixos-unstable -i python3 -p python3 -p python3.pkgs.lxml
+
+"""
+Pandoc will strip any markup within code elements so
+let’s escape them so that they can be handled manually.
+"""
+
+import lxml.etree as ET
+import re
+import sys
+
+def replace_element_by_text(el: ET.Element, text: str) -> None:
+    """
+    Author: bernulf
+    Source: https://stackoverflow.com/a/10520552/160386
+    SPDX-License-Identifier: CC-BY-SA-3.0
+    """
+    text = text + (el.tail or "")
+    parent = el.getparent()
+    if parent is not None:
+        previous = el.getprevious()
+        if previous is not None:
+            previous.tail = (previous.tail or "") + text
+        else:
+            parent.text = (parent.text or "") + text
+        parent.remove(el)
+
+DOCBOOK_NS = "http://docbook.org/ns/docbook"
+
+# List of elements that pandoc’s DocBook reader strips markup from.
+# https://github.com/jgm/pandoc/blob/master/src/Text/Pandoc/Readers/DocBook.hs
+code_elements = [
+    # CodeBlock
+    "literallayout",
+    "screen",
+    "programlisting",
+    # Code (inline)
+    "classname",
+    "code",
+    "filename",
+    "envar",
+    "literal",
+    "computeroutput",
+    "prompt",
+    "parameter",
+    "option",
+    "markup",
+    "wordasword",
+    "command",
+    "varname",
+    "function",
+    "type",
+    "symbol",
+    "constant",
+    "userinput",
+    "systemitem",
+]
+
+XMLNS_REGEX = re.compile(r'\s+xmlns(?::[^=]+)?="[^"]*"')
+ROOT_ELEMENT_REGEX = re.compile(r'^\s*<[^>]+>')
+
+def remove_xmlns(match: re.Match) -> str:
+    """
+    Removes xmlns attributes.
+
+    Expects a match containing an opening tag.
+    """
+    return XMLNS_REGEX.sub('', match.group(0))
+
+if __name__ == '__main__':
+    assert len(sys.argv) >= 3, "usage: escape-code-markup.py <input> <output>"
+
+    tree = ET.parse(sys.argv[1])
+    name_predicate = " or ".join([f"local-name()='{el}'" for el in code_elements])
+
+    for markup in tree.xpath(f"//*[({name_predicate}) and namespace-uri()='{DOCBOOK_NS}']/*"):
+        text = ET.tostring(markup, encoding=str)
+
+        # tostring adds xmlns attributes to the element we want to stringify
+        # as if it was supposed to be usable standalone.
+        # We are just converting it to CDATA so we do not care.
+        # Let’s strip the namespace declarations to keep the code clean.
+        #
+        # Note that this removes even namespaces that were potentially
+        # in the original file. Though, that should be very rare –
+        # most of the time, we will stringify empty DocBook elements
+        # like <xref> or <co> or, at worst, <link> with xlink:href attribute.
+        #
+        # Also note that the regex expects the root element to be first
+        # thing in the string. But that should be fine, the tostring method
+        # does not produce XML declaration or doctype by default.
+        text = ROOT_ELEMENT_REGEX.sub(remove_xmlns, text)
+
+        replace_element_by_text(markup, text)
+
+    tree.write(sys.argv[2])