Template loaders defined before local settings

TEMPLATE_LOADERS was defined after loading local_settings.py and was sqashing
any attempt to customize the template loaders.
This patch adds CACHED_TEMPLATE_LOADERS, ADD_TEMPLATE_LOADERS and a doc
defining how to use them in order to ease customization process.

Change-Id: I2544529ee965ef01c6ac4973056801ebee50be6d
Closes-Bug: #1568764

doc/source/topics/settings.rst

@@ -1582,10 +1582,42 @@ Default: ``True``
 Controls whether unhandled exceptions should generate a generic 500 response
 or present the user with a pretty-formatted debug information page.
 
+When set, CACHED_TEMPLATE_LOADERS will not be cached.
+
 This setting should **always** be set to ``False`` for production deployments
 as the debug page can display sensitive information to users and attackers
 alike.
 
+``TEMPLATE_LOADERS``
+---------------------------
+
+.. versionadded:: 10.0.0(Newton)
+
+These template loaders will be the first loaders and get loaded before the
+CACHED_TEMPLATE_LOADERS. Use ADD_TEMPLATE_LOADERS if you want to add loaders at
+the end and not cache loaded templates.
+After the whole settings process has gone through, TEMPLATE_LOADERS will be:
+
+    TEMPLATE_LOADERS += (
+            ('django.template.loaders.cached.Loader', CACHED_TEMPLATE_LOADERS),
+        ) + tuple(ADD_TEMPLATE_LOADERS)
+
+``CACHED_TEMPLATE_LOADERS``
+---------------------------
+
+.. versionadded:: 10.0.0(Newton)
+
+Template loaders defined here will have their output cached if DEBUG
+is set to False.
+
+``ADD_TEMPLATE_LOADERS``
+---------------------------
+
+.. versionadded:: 10.0.0(Newton)
+
+Template loaders defined here will be be loaded at the end of TEMPLATE_LOADERS,
+after the CACHED_TEMPLATE_LOADERS and will never have a cached output.
+
 ``SECRET_KEY``
 --------------
 

openstack_dashboard/settings.py

@@ -123,6 +123,15 @@ TEMPLATE_CONTEXT_PROCESSORS = (
     'openstack_dashboard.context_processors.openstack',
 )
 
+TEMPLATE_LOADERS = ('horizon.themes.ThemeTemplateLoader',)
+
+CACHED_TEMPLATE_LOADERS = (
+    'django.template.loaders.filesystem.Loader',
+    'django.template.loaders.app_directories.Loader',
+    'horizon.loaders.TemplateLoader',)
+
+ADD_TEMPLATE_LOADERS = []
+
 TEMPLATE_DIRS = (
     os.path.join(ROOT_PATH, 'templates'),
 )
@@ -283,15 +292,12 @@ try:
 except ImportError:
     logging.warning("No local_settings file found.")
 
-_LOADERS = ('django.template.loaders.filesystem.Loader',
-            'django.template.loaders.app_directories.Loader',
-            'horizon.loaders.TemplateLoader',)
-
 if DEBUG:
-    TEMPLATE_LOADERS = ('horizon.themes.ThemeTemplateLoader',) + _LOADERS
+    TEMPLATE_LOADERS += CACHED_TEMPLATE_LOADERS + tuple(ADD_TEMPLATE_LOADERS)
 else:
-    TEMPLATE_LOADERS = ('horizon.themes.ThemeTemplateLoader',
+    TEMPLATE_LOADERS += (
-                        ('django.template.loaders.cached.Loader', _LOADERS),)
+        ('django.template.loaders.cached.Loader', CACHED_TEMPLATE_LOADERS),
+    ) + tuple(ADD_TEMPLATE_LOADERS)
 
 # allow to drop settings snippets into a local_settings_dir
 LOCAL_SETTINGS_DIR_PATH = os.path.join(ROOT_PATH, "local", "local_settings.d")

releasenotes/notes/bug-1568764-cached-template-loaders-3536f35e11099eba.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - The final django TEMPLATE_LOADERS configuration will now be generated from
+    TEMPLATE_LOADERS, CACHED_TEMPLATE_LOADERS and ADD_TEMPLATE_LOADERS
+    settings. See the settings documentation for more information.