flush_memcached: Respect DJANGO_SETTINGS_MODULE.

We don’t need to flush anything for zproject.test_settings, which
disables memcached.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

scripts/setup/flush-memcached

@@ -4,19 +4,24 @@ import sys
 
 BASE_DIR = os.path.join(os.path.dirname(os.path.abspath(__file__)), "../..")
 sys.path.append(BASE_DIR)
+os.environ.setdefault("DJANGO_SETTINGS_MODULE", "zproject.settings")
 
 from scripts.lib.setup_path import setup_path
 
 setup_path()
 
 import bmemcached
-
-from zproject import settings
+from django.conf import settings
 
 cache = settings.CACHES["default"]
-assert isinstance(cache, dict)  # for mypy
-client = bmemcached.Client((cache["LOCATION"],), **cache["OPTIONS"])
-try:
-    client.flush_all()
-finally:
-    client.disconnect_all()
+match cache["BACKEND"]:
+    case "django.core.cache.backends.locmem.LocMemCache":
+        pass
+    case "zerver.lib.singleton_bmemcached.SingletonBMemcached":
+        client = bmemcached.Client((cache["LOCATION"],), **cache["OPTIONS"])
+        try:
+            client.flush_all()
+        finally:
+            client.disconnect_all()
+    case _:
+        raise ValueError(f"Unrecognized cache backend {cache['BACKEND']}")