"""
Middleware to serve assets.
"""
import logging
import datetime
log = logging.getLogger(__name__)
try:
import newrelic.agent
except ImportError:
newrelic = None # pylint: disable=invalid-name
from django.http import (
HttpResponse, HttpResponseNotModified, HttpResponseForbidden,
HttpResponseBadRequest, HttpResponseNotFound, HttpResponsePermanentRedirect)
from student.models import CourseEnrollment
from xmodule.assetstore.assetmgr import AssetManager
from xmodule.contentstore.content import StaticContent, XASSET_LOCATION_TAG
from xmodule.modulestore import InvalidLocationError
from opaque_keys import InvalidKeyError
from opaque_keys.edx.locator import AssetLocator
from openedx.core.djangoapps.header_control import force_header_for_response
from .caching import get_cached_content, set_cached_content
from xmodule.modulestore.exceptions import ItemNotFoundError
from xmodule.exceptions import NotFoundError
from .models import CourseAssetCacheTtlConfig, CdnUserAgentsConfig
# 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(object):
"""
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"""
return (
request.path.startswith('/' + XASSET_LOCATION_TAG + '/')
or
request.path.startswith('/' + AssetLocator.CANONICAL_NAMESPACE)
or
StaticContent.is_versioned_asset_path(request.path)
)
# pylint: disable=too-many-statements
def process_request(self, request):
"""Process the given request"""
asset_path = request.path
if self.is_asset_request(request):
# Make sure we can convert this request into a location.
if AssetLocator.CANONICAL_NAMESPACE in asset_path:
asset_path = asset_path.replace('block/', 'block@', 1)
# If this is a versioned request, pull out the digest and chop off the prefix.
requested_digest = None
if StaticContent.is_versioned_asset_path(asset_path):
requested_digest, asset_path = StaticContent.parse_versioned_asset_path(asset_path)
# Make sure we have a valid location value for this asset.
try:
loc = StaticContent.get_location_from_path(asset_path)
except (InvalidLocationError, InvalidKeyError):
return HttpResponseBadRequest()
# Attempt to load the asset to make sure it exists, and grab the asset digest
# if we're able to load it.
actual_digest = None
try:
content = self.load_asset_from_location(loc)
actual_digest = getattr(content, "content_digest", None)
except (ItemNotFoundError, NotFoundError):
return HttpResponseNotFound()
# If this was a versioned asset, and the digest doesn't match, redirect
# them to the actual version.
if requested_digest is not None and actual_digest is not None and (actual_digest != requested_digest):
actual_asset_path = StaticContent.add_version_to_asset_path(asset_path, actual_digest)
return HttpResponsePermanentRedirect(actual_asset_path)
# Set the basics for this request. Make sure that the course key for this
# asset has a run, which old-style courses do not. Otherwise, this will
# explode when the key is serialized to be sent to NR.
safe_course_key = loc.course_key
if safe_course_key.run is None:
safe_course_key = safe_course_key.replace(run='only')
if newrelic:
newrelic.agent.add_custom_parameter('course_id', safe_course_key)
newrelic.agent.add_custom_parameter('org', loc.org)
newrelic.agent.add_custom_parameter('contentserver.path', loc.path)
# Figure out if this is a CDN using us as the origin.
is_from_cdn = StaticContentServer.is_cdn_request(request)
newrelic.agent.add_custom_parameter('contentserver.from_cdn', is_from_cdn)
# Check if this content is locked or not.
locked = self.is_content_locked(content)
newrelic.agent.add_custom_parameter('contentserver.locked', locked)
# Check that user has access to the content.
if not self.is_user_authorized(request, content, loc):
return HttpResponseForbidden('Unauthorized')
# Figure out if the client sent us a conditional request, and let them know
# if this asset has changed since then.
last_modified_at_str = content.last_modified_at.strftime(HTTP_DATE_FORMAT)
if 'HTTP_IF_MODIFIED_SINCE' in request.META:
if_modified_since = request.META['HTTP_IF_MODIFIED_SINCE']
if if_modified_since == last_modified_at_str:
return HttpResponseNotModified()
# *** File streaming within a byte range ***
# If a Range is provided, parse Range attribute of the request
# Add Content-Range in the response if Range is structurally correct
# Request -> Range attribute structure: "Range: bytes=first-[last]"
# Response -> Content-Range attribute structure: "Content-Range: bytes first-last/totalLength"
# http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35
response = None
if request.META.get('HTTP_RANGE'):
# If we have a StaticContent, get a StaticContentStream. Can't manipulate the bytes otherwise.
if isinstance(content, StaticContent):
content = AssetManager.find(loc, as_stream=True)
header_value = request.META['HTTP_RANGE']
try:
unit, ranges = parse_range_header(header_value, content.length)
except ValueError as exception:
# If the header field is syntactically invalid it should be ignored.
log.exception(
u"%s in Range header: %s for content: %s", exception.message, header_value, unicode(loc)
)
else:
if unit != 'bytes':
# Only accept ranges in bytes
log.warning(u"Unknown unit in Range header: %s for content: %s", header_value, unicode(loc))
elif len(ranges) > 1:
# According to Http/1.1 spec content for multiple ranges should be sent as a multipart message.
# http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.16
# But we send back the full content.
log.warning(
u"More than 1 ranges in Range header: %s for content: %s", header_value, unicode(loc)
)
else:
first, last = ranges[0]
if 0 <= first <= last < content.length:
# If the byte range is satisfiable
response = HttpResponse(content.stream_data_in_range(first, last))
response['Content-Range'] = 'bytes {first}-{last}/{length}'.format(
first=first, last=last, length=content.length
)
response['Content-Length'] = str(last - first + 1)
response.status_code = 206 # Partial Content
if newrelic:
newrelic.agent.add_custom_parameter('contentserver.ranged', True)
else:
log.warning(
u"Cannot satisfy ranges in Range header: %s for content: %s", header_value, unicode(loc)
)
return HttpResponse(status=416) # Requested Range Not Satisfiable
# If Range header is absent or syntactically invalid return a full content response.
if response is None:
response = HttpResponse(content.stream_data())
response['Content-Length'] = content.length
if newrelic:
newrelic.agent.add_custom_parameter('contentserver.content_len', content.length)
newrelic.agent.add_custom_parameter('contentserver.content_type', content.content_type)
# "Accept-Ranges: bytes" tells the user that only "bytes" ranges are allowed
response['Accept-Ranges'] = 'bytes'
response['Content-Type'] = content.content_type
# Set any caching headers, and do any response cleanup needed. Based on how much
# middleware we have in place, there's no easy way to use the built-in Django
# utilities and properly sanitize and modify a response to ensure that it is as
# cacheable as possible, which is why we do it ourselves.
self.set_caching_headers(content, response)
return response
def set_caching_headers(self, content, response):
"""
Sets caching headers based on whether or not the asset is locked.
"""
is_locked = getattr(content, "locked", False)
# We want to signal to the end user's browser, and to any intermediate proxies/caches,
# whether or not this asset is cacheable. If we have a TTL configured, we inform the
# caller, for unlocked assets, how long they are allowed to cache it. Since locked
# assets should be restricted to enrolled students, we simply send headers that
# indicate there should be no caching whatsoever.
cache_ttl = CourseAssetCacheTtlConfig.get_cache_ttl()
if cache_ttl > 0 and not is_locked:
if newrelic:
newrelic.agent.add_custom_parameter('contentserver.cacheable', True)
response['Expires'] = StaticContentServer.get_expiration_value(datetime.datetime.utcnow(), cache_ttl)
response['Cache-Control'] = "public, max-age={ttl}, s-maxage={ttl}".format(ttl=cache_ttl)
elif is_locked:
if newrelic:
newrelic.agent.add_custom_parameter('contentserver.cacheable', False)
response['Cache-Control'] = "private, no-cache, no-store"
response['Last-Modified'] = content.last_modified_at.strftime(HTTP_DATE_FORMAT)
# Force the Vary header to only vary responses on Origin, so that XHR and browser requests get cached
# separately and don't screw over one another. i.e. a browser request that doesn't send Origin, and
# caches a version of the response without CORS headers, in turn breaking XHR requests.
force_header_for_response(response, 'Vary', 'Origin')
@staticmethod
def is_cdn_request(request):
"""
Attempts to determine whether or not the given request is coming from a CDN.
Currently, this is a static check because edx.org only uses CloudFront, but may
be expanded in the future.
"""
cdn_user_agents = CdnUserAgentsConfig.get_cdn_user_agents()
user_agent = request.META.get('HTTP_USER_AGENT', '')
if user_agent in cdn_user_agents:
# This is a CDN request.
return True
return False
@staticmethod
def get_expiration_value(now, cache_ttl):
"""Generates an RFC1123 datetime string based on a future offset."""
expire_dt = now + datetime.timedelta(seconds=cache_ttl)
return expire_dt.strftime(HTTP_DATE_FORMAT)
def is_content_locked(self, content):
"""
Determines whether or not the given content is locked.
"""
return bool(getattr(content, "locked", False))
def is_user_authorized(self, request, content, location):
"""
Determines whether or not the user for this request is authorized to view the given asset.
"""
if not self.is_content_locked(content):
return True
if not hasattr(request, "user") or not request.user.is_authenticated():
return False
if not request.user.is_staff:
deprecated = getattr(location, 'deprecated', False)
if deprecated and not CourseEnrollment.is_enrolled_by_partial(request.user, location.course_key):
return False
if not deprecated and not CourseEnrollment.is_enrolled(request.user, location.course_key):
return False
return True
def load_asset_from_location(self, location):
"""
Loads an asset based on its location, either retrieving it from a cache
or loading it directly from the contentstore.
"""
# See if we can load this item from cache.
content = get_cached_content(location)
if content is None:
# Not in cache, so just try and load it from the asset manager.
try:
content = AssetManager.find(location, as_stream=True)
except (ItemNotFoundError, NotFoundError):
raise
# Now that we fetched it, let's go ahead and try to cache it. We cap this at 1MB
# because it's the default for memcached and also we don't want to do too much
# buffering in memory when we're serving an actual request.
if content.length is not None and content.length < 1048576:
content = content.copy_to_in_mem()
set_cached_content(content)
return content
def parse_range_header(header_value, content_length):
"""
Returns the unit and a list of (start, end) tuples of ranges.
Raises ValueError if header is syntactically invalid or does not contain a range.
See spec for details: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35
"""
unit = None
ranges = []
if '=' in header_value:
unit, byte_ranges_string = header_value.split('=')
# Parse the byte ranges.
for byte_range_string in byte_ranges_string.split(','):
byte_range_string = byte_range_string.strip()
# Case 0:
if '-' not in byte_range_string: # Invalid syntax of header value.
raise ValueError('Invalid syntax.')
# Case 1: -500
elif byte_range_string.startswith('-'):
first = max(0, (content_length + int(byte_range_string)))
last = content_length - 1
# Case 2: 500-
elif byte_range_string.endswith('-'):
first = int(byte_range_string[0:-1])
last = content_length - 1
# Case 3: 500-999
else:
first, last = byte_range_string.split('-')
first = int(first)
last = min(int(last), content_length - 1)
ranges.append((first, last))
if len(ranges) == 0:
raise ValueError('Invalid syntax')
return unit, ranges