xblock-external-ui: Add XBlock API call to render XBlock views

Included commits:
* xblock-external-ui: Include CSRF token in the API answer
* xblock-external-ui: Adds support for CORS headers (cross-domain request)
* xblock-external-ui: Include full path when building local_url
* xblock-external-ui: Fix TestHandleXBlockCallback & bok_choy, add tests
* xblock-external-ui: Only return `instance` in `_invoke_xblock_handler()`
* xblock-external-ui: Group resources by hash tag to avoid duplicate loads
* xblock-external-ui: Alternate referer check for CORS requests

common/djangoapps/cors_csrf/middleware.py

+"""
+Middleware for handling CSRF checks with CORS requests
+
+When processing HTTPS requests, the default CSRF middleware checks that the referer
+domain and protocol is the same as the request's domain and protocol. This is meant
+to avoid a type of attack for sites which serve their content with both HTTP and HTTPS,
+with a man in the middle on the HTTP requests.
+
+https://github.com/django/django/blob/b91c385e324f1cb94d20e2ad146372c259d51d3b/django/middleware/csrf.py#L117
+
+This doesn't work well with CORS requests, which aren't vulnerable to this attack when
+the server from which the request is coming uses HTTPS too, as it prevents the man in the
+middle attack vector.
+
+We thus do the CSRF check of requests coming from an authorized CORS host separately
+in this middleware, applying the same protections as the default CSRF middleware, but
+without the referrer check, when both the request and the referer use HTTPS.
+"""
+
+import logging
+import urlparse
+
+from django.conf import settings
+from django.middleware.csrf import CsrfViewMiddleware
+
+log = logging.getLogger(__name__)
+
+
+class CorsCSRFMiddleware(CsrfViewMiddleware):
+    def is_enabled(self, request):
+        if not settings.FEATURES.get('ENABLE_CORS_HEADERS'):
+            return False
+
+        referer = request.META.get('HTTP_REFERER')
+        if referer is None or referer == '':
+            return False
+        referer_parts = urlparse.urlparse(referer)
+
+        if referer_parts.hostname not in getattr(settings, 'CORS_ORIGIN_WHITELIST', []):
+            return False
+        if not request.is_secure() or referer_parts.scheme != 'https':
+            return False
+
+        return True
+
+    def process_view(self, request, callback, callback_args, callback_kwargs):
+        log.warn('oooooo')
+        if not self.is_enabled(request):
+            return
+
+        is_secure_default = request.is_secure
+        def is_secure_patched():
+            return False
+        request.is_secure = is_secure_patched
+
+        res = super(CorsCSRFMiddleware, self).process_view(request, callback, callback_args, callback_kwargs)
+        request.is_secure = is_secure_default
+        return res

lms/djangoapps/courseware/module_render.py

+import hashlib
 import json
 import logging
 import mimetypes
 
 import static_replace
 
+from collections import OrderedDict
 from functools import partial
 from requests.auth import HTTPBasicAuth
 from dogapi import dog_stats_api
@@ -11,6 +13,7 @@ from dogapi import dog_stats_api
 from django.conf import settings
 from django.contrib.auth.models import User
 from django.core.cache import cache
+from django.core.context_processors import csrf
 from django.core.exceptions import PermissionDenied
 from django.core.urlresolvers import reverse
 from django.http import Http404, HttpResponse
@@ -29,7 +32,7 @@ from student.models import anonymous_id_for_user, user_by_anonymous_id
 from xblock.core import XBlock
 from xblock.fields import Scope
 from xblock.runtime import KvsFieldData, KeyValueStore
-from xblock.exceptions import NoSuchHandlerError
+from xblock.exceptions import NoSuchHandlerError, NoSuchViewError
 from xblock.django.request import django_to_webob_request, webob_to_django_response
 from xmodule.error_module import ErrorDescriptor, NonStaffErrorDescriptor
 from xmodule.exceptions import NotFoundError, ProcessingError
@@ -562,23 +565,19 @@ def xblock_resource(request, block_type, uri):  # pylint: disable=unused-argumen
     return HttpResponse(content, mimetype=mimetype)
 
 
-def _invoke_xblock_handler(request, course_id, usage_id, handler, suffix, user):
+def _get_module_by_usage_id(request, course_id, usage_id):
     """
-    Invoke an XBlock handler, either authenticated or not.
+    Gets a module instance based on its `usage_id` in a course, for a given request/user
 
+    Returns (location, descriptor, instance)
     """
     location = unquote_slashes(usage_id)
+    user = request.user
 
     # Check parameters and fail fast if there's a problem
     if not Location.is_valid(location):
         raise Http404("Invalid location")
 
-    # Check submitted files
-    files = request.FILES or {}
-    error_msg = _check_files_limits(files)
-    if error_msg:
-        return HttpResponse(json.dumps({'success': error_msg}))
-
     try:
         descriptor = modulestore().get_instance(course_id, location)
     except ItemNotFoundError:
@@ -590,13 +589,6 @@ def _invoke_xblock_handler(request, course_id, usage_id, handler, suffix, user):
         )
         raise Http404
 
-    tracking_context_name = 'module_callback_handler'
-    tracking_context = {
-        'module': {
-            'display_name': descriptor.display_name_with_default,
-        }
-    }
-
     field_data_cache = FieldDataCache.cache_for_descriptor_descendents(
         course_id,
         user,
@@ -609,6 +601,30 @@ def _invoke_xblock_handler(request, course_id, usage_id, handler, suffix, user):
         log.debug("No module %s for user %s -- access denied?", location, user)
         raise Http404
 
+    return instance
+
+
+def _invoke_xblock_handler(request, course_id, usage_id, handler, suffix, user):
+    """
+    Invoke an XBlock handler, either authenticated or not.
+
+    """
+
+    # Check submitted files
+    files = request.FILES or {}
+    error_msg = _check_files_limits(files)
+    if error_msg:
+        return HttpResponse(json.dumps({'success': error_msg}))
+
+    instance = _get_module_by_usage_id(request, course_id, usage_id)
+
+    tracking_context_name = 'module_callback_handler'
+    tracking_context = {
+        'module': {
+            'display_name': instance.display_name_with_default,
+        }
+    }
+
     req = django_to_webob_request(request)
     try:
         with tracker.get_tracker().context(tracking_context_name, tracking_context):
@@ -637,6 +653,44 @@ def _invoke_xblock_handler(request, course_id, usage_id, handler, suffix, user):
     return webob_to_django_response(resp)
 
 
+def hash_resource(resource):
+    """
+    Hash a :class:`xblock.fragment.FragmentResource
+    """
+    md5 = hashlib.md5()
+    for data in resource:
+        md5.update(data)
+    return md5.hexdigest()
+
+
+def xblock_view(request, course_id, usage_id, view_name):
+    """
+    Returns the rendered view of a given XBlock, with related resources
+
+    Returns a json object containing two keys:
+        html: The rendered html of the view
+        resources: A list of tuples where the first element is the resource hash, and
+            the second is the resource description
+    """
+    instance = _get_module_by_usage_id(request, course_id, usage_id)
+
+    try:
+        fragment = instance.render(view_name)
+    except NoSuchViewError:
+        log.exception("Attempt to render missing view on %s: %s", instance, view_name)
+        raise Http404
+
+    hashed_resources = OrderedDict()
+    for resource in fragment.resources:
+        hashed_resources[hash_resource(resource)] = resource
+
+    return JsonResponse({
+        'html': fragment.content,
+        'resources': hashed_resources.items(),
+        'csrf_token': str(csrf(request)['csrf_token']),
+    })
+
+
 def get_score_bucket(grade, max_grade):
     """
     Function to split arbitrary score ranges into 3 buckets.

lms/djangoapps/courseware/tests/test_module_render.py

@@ -306,6 +306,28 @@ class TestHandleXBlockCallback(ModuleStoreTestCase, LoginEnrollmentTestCase):
                 'bad_dispatch',
             )
 
+    def test_xblock_view_handler(self):
+        args=[
+            'edX/toy/2012_Fall',
+            quote_slashes('i4x://edX/toy/videosequence/Toy_Videos'),
+            'student_view'
+        ]
+        xblock_view_url = reverse(
+            'xblock_view',
+            args=args
+        )
+
+        request = self.request_factory.get(xblock_view_url)
+        request.user = self.mock_user
+        response = render.xblock_view(request, *args)
+        self.assertEquals(200, response.status_code)
+
+        expected = ['csrf_token',  'html', 'resources']
+        content = json.loads(response.content)
+        for section in expected:
+            self.assertIn(section, content)
+        self.assertIn('<div class="xblock xblock-student_view xmodule_display', content['html'])
+
 
 @override_settings(MODULESTORE=TEST_DATA_MIXED_MODULESTORE)
 class TestTOC(TestCase):

lms/envs/aws.py

@@ -276,6 +276,17 @@ if FEATURES.get('AUTH_USE_CAS'):
 
 HOSTNAME_MODULESTORE_DEFAULT_MAPPINGS = ENV_TOKENS.get('HOSTNAME_MODULESTORE_DEFAULT_MAPPINGS',{})
 
+############# CORS headers for cross-domain requests #################
+
+if FEATURES.get('ENABLE_CORS_HEADERS'):
+    INSTALLED_APPS += ('corsheaders', 'cors_csrf')
+    MIDDLEWARE_CLASSES = (
+        'corsheaders.middleware.CorsMiddleware',
+        'cors_csrf.middleware.CorsCSRFMiddleware',
+    ) + MIDDLEWARE_CLASSES
+    CORS_ALLOW_CREDENTIALS = True
+    CORS_ORIGIN_WHITELIST = ENV_TOKENS.get('CORS_ORIGIN_WHITELIST', ())
+
 ############################## SECURE AUTH ITEMS ###############
 # Secret things: passwords, access keys, etc.
 

lms/envs/bok_choy.env.json

@@ -88,7 +88,7 @@
     "SEGMENT_IO_LMS": true,
     "SERVER_EMAIL": "devops@example.com",
     "SESSION_COOKIE_DOMAIN": null,
-    "SITE_NAME": "localhost",
+    "SITE_NAME": "localhost:8003",
     "STATIC_ROOT_BASE": "** OVERRIDDEN **",
     "STATIC_URL_BASE": "/static/",
     "SYSLOG_SERVER": "",

lms/envs/common.py

@@ -113,6 +113,9 @@ FEATURES = {
     # with Shib.  Feature was requested by Stanford's office of general counsel
     'SHIB_DISABLE_TOS': False,
 
+    # Allows to configure the LMS to provide CORS headers to serve requests from other domains
+    'ENABLE_CORS_HEADERS': False,
+
     # Can be turned off if course lists need to be hidden. Effects views and templates.
     'COURSES_ARE_BROWSABLE': True,
 
@@ -1275,6 +1278,17 @@ if FEATURES.get('AUTH_USE_CAS'):
     INSTALLED_APPS += ('django_cas',)
     MIDDLEWARE_CLASSES += ('django_cas.middleware.CASMiddleware',)
 
+############# CORS headers for cross-domain requests #################
+
+if FEATURES.get('ENABLE_CORS_HEADERS'):
+    INSTALLED_APPS += ('corsheaders', 'cors_csrf')
+    MIDDLEWARE_CLASSES = (
+        'corsheaders.middleware.CorsMiddleware',
+        'cors_csrf.middleware.CorsCSRFMiddleware',
+    ) + MIDDLEWARE_CLASSES
+    CORS_ALLOW_CREDENTIALS = True
+    CORS_ORIGIN_WHITELIST = ()
+
 ###################### Registration ##################################
 
 # For each of the fields, give one of the following values:

lms/lib/xblock/runtime.py

@@ -4,6 +4,7 @@ Module implementing `xblock.runtime.Runtime` functionality for the LMS
 
 import re
 
+from django.conf import settings
 from django.core.urlresolvers import reverse
 
 from user_api import user_service
@@ -106,10 +107,11 @@ class LmsHandlerUrls(object):
         """
         local_resource_url for Studio
         """
-        return reverse('xblock_resource_url', kwargs={
+        path = reverse('xblock_resource_url', kwargs={
             'block_type': block.scope_ids.block_type,
             'uri': uri,
         })
+        return '//{}{}'.format(settings.SITE_NAME, path)
 
 
 class LmsPartitionService(PartitionService):

lms/urls.py

@@ -190,6 +190,9 @@ if settings.COURSEWARE_ENABLED:
         url(r'^courses/(?P<course_id>[^/]+/[^/]+/[^/]+)/xblock/(?P<usage_id>[^/]*)/handler/(?P<handler>[^/]*)(?:/(?P<suffix>.*))?$',
             'courseware.module_render.handle_xblock_callback',
             name='xblock_handler'),
+        url(r'^courses/(?P<course_id>[^/]+/[^/]+/[^/]+)/xblock/(?P<usage_id>[^/]*)/view/(?P<view_name>[^/]*)$',
+            'courseware.module_render.xblock_view',
+            name='xblock_view'),
         url(r'^courses/(?P<course_id>[^/]+/[^/]+/[^/]+)/xblock/(?P<usage_id>[^/]*)/handler_noauth/(?P<handler>[^/]*)(?:/(?P<suffix>.*))?$',
             'courseware.module_render.handle_xblock_callback_noauth',
             name='xblock_handler_noauth'),

requirements/edx/base.txt

@@ -91,6 +91,9 @@ sphinx_rtd_theme==0.1.5
 Babel==1.3
 transifex-client==0.10
 
+# Used to allow to configure CORS headers for cross-domain requests
+django-cors-headers==0.12
+
 # We've tried several times to update the debug toolbar to version 1.0.1,
 # and had problems each time, resulting in us rolling back to 0.9.4. Before
 # submitting another pull request to do this update, check the following: