Trim cached websocket event payload once per event instead of per subscriber (#175569)

Co-authored-by: Claude <noreply@anthropic.com>

homeassistant/components/websocket_api/messages.py

@@ -131,7 +131,7 @@ def cached_event_message(message_id_as_bytes: bytes, event: Event) -> bytes:
     """
     return b"".join(
         (
-            _partial_cached_event_message(event)[:-1],
+            _partial_cached_event_message(event),
             b',"id":',
             message_id_as_bytes,
             b"}",
@@ -143,13 +143,14 @@ def cached_event_message(message_id_as_bytes: bytes, event: Event) -> bytes:
 def _partial_cached_event_message(event: Event) -> bytes:
     """Cache and serialize the event to json.
 
-    The message is constructed without the id which appended
-    in cached_event_message.
+    The message is cached without the trailing "}" and without the id, both of
+    which are appended in cached_event_message. Trimming here means the slice
+    happens once per event instead of once per subscriber.
     """
     return (
         _message_to_json_bytes_or_none({"type": "event", "event": event.json_fragment})
         or INVALID_JSON_PARTIAL_MESSAGE
-    )
+    )[:-1]
 
 
 def cached_state_diff_message(
@@ -165,7 +166,7 @@ def cached_state_diff_message(
     """
     return b"".join(
         (
-            _partial_cached_state_diff_message(event)[:-1],
+            _partial_cached_state_diff_message(event),
             b',"id":',
             message_id_as_bytes,
             b"}",
@@ -177,15 +178,16 @@ def cached_state_diff_message(
 def _partial_cached_state_diff_message(event: Event[EventStateChangedData]) -> bytes:
     """Cache and serialize the event to json.
 
-    The message is constructed without the id which
-    will be appended in cached_state_diff_message
+    The message is cached without the trailing "}" and without the id, both of
+    which are appended in cached_state_diff_message. Trimming here means the
+    slice happens once per event instead of once per subscriber.
     """
     return (
         _message_to_json_bytes_or_none(
             {"type": "event", "event": _state_diff_event(event)}
         )
         or INVALID_JSON_PARTIAL_MESSAGE
-    )
+    )[:-1]
 
 
 def _state_diff_event(

tests/components/websocket_api/test_messages.py

@@ -10,6 +10,7 @@ from homeassistant.components.websocket_api.messages import (
 )
 from homeassistant.const import EVENT_STATE_CHANGED
 from homeassistant.core import Context, Event, HomeAssistant, State, callback
+from homeassistant.util.json import json_loads
 
 from tests.common import async_capture_events
 
@@ -52,6 +53,19 @@ async def test_cached_event_message(hass: HomeAssistant) -> None:
     assert cache_info.currsize == 2
 
 
+async def test_cached_event_message_is_valid_json(hass: HomeAssistant) -> None:
+    """Test the cached event message is valid JSON with the id appended."""
+    events = async_capture_events(hass, EVENT_STATE_CHANGED)
+    hass.states.async_set("light.window", "on")
+    await hass.async_block_till_done()
+
+    parsed = json_loads(cached_event_message(b"2", events[0]))
+
+    assert parsed["id"] == 2
+    assert parsed["type"] == "event"
+    assert parsed["event"]["event_type"] == EVENT_STATE_CHANGED
+
+
 async def test_cached_event_message_with_different_idens(hass: HomeAssistant) -> None:
     """Test that we cache event messages when the subscrition idens differ."""