meson: Pass -j option to sphinx

Save a bit of build time by passing the number of jobs option to
sphinx.

We cannot use  the -j option from make because  meson does not support
setting build time parameters for custom targets. Use nproc instead or
the equivalent sphinx option "-j  auto", if that is available (version
>=1.7.0).

Also make sure our plugins support parallelism and report it properly
to sphinx. Particularly, implement the merge_domaindata method in
DBusDomain that is used to merge in data from other subprocesses.

Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20230503203947.3417-2-farosas@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/meson.build

@@ -10,6 +10,18 @@ if sphinx_build.found()
     SPHINX_ARGS += [ '-W', '-Dkerneldoc_werror=1' ]
   endif
 
+  sphinx_version = run_command(SPHINX_ARGS + ['--version'],
+                               check: true).stdout().split()[1]
+  if sphinx_version.version_compare('>=1.7.0')
+    SPHINX_ARGS += ['-j', 'auto']
+  else
+    nproc = find_program('nproc')
+    if nproc.found()
+      jobs = run_command(nproc, check: true).stdout()
+      SPHINX_ARGS += ['-j', jobs]
+    endif
+  endif
+
   # This is a bit awkward but works: create a trivial document and
   # try to run it with our configuration file (which enforces a
   # version requirement). This will fail if sphinx-build is too old.

docs/sphinx/dbusdomain.py

@@ -400,6 +400,10 @@ class DBusDomain(Domain):
         for refname, obj in self.objects.items():
             yield (refname, refname, obj.objtype, obj.docname, obj.node_id, 1)
 
+    def merge_domaindata(self, docnames, otherdata):
+        for name, obj in otherdata['objects'].items():
+            if obj.docname in docnames:
+                self.data['objects'][name] = obj
 
 def setup(app):
     app.add_domain(DBusDomain)

docs/sphinx/fakedbusdoc.py

@@ -23,3 +23,8 @@ class FakeDBusDocDirective(Directive):
 def setup(app: Sphinx) -> Dict[str, Any]:
     """Register a fake dbus-doc directive with Sphinx"""
     app.add_directive("dbus-doc", FakeDBusDocDirective)
+
+    return dict(
+        parallel_read_safe = True,
+        parallel_write_safe = True
+    )

docs/sphinx/qmp_lexer.py

@@ -41,3 +41,8 @@ def setup(sphinx):
         sphinx.add_lexer('QMP', QMPExampleLexer)
     except errors.VersionRequirementError:
         sphinx.add_lexer('QMP', QMPExampleLexer())
+
+    return dict(
+        parallel_read_safe = True,
+        parallel_write_safe = True
+    )