01a9ad2369
This introduces a mechanism to control the time-to-live for an unlocked course asset, which will allow browsers and intermediate proxies/caches to cache these course assets, determinstically. Locked assets, with their nature of requiring authorization, are not eligible for caching.
251 lines
11 KiB
Python
251 lines
11 KiB
Python
"""
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Middleware to serve assets.
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"""
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import logging
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import datetime
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from django.http import (
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HttpResponse, HttpResponseNotModified, HttpResponseForbidden,
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HttpResponseBadRequest, HttpResponseNotFound)
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from student.models import CourseEnrollment
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from contentserver.models import CourseAssetCacheTtlConfig
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from clean_headers import remove_headers_from_response
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from xmodule.assetstore.assetmgr import AssetManager
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from xmodule.contentstore.content import StaticContent, XASSET_LOCATION_TAG
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from xmodule.modulestore import InvalidLocationError
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from opaque_keys import InvalidKeyError
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from opaque_keys.edx.locator import AssetLocator
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from cache_toolbox.core import get_cached_content, set_cached_content
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from xmodule.modulestore.exceptions import ItemNotFoundError
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from xmodule.exceptions import NotFoundError
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# TODO: Soon as we have a reasonable way to serialize/deserialize AssetKeys, we need
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# to change this file so instead of using course_id_partial, we're just using asset keys
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log = logging.getLogger(__name__)
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HTTP_DATE_FORMAT = "%a, %d %b %Y %H:%M:%S GMT"
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class StaticContentServer(object):
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def is_asset_request(self, request):
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"""Determines whether the given request is an asset request"""
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return (
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request.path.startswith('/' + XASSET_LOCATION_TAG + '/')
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or
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request.path.startswith('/' + AssetLocator.CANONICAL_NAMESPACE)
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)
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def process_request(self, request):
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"""Process the given request"""
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if self.is_asset_request(request):
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# Make sure we can convert this request into a location.
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if AssetLocator.CANONICAL_NAMESPACE in request.path:
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request.path = request.path.replace('block/', 'block@', 1)
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try:
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loc = StaticContent.get_location_from_path(request.path)
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except (InvalidLocationError, InvalidKeyError):
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return HttpResponseBadRequest()
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# Try and load the asset.
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content = None
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try:
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content = self.load_asset_from_location(loc)
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except (ItemNotFoundError, NotFoundError):
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return HttpResponseNotFound()
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# Check that user has access to the content.
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if not self.is_user_authorized(request, content, loc):
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return HttpResponseForbidden('Unauthorized')
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# Figure out if the client sent us a conditional request, and let them know
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# if this asset has changed since then.
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last_modified_at_str = content.last_modified_at.strftime(HTTP_DATE_FORMAT)
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if 'HTTP_IF_MODIFIED_SINCE' in request.META:
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if_modified_since = request.META['HTTP_IF_MODIFIED_SINCE']
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if if_modified_since == last_modified_at_str:
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return HttpResponseNotModified()
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# *** File streaming within a byte range ***
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# If a Range is provided, parse Range attribute of the request
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# Add Content-Range in the response if Range is structurally correct
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# Request -> Range attribute structure: "Range: bytes=first-[last]"
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# Response -> Content-Range attribute structure: "Content-Range: bytes first-last/totalLength"
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# http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35
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response = None
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if request.META.get('HTTP_RANGE'):
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# If we have a StaticContent, get a StaticContentStream. Can't manipulate the bytes otherwise.
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if type(content) == StaticContent:
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content = AssetManager.find(loc, as_stream=True)
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header_value = request.META['HTTP_RANGE']
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try:
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unit, ranges = parse_range_header(header_value, content.length)
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except ValueError as exception:
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# If the header field is syntactically invalid it should be ignored.
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log.exception(
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u"%s in Range header: %s for content: %s", exception.message, header_value, unicode(loc)
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)
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else:
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if unit != 'bytes':
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# Only accept ranges in bytes
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log.warning(u"Unknown unit in Range header: %s for content: %s", header_value, unicode(loc))
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elif len(ranges) > 1:
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# According to Http/1.1 spec content for multiple ranges should be sent as a multipart message.
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# http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.16
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# But we send back the full content.
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log.warning(
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u"More than 1 ranges in Range header: %s for content: %s", header_value, unicode(loc)
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)
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else:
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first, last = ranges[0]
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if 0 <= first <= last < content.length:
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# If the byte range is satisfiable
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response = HttpResponse(content.stream_data_in_range(first, last))
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response['Content-Range'] = 'bytes {first}-{last}/{length}'.format(
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first=first, last=last, length=content.length
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)
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response['Content-Length'] = str(last - first + 1)
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response.status_code = 206 # Partial Content
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else:
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log.warning(
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u"Cannot satisfy ranges in Range header: %s for content: %s", header_value, unicode(loc)
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)
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return HttpResponse(status=416) # Requested Range Not Satisfiable
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# If Range header is absent or syntactically invalid return a full content response.
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if response is None:
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response = HttpResponse(content.stream_data())
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response['Content-Length'] = content.length
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# "Accept-Ranges: bytes" tells the user that only "bytes" ranges are allowed
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response['Accept-Ranges'] = 'bytes'
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response['Content-Type'] = content.content_type
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# Set any caching headers, and do any response cleanup needed. Based on how much
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# middleware we have in place, there's no easy way to use the built-in Django
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# utilities and properly sanitize and modify a response to ensure that it is as
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# cacheable as possible, which is why we do it ourselves.
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self.set_caching_headers(content, response)
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return response
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def set_caching_headers(self, content, response):
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"""
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Sets caching headers based on whether or not the asset is locked.
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"""
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is_locked = getattr(content, "locked", False)
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# We want to signal to the end user's browser, and to any intermediate proxies/caches,
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# whether or not this asset is cacheable. If we have a TTL configured, we inform the
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# caller, for unlocked assets, how long they are allowed to cache it. Since locked
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# assets should be restricted to enrolled students, we simply send headers that
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# indicate there should be no caching whatsoever.
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cache_ttl = CourseAssetCacheTtlConfig.get_cache_ttl()
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if cache_ttl > 0 and not is_locked:
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response['Expires'] = StaticContentServer.get_expiration_value(datetime.datetime.utcnow(), cache_ttl)
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response['Cache-Control'] = "public, max-age={ttl}, s-maxage={ttl}".format(ttl=cache_ttl)
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elif is_locked:
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response['Cache-Control'] = "private, no-cache, no-store"
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response['Last-Modified'] = content.last_modified_at.strftime(HTTP_DATE_FORMAT)
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remove_headers_from_response(response, "Vary")
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@staticmethod
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def get_expiration_value(now, cache_ttl):
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"""Generates an RFC1123 datetime string based on a future offset."""
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expire_dt = now + datetime.timedelta(seconds=cache_ttl)
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return expire_dt.strftime(HTTP_DATE_FORMAT)
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def is_user_authorized(self, request, content, location):
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"""
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Determines whether or not the user for this request is authorized to view the given asset.
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"""
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is_locked = getattr(content, "locked", False)
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if not is_locked:
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return True
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if not hasattr(request, "user") or not request.user.is_authenticated():
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return False
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if not request.user.is_staff:
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deprecated = getattr(location, 'deprecated', False)
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if deprecated and not CourseEnrollment.is_enrolled_by_partial(request.user, location.course_key):
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return False
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if not deprecated and not CourseEnrollment.is_enrolled(request.user, location.course_key):
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return False
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return True
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def load_asset_from_location(self, location):
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"""
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Loads an asset based on its location, either retrieving it from a cache
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or loading it directly from the contentstore.
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"""
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# See if we can load this item from cache.
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content = get_cached_content(location)
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if content is None:
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# Not in cache, so just try and load it from the asset manager.
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try:
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content = AssetManager.find(location, as_stream=True)
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except (ItemNotFoundError, NotFoundError):
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raise
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# Now that we fetched it, let's go ahead and try to cache it. We cap this at 1MB
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# because it's the default for memcached and also we don't want to do too much
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# buffering in memory when we're serving an actual request.
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if content.length is not None and content.length < 1048576:
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content = content.copy_to_in_mem()
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set_cached_content(content)
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return content
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def parse_range_header(header_value, content_length):
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"""
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Returns the unit and a list of (start, end) tuples of ranges.
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Raises ValueError if header is syntactically invalid or does not contain a range.
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See spec for details: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35
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"""
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unit = None
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ranges = []
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if '=' in header_value:
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unit, byte_ranges_string = header_value.split('=')
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# Parse the byte ranges.
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for byte_range_string in byte_ranges_string.split(','):
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byte_range_string = byte_range_string.strip()
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# Case 0:
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if '-' not in byte_range_string: # Invalid syntax of header value.
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raise ValueError('Invalid syntax.')
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# Case 1: -500
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elif byte_range_string.startswith('-'):
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first = max(0, (content_length + int(byte_range_string)))
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last = content_length - 1
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# Case 2: 500-
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elif byte_range_string.endswith('-'):
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first = int(byte_range_string[0:-1])
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last = content_length - 1
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# Case 3: 500-999
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else:
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first, last = byte_range_string.split('-')
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first = int(first)
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last = min(int(last), content_length - 1)
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ranges.append((first, last))
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if len(ranges) == 0:
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raise ValueError('Invalid syntax')
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return unit, ranges
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