docs/qapidoc: add QMP highlighting to annotated qmp-example blocks

For any code literal blocks inside of a qmp-example directive, apply and
enforce the QMP lexer/highlighter to those blocks.

This way, you won't need to write:

```
.. qmp-example::
   :annotated:

   Blah blah

   .. code-block:: QMP

      -> { "lorem": "ipsum" }
```

But instead, simply:

```
.. qmp-example::
   :annotated:

   Blah blah::

     -> { "lorem": "ipsum" }
```

Once the directive block is exited, whatever the previous default
highlight language was will be restored; localizing the forced QMP
lexing to exclusively this directive.

Note, if the default language is *already* QMP, this directive will not
generate and restore redundant highlight configuration nodes. We may
well decide that the default language ought to be QMP for any QAPI
reference pages, but this way the directive behaves consistently no
matter where it is used.

Signed-off-by: John Snow <jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240717021312.606116-5-jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/sphinx/qapidoc.py

@@ -26,6 +26,7 @@ https://www.sphinx-doc.org/en/master/development/index.html
 
 import os
 import re
+import sys
 import textwrap
 from typing import List
 
@@ -36,6 +37,7 @@ from qapi.error import QAPIError, QAPISemError
 from qapi.gen import QAPISchemaVisitor
 from qapi.schema import QAPISchema
 
+from sphinx import addnodes
 from sphinx.directives.code import CodeBlock
 from sphinx.errors import ExtensionError
 from sphinx.util.docutils import switch_source_input
@@ -545,10 +547,10 @@ class QMPExample(CodeBlock, NestedDirective):
     Custom admonition for QMP code examples.
 
     When the :annotated: option is present, the body of this directive
-    is parsed as normal rST instead. Code blocks must be explicitly
-    written by the user, but this allows for intermingling explanatory
-    paragraphs with arbitrary rST syntax and code blocks for more
-    involved examples.
+    is parsed as normal rST, but with any '::' code blocks set to use
+    the QMP lexer. Code blocks must be explicitly written by the user,
+    but this allows for intermingling explanatory paragraphs with
+    arbitrary rST syntax and code blocks for more involved examples.
 
     When :annotated: is absent, the directive body is treated as a
     simple standalone QMP code block literal.
@@ -562,6 +564,33 @@ class QMPExample(CodeBlock, NestedDirective):
         "title": directives.unchanged,
     }
 
+    def _highlightlang(self) -> addnodes.highlightlang:
+        """Return the current highlightlang setting for the document"""
+        node = None
+        doc = self.state.document
+
+        if hasattr(doc, "findall"):
+            # docutils >= 0.18.1
+            for node in doc.findall(addnodes.highlightlang):
+                pass
+        else:
+            for elem in doc.traverse():
+                if isinstance(elem, addnodes.highlightlang):
+                    node = elem
+
+        if node:
+            return node
+
+        # No explicit directive found, use defaults
+        node = addnodes.highlightlang(
+            lang=self.env.config.highlight_language,
+            force=False,
+            # Yes, Sphinx uses this value to effectively disable line
+            # numbers and not 0 or None or -1 or something. ¯\_(ツ)_/¯
+            linenothreshold=sys.maxsize,
+        )
+        return node
+
     def admonition_wrap(self, *content) -> List[nodes.Node]:
         title = "Example:"
         if "title" in self.options:
@@ -576,8 +605,24 @@ class QMPExample(CodeBlock, NestedDirective):
         return [admon]
 
     def run_annotated(self) -> List[nodes.Node]:
+        lang_node = self._highlightlang()
+
         content_node: nodes.Element = nodes.section()
+
+        # Configure QMP highlighting for "::" blocks, if needed
+        if lang_node["lang"] != "QMP":
+            content_node += addnodes.highlightlang(
+                lang="QMP",
+                force=False,  # "True" ignores lexing errors
+                linenothreshold=lang_node["linenothreshold"],
+            )
+
         self.do_parse(self.content, content_node)
+
+        # Restore prior language highlighting, if needed
+        if lang_node["lang"] != "QMP":
+            content_node += addnodes.highlightlang(**lang_node.attributes)
+
         return content_node.children
 
     def run(self) -> List[nodes.Node]: