[PATCH v2 16/21] docs/qapidoc: add QMP highlighting to annotated qmp-example blocks

[John Snow <jsnow@redhat.com>]

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.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 docs/sphinx/qapidoc.py | 53 ++++++++++++++++++++++++++++++++++++++----
 1 file changed, 49 insertions(+), 4 deletions(-)

diff --git a/docs/sphinx/qapidoc.py b/docs/sphinx/qapidoc.py
index a2fa05fc491..c8c404ad1b0 100644
--- a/docs/sphinx/qapidoc.py
+++ b/docs/sphinx/qapidoc.py
@@ -26,6 +26,7 @@
 
 import os
 import re
+import sys
 import textwrap
 from typing import List
 
@@ -37,6 +38,7 @@
 from qapi.schema import QAPISchema
 
 import sphinx
+from sphinx import addnodes
 from sphinx.directives.code import CodeBlock
 from sphinx.errors import ExtensionError
 from sphinx.util.docutils import SphinxDirective
@@ -575,10 +577,10 @@ 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.
@@ -592,6 +594,33 @@ 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:
@@ -606,8 +635,24 @@ def admonition_wrap(self, *content) -> List[nodes.Node]:
         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]:
-- 
2.45.0