238 lines
9.4 KiB
Python
238 lines
9.4 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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http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.44
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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 get_cache
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from django.utils.encoding import smart_str, iri_to_uri
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from django.utils.http import http_date
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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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This function patches the Cache-Control header by adding all
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keyword arguments to it. 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 t[0] + '=' + smart_str(t[1])
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if response.has_header('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(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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Returns 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
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cc_delim_re.split(response['Cache-Control'])])
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if 'max-age' in cc:
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try:
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return int(cc['max-age'])
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except (ValueError, TypeError):
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pass
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def patch_response_headers(response, cache_timeout=None):
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"""
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Adds some useful headers to the given HttpResponse object:
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ETag, Last-Modified, Expires and 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 settings.USE_ETAGS and not response.has_header('ETag'):
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response['ETag'] = '"%s"' % hashlib.md5(response.content).hexdigest()
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if not response.has_header('Last-Modified'):
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response['Last-Modified'] = http_date()
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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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Adds 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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def patch_vary_headers(response, newheaders):
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"""
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Adds (or updates) the "Vary" header in the given HttpResponse object.
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newheaders is a list of header names that should be in "Vary". Existing
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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 = set([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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response['Vary'] = ', '.join(vary_headers + additional_headers)
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def has_vary_header(response, header_query):
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"""
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Checks 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 = set([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 enabled, returns the cache key ending with a locale."""
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if settings.USE_I18N:
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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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return cache_key
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def _generate_cache_key(request, method, headerlist, key_prefix):
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"""Returns 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, None)
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if value is not None:
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ctx.update(value)
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path = hashlib.md5(iri_to_uri(request.get_full_path()))
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cache_key = 'views.decorators.cache.cache_page.%s.%s.%s.%s' % (
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key_prefix, request.method, path.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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"""Returns a cache key for the header cache."""
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path = hashlib.md5(iri_to_uri(request.get_full_path()))
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cache_key = 'views.decorators.cache.cache_header.%s.%s' % (
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key_prefix, path.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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Returns a cache key based on the request path 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 path registry and uses those to build a cache key
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to check against.
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If there is no headerlist stored, the page needs to be rebuilt, so this
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function returns None.
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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 = get_cache(settings.CACHE_MIDDLEWARE_ALIAS)
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headerlist = cache.get(cache_key, None)
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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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Learns what headers to take into account for some request path from the
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response object. It stores those headers in a global path registry so that
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later access to that path 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 = get_cache(settings.CACHE_MIDDLEWARE_ALIAS)
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if response.has_header('Vary'):
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headerlist = ['HTTP_'+header.upper().replace('-', '_')
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for header in cc_delim_re.split(response['Vary'])]
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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.get_full_path()
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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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