Ensure websocket event serializer cache is effective if subscription iden differs (#42226)

Since someone websocket subscriptions will use an iden of 2 for
state_changed event (most comment), and some will use another
number for all events, the cache would not be used because the
iden number was different.  We now cache only the event and
use a fast replace to insert the iden number into the serailized
response.

homeassistant/components/websocket_api/messages.py

@@ -12,6 +12,7 @@ from homeassistant.util.json import (
     find_paths_unserializable_data,
     format_unserializable_data,
 )
+from homeassistant.util.yaml.loader import JSON_TYPE
 
 from . import const
 
@@ -27,6 +28,9 @@ MINIMAL_MESSAGE_SCHEMA = vol.Schema(
 # Base schema to extend by message handlers
 BASE_COMMAND_MESSAGE_SCHEMA = vol.Schema({vol.Required("id"): cv.positive_int})
 
+IDEN_TEMPLATE = "__IDEN__"
+IDEN_JSON_TEMPLATE = '"__IDEN__"'
+
 
 def result_message(iden: int, result: Any = None) -> Dict:
     """Return a success result message."""
@@ -43,12 +47,11 @@ def error_message(iden: int, code: str, message: str) -> Dict:
     }
 
 
-def event_message(iden: int, event: Any) -> Dict:
+def event_message(iden: JSON_TYPE, event: Any) -> Dict:
     """Return an event message."""
     return {"id": iden, "type": "event", "event": event}
 
 
-@lru_cache(maxsize=128)
 def cached_event_message(iden: int, event: Event) -> str:
     """Return an event message.
 
@@ -58,7 +61,17 @@ def cached_event_message(iden: int, event: Event) -> str:
     all getting many of the same events (mostly state changed)
     we can avoid serializing the same data for each connection.
     """
-    return message_to_json(event_message(iden, event))
+    return _cached_event_message(event).replace(IDEN_JSON_TEMPLATE, str(iden), 1)
+
+
+@lru_cache(maxsize=128)
+def _cached_event_message(event: Event) -> str:
+    """Cache and serialize the event to json.
+
+    The IDEN_TEMPLATE is used which will be replaced
+    with the actual iden in cached_event_message
+    """
+    return message_to_json(event_message(IDEN_TEMPLATE, event))
 
 
 def message_to_json(message: Any) -> str: