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feat: Start conversion of StaticContentServer from middleware into vi…
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…ew (#34703)

See #34702

This necessarily involves switching from calling
`StaticContent.is_versioned_asset_path` to determine whether to handle the
request to having a hardcoded urlpattern. I've made the choice to hardcode
the other two patterns similarly rather than using imported constants. The
mapping of URL patterns to database records should be explicit (even though
we don't expect those constants to change out from under us.)

I've renamed the middleware rather than choosing a new name for the
implementation because there are other references in tests and other code.
This was the smaller change.

A note on HTTP methods: The middleware currently completely ignores the
request's HTTP method, so I wanted to confirm that only GETs were being
used in practice. This query reveals that 99.8% of requests that this
middleware handles are GET, with just a smattering of PROPFIND and OPTIONS
and a tiny number of HEAD and POST:
```
from Transaction select count(*) facet request.method
where name = 'WebTransaction/Function/openedx.core.djangoapps.contentserver.middleware:StaticContentServer'
since 4 weeks ago
```
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timmc-edx authored Jun 10, 2024
1 parent 5a25e7c commit 0eb61e2
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Showing 7 changed files with 113 additions and 3 deletions.
2 changes: 1 addition & 1 deletion cms/envs/common.py
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Expand Up @@ -948,7 +948,7 @@
'openedx.core.djangoapps.cache_toolbox.middleware.CacheBackedAuthenticationMiddleware',

'common.djangoapps.student.middleware.UserStandingMiddleware',
'openedx.core.djangoapps.contentserver.middleware.StaticContentServer',
'openedx.core.djangoapps.contentserver.middleware.StaticContentServerMiddleware',

'django.contrib.messages.middleware.MessageMiddleware',
'common.djangoapps.track.middleware.TrackMiddleware',
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3 changes: 3 additions & 0 deletions cms/urls.py
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Expand Up @@ -74,6 +74,9 @@
path('heartbeat', include('openedx.core.djangoapps.heartbeat.urls')),
path('i18n/', include('django.conf.urls.i18n')),

# Course assets
path('', include('openedx.core.djangoapps.contentserver.urls')),

# User API endpoints
path('api/user/', include('openedx.core.djangoapps.user_api.urls')),

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2 changes: 1 addition & 1 deletion lms/envs/common.py
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Expand Up @@ -2287,7 +2287,7 @@ def _make_locale_paths(settings): # pylint: disable=missing-function-docstring
'openedx.core.djangoapps.safe_sessions.middleware.EmailChangeMiddleware',

'common.djangoapps.student.middleware.UserStandingMiddleware',
'openedx.core.djangoapps.contentserver.middleware.StaticContentServer',
'openedx.core.djangoapps.contentserver.middleware.StaticContentServerMiddleware',

# Adds user tags to tracking events
# Must go before TrackMiddleware, to get the context set up
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3 changes: 3 additions & 0 deletions lms/urls.py
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Expand Up @@ -111,6 +111,9 @@

path('i18n/', include('django.conf.urls.i18n')),

# Course assets
path('', include('openedx.core.djangoapps.contentserver.urls')),

# Enrollment API RESTful endpoints
path('api/enrollment/v1/', include('openedx.core.djangoapps.enrollments.urls')),

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32 changes: 31 additions & 1 deletion openedx/core/djangoapps/contentserver/middleware.py
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Expand Up @@ -16,6 +16,7 @@
)
from django.utils.deprecation import MiddlewareMixin
from edx_django_utils.monitoring import set_custom_attribute
from edx_toggles.toggles import WaffleFlag
from opaque_keys import InvalidKeyError
from opaque_keys.edx.locator import AssetLocator

Expand All @@ -32,19 +33,45 @@

log = logging.getLogger(__name__)

# .. toggle_name: content_server.use_view
# .. toggle_implementation: WaffleFlag
# .. toggle_default: False
# .. toggle_description: Deployment flag for switching asset serving from a middleware
# to a view. Intended to be used once in each environment to test the cutover and
# ensure there are no errors or changes in behavior. Once this has been tested,
# the middleware can be fully converted to a view.
# .. toggle_use_cases: temporary
# .. toggle_creation_date: 2024-05-02
# .. toggle_target_removal_date: 2024-07-01
# .. toggle_tickets: https://github.com/openedx/edx-platform/issues/34702
CONTENT_SERVER_USE_VIEW = WaffleFlag('content_server.use_view', module_name=__name__)

# TODO: Soon as we have a reasonable way to serialize/deserialize AssetKeys, we need
# to change this file so instead of using course_id_partial, we're just using asset keys

HTTP_DATE_FORMAT = "%a, %d %b %Y %H:%M:%S GMT"


class StaticContentServer(MiddlewareMixin):
class StaticContentServerMiddleware(MiddlewareMixin):
"""
Shim to maintain old pattern of serving course assets from a middleware. See views.py.
"""
def process_request(self, request):
"""Intercept asset request or allow view to handle it, depending on config."""
if CONTENT_SERVER_USE_VIEW.is_enabled():
return
else:
set_custom_attribute('content_server.handled_by.middleware', True)
return IMPL.process_request(request)


class StaticContentServer():
"""
Serves course assets to end users. Colloquially referred to as "contentserver."
"""
def is_asset_request(self, request):
"""Determines whether the given request is an asset request"""
# Don't change this without updating urls.py! See docstring of views.py.
return (
request.path.startswith('/' + XASSET_LOCATION_TAG + '/')
or
Expand Down Expand Up @@ -295,6 +322,9 @@ def load_asset_from_location(self, location):
return content


IMPL = StaticContentServer()


def parse_range_header(header_value, content_length):
"""
Returns the unit and a list of (start, end) tuples of ranges.
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16 changes: 16 additions & 0 deletions openedx/core/djangoapps/contentserver/urls.py
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@@ -0,0 +1,16 @@
"""
URL patterns for course asset serving.
"""

from django.urls import path, re_path

from . import views

# These patterns are incomplete and do not capture the variable
# components of the URLs. That's because the view itself is separately
# parsing the paths, for historical reasons. See docstring on views.py.
urlpatterns = [
path("c4x/", views.course_assets_view),
re_path("^asset-v1:", views.course_assets_view),
re_path("^assets/courseware/", views.course_assets_view),
]
58 changes: 58 additions & 0 deletions openedx/core/djangoapps/contentserver/views.py
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@@ -0,0 +1,58 @@
"""
Views for serving course assets.
Historically, this was implemented as a *middleware* (StaticContentServer) that
intercepted requests with paths matching certain patterns, rather than using
urlpatterns and a view. There wasn't any good reason for this, as far as I can
tell. It causes some problems for telemetry: When the code-owner middleware asks
Django what view handled the request, it does so by looking at the result of the
`resolve` utility, but these URLs get a Resolver404 (because there's no
registered urlpattern).
We'd like to turn this into a proper view:
https://github.com/openedx/edx-platform/issues/34702
The first step, seen here, is to have urlpatterns (redundant with the
middleware's `is_asset_request` method) and a view, but the view just calls into
the same code the middleware uses. The implementation of the middleware has been
moved into StaticContentServerImpl, leaving the middleware as just a shell
around the latter.
A waffle flag chooses whether to allow the middleware to handle the request, or
whether to pass the request along to the view. Why? Because we might be relying
by accident on some weird behavior inherent to misusing a middleware this way,
and we need a way to quickly switch back if we encounter problems.
If the view works, we can move all of StaticContentServerImpl directly into the
view and drop the middleware and the waffle flag.
"""
from django.http import HttpResponseNotFound
from django.views.decorators.http import require_safe
from edx_django_utils.monitoring import set_custom_attribute

from .middleware import CONTENT_SERVER_USE_VIEW, IMPL


@require_safe
def course_assets_view(request):
"""
Serve course assets to end users. Colloquially referred to as "contentserver."
"""
set_custom_attribute('content_server.handled_by.view', True)

if not CONTENT_SERVER_USE_VIEW.is_enabled():
# Should never happen; keep track of occurrences.
set_custom_attribute('content_server.view.called_when_disabled', True)
# But handle the request anyhow.

# We'll delegate request handling to an instance of the middleware
# until we can verify that the behavior is identical when requests
# come all the way through to the view.
response = IMPL.process_request(request)

if response is None:
# Shouldn't happen
set_custom_attribute('content_server.view.no_response_from_impl', True)
return HttpResponseNotFound()
else:
return response

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