docs/qapi-domain: Fix error context reporting in Sphinx 5.x and 6.x

Sphinx 5.3.0 to Sphinx 6.2.0 has a bug where nested content in an
ObjectDescription content block has its error position reported
incorrectly due to an oversight when they added nested section support
to this directive.

(This bug is present in Sphinx's own Python and C domains; test it
yourself by creating a py:func directive and creating a syntax error in
the directive's content block. The reporting will be incorrect.)

To avoid overriding and re-implementing the entirety of the run()
method, a workaround is employed where we parse the content block
ourselves in before_content(), then null the content block to make
Sphinx's own parsing a no-op. Then, in transform_content (which occurs
after Sphinx's nested parse), we simply swap our own parsed content tree
back in for Sphinx's.

It appears a little tricky, but it's the nicest solution I can find.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-32-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

docs/sphinx/compat.py

@@ -4,6 +4,7 @@ Sphinx cross-version compatibility goop
 
 import re
 from typing import (
+    TYPE_CHECKING,
     Any,
     Callable,
     Optional,
@@ -12,9 +13,11 @@ from typing import (
 
 from docutils import nodes
 from docutils.nodes import Element, Node, Text
+from docutils.statemachine import StringList
 
 import sphinx
 from sphinx import addnodes, util
+from sphinx.directives import ObjectDescription
 from sphinx.environment import BuildEnvironment
 from sphinx.roles import XRefRole
 from sphinx.util import docfields
@@ -172,3 +175,56 @@ class CompatGroupedField(docfields.GroupedField):
 class CompatTypedField(docfields.TypedField):
     if MAKE_XREF_WORKAROUND:
         make_xref = _compat_make_xref
+
+
+# ################################################################
+# Nested parsing error location fix for Sphinx 5.3.0 < x < 6.2.0 #
+# ################################################################
+
+# When we require Sphinx 4.x, the TYPE_CHECKING hack where we avoid
+# subscripting ObjectDescription at runtime can be removed in favor of
+# just always subscripting the class.
+
+# When we require Sphinx > 6.2.0, the rest of this compatibility hack
+# can be dropped and QAPIObject can just inherit directly from
+# ObjectDescription[Signature].
+
+SOURCE_LOCATION_FIX = (5, 3, 0) <= sphinx.version_info[:3] < (6, 2, 0)
+
+Signature = str
+
+
+if TYPE_CHECKING:
+    _BaseClass = ObjectDescription[Signature]
+else:
+    _BaseClass = ObjectDescription
+
+
+class ParserFix(_BaseClass):
+
+    _temp_content: StringList
+    _temp_offset: int
+    _temp_node: Optional[addnodes.desc_content]
+
+    def before_content(self) -> None:
+        # Work around a sphinx bug and parse the content ourselves.
+        self._temp_content = self.content
+        self._temp_offset = self.content_offset
+        self._temp_node = None
+
+        if SOURCE_LOCATION_FIX:
+            self._temp_node = addnodes.desc_content()
+            self.state.nested_parse(
+                self.content, self.content_offset, self._temp_node
+            )
+            # Sphinx will try to parse the content block itself,
+            # Give it nothingness to parse instead.
+            self.content = StringList()
+            self.content_offset = 0
+
+    def transform_content(self, content_node: addnodes.desc_content) -> None:
+        # Sphinx workaround: Inject our parsed content and restore state.
+        if self._temp_node:
+            content_node += self._temp_node.children
+            self.content = self._temp_content
+            self.content_offset = self._temp_offset