cache: Instantiate only one BMemcached cache backend.

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

pyproject.toml

@@ -61,6 +61,7 @@ module = [
     "disposable_email_domains.*",
     "django.*",
     "django_auth_ldap.*",
+    "django_bmemcached.*",
     "django_cte.*",
     "django_otp.*",
     "django_scim.*",

tools/test-backend

@@ -140,6 +140,7 @@ not_yet_fully_covered = [
     "zerver/webhooks/zapier/view.py",
     # Cannot have coverage, as tests run in a transaction
     "zerver/lib/safe_session_cached_db.py",
+    "zerver/lib/singleton_bmemcached.py",
 ]
 
 enforce_fully_covered = sorted(

zerver/lib/singleton_bmemcached.py

@@ -0,0 +1,19 @@
+import pickle
+from functools import lru_cache
+from typing import Any, Dict
+
+from django_bmemcached.memcached import BMemcached
+
+
+@lru_cache(None)
+def _get_bmemcached(location: str, params: bytes) -> BMemcached:
+    return BMemcached(location, pickle.loads(params))
+
+
+def SingletonBMemcached(location: str, params: Dict[str, Any]) -> BMemcached:
+    # Django instantiates the cache backend per-task to guard against
+    # thread safety issues, but BMemcached is already thread-safe and
+    # does its own per-thread pooling, so make sure we instantiate only
+    # one to avoid extra connections.
+
+    return _get_bmemcached(location, pickle.dumps(params))

zilencer/management/commands/populate_db.py

@@ -96,7 +96,7 @@ def clear_database() -> None:
     # With `zproject.test_settings`, we aren't using real memcached
     # and; we only need to flush memcached if we're populating a
     # database that would be used with it (i.e. zproject.dev_settings).
-    if default_cache["BACKEND"] == "django_bmemcached.memcached.BMemcached":
+    if default_cache["BACKEND"] == "zerver.lib.singleton_bmemcached.SingletonBMemcached":
         bmemcached.Client(
             (default_cache["LOCATION"],),
             **default_cache["OPTIONS"],

zproject/computed_settings.py

@@ -340,7 +340,7 @@ MEMCACHED_PASSWORD = get_secret("memcached_password")
 
 CACHES = {
     "default": {
-        "BACKEND": "django_bmemcached.memcached.BMemcached",
+        "BACKEND": "zerver.lib.singleton_bmemcached.SingletonBMemcached",
         "LOCATION": MEMCACHED_LOCATION,
         "OPTIONS": {
             "socket_timeout": 3600,