perf: add lru_cache to improve performance with multiple themes

These changes should improve the performance caused by the file I/O
when it's running in docker, using lru_cache to save thousands of calls to listdir
when running with a handful of themes defined in COMPREHENSIVE_THEME_DIRS.

lms/djangoapps/courseware/tests/test_comprehensive_theming.py

@@ -8,6 +8,8 @@
 
 from common.djangoapps import edxmako
 from openedx.core.djangoapps.theming.tests.test_util import with_comprehensive_theme
+from openedx.core.djangoapps.theming.helpers import get_themes
+from openedx.core.djangoapps.theming.helpers_dirs import get_theme_dirs
 from openedx.core.lib.tempdir import create_symlink, delete_symlink, mkdtemp_clean
 
 
@@ -20,6 +22,10 @@ def setUp(self):
         # Clear the internal staticfiles caches, to get test isolation.
         staticfiles.finders.get_finder.cache_clear()
 
+        # Clear cache on get_theme methods.
+        get_themes.cache_clear()
+        get_theme_dirs.cache_clear()
+
     @with_comprehensive_theme('red-theme')
     def test_red_footer(self):
         """

openedx/core/djangoapps/theming/helpers.py

@@ -23,6 +23,7 @@
     get_themes_unchecked
 )
 from openedx.core.lib.cache_utils import request_cached
+from functools import lru_cache
 
 logger = getLogger(__name__)  # pylint: disable=invalid-name
 
@@ -255,6 +256,7 @@ def theme_exists(theme_name, themes_dir=None):
     return False
 
 
+@lru_cache
 def get_themes(themes_dir=None):
     """
     get a list of all themes known to the system.

openedx/core/djangoapps/theming/helpers_dirs.py

@@ -7,6 +7,7 @@
 import os
 
 from path import Path
+from functools import lru_cache
 
 
 def get_theme_base_dirs_from_settings(theme_base_dirs=None):
@@ -49,6 +50,7 @@ def get_themes_unchecked(themes_base_dirs, project_root=None):
     return themes
 
 
+@lru_cache
 def get_theme_dirs(themes_base_dir=None):
     """
     Get all the theme dirs directly under a given base dir.

openedx/core/djangoapps/theming/tests/test_helpers.py

@@ -16,13 +16,23 @@
     get_themes,
     strip_site_theme_templates_path
 )
+from openedx.core.djangoapps.theming.helpers_dirs import get_theme_dirs
 from openedx.core.djangoapps.theming.tests.test_util import with_comprehensive_theme
 from openedx.core.djangolib.testing.utils import skip_unless_cms, skip_unless_lms
 
 
 class TestHelpers(TestCase):
     """Test comprehensive theming helper functions."""
 
+    def setUp(self):
+        """
+        Clear cache on get_theme methods.
+        """
+        super().setUp()
+
+        get_themes.cache_clear()
+        get_theme_dirs.cache_clear()
+
     def test_get_themes(self):
         """
         Tests template paths are returned from enabled theme.