Feature flag to make `readthedocs` theme default on MkDocs docs

Historically, we were using `readthedocs` as default theme for MkDocs
but in https://github.com/rtfd/readthedocs.org/pull/4556 we decided to
change it to get the same behavior when building locally than in Read
the Docs.

This commit adds a Feature flag to keep having the old behavior for
some particular projects so we can add these project and do not break
their documentation (change the theme without asking/reason).

readthedocs/doc_builder/backends/mkdocs.py

@@ -16,6 +16,7 @@ from django.template import loader as template_loader
 
 from readthedocs.doc_builder.base import BaseBuilder
 from readthedocs.doc_builder.exceptions import BuildEnvironmentError
+from readthedocs.projects.models import Feature
 
 log = logging.getLogger(__name__)
 
@@ -50,6 +51,23 @@ class BaseMkdocs(BaseBuilder):
         self.root_path = self.version.project.checkout_path(self.version.slug)
         self.yaml_file = self.get_yaml_config()
 
+        # README: historically, the default theme was ``readthedocs`` but in
+        # https://github.com/rtfd/readthedocs.org/pull/4556 we change it to
+        # ``mkdocs`` to maintain the same behavior in Read the Docs than
+        # building locally. Although, we can't apply the this into the Corporate
+        # site. To keep the same default theme there, we created a Feature flag
+        # for these project that were building with MkDocs in the Corporate
+        # site.
+        if self.project.has_feature(Feature.MKDOCS_THEME_RTD):
+            self.DEFAULT_THEME_NAME = 'readthedocs'
+            log.warning(
+                'Project using readthedocs theme as default for MkDocs: slug=%s',
+                self.project.slug,
+            )
+        else:
+            self.DEFAULT_THEME_NAME = 'mkdocs'
+
+
     def get_yaml_config(self):
         """Find the ``mkdocs.yml`` file in the project root."""
         mkdoc_path = self.config.mkdocs.configuration
@@ -130,6 +148,13 @@ class BaseMkdocs(BaseBuilder):
         # This supports using RTD's privacy improvements around analytics
         user_config['google_analytics'] = None
 
+        # README: make MkDocs to use ``readthedocs`` theme as default if the
+        # user didn't specify a specific theme manually
+        if self.project.has_feature(Feature.MKDOCS_THEME_RTD):
+            if 'theme' not in user_config:
+                # mkdocs<0.17 syntax
+                user_config['theme'] = self.DEFAULT_THEME_NAME
+
         # Write the modified mkdocs configuration
         yaml.safe_dump(
             user_config,

readthedocs/projects/migrations/0028_feature-flag-mkdocs-theme.py

@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.16 on 2018-10-24 07:43
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+FEATURE_ID = 'mkdocs_theme_rtd'
+
+
+def forward_add_feature(apps, schema_editor):
+    Feature = apps.get_model('projects', 'Feature')
+    Feature.objects.create(
+        feature_id=FEATURE_ID,
+        # Not using ``default_true=True`` because we will do this manually in
+        # the database from the Corporate site only, since this is not required
+        # in the Community site
+        # default_true=True,
+    )
+
+
+def reverse_add_feature(apps, schema_editor):
+    Feature = apps.get_model('projects', 'Feature')
+    Feature.objects.filter(feature_id=FEATURE_ID).delete()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('projects', '0027_remove_json_with_html_feature'),
+    ]
+
+    operations = [
+        migrations.RunPython(forward_add_feature, reverse_add_feature),
+    ]

readthedocs/projects/models.py

@@ -1060,6 +1060,7 @@ class Feature(models.Model):
     SKIP_SUBMODULES = 'skip_submodules'
     DONT_OVERWRITE_SPHINX_CONTEXT = 'dont_overwrite_sphinx_context'
     ALLOW_V2_CONFIG_FILE = 'allow_v2_config_file'
+    MKDOCS_THEME_RTD = 'mkdocs_theme_rtd'
 
     FEATURES = (
         (USE_SPHINX_LATEST, _('Use latest version of Sphinx')),
@@ -1071,6 +1072,7 @@ class Feature(models.Model):
             'Do not overwrite context vars in conf.py with Read the Docs context',)),
         (ALLOW_V2_CONFIG_FILE, _(
             'Allow to use the v2 of the configuration file')),
+        (MKDOCS_THEME_RTD, _('Use Read the Docs theme for MkDocs as default theme')),
     )
 
     projects = models.ManyToManyField(

readthedocs/rtd_tests/tests/test_doc_builder.py

@@ -20,7 +20,7 @@ from readthedocs.doc_builder.backends.mkdocs import MkdocsHTML
 from readthedocs.doc_builder.backends.sphinx import BaseSphinx
 from readthedocs.doc_builder.python_environments import Virtualenv
 from readthedocs.projects.exceptions import ProjectConfigurationError
-from readthedocs.projects.models import Project
+from readthedocs.projects.models import Feature, Project
 
 
 class SphinxBuilderTest(TestCase):
@@ -224,6 +224,67 @@ class MkdocsBuilderTest(TestCase):
         }
         self.assertEqual(builder.get_theme_name(config), 'mydir')
 
+    @patch('readthedocs.doc_builder.base.BaseBuilder.run')
+    @patch('readthedocs.projects.models.Project.checkout_path')
+    def test_get_theme_name_with_feature_flag(self, checkout_path, run):
+        tmpdir = tempfile.mkdtemp()
+        checkout_path.return_value = tmpdir
+        Feature.objects.get(
+            feature_id=Feature.MKDOCS_THEME_RTD,
+        ).projects.add(self.project)
+
+        python_env = Virtualenv(
+            version=self.version,
+            build_env=self.build_env,
+            config=None,
+        )
+        builder = MkdocsHTML(
+            build_env=self.build_env,
+            python_env=python_env,
+        )
+        self.assertEqual(builder.get_theme_name({}), 'readthedocs')
+        with patch('readthedocs.doc_builder.backends.mkdocs.yaml') as mock_yaml:
+            with patch('readthedocs.doc_builder.backends.mkdocs.MkdocsHTML.load_yaml_config') as mock_load_yaml_config:
+                mock_load_yaml_config.return_value = {'site_name': self.project.name}
+                builder.append_conf()
+
+            mock_yaml.safe_dump.assert_called_once_with(
+                {
+                    'site_name': mock.ANY,
+                    'docs_dir': mock.ANY,
+                    'extra_javascript': mock.ANY,
+                    'extra_css': mock.ANY,
+                    'google_analytics': mock.ANY,
+                    'theme': 'readthedocs',
+                },
+                mock.ANY,
+            )
+            mock_yaml.reset_mock()
+
+            config = {
+                'theme': 'customtheme',
+            }
+            self.assertEqual(builder.get_theme_name(config), 'customtheme')
+            with patch('readthedocs.doc_builder.backends.mkdocs.MkdocsHTML.load_yaml_config') as mock_load_yaml_config:
+                mock_load_yaml_config.return_value = {
+                    'site_name': self.project.name,
+                    'theme': 'customtheme',
+                }
+                builder.append_conf()
+
+            mock_yaml.safe_dump.assert_called_once_with(
+                {
+                    'site_name': mock.ANY,
+                    'docs_dir': mock.ANY,
+                    'extra_javascript': mock.ANY,
+                    'extra_css': mock.ANY,
+                    'google_analytics': mock.ANY,
+                    'theme': 'customtheme',
+                },
+                mock.ANY,
+            )
+
+
     @patch('readthedocs.doc_builder.base.BaseBuilder.run')
     @patch('readthedocs.projects.models.Project.checkout_path')
     def test_append_conf_create_yaml(self, checkout_path, run):