cache: Log a warning when attempting to store a whole QuerySet.

As noted in the previous commit, this causes bloat in memcached, for
no purpose.  Log a warning when `cache_with_key` sees a QuerySet
returned from the function it is decorating.

zerver/lib/cache.py

@@ -26,6 +26,7 @@ from django.conf import settings
 from django.core.cache import caches
 from django.core.cache.backends.base import BaseCache
 from django.db.models import Q
+from django.db.models.query import QuerySet
 from django.http import HttpRequest
 from typing_extensions import ParamSpec
 
@@ -173,6 +174,12 @@ def cache_with_key(
                 return val[0]
 
             val = func(*args, **kwargs)
+            if isinstance(val, QuerySet):  # type: ignore[misc] # https://github.com/typeddjango/django-stubs/issues/704
+                logging.warning(
+                    "cache_with_key attempted to store a full QuerySet object -- flattening using list()",
+                    stack_info=True,
+                )
+                val = list(val)
 
             cache_set(key, val, cache_name=cache_name, timeout=timeout)