Add support for Vary

3 files changed, 49 insertions, 7 deletions

diff --git a/requests_cache/backends/base.py b/requests_cache/backends/base.py
index 5e00db0..2c5b0b6 100644
--- a/requests_cache/backends/base.py
+++ b/requests_cache/backends/base.py
@@ -104,13 +104,15 @@ class BaseCache:
         self.responses.close()
         self.redirects.close()
 
-    def create_key(self, request: AnyRequest = None, **kwargs) -> str:
+    def create_key(
+        self, request: AnyRequest = None, match_headers: Iterable[str] = None, **kwargs
+    ) -> str:
         """Create a normalized cache key from a request object"""
         key_fn = self._settings.key_fn or create_key
         return key_fn(
             request=request,
             ignored_parameters=self._settings.ignored_parameters,
-            match_headers=self._settings.match_headers,
+            match_headers=match_headers or self._settings.match_headers,
             serializer=self.responses.serializer,
             **kwargs,
         )
diff --git a/requests_cache/policy/actions.py b/requests_cache/policy/actions.py
index 707b0d7..ad201f9 100644
--- a/requests_cache/policy/actions.py
+++ b/requests_cache/policy/actions.py
@@ -17,7 +17,7 @@ from . import (
     get_expiration_seconds,
     get_url_expiration,
 )
-from .settings import CacheSettings
+from .settings import CacheSettings, KeyCallback
 
 if TYPE_CHECKING:
     from ..models import CachedResponse
@@ -64,6 +64,7 @@ class CacheActions(RichMixin):
     # Temporary attributes
     _only_if_cached: bool = field(default=False, repr=False)
     _refresh: bool = field(default=False, repr=False)
+    _request: PreparedRequest = field(default=None, repr=False)
     _stale_if_error: Union[bool, ExpirationTime] = field(default=None, repr=False)
     _stale_while_revalidate: Union[bool, ExpirationTime] = field(default=None, repr=False)
     _validation_headers: Dict[str, str] = field(factory=dict, repr=False)
@@ -76,6 +77,10 @@ class CacheActions(RichMixin):
         indicate a user-requested refresh. Typically that's only used in response headers, and
         `max-age=0` would be used by a client to request a refresh. However, this would conflict
         with the `expire_after` option provided in :py:meth:`.CachedSession.request`.
+
+        Args:
+            request: The outgoing request
+            settings: Session-level cache settings
         """
         settings = settings or CacheSettings()
         directives = CacheDirectives.from_headers(request.headers)
@@ -107,15 +112,16 @@ class CacheActions(RichMixin):
 
         actions = cls(
             cache_key=cache_key,
+            directives=directives,
             expire_after=expire_after,
             only_if_cached=only_if_cached,
             refresh=refresh,
+            request=request,
+            settings=settings,
             skip_read=any(read_criteria.values()),
             skip_write=directives.no_store,
             stale_if_error=stale_if_error,
             stale_while_revalidate=stale_while_revalidate,
-            directives=directives,
-            settings=settings,
         )
         return actions
 
@@ -155,11 +161,18 @@ class CacheActions(RichMixin):
 
         return datetime.utcnow() < cached_response.expires + offset
 
-    def update_from_cached_response(self, cached_response: 'CachedResponse'):
+    def update_from_cached_response(
+        self, cached_response: 'CachedResponse', create_key: KeyCallback = None, **key_kwargs
+    ):
         """Determine if we can reuse a cached response, or set headers for a conditional request
         if possible.
 
         Used after fetching a cached response, but before potentially sending a new request.
+
+        Args:
+            cached_response: Cached response to examine
+            create_key: Cache key function, used for validating ``Vary`` headers
+            key_kwargs: Additional keyword arguments for ``create_key``.
         """
         usable_response = self.is_usable(cached_response)
         usable_if_error = self.is_usable(cached_response, error=True)
@@ -170,6 +183,9 @@ class CacheActions(RichMixin):
         # Send the request for the first time
         elif cached_response is None:
             self.send_request = True
+        # If response contains Vary and doesn't match, consider it a cache miss
+        elif create_key and not self._validate_vary(cached_response, create_key, **key_kwargs):
+            self.send_request = True
         # Resend the request, unless settings permit a stale response
         elif not usable_response and not (self._only_if_cached and usable_if_error):
             self.resend_request = True
@@ -262,6 +278,21 @@ class CacheActions(RichMixin):
             self.send_request = True
             self.resend_request = False
 
+    def _validate_vary(
+        self, cached_response: 'CachedResponse', create_key: KeyCallback, **key_kwargs
+    ) -> bool:
+        """If the cached response contains Vary, check that the specified request headers match"""
+        vary = cached_response.headers.get('Vary')
+        if not vary:
+            return True
+        elif vary == '*':
+            return False
+
+        # Generate a secondary cache key based on Vary for both the cached request and new request
+        key_kwargs['match_headers'] = [k.strip() for k in vary.split(',')]
+        vary_cache_key = create_key(cached_response.request, **key_kwargs)
+        return create_key(self._request, **key_kwargs) == vary_cache_key
+
 
 def _log_cache_criteria(operation: str, criteria: Dict):
     """Log details on any failed checks for cache read or write"""
diff --git a/requests_cache/session.py b/requests_cache/session.py
index b65da1e..f3dec60 100644
--- a/requests_cache/session.py
+++ b/requests_cache/session.py
@@ -192,7 +192,16 @@ class CacheMixin(MIXIN_BASE):
         cached_response: Optional[CachedResponse] = None
         if not actions.skip_read:
             cached_response = self.cache.get_response(actions.cache_key)
-        actions.update_from_cached_response(cached_response)
+        actions.update_from_cached_response(cached_response, self.cache.create_key, **kwargs)
+
+        # TODO: Does this fit better here, or in CacheActions?
+        # If response contains Vary, check that the specified request headers match
+        # if cached_response and cached_response.headers.get('Vary'):
+        #     vary = cached_response.headers['Vary']
+        #     new_cache_key = self.cache.create_key(request, match_headers=vary)
+        #     vary_cache_key = self.cache.create_key(cached_response.request, match_headers=vary)
+        #     if new_cache_key != vary_cache_key:
+        #         cached_response = None
 
         # Handle missing and expired responses based on settings and headers
         if actions.error_504: