"""

Cache middleware. If enabled, each Django-powered page will be cached based on

URL. The canonical way to enable cache middleware is to set

``UpdateCacheMiddleware`` as your first piece of middleware, and

``FetchFromCacheMiddleware`` as the last::

    MIDDLEWARE_CLASSES = [

        'django.middleware.cache.UpdateCacheMiddleware',

        ...

        'django.middleware.cache.FetchFromCacheMiddleware'

    ]

This is counter-intuitive, but correct: ``UpdateCacheMiddleware`` needs to run

last during the response phase, which processes middleware bottom-up;

``FetchFromCacheMiddleware`` needs to run last during the request phase, which

processes middleware top-down.

The single-class ``CacheMiddleware`` can be used for some simple sites.

However, if any other piece of middleware needs to affect the cache key, you'll

need to use the two-part ``UpdateCacheMiddleware`` and

``FetchFromCacheMiddleware``. This'll most often happen when you're using

Django's ``LocaleMiddleware``.

More details about how the caching works:

* Only parameter-less GET or HEAD-requests with status code 200 are cached.

* The number of seconds each page is stored for is set by the "max-age" section

  of the response's "Cache-Control" header, falling back to the

  CACHE_MIDDLEWARE_SECONDS setting if the section was not found.

* If CACHE_MIDDLEWARE_ANONYMOUS_ONLY is set to True, only anonymous requests

  (i.e., those not made by a logged-in user) will be cached. This is a simple

  and effective way of avoiding the caching of the Django admin (and any other

  user-specific content).

* This middleware expects that a HEAD request is answered with a response

  exactly like the corresponding GET request.

* When a hit occurs, a shallow copy of the original response object is returned

  from process_request.

* Pages will be cached based on the contents of the request headers listed in

  the response's "Vary" header.

* This middleware also sets ETag, Last-Modified, Expires and Cache-Control

  headers on the response object.

"""

from django.conf import settings

from django.core.cache import cache

from django.utils.cache import get_cache_key, learn_cache_key, patch_response_headers, get_max_age

class UpdateCacheMiddleware(object):

    """

    Response-phase cache middleware that updates the cache if the response is

    cacheable.

    Must be used as part of the two-part update/fetch cache middleware.

    UpdateCacheMiddleware must be the first piece of middleware in

    MIDDLEWARE_CLASSES so that it'll get called last during the response phase.

    """

    def __init__(self):

        self.cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS

        self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX

        self.cache_anonymous_only = getattr(settings, 'CACHE_MIDDLEWARE_ANONYMOUS_ONLY', False)

    def process_response(self, request, response):

        """Sets the cache, if needed."""

        if not hasattr(request, '_cache_update_cache') or not request._cache_update_cache:

            # We don't need to update the cache, just return.

            return response

        if request.method != 'GET':

            # This is a stronger requirement than above. It is needed

            # because of interactions between this middleware and the

            # HTTPMiddleware, which throws the body of a HEAD-request

            # away before this middleware gets a chance to cache it.

            return response

        if not response.status_code == 200:

            return response

        # Try to get the timeout from the "max-age" section of the "Cache-

        # Control" header before reverting to using the default cache_timeout

        # length.

        timeout = get_max_age(response)

        if timeout == None:

            timeout = self.cache_timeout

        elif timeout == 0:

            # max-age was set to 0, don't bother caching.

            return response

        patch_response_headers(response, timeout)

        if timeout:

            cache_key = learn_cache_key(request, response, timeout, self.key_prefix)

            cache.set(cache_key, response, timeout)

        return response

class FetchFromCacheMiddleware(object):

    """

    Request-phase cache middleware that fetches a page from the cache.

    Must be used as part of the two-part update/fetch cache middleware.

    FetchFromCacheMiddleware must be the last piece of middleware in

    MIDDLEWARE_CLASSES so that it'll get called last during the request phase.

    """

    def __init__(self):

        self.cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS

        self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX

        self.cache_anonymous_only = getattr(settings, 'CACHE_MIDDLEWARE_ANONYMOUS_ONLY', False)

    def process_request(self, request):

        """

        Checks whether the page is already cached and returns the cached

        version if available.

        """

        if self.cache_anonymous_only:

            assert hasattr(request, 'user'), "The Django cache middleware with CACHE_MIDDLEWARE_ANONYMOUS_ONLY=True requires authentication middleware to be installed. Edit your MIDDLEWARE_CLASSES setting to insert 'django.contrib.auth.middleware.AuthenticationMiddleware' before the CacheMiddleware."

        if not request.method in ('GET', 'HEAD') or request.GET:

            request._cache_update_cache = False

            return None # Don't bother checking the cache.

        if self.cache_anonymous_only and request.user.is_authenticated():

            request._cache_update_cache = False

            return None # Don't cache requests from authenticated users.

        cache_key = get_cache_key(request, self.key_prefix)

        if cache_key is None:

            request._cache_update_cache = True

            return None # No cache information available, need to rebuild.

        response = cache.get(cache_key, None)

        if response is None:

            request._cache_update_cache = True

            return None # No cache information available, need to rebuild.

        request._cache_update_cache = False

        return response

class CacheMiddleware(UpdateCacheMiddleware, FetchFromCacheMiddleware):

    """

    Cache middleware that provides basic behavior for many simple sites.

    Also used as the hook point for the cache decorator, which is generated

    using the decorator-from-middleware utility.

    """

    def __init__(self, cache_timeout=None, key_prefix=None, cache_anonymous_only=None):

        self.cache_timeout = cache_timeout

        if cache_timeout is None:

            self.cache_timeout = settings.CACHE_MIDDLEWARE_SECONDS

        self.key_prefix = key_prefix

        if key_prefix is None:

            self.key_prefix = settings.CACHE_MIDDLEWARE_KEY_PREFIX

        if cache_anonymous_only is None:

            self.cache_anonymous_only = getattr(settings, 'CACHE_MIDDLEWARE_ANONYMOUS_ONLY', False)

        else:

            self.cache_anonymous_only = cache_anonymous_only