[PATCH v3] meson: Pass -j option to sphinx

[Fabiano Rosas]

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.

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.

before:
  $ time make man html
  ...
  [1/2] Generating docs/QEMU manual with a custom command
  [2/2] Generating docs/QEMU man pages with a custom command

  real    0m43.157s
  user    0m42.642s
  sys     0m0.576s

after:
  $ time make man html
  ...
  [1/2] Generating docs/QEMU manual with a custom command
  [2/2] Generating docs/QEMU man pages with a custom command

  real    0m25.014s
  user    0m51.288s
  sys     0m2.085s

Tested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
---
 docs/meson.build           | 12 ++++++++++++
 docs/sphinx/dbusdomain.py  |  4 ++++
 docs/sphinx/fakedbusdoc.py |  5 +++++
 docs/sphinx/qmp_lexer.py   |  5 +++++
 4 files changed, 26 insertions(+)

diff --git a/docs/meson.build b/docs/meson.build
index f220800e3e..138ec6ce6f 100644
--- a/docs/meson.build
+++ b/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('>=5.1.2')
+    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.
diff --git a/docs/sphinx/dbusdomain.py b/docs/sphinx/dbusdomain.py
index 2ea95af623..9872fd5bf6 100644
--- a/docs/sphinx/dbusdomain.py
+++ b/docs/sphinx/dbusdomain.py
@@ -400,6 +400,10 @@ def get_objects(self) -> Iterator[Tuple[str, str, str, 
str, str, int]]:
         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)
diff --git a/docs/sphinx/fakedbusdoc.py b/docs/sphinx/fakedbusdoc.py
index d2c5079046..2d2e6ef640 100644
--- a/docs/sphinx/fakedbusdoc.py
+++ b/docs/sphinx/fakedbusdoc.py
@@ -23,3 +23,8 @@ def run(self):
 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
+    )
diff --git a/docs/sphinx/qmp_lexer.py b/docs/sphinx/qmp_lexer.py
index f7e4c0e198..a59de8a079 100644
--- a/docs/sphinx/qmp_lexer.py
+++ b/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
+    )
-- 
2.35.3