Support immediate expiration + revalidation for Cache-Control: max-age=0 and Expires: 0

4 files changed, 56 insertions, 4 deletions

diff --git a/HISTORY.md b/HISTORY.md
index f86478f..ccbfe96 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -7,6 +7,8 @@
 * Make per-request expiration thread-safe for both `CachedSession.request()` and `CachedSession.send()`
 * Handle some additional corner cases when normalizing request data
 * Some micro-optimizations for request matching
+* Fix issue with cache headers not being used correctly if `cache_control=True` is used with an `expire_after` value
+* Support immediate expiration + revalidation for `Cache-Control: max-age=0` and `Expires: 0`
 * Fix license metadata as shown on PyPI
 
 ## 0.8.1 (2021-09-15)
diff --git a/requests_cache/cache_control.py b/requests_cache/cache_control.py
index 909ff5c..2cad7b4 100644
--- a/requests_cache/cache_control.py
+++ b/requests_cache/cache_control.py
@@ -136,12 +136,23 @@ class CacheActions:
         directives = get_cache_directives(response.headers)
         logger.debug(f'Cache directives from response headers: {directives}')
 
-        # Check expiration headers and conditions for writing to the cache
+        # Check expiration headers
         self.expire_after = coalesce(
             directives.get('max-age'), directives.get('expires'), self.expire_after
         )
+
+        # If expiration is 0 and there's a validator, save it to the cache and revalidate on use
+        has_validator = response.headers.get('ETag') or response.headers.get('Last-Modified')
+        if self.expire_after == DO_NOT_CACHE and has_validator:
+            self.expire_after = datetime.utcnow()
+
+        # Check conditions for writing to the cache
         self.skip_write = any(
-            [self.expire_after == DO_NOT_CACHE, 'no-store' in directives, self.skip_write]
+            [
+                self.expire_after == DO_NOT_CACHE,
+                'no-store' in directives,
+                self.skip_write,
+            ]
         )
 
 
diff --git a/tests/integration/base_cache_test.py b/tests/integration/base_cache_test.py
index b435b4f..8f1c5f2 100644
--- a/tests/integration/base_cache_test.py
+++ b/tests/integration/base_cache_test.py
@@ -5,11 +5,12 @@ from io import BytesIO
 from threading import Thread
 from time import sleep, time
 from typing import Dict, Type
+from unittest.mock import MagicMock, patch
 from urllib.parse import parse_qs, urlparse
 
 import pytest
 import requests
-from requests.models import PreparedRequest
+from requests import PreparedRequest, Session
 
 from requests_cache import ALL_METHODS, CachedResponse, CachedSession
 from requests_cache.backends.base import BaseCache
@@ -168,7 +169,7 @@ class BaseCacheTest:
             ({'ETag': ETAG, 'Last-Modified': LAST_MODIFIED}, True),
         ],
     )
-    def test_304_not_modified(self, cached_response_headers, expected_from_cache):
+    def test_conditional_request(self, cached_response_headers, expected_from_cache):
         """Test behavior of ETag and Last-Modified headers and 304 responses.
 
         When a cached response contains one of these headers, corresponding request headers should
@@ -184,6 +185,29 @@ class BaseCacheTest:
         response = session.get(httpbin('cache'))
         assert response.from_cache == expected_from_cache
 
+    def test_conditional_request__max_age_0(self):
+        """With both max-age=0 and a validator, the response should be saved and revalidated on next
+        request
+        """
+        url = httpbin('response-headers')
+        response_headers = {'Cache-Control': 'max-age=0', 'ETag': ETAG}
+        session = self.init_session(cache_control=True)
+
+        # This endpoint returns request params as response headers
+        session.get(url, params=response_headers)
+
+        # It doesn't respond to conditional requests, but let's just pretend it does
+        with patch.object(Session, 'send', return_value=MagicMock(status_code=304)):
+            response = session.get(url, params=response_headers)
+
+        assert response.from_cache is True
+        assert response.is_expired is True
+
+        # cache_key = list(session.cache.responses.keys())[0]
+        # response = session.cache.responses[cache_key]
+        # response.request.url = httpbin(f'etag/{ETAG}')
+        # response = session.get(httpbin('response-headers'), params=response_headers)
+
     @pytest.mark.parametrize('stream', [True, False])
     def test_response_decode(self, stream):
         """Test that gzip-compressed raw responses (including streamed responses) can be manually
diff --git a/tests/unit/test_cache_control.py b/tests/unit/test_cache_control.py
index d2c6233..2388f3d 100644
--- a/tests/unit/test_cache_control.py
+++ b/tests/unit/test_cache_control.py
@@ -238,6 +238,21 @@ def test_update_from_response__ignored():
     assert actions.expire_after is None
 
 
+@patch('requests_cache.cache_control.datetime')
+def test_update_from_response__revalidate(mock_datetime):
+    """If expiration is 0 and there's a validator the response should be cached and revalidated"""
+    url = 'https://img.site.com/base/img.jpg'
+    actions = CacheActions.from_request(
+        cache_key='key',
+        request=MagicMock(url=url),
+        cache_control=True,
+    )
+    actions.update_from_response(
+        MagicMock(url=url, headers={'Cache-Control': 'max-age=0', 'ETag': ETAG})
+    )
+    assert actions.expire_after == mock_datetime.utcnow()
+
+
 @pytest.mark.parametrize('directive', IGNORED_DIRECTIVES)
 def test_ignored_headers(directive):
     """Ensure that currently unimplemented Cache-Control headers do not affect behavior"""