session: Enforce that changes cannot happen in a transaction.

tools/test-backend

@@ -140,6 +140,8 @@ not_yet_fully_covered = [
     "zerver/webhooks/teamcity/view.py",
     "zerver/webhooks/travis/view.py",
     "zerver/webhooks/zapier/view.py",
+    # Cannot have coverage, as tests run in a transaction
+    "zerver/lib/safe_session_cached_db.py",
 ]
 
 enforce_fully_covered = sorted(

zerver/lib/cache_helpers.py

@@ -56,7 +56,7 @@ def huddle_cache_items(items_for_remote_cache: Dict[str, Tuple[Huddle]], huddle:
 
 
 def session_cache_items(items_for_remote_cache: Dict[str, str], session: Session) -> None:
-    if settings.SESSION_ENGINE != "django.contrib.sessions.backends.cached_db":
+    if settings.SESSION_ENGINE != "zerver.lib.safe_session_cached_db":
         # If we're not using the cached_db session engine, we there
         # will be no store.cache_key attribute, and in any case we
         # don't need to fill the cache, since it won't exist.

zerver/lib/safe_session_cached_db.py

@@ -0,0 +1,25 @@
+from typing import Optional
+
+from django.contrib.sessions.backends.cached_db import SessionStore as CachedDbSessionStore
+from django.db.transaction import get_connection
+
+
+class SessionStore(CachedDbSessionStore):
+    """Caching session object which does not leak into the cache.
+
+    django.contrib.sessions.backends.cached_db does write-through to
+    the cache and the backing database.  If the database is in a
+    transaction, this may leak not-yet-committed changes to the cache,
+    which can lead to inconsistent state.  This class wraps changes to
+    the session in assertions which enforce that the database cannot
+    be in a transaction before writing.
+
+    """
+
+    def save(self, must_create: bool = False) -> None:
+        assert not get_connection().in_atomic_block
+        super().save(must_create)
+
+    def delete(self, session_key: Optional[str] = None) -> None:
+        assert not get_connection().in_atomic_block
+        super().delete(session_key)

zproject/computed_settings.py

@@ -334,7 +334,7 @@ RABBITMQ_PASSWORD = get_secret("rabbitmq_password")
 # CACHING CONFIGURATION
 ########################################################################
 
-SESSION_ENGINE = "django.contrib.sessions.backends.cached_db"
+SESSION_ENGINE = "zerver.lib.safe_session_cached_db"
 
 MEMCACHED_PASSWORD = get_secret("memcached_password")