[PATCH 27/57] docs/qapi-domain: add XREF compatibility goop for Sphinx < 4.1

[John Snow <jsnow@redhat.com>]

Sphinx < 4.1 handles cross-references ... differently. Factor out and
isolate the compatibility goop we need to make cross references work
properly in old versions of Sphinx.

Yes, it's ugly. Yes, it works. No, I don't want to talk about
it.

Understand that this patch exists because of the overflowing love in my
heart.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 docs/sphinx/compat.py      | 129 ++++++++++++++++++++++++++++++++++++-
 docs/sphinx/qapi_domain.py |  24 ++++---
 2 files changed, 142 insertions(+), 11 deletions(-)

diff --git a/docs/sphinx/compat.py b/docs/sphinx/compat.py
index 1f9f1f42ef7..7e8d72c9b1f 100644
--- a/docs/sphinx/compat.py
+++ b/docs/sphinx/compat.py
@@ -2,14 +2,32 @@
 Sphinx cross-version compatibility goop
 """
 
-from typing import Callable
+import re
+from typing import (
+    Any,
+    Callable,
+    Optional,
+    Type,
+)
 
+from docutils import nodes
 from docutils.nodes import Element, Node, Text
 
 import sphinx
 from sphinx import addnodes
-from sphinx.util.docutils import SphinxDirective, switch_source_input
+from sphinx.environment import BuildEnvironment
+from sphinx.roles import XRefRole
+from sphinx.util import docfields
+from sphinx.util.docutils import (
+    ReferenceRole,
+    SphinxDirective,
+    switch_source_input,
+)
 from sphinx.util.nodes import nested_parse_with_titles
+from sphinx.util.typing import TextlikeNode
+
+
+MAKE_XREF_WORKAROUND = sphinx.version_info[:3] < (4, 1, 0)
 
 
 SpaceNode: Callable[[str], Node]
@@ -46,3 +64,110 @@ def nested_parse(directive: SphinxDirective, content_node: Element) -> None:
             nested_parse_with_titles(
                 directive.state, directive.content, content_node
             )
+
+
+# ###########################################
+# xref compatibility hacks for Sphinx < 4.1 #
+# ###########################################
+
+# When we require >= Sphinx 4.1, the following function and the
+# subsequent 3 compatibility classes can be removed. Anywhere in
+# qapi_domain that uses one of these Compat* types can be switched to
+# using the garden-variety lib-provided classes with no trickery.
+
+
+def _compat_make_xref(  # pylint: disable=unused-argument
+    self: sphinx.util.docfields.Field,
+    rolename: str,
+    domain: str,
+    target: str,
+    innernode: Type[TextlikeNode] = addnodes.literal_emphasis,
+    contnode: Optional[Node] = None,
+    env: Optional[BuildEnvironment] = None,
+    inliner: Any = None,
+    location: Any = None,
+) -> Node:
+    """
+    Compatibility workaround for Sphinx versions prior to 4.1.0.
+
+    Older sphinx versions do not use the domain's XRefRole for parsing
+    and formatting cross-references, so we need to perform this magick
+    ourselves to avoid needing to write the parser/formatter in two
+    separate places.
+
+    This workaround isn't brick-for-brick compatible with modern Sphinx
+    versions, because we do not have access to the parent directive's
+    state during this parsing like we do in more modern versions.
+
+    It's no worse than what pre-Sphinx 4.1.0 does, so... oh well!
+    """
+
+    # Yes, this function is gross. Pre-4.1 support is a miracle.
+    # pylint: disable=too-many-locals
+
+    assert env
+    # Note: Sphinx's own code ignores the type warning here, too.
+    if not rolename:
+        return contnode or innernode(target, target)  # type: ignore[call-arg]
+
+    # Get the role instance, but don't *execute it* - we lack the
+    # correct state to do so. Instead, we'll just use its public
+    # methods to do our reference formatting, and emulate the rest.
+    role = env.get_domain(domain).roles[rolename]
+    assert isinstance(role, XRefRole)
+
+    # XRefRole features not supported by this compatibility shim;
+    # these were not supported in Sphinx 3.x either, so nothing of
+    # value is really lost.
+    assert not target.startswith("!")
+    assert not re.match(ReferenceRole.explicit_title_re, target)
+    assert not role.lowercase
+    assert not role.fix_parens
+
+    # Code below based mostly on sphinx.roles.XRefRole; run() and
+    # create_xref_node()
+    options = {
+        "refdoc": env.docname,
+        "refdomain": domain,
+        "reftype": rolename,
+        "refexplicit": False,
+        "refwarn": role.warn_dangling,
+    }
+    refnode = role.nodeclass(target, **options)
+    title, target = role.process_link(env, refnode, False, target, target)
+    refnode["reftarget"] = target
+    classes = ["xref", domain, f"{domain}-{rolename}"]
+    refnode += role.innernodeclass(target, title, classes=classes)
+
+    # This is the very gross part of the hack. Normally,
+    # result_nodes takes a document object to which we would pass
+    # self.inliner.document. Prior to Sphinx 4.1, we don't *have* an
+    # inliner to pass, so we have nothing to pass here. However, the
+    # actual implementation of role.result_nodes in this case
+    # doesn't actually use that argument, so this winds up being
+    # ... fine. Rest easy at night knowing this code only runs under
+    # old versions of Sphinx, so at least it won't change in the
+    # future on us and lead to surprising new failures.
+    # Gross, I know.
+    result_nodes, _messages = role.result_nodes(
+        None,  # type: ignore
+        env,
+        refnode,
+        is_ref=True,
+    )
+    return nodes.inline(target, "", *result_nodes)
+
+
+class CompatField(docfields.Field):
+    if MAKE_XREF_WORKAROUND:
+        make_xref = _compat_make_xref
+
+
+class CompatGroupedField(docfields.GroupedField):
+    if MAKE_XREF_WORKAROUND:
+        make_xref = _compat_make_xref
+
+
+class CompatTypedField(docfields.TypedField):
+    if MAKE_XREF_WORKAROUND:
+        make_xref = _compat_make_xref
diff --git a/docs/sphinx/qapi_domain.py b/docs/sphinx/qapi_domain.py
index 753e07024f5..d3487c5dfeb 100644
--- a/docs/sphinx/qapi_domain.py
+++ b/docs/sphinx/qapi_domain.py
@@ -26,7 +26,14 @@
 from docutils import nodes
 from docutils.parsers.rst import directives
 
-from compat import KeywordNode, SpaceNode, nested_parse
+from compat import (
+    CompatField,
+    CompatGroupedField,
+    CompatTypedField,
+    KeywordNode,
+    SpaceNode,
+    nested_parse,
+)
 from sphinx import addnodes
 from sphinx.addnodes import desc_signature, pending_xref
 from sphinx.directives import ObjectDescription
@@ -39,7 +46,6 @@
 from sphinx.locale import _, __
 from sphinx.roles import XRefRole
 from sphinx.util import logging
-from sphinx.util.docfields import Field, GroupedField, TypedField
 from sphinx.util.docutils import SphinxDirective
 from sphinx.util.nodes import make_id, make_refnode
 
@@ -187,7 +193,7 @@ class QAPIObject(ObjectDescription[Signature]):
 
     doc_field_types = [
         # :feat name: descr
-        GroupedField(
+        CompatGroupedField(
             "feature",
             label=_("Features"),
             names=("feat",),
@@ -444,7 +450,7 @@ class QAPICommand(QAPIObject):
     doc_field_types.extend(
         [
             # :arg TypeName ArgName: descr
-            TypedField(
+            CompatTypedField(
                 "argument",
                 label=_("Arguments"),
                 names=("arg",),
@@ -452,14 +458,14 @@ class QAPICommand(QAPIObject):
                 can_collapse=False,
             ),
             # :error: descr
-            Field(
+            CompatField(
                 "error",
                 label=_("Errors"),
                 names=("error", "errors"),
                 has_arg=False,
             ),
             # :returns TypeName: descr
-            GroupedField(
+            CompatGroupedField(
                 "returnvalue",
                 label=_("Returns"),
                 rolename="type",
@@ -477,7 +483,7 @@ class QAPIEnum(QAPIObject):
     doc_field_types.extend(
         [
             # :value name: descr
-            GroupedField(
+            CompatGroupedField(
                 "value",
                 label=_("Values"),
                 names=("value",),
@@ -494,7 +500,7 @@ class QAPIAlternate(QAPIObject):
     doc_field_types.extend(
         [
             # :choice type name: descr
-            TypedField(
+            CompatTypedField(
                 "choice",
                 label=_("Choices"),
                 names=("choice",),
@@ -512,7 +518,7 @@ class QAPIObjectWithMembers(QAPIObject):
     doc_field_types.extend(
         [
             # :member type name: descr
-            TypedField(
+            CompatTypedField(
                 "member",
                 label=_("Members"),
                 names=("memb",),
-- 
2.48.1