docs/sphinx: Add new qapi-doc Sphinx extension

Some of our documentation is auto-generated from documentation
comments in the JSON schema.

For Sphinx, rather than creating a file to include, the most natural
way to handle this is to have a small custom Sphinx extension which
processes the JSON file and inserts documentation into the rST
file being processed.

This is the same approach that kerneldoc and hxtool use.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200925162316.21205-8-peter.maydell@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Comment and doc string formatting tweaked, unused method dropped,
a few line breaks tweaked to follow PEP 8 more closely, MAINTAINERS
section QAPI updated]
Signed-off-by: Markus Armbruster <armbru@redhat.com>

3 files changed, 556 insertions, 1 deletions

diff --git a/MAINTAINERS b/MAINTAINERS
index be0a8e648c..ac3e4e465c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2387,6 +2387,7 @@ F: tests/test-qmp-*.c
 F: tests/test-visitor-serialization.c
 F: scripts/qapi-gen.py
 F: scripts/qapi/*
+F: docs/sphinx/qapidoc.py
 F: docs/devel/qapi*
 T: git https://repo.or.cz/qemu/armbru.git qapi-next
 
@@ -3151,6 +3152,7 @@ M: Peter Maydell <peter.maydell@linaro.org>
 S: Maintained
 F: docs/conf.py
 F: docs/*/conf.py
+F: docs/sphinx/
 
 Miscellaneous
 -------------
diff --git a/docs/conf.py b/docs/conf.py
index 0dbd90dc11..606f623211 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -52,7 +52,10 @@ except NameError:
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use an absolute path starting from qemu_docdir.
 #
+# Our extensions are in docs/sphinx; the qapidoc extension requires
+# the QAPI modules from scripts/.
 sys.path.insert(0, os.path.join(qemu_docdir, "sphinx"))
+sys.path.insert(0, os.path.join(qemu_docdir, "../scripts"))
 
 
 # -- General configuration ------------------------------------------------
@@ -67,7 +70,7 @@ needs_sphinx = '1.6'
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['kerneldoc', 'qmp_lexer', 'hxtool', 'depfile']
+extensions = ['kerneldoc', 'qmp_lexer', 'hxtool', 'depfile', 'qapidoc']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -241,3 +244,4 @@ texinfo_documents = [
 kerneldoc_bin = os.path.join(qemu_docdir, '../scripts/kernel-doc')
 kerneldoc_srctree = os.path.join(qemu_docdir, '..')
 hxtool_srctree = os.path.join(qemu_docdir, '..')
+qapidoc_srctree = os.path.join(qemu_docdir, '..')
diff --git a/docs/sphinx/qapidoc.py b/docs/sphinx/qapidoc.py
new file mode 100644
index 0000000000..6944ffa6aa
--- /dev/null
+++ b/docs/sphinx/qapidoc.py
@@ -0,0 +1,549 @@
+# coding=utf-8
+#
+# QEMU qapidoc QAPI file parsing extension
+#
+# Copyright (c) 2020 Linaro
+#
+# This work is licensed under the terms of the GNU GPLv2 or later.
+# See the COPYING file in the top-level directory.
+
+"""
+qapidoc is a Sphinx extension that implements the qapi-doc directive
+
+The purpose of this extension is to read the documentation comments
+in QAPI schema files, and insert them all into the current document.
+
+It implements one new rST directive, "qapi-doc::".
+Each qapi-doc:: directive takes one argument, which is the
+pathname of the schema file to process, relative to the source tree.
+
+The docs/conf.py file must set the qapidoc_srctree config value to
+the root of the QEMU source tree.
+
+The Sphinx documentation on writing extensions is at:
+https://www.sphinx-doc.org/en/master/development/index.html
+"""
+
+import os
+import re
+
+from docutils import nodes
+from docutils.statemachine import ViewList
+from docutils.parsers.rst import directives, Directive
+from sphinx.errors import ExtensionError
+from sphinx.util.nodes import nested_parse_with_titles
+import sphinx
+from qapi.gen import QAPISchemaVisitor
+from qapi.schema import QAPIError, QAPISemError, QAPISchema
+
+
+# Sphinx up to 1.6 uses AutodocReporter; 1.7 and later
+# use switch_source_input. Check borrowed from kerneldoc.py.
+Use_SSI = sphinx.__version__[:3] >= '1.7'
+if Use_SSI:
+    from sphinx.util.docutils import switch_source_input
+else:
+    from sphinx.ext.autodoc import AutodocReporter
+
+
+__version__ = '1.0'
+
+
+# Function borrowed from pydash, which is under the MIT license
+def intersperse(iterable, separator):
+    """Yield the members of *iterable* interspersed with *separator*."""
+    iterable = iter(iterable)
+    yield next(iterable)
+    for item in iterable:
+        yield separator
+        yield item
+
+
+class QAPISchemaGenRSTVisitor(QAPISchemaVisitor):
+    """A QAPI schema visitor which generates docutils/Sphinx nodes
+
+    This class builds up a tree of docutils/Sphinx nodes corresponding
+    to documentation for the various QAPI objects. To use it, first
+    create a QAPISchemaGenRSTVisitor object, and call its
+    visit_begin() method.  Then you can call one of the two methods
+    'freeform' (to add documentation for a freeform documentation
+    chunk) or 'symbol' (to add documentation for a QAPI symbol). These
+    will cause the visitor to build up the tree of document
+    nodes. Once you've added all the documentation via 'freeform' and
+    'symbol' method calls, you can call 'get_document_nodes' to get
+    the final list of document nodes (in a form suitable for returning
+    from a Sphinx directive's 'run' method).
+    """
+    def __init__(self, sphinx_directive):
+        self._cur_doc = None
+        self._sphinx_directive = sphinx_directive
+        self._top_node = nodes.section()
+        self._active_headings = [self._top_node]
+
+    def _make_dlitem(self, term, defn):
+        """Return a dlitem node with the specified term and definition.
+
+        term should be a list of Text and literal nodes.
+        defn should be one of:
+        - a string, which will be handed to _parse_text_into_node
+        - a list of Text and literal nodes, which will be put into
+          a paragraph node
+        """
+        dlitem = nodes.definition_list_item()
+        dlterm = nodes.term('', '', *term)
+        dlitem += dlterm
+        if defn:
+            dldef = nodes.definition()
+            if isinstance(defn, list):
+                dldef += nodes.paragraph('', '', *defn)
+            else:
+                self._parse_text_into_node(defn, dldef)
+            dlitem += dldef
+        return dlitem
+
+    def _make_section(self, title):
+        """Return a section node with optional title"""
+        section = nodes.section(ids=[self._sphinx_directive.new_serialno()])
+        if title:
+            section += nodes.title(title, title)
+        return section
+
+    def _nodes_for_ifcond(self, ifcond, with_if=True):
+        """Return list of Text, literal nodes for the ifcond
+
+        Return a list which gives text like ' (If: cond1, cond2, cond3)', where
+        the conditions are in literal-text and the commas are not.
+        If with_if is False, we don't return the "(If: " and ")".
+        """
+        condlist = intersperse([nodes.literal('', c) for c in ifcond],
+                               nodes.Text(', '))
+        if not with_if:
+            return condlist
+
+        nodelist = [nodes.Text(' ('), nodes.strong('', 'If: ')]
+        nodelist.extend(condlist)
+        nodelist.append(nodes.Text(')'))
+        return nodelist
+
+    def _nodes_for_one_member(self, member):
+        """Return list of Text, literal nodes for this member
+
+        Return a list of doctree nodes which give text like
+        'name: type (optional) (If: ...)' suitable for use as the
+        'term' part of a definition list item.
+        """
+        term = [nodes.literal('', member.name)]
+        if member.type.doc_type():
+            term.append(nodes.Text(': '))
+            term.append(nodes.literal('', member.type.doc_type()))
+        if member.optional:
+            term.append(nodes.Text(' (optional)'))
+        if member.ifcond:
+            term.extend(self._nodes_for_ifcond(member.ifcond))
+        return term
+
+    def _nodes_for_variant_when(self, variants, variant):
+        """Return list of Text, literal nodes for variant 'when' clause
+
+        Return a list of doctree nodes which give text like
+        'when tagname is variant (If: ...)' suitable for use in
+        the 'variants' part of a definition list.
+        """
+        term = [nodes.Text(' when '),
+                nodes.literal('', variants.tag_member.name),
+                nodes.Text(' is '),
+                nodes.literal('', '"%s"' % variant.name)]
+        if variant.ifcond:
+            term.extend(self._nodes_for_ifcond(variant.ifcond))
+        return term
+
+    def _nodes_for_members(self, doc, what, base=None, variants=None):
+        """Return list of doctree nodes for the table of members"""
+        dlnode = nodes.definition_list()
+        for section in doc.args.values():
+            term = self._nodes_for_one_member(section.member)
+            # TODO drop fallbacks when undocumented members are outlawed
+            if section.text:
+                defn = section.text
+            elif (variants and variants.tag_member == section.member
+                  and not section.member.type.doc_type()):
+                values = section.member.type.member_names()
+                defn = [nodes.Text('One of ')]
+                defn.extend(intersperse([nodes.literal('', v) for v in values],
+                                        nodes.Text(', ')))
+            else:
+                defn = [nodes.Text('Not documented')]
+
+            dlnode += self._make_dlitem(term, defn)
+
+        if base:
+            dlnode += self._make_dlitem([nodes.Text('The members of '),
+                                         nodes.literal('', base.doc_type())],
+                                        None)
+
+        if variants:
+            for v in variants.variants:
+                if v.type.is_implicit():
+                    assert not v.type.base and not v.type.variants
+                    for m in v.type.local_members:
+                        term = self._nodes_for_one_member(m)
+                        term.extend(self._nodes_for_variant_when(variants, v))
+                        dlnode += self._make_dlitem(term, None)
+                else:
+                    term = [nodes.Text('The members of '),
+                            nodes.literal('', v.type.doc_type())]
+                    term.extend(self._nodes_for_variant_when(variants, v))
+                    dlnode += self._make_dlitem(term, None)
+
+        if not dlnode.children:
+            return []
+
+        section = self._make_section(what)
+        section += dlnode
+        return [section]
+
+    def _nodes_for_enum_values(self, doc):
+        """Return list of doctree nodes for the table of enum values"""
+        seen_item = False
+        dlnode = nodes.definition_list()
+        for section in doc.args.values():
+            termtext = [nodes.literal('', section.member.name)]
+            if section.member.ifcond:
+                termtext.extend(self._nodes_for_ifcond(section.member.ifcond))
+            # TODO drop fallbacks when undocumented members are outlawed
+            if section.text:
+                defn = section.text
+            else:
+                defn = [nodes.Text('Not documented')]
+
+            dlnode += self._make_dlitem(termtext, defn)
+            seen_item = True
+
+        if not seen_item:
+            return []
+
+        section = self._make_section('Values')
+        section += dlnode
+        return [section]
+
+    def _nodes_for_arguments(self, doc, boxed_arg_type):
+        """Return list of doctree nodes for the arguments section"""
+        if boxed_arg_type:
+            assert not doc.args
+            section = self._make_section('Arguments')
+            dlnode = nodes.definition_list()
+            dlnode += self._make_dlitem(
+                [nodes.Text('The members of '),
+                 nodes.literal('', boxed_arg_type.name)],
+                None)
+            section += dlnode
+            return [section]
+
+        return self._nodes_for_members(doc, 'Arguments')
+
+    def _nodes_for_features(self, doc):
+        """Return list of doctree nodes for the table of features"""
+        seen_item = False
+        dlnode = nodes.definition_list()
+        for section in doc.features.values():
+            dlnode += self._make_dlitem([nodes.literal('', section.name)],
+                                        section.text)
+            seen_item = True
+
+        if not seen_item:
+            return []
+
+        section = self._make_section('Features')
+        section += dlnode
+        return [section]
+
+    def _nodes_for_example(self, exampletext):
+        """Return list of doctree nodes for a code example snippet"""
+        return [nodes.literal_block(exampletext, exampletext)]
+
+    def _nodes_for_sections(self, doc):
+        """Return list of doctree nodes for additional sections"""
+        nodelist = []
+        for section in doc.sections:
+            snode = self._make_section(section.name)
+            if section.name and section.name.startswith('Example'):
+                snode += self._nodes_for_example(section.text)
+            else:
+                self._parse_text_into_node(section.text, snode)
+            nodelist.append(snode)
+        return nodelist
+
+    def _nodes_for_if_section(self, ifcond):
+        """Return list of doctree nodes for the "If" section"""
+        nodelist = []
+        if ifcond:
+            snode = self._make_section('If')
+            snode += self._nodes_for_ifcond(ifcond, with_if=False)
+            nodelist.append(snode)
+        return nodelist
+
+    def _add_doc(self, typ, sections):
+        """Add documentation for a command/object/enum...
+
+        We assume we're documenting the thing defined in self._cur_doc.
+        typ is the type of thing being added ("Command", "Object", etc)
+
+        sections is a list of nodes for sections to add to the definition.
+        """
+
+        doc = self._cur_doc
+        snode = nodes.section(ids=[self._sphinx_directive.new_serialno()])
+        snode += nodes.title('', '', *[nodes.literal(doc.symbol, doc.symbol),
+                                       nodes.Text(' (' + typ + ')')])
+        self._parse_text_into_node(doc.body.text, snode)
+        for s in sections:
+            if s is not None:
+                snode += s
+        self._add_node_to_current_heading(snode)
+
+    def visit_enum_type(self, name, info, ifcond, features, members, prefix):
+        doc = self._cur_doc
+        self._add_doc('Enum',
+                      self._nodes_for_enum_values(doc)
+                      + self._nodes_for_features(doc)
+                      + self._nodes_for_sections(doc)
+                      + self._nodes_for_if_section(ifcond))
+
+    def visit_object_type(self, name, info, ifcond, features,
+                          base, members, variants):
+        doc = self._cur_doc
+        if base and base.is_implicit():
+            base = None
+        self._add_doc('Object',
+                      self._nodes_for_members(doc, 'Members', base, variants)
+                      + self._nodes_for_features(doc)
+                      + self._nodes_for_sections(doc)
+                      + self._nodes_for_if_section(ifcond))
+
+    def visit_alternate_type(self, name, info, ifcond, features, variants):
+        doc = self._cur_doc
+        self._add_doc('Alternate',
+                      self._nodes_for_members(doc, 'Members')
+                      + self._nodes_for_features(doc)
+                      + self._nodes_for_sections(doc)
+                      + self._nodes_for_if_section(ifcond))
+
+    def visit_command(self, name, info, ifcond, features, arg_type,
+                      ret_type, gen, success_response, boxed, allow_oob,
+                      allow_preconfig):
+        doc = self._cur_doc
+        self._add_doc('Command',
+                      self._nodes_for_arguments(doc,
+                                                arg_type if boxed else None)
+                      + self._nodes_for_features(doc)
+                      + self._nodes_for_sections(doc)
+                      + self._nodes_for_if_section(ifcond))
+
+    def visit_event(self, name, info, ifcond, features, arg_type, boxed):
+        doc = self._cur_doc
+        self._add_doc('Event',
+                      self._nodes_for_arguments(doc,
+                                                arg_type if boxed else None)
+                      + self._nodes_for_features(doc)
+                      + self._nodes_for_sections(doc)
+                      + self._nodes_for_if_section(ifcond))
+
+    def symbol(self, doc, entity):
+        """Add documentation for one symbol to the document tree
+
+        This is the main entry point which causes us to add documentation
+        nodes for a symbol (which could be a 'command', 'object', 'event',
+        etc). We do this by calling 'visit' on the schema entity, which
+        will then call back into one of our visit_* methods, depending
+        on what kind of thing this symbol is.
+        """
+        self._cur_doc = doc
+        entity.visit(self)
+        self._cur_doc = None
+
+    def _start_new_heading(self, heading, level):
+        """Start a new heading at the specified heading level
+
+        Create a new section whose title is 'heading' and which is placed
+        in the docutils node tree as a child of the most recent level-1
+        heading. Subsequent document sections (commands, freeform doc chunks,
+        etc) will be placed as children of this new heading section.
+        """
+        if len(self._active_headings) < level:
+            raise QAPISemError(self._cur_doc.info,
+                               'Level %d subheading found outside a '
+                               'level %d heading'
+                               % (level, level - 1))
+        snode = self._make_section(heading)
+        self._active_headings[level - 1] += snode
+        self._active_headings = self._active_headings[:level]
+        self._active_headings.append(snode)
+
+    def _add_node_to_current_heading(self, node):
+        """Add the node to whatever the current active heading is"""
+        self._active_headings[-1] += node
+
+    def freeform(self, doc):
+        """Add a piece of 'freeform' documentation to the document tree
+
+        A 'freeform' document chunk doesn't relate to any particular
+        symbol (for instance, it could be an introduction).
+
+        If the freeform document starts with a line of the form
+        '= Heading text', this is a section or subsection heading, with
+        the heading level indicated by the number of '=' signs.
+        """
+
+        # QAPIDoc documentation says free-form documentation blocks
+        # must have only a body section, nothing else.
+        assert not doc.sections
+        assert not doc.args
+        assert not doc.features
+        self._cur_doc = doc
+
+        text = doc.body.text
+        if re.match(r'=+ ', text):
+            # Section/subsection heading (if present, will always be
+            # the first line of the block)
+            (heading, _, text) = text.partition('\n')
+            (leader, _, heading) = heading.partition(' ')
+            self._start_new_heading(heading, len(leader))
+            if text == '':
+                return
+
+        node = self._make_section(None)
+        self._parse_text_into_node(text, node)
+        self._add_node_to_current_heading(node)
+        self._cur_doc = None
+
+    def _parse_text_into_node(self, doctext, node):
+        """Parse a chunk of QAPI-doc-format text into the node
+
+        The doc comment can contain most inline rST markup, including
+        bulleted and enumerated lists.
+        As an extra permitted piece of markup, @var will be turned
+        into ``var``.
+        """
+
+        # Handle the "@var means ``var`` case
+        doctext = re.sub(r'@([\w-]+)', r'``\1``', doctext)
+
+        rstlist = ViewList()
+        for line in doctext.splitlines():
+            # The reported line number will always be that of the start line
+            # of the doc comment, rather than the actual location of the error.
+            # Being more precise would require overhaul of the QAPIDoc class
+            # to track lines more exactly within all the sub-parts of the doc
+            # comment, as well as counting lines here.
+            rstlist.append(line, self._cur_doc.info.fname,
+                           self._cur_doc.info.line)
+        # Append a blank line -- in some cases rST syntax errors get
+        # attributed to the line after one with actual text, and if there
+        # isn't anything in the ViewList corresponding to that then Sphinx
+        # 1.6's AutodocReporter will then misidentify the source/line location
+        # in the error message (usually attributing it to the top-level
+        # .rst file rather than the offending .json file). The extra blank
+        # line won't affect the rendered output.
+        rstlist.append("", self._cur_doc.info.fname, self._cur_doc.info.line)
+        self._sphinx_directive.do_parse(rstlist, node)
+
+    def get_document_nodes(self):
+        """Return the list of docutils nodes which make up the document"""
+        return self._top_node.children
+
+
+class QAPISchemaGenDepVisitor(QAPISchemaVisitor):
+    """A QAPI schema visitor which adds Sphinx dependencies each module
+
+    This class calls the Sphinx note_dependency() function to tell Sphinx
+    that the generated documentation output depends on the input
+    schema file associated with each module in the QAPI input.
+    """
+    def __init__(self, env, qapidir):
+        self._env = env
+        self._qapidir = qapidir
+
+    def visit_module(self, name):
+        if name is not None:
+            qapifile = self._qapidir + '/' + name
+            self._env.note_dependency(os.path.abspath(qapifile))
+        super().visit_module(name)
+
+
+class QAPIDocDirective(Directive):
+    """Extract documentation from the specified QAPI .json file"""
+    required_argument = 1
+    optional_arguments = 1
+    option_spec = {
+        'qapifile': directives.unchanged_required
+    }
+    has_content = False
+
+    def new_serialno(self):
+        """Return a unique new ID string suitable for use as a node's ID"""
+        env = self.state.document.settings.env
+        return 'qapidoc-%d' % env.new_serialno('qapidoc')
+
+    def run(self):
+        env = self.state.document.settings.env
+        qapifile = env.config.qapidoc_srctree + '/' + self.arguments[0]
+        qapidir = os.path.dirname(qapifile)
+
+        try:
+            schema = QAPISchema(qapifile)
+
+            # First tell Sphinx about all the schema files that the
+            # output documentation depends on (including 'qapifile' itself)
+            schema.visit(QAPISchemaGenDepVisitor(env, qapidir))
+
+            vis = QAPISchemaGenRSTVisitor(self)
+            vis.visit_begin(schema)
+            for doc in schema.docs:
+                if doc.symbol:
+                    vis.symbol(doc, schema.lookup_entity(doc.symbol))
+                else:
+                    vis.freeform(doc)
+            return vis.get_document_nodes()
+        except QAPIError as err:
+            # Launder QAPI parse errors into Sphinx extension errors
+            # so they are displayed nicely to the user
+            raise ExtensionError(str(err))
+
+    def do_parse(self, rstlist, node):
+        """Parse rST source lines and add them to the specified node
+
+        Take the list of rST source lines rstlist, parse them as
+        rST, and add the resulting docutils nodes as children of node.
+        The nodes are parsed in a way that allows them to include
+        subheadings (titles) without confusing the rendering of
+        anything else.
+        """
+        # This is from kerneldoc.py -- it works around an API change in
+        # Sphinx between 1.6 and 1.7. Unlike kerneldoc.py, we use
+        # sphinx.util.nodes.nested_parse_with_titles() rather than the
+        # plain self.state.nested_parse(), and so we can drop the saving
+        # of title_styles and section_level that kerneldoc.py does,
+        # because nested_parse_with_titles() does that for us.
+        if Use_SSI:
+            with switch_source_input(self.state, rstlist):
+                nested_parse_with_titles(self.state, rstlist, node)
+        else:
+            save = self.state.memo.reporter
+            self.state.memo.reporter = AutodocReporter(
+                rstlist, self.state.memo.reporter)
+            try:
+                nested_parse_with_titles(self.state, rstlist, node)
+            finally:
+                self.state.memo.reporter = save
+
+
+def setup(app):
+    """ Register qapi-doc directive with Sphinx"""
+    app.add_config_value('qapidoc_srctree', None, 'env')
+    app.add_directive('qapi-doc', QAPIDocDirective)
+
+    return dict(
+        version=__version__,
+        parallel_read_safe=True,
+        parallel_write_safe=True
+    )