refactor: always define CORS_ALLOW_HEADERS centrally

The LMS and Studio need to set values for CORS_ALLOW_HEADERS so that the
MFEs can work properly, since preflight requests will need to send over
extra headers. Prior to this commit, CORS_ALLOW_HEADERS was being
redefined in multiple places in edx-platform and again in Tutor's config
because it was only being conditionally set if ENABLE_CORS_HEADERS was
True (which was a policy setting). But CORS_ALLOW_HEADERS is application
logic in that the value is determined by what the view needs, and won't
vary by deployment.

By consolidating this to always be defined in the common.py files, we
make sure that deployment environments don't have to define it. An
example of where this bit us was when course import in the course
authoring MFE did not work because Tutor was using an outdated value for
this setting.

A followup to this would be to just rip out the ENABLE_CORS_HEADERS
setting entirely, and just always have it on. But that would benefit
from a little more discovery to make sure there's no weird use case that
still requires it to be False (maybe something in the test suite?).

cms/envs/common.py

@@ -2575,11 +2575,15 @@
     CORS_ORIGIN_WHITELIST = ()
     CORS_ORIGIN_ALLOW_ALL = False
     CORS_ALLOW_INSECURE = False
-    CORS_ALLOW_HEADERS = corsheaders_default_headers + (
-        'use-jwt-cookie',
-        'content-range',
-        'content-disposition',
-    )
+
+# Set CORS_ALLOW_HEADERS regardless of whether we've enabled ENABLE_CORS_HEADERS
+# because that decision might happen in a later config file. (The headers to
+# allow is an application logic, and not site policy.)
+CORS_ALLOW_HEADERS = corsheaders_default_headers + (
+    'use-jwt-cookie',
+    'content-range',
+    'content-disposition',
+)
 
 LOGIN_REDIRECT_WHITELIST = []
 

cms/envs/devstack.py

@@ -5,7 +5,6 @@
 
 import logging
 from os.path import abspath, dirname, join
-from corsheaders.defaults import default_headers as corsheaders_default_headers
 
 from .production import *  # pylint: disable=wildcard-import, unused-wildcard-import
 
@@ -256,11 +255,6 @@ def should_show_debug_toolbar(request):  # lint-amnesty, pylint: disable=missing
 FEATURES['ENABLE_CORS_HEADERS'] = True
 CORS_ALLOW_CREDENTIALS = True
 CORS_ORIGIN_ALLOW_ALL = True
-CORS_ALLOW_HEADERS = corsheaders_default_headers + (
-    'use-jwt-cookie',
-    'content-range',
-    'content-disposition',
-)
 
 ################### Special Exams (Proctoring) and Prereqs ###################
 FEATURES['ENABLE_SPECIAL_EXAMS'] = True

cms/envs/production.py

@@ -13,7 +13,6 @@
 import warnings
 import yaml
 
-from corsheaders.defaults import default_headers as corsheaders_default_headers
 import django
 from django.core.exceptions import ImproperlyConfigured
 from django.urls import reverse_lazy
@@ -598,11 +597,6 @@ def get_env_setting(setting):
 
     CORS_ORIGIN_ALLOW_ALL = ENV_TOKENS.get('CORS_ORIGIN_ALLOW_ALL', False)
     CORS_ALLOW_INSECURE = ENV_TOKENS.get('CORS_ALLOW_INSECURE', False)
-    CORS_ALLOW_HEADERS = corsheaders_default_headers + (
-        'use-jwt-cookie',
-        'content-range',
-        'content-disposition',
-    )
 
 ################# Settings for brand logos. #################
 LOGO_URL = ENV_TOKENS.get('LOGO_URL', LOGO_URL)

lms/envs/common.py

@@ -3675,9 +3675,13 @@ def _make_locale_paths(settings):  # pylint: disable=missing-function-docstring
     CORS_ORIGIN_WHITELIST = ()
     CORS_ORIGIN_ALLOW_ALL = False
     CORS_ALLOW_INSECURE = False
-    CORS_ALLOW_HEADERS = corsheaders_default_headers + (
-        'use-jwt-cookie',
-    )
+
+# Set CORS_ALLOW_HEADERS regardless of whether we've enabled ENABLE_CORS_HEADERS
+# because that decision might happen in a later config file. (The headers to
+# allow is an application logic, and not site policy.)
+CORS_ALLOW_HEADERS = corsheaders_default_headers + (
+    'use-jwt-cookie',
+)
 
 # Default cache expiration for the cross-domain proxy HTML page.
 # This is a static page that can be iframed into an external page

lms/envs/devstack.py

@@ -7,8 +7,6 @@
 import logging
 from os.path import abspath, dirname, join
 
-from corsheaders.defaults import default_headers as corsheaders_default_headers
-
 # pylint: enable=unicode-format-string  # lint-amnesty, pylint: disable=bad-option-value
 #####################################################################
 from edx_django_utils.plugins import add_plugins
@@ -295,9 +293,6 @@ def should_show_debug_toolbar(request):  # lint-amnesty, pylint: disable=missing
 CORS_ALLOW_CREDENTIALS = True
 CORS_ORIGIN_WHITELIST = ()
 CORS_ORIGIN_ALLOW_ALL = True
-CORS_ALLOW_HEADERS = corsheaders_default_headers + (
-    'use-jwt-cookie',
-)
 
 LOGIN_REDIRECT_WHITELIST.extend([
     CMS_BASE,

lms/envs/production.py

@@ -22,7 +22,6 @@
 import os
 
 import yaml
-from corsheaders.defaults import default_headers as corsheaders_default_headers
 import django
 from django.core.exceptions import ImproperlyConfigured
 from edx_django_utils.plugins import add_plugins
@@ -382,9 +381,6 @@ def get_env_setting(setting):
 
     CORS_ORIGIN_ALLOW_ALL = ENV_TOKENS.get('CORS_ORIGIN_ALLOW_ALL', False)
     CORS_ALLOW_INSECURE = ENV_TOKENS.get('CORS_ALLOW_INSECURE', False)
-    CORS_ALLOW_HEADERS = corsheaders_default_headers + (
-        'use-jwt-cookie',
-    )
 
     # If setting a cross-domain cookie, it's really important to choose
     # a name for the cookie that is DIFFERENT than the cookies used