393 lines
15 KiB
Python
393 lines
15 KiB
Python
"""
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This module contains helper functions for controlling caching. It does so by
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managing the "Vary" header of responses. It includes functions to patch the
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header of response objects directly and decorators that change functions to do
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that header-patching themselves.
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For information on the Vary header, see:
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https://tools.ietf.org/html/rfc7231#section-7.1.4
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Essentially, the "Vary" HTTP header defines which headers a cache should take
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into account when building its cache key. Requests with the same path but
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different header content for headers named in "Vary" need to get different
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cache keys to prevent delivery of wrong content.
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An example: i18n middleware would need to distinguish caches by the
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"Accept-language" header.
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"""
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import hashlib
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import re
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import time
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from django.conf import settings
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from django.core.cache import caches
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from django.http import HttpResponse, HttpResponseNotModified
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from django.utils.encoding import iri_to_uri
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from django.utils.http import (
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http_date, parse_etags, parse_http_date_safe, quote_etag,
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)
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from django.utils.log import log_response
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from django.utils.timezone import get_current_timezone_name
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from django.utils.translation import get_language
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cc_delim_re = re.compile(r'\s*,\s*')
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def patch_cache_control(response, **kwargs):
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"""
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Patch the Cache-Control header by adding all keyword arguments to it.
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The transformation is as follows:
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* All keyword parameter names are turned to lowercase, and underscores
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are converted to hyphens.
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* If the value of a parameter is True (exactly True, not just a
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true value), only the parameter name is added to the header.
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* All other parameters are added with their value, after applying
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str() to it.
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"""
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def dictitem(s):
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t = s.split('=', 1)
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if len(t) > 1:
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return (t[0].lower(), t[1])
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else:
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return (t[0].lower(), True)
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def dictvalue(t):
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if t[1] is True:
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return t[0]
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else:
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return '%s=%s' % (t[0], t[1])
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if response.get('Cache-Control'):
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cc = cc_delim_re.split(response['Cache-Control'])
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cc = dict(dictitem(el) for el in cc)
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else:
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cc = {}
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# If there's already a max-age header but we're being asked to set a new
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# max-age, use the minimum of the two ages. In practice this happens when
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# a decorator and a piece of middleware both operate on a given view.
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if 'max-age' in cc and 'max_age' in kwargs:
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kwargs['max_age'] = min(int(cc['max-age']), kwargs['max_age'])
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# Allow overriding private caching and vice versa
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if 'private' in cc and 'public' in kwargs:
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del cc['private']
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elif 'public' in cc and 'private' in kwargs:
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del cc['public']
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for (k, v) in kwargs.items():
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cc[k.replace('_', '-')] = v
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cc = ', '.join(dictvalue(el) for el in cc.items())
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response['Cache-Control'] = cc
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def get_max_age(response):
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"""
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Return the max-age from the response Cache-Control header as an integer,
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or None if it wasn't found or wasn't an integer.
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"""
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if not response.has_header('Cache-Control'):
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return
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cc = dict(_to_tuple(el) for el in cc_delim_re.split(response['Cache-Control']))
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try:
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return int(cc['max-age'])
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except (ValueError, TypeError, KeyError):
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pass
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def set_response_etag(response):
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if not response.streaming:
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response['ETag'] = quote_etag(hashlib.md5(response.content).hexdigest())
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return response
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def _precondition_failed(request):
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response = HttpResponse(status=412)
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log_response(
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'Precondition Failed: %s', request.path,
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response=response,
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request=request,
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)
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return response
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def _not_modified(request, response=None):
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new_response = HttpResponseNotModified()
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if response:
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# Preserve the headers required by Section 4.1 of RFC 7232, as well as
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# Last-Modified.
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for header in ('Cache-Control', 'Content-Location', 'Date', 'ETag', 'Expires', 'Last-Modified', 'Vary'):
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if header in response:
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new_response[header] = response[header]
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# Preserve cookies as per the cookie specification: "If a proxy server
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# receives a response which contains a Set-cookie header, it should
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# propagate the Set-cookie header to the client, regardless of whether
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# the response was 304 (Not Modified) or 200 (OK).
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# https://curl.haxx.se/rfc/cookie_spec.html
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new_response.cookies = response.cookies
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return new_response
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def get_conditional_response(request, etag=None, last_modified=None, response=None):
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# Only return conditional responses on successful requests.
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if response and not (200 <= response.status_code < 300):
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return response
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# Get HTTP request headers.
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if_match_etags = parse_etags(request.META.get('HTTP_IF_MATCH', ''))
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if_unmodified_since = request.META.get('HTTP_IF_UNMODIFIED_SINCE')
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if_unmodified_since = if_unmodified_since and parse_http_date_safe(if_unmodified_since)
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if_none_match_etags = parse_etags(request.META.get('HTTP_IF_NONE_MATCH', ''))
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if_modified_since = request.META.get('HTTP_IF_MODIFIED_SINCE')
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if_modified_since = if_modified_since and parse_http_date_safe(if_modified_since)
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# Step 1 of section 6 of RFC 7232: Test the If-Match precondition.
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if if_match_etags and not _if_match_passes(etag, if_match_etags):
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return _precondition_failed(request)
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# Step 2: Test the If-Unmodified-Since precondition.
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if (not if_match_etags and if_unmodified_since and
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not _if_unmodified_since_passes(last_modified, if_unmodified_since)):
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return _precondition_failed(request)
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# Step 3: Test the If-None-Match precondition.
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if if_none_match_etags and not _if_none_match_passes(etag, if_none_match_etags):
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if request.method in ('GET', 'HEAD'):
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return _not_modified(request, response)
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else:
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return _precondition_failed(request)
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# Step 4: Test the If-Modified-Since precondition.
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if (not if_none_match_etags and if_modified_since and
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not _if_modified_since_passes(last_modified, if_modified_since)):
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if request.method in ('GET', 'HEAD'):
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return _not_modified(request, response)
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# Step 5: Test the If-Range precondition (not supported).
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# Step 6: Return original response since there isn't a conditional response.
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return response
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def _if_match_passes(target_etag, etags):
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"""
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Test the If-Match comparison as defined in section 3.1 of RFC 7232.
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"""
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if not target_etag:
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# If there isn't an ETag, then there can't be a match.
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return False
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elif etags == ['*']:
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# The existence of an ETag means that there is "a current
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# representation for the target resource", even if the ETag is weak,
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# so there is a match to '*'.
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return True
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elif target_etag.startswith('W/'):
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# A weak ETag can never strongly match another ETag.
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return False
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else:
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# Since the ETag is strong, this will only return True if there's a
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# strong match.
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return target_etag in etags
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def _if_unmodified_since_passes(last_modified, if_unmodified_since):
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"""
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Test the If-Unmodified-Since comparison as defined in section 3.4 of
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RFC 7232.
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"""
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return last_modified and last_modified <= if_unmodified_since
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def _if_none_match_passes(target_etag, etags):
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"""
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Test the If-None-Match comparison as defined in section 3.2 of RFC 7232.
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"""
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if not target_etag:
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# If there isn't an ETag, then there isn't a match.
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return True
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elif etags == ['*']:
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# The existence of an ETag means that there is "a current
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# representation for the target resource", so there is a match to '*'.
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return False
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else:
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# The comparison should be weak, so look for a match after stripping
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# off any weak indicators.
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target_etag = target_etag.strip('W/')
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etags = (etag.strip('W/') for etag in etags)
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return target_etag not in etags
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def _if_modified_since_passes(last_modified, if_modified_since):
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"""
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Test the If-Modified-Since comparison as defined in section 3.3 of RFC 7232.
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"""
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return not last_modified or last_modified > if_modified_since
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def patch_response_headers(response, cache_timeout=None):
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"""
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Add HTTP caching headers to the given HttpResponse: Expires and
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Cache-Control.
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Each header is only added if it isn't already set.
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cache_timeout is in seconds. The CACHE_MIDDLEWARE_SECONDS setting is used
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by default.
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"""
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if cache_timeout is None:
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cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
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if cache_timeout < 0:
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cache_timeout = 0 # Can't have max-age negative
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if not response.has_header('Expires'):
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response['Expires'] = http_date(time.time() + cache_timeout)
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patch_cache_control(response, max_age=cache_timeout)
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def add_never_cache_headers(response):
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"""
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Add headers to a response to indicate that a page should never be cached.
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"""
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patch_response_headers(response, cache_timeout=-1)
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patch_cache_control(response, no_cache=True, no_store=True, must_revalidate=True, private=True)
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def patch_vary_headers(response, newheaders):
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"""
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Add (or update) the "Vary" header in the given HttpResponse object.
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newheaders is a list of header names that should be in "Vary". If headers
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contains an asterisk, then "Vary" header will consist of a single asterisk
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'*'. Otherwise, existing headers in "Vary" aren't removed.
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"""
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# Note that we need to keep the original order intact, because cache
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# implementations may rely on the order of the Vary contents in, say,
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# computing an MD5 hash.
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if response.has_header('Vary'):
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vary_headers = cc_delim_re.split(response['Vary'])
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else:
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vary_headers = []
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# Use .lower() here so we treat headers as case-insensitive.
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existing_headers = {header.lower() for header in vary_headers}
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additional_headers = [newheader for newheader in newheaders
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if newheader.lower() not in existing_headers]
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vary_headers += additional_headers
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if '*' in vary_headers:
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response['Vary'] = '*'
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else:
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response['Vary'] = ', '.join(vary_headers)
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def has_vary_header(response, header_query):
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"""
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Check to see if the response has a given header name in its Vary header.
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"""
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if not response.has_header('Vary'):
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return False
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vary_headers = cc_delim_re.split(response['Vary'])
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existing_headers = {header.lower() for header in vary_headers}
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return header_query.lower() in existing_headers
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def _i18n_cache_key_suffix(request, cache_key):
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"""If necessary, add the current locale or time zone to the cache key."""
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if settings.USE_I18N or settings.USE_L10N:
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# first check if LocaleMiddleware or another middleware added
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# LANGUAGE_CODE to request, then fall back to the active language
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# which in turn can also fall back to settings.LANGUAGE_CODE
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cache_key += '.%s' % getattr(request, 'LANGUAGE_CODE', get_language())
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if settings.USE_TZ:
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cache_key += '.%s' % get_current_timezone_name()
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return cache_key
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def _generate_cache_key(request, method, headerlist, key_prefix):
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"""Return a cache key from the headers given in the header list."""
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ctx = hashlib.md5()
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for header in headerlist:
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value = request.META.get(header)
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if value is not None:
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ctx.update(value.encode())
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url = hashlib.md5(iri_to_uri(request.build_absolute_uri()).encode('ascii'))
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cache_key = 'views.decorators.cache.cache_page.%s.%s.%s.%s' % (
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key_prefix, method, url.hexdigest(), ctx.hexdigest())
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return _i18n_cache_key_suffix(request, cache_key)
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def _generate_cache_header_key(key_prefix, request):
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"""Return a cache key for the header cache."""
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url = hashlib.md5(iri_to_uri(request.build_absolute_uri()).encode('ascii'))
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cache_key = 'views.decorators.cache.cache_header.%s.%s' % (
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key_prefix, url.hexdigest())
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return _i18n_cache_key_suffix(request, cache_key)
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def get_cache_key(request, key_prefix=None, method='GET', cache=None):
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"""
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Return a cache key based on the request URL and query. It can be used
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in the request phase because it pulls the list of headers to take into
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account from the global URL registry and uses those to build a cache key
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to check against.
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If there isn't a headerlist stored, return None, indicating that the page
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needs to be rebuilt.
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"""
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if key_prefix is None:
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key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
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cache_key = _generate_cache_header_key(key_prefix, request)
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if cache is None:
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cache = caches[settings.CACHE_MIDDLEWARE_ALIAS]
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headerlist = cache.get(cache_key)
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if headerlist is not None:
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return _generate_cache_key(request, method, headerlist, key_prefix)
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else:
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return None
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def learn_cache_key(request, response, cache_timeout=None, key_prefix=None, cache=None):
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"""
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Learn what headers to take into account for some request URL from the
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response object. Store those headers in a global URL registry so that
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later access to that URL will know what headers to take into account
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without building the response object itself. The headers are named in the
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Vary header of the response, but we want to prevent response generation.
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The list of headers to use for cache key generation is stored in the same
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cache as the pages themselves. If the cache ages some data out of the
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cache, this just means that we have to build the response once to get at
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the Vary header and so at the list of headers to use for the cache key.
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"""
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if key_prefix is None:
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key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX
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if cache_timeout is None:
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cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS
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cache_key = _generate_cache_header_key(key_prefix, request)
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if cache is None:
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cache = caches[settings.CACHE_MIDDLEWARE_ALIAS]
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if response.has_header('Vary'):
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is_accept_language_redundant = settings.USE_I18N or settings.USE_L10N
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# If i18n or l10n are used, the generated cache key will be suffixed
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# with the current locale. Adding the raw value of Accept-Language is
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# redundant in that case and would result in storing the same content
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# under multiple keys in the cache. See #18191 for details.
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headerlist = []
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for header in cc_delim_re.split(response['Vary']):
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header = header.upper().replace('-', '_')
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if header != 'ACCEPT_LANGUAGE' or not is_accept_language_redundant:
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headerlist.append('HTTP_' + header)
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headerlist.sort()
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cache.set(cache_key, headerlist, cache_timeout)
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return _generate_cache_key(request, request.method, headerlist, key_prefix)
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else:
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# if there is no Vary header, we still need a cache key
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# for the request.build_absolute_uri()
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cache.set(cache_key, [], cache_timeout)
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return _generate_cache_key(request, request.method, [], key_prefix)
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def _to_tuple(s):
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t = s.split('=', 1)
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if len(t) == 2:
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return t[0].lower(), t[1]
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return t[0].lower(), True
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