doc: fix parallel builds

Several of our extensions don't declare they are parallel read or
write safe. Upon inspection, they are.

Not declaring parallel read safety defeats a lot of the speed ups that
are possible when using SPHINXOPTS="-j=auto", so mark the extensions
safe and get the performance back.

Signed-off-by: Marti Bolivar <marti@foundries.io>

Author: Marti Bolivar

doc/extensions/only/eager_only.py

@@ -31,7 +31,7 @@ def run(self, *args):
         # Evaluate the condition eagerly, and if false return no nodes right away
         env = self.state.document.settings.env
         env.app.builder.tags.add('TRUE')
-        #print(repr(self.arguments[0]))
+
         if not env.app.builder.tags.eval_condition(self.arguments[0]):
             return []
 
@@ -44,3 +44,13 @@ def run(self, *args):
 
 def setup(app):
     directives.register_directive('only', EagerOnly)
+
+    # The tags.add call above is setting tags.tags['TRUE'] = True.
+    # The operation is idempotent and will have taken effect before
+    # the next eval_condition() which may rely on it so this is thread
+    # safe both for read and writes (all other operations are local to
+    # the local nodes variable).
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+        }

doc/extensions/zephyr/html_redirects.py

@@ -43,6 +43,13 @@ def setup(app):
     app.add_config_value('html_redirect_pages', [], 'html')
     app.connect('build-finished', create_redirect_pages)
 
+    # Since we're just setting up a build-finished hook, which runs
+    # after both reading and writing, this extension is safe for both.
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }
+
 
 def create_redirect_pages(app, docname):
     if not isinstance(app.builder, StandaloneHTMLBuilder):

doc/extensions/zephyr/link-roles.py

@@ -28,6 +28,13 @@ def setup(app):
     app.add_role('zephyr_file', autolink('{}/blob/{}/%s'.format(baseurl, rev)))
     app.add_role('zephyr_raw', autolink('{}/raw/{}/%s'.format(baseurl, rev)))
 
+    # The role just creates new nodes based on information in the
+    # arguments; its behavior doesn't depend on any other documents.
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }
+
 
 def autolink(pattern):
     def role(name, rawtext, text, lineno, inliner, options={}, content=[]):