Serialize websocket event message once (#40453)

Since most of the json serialize work for the websocket was done
multiple times for the same message, we can avoid the overhead
of serializing the same message many times (once per websocket
client) with a cache.

tests/components/websocket_api/test_messages.py

@@ -0,0 +1,65 @@
+"""Test Websocket API messages module."""
+
+from homeassistant.components.websocket_api.messages import (
+    cached_event_message,
+    message_to_json,
+)
+from homeassistant.const import EVENT_STATE_CHANGED
+from homeassistant.core import callback
+
+
+async def test_cached_event_message(hass):
+    """Test that we cache event messages."""
+
+    events = []
+
+    @callback
+    def _event_listener(event):
+        events.append(event)
+
+    hass.bus.async_listen(EVENT_STATE_CHANGED, _event_listener)
+
+    hass.states.async_set("light.window", "on")
+    hass.states.async_set("light.window", "off")
+    await hass.async_block_till_done()
+
+    assert len(events) == 2
+
+    msg0 = cached_event_message(2, events[0])
+    assert msg0 == cached_event_message(2, events[0])
+
+    msg1 = cached_event_message(2, events[1])
+    assert msg1 == cached_event_message(2, events[1])
+
+    assert msg0 != msg1
+
+    cache_info = cached_event_message.cache_info()
+    assert cache_info.hits == 2
+    assert cache_info.misses == 2
+    assert cache_info.currsize == 2
+
+    cached_event_message(2, events[1])
+    cache_info = cached_event_message.cache_info()
+    assert cache_info.hits == 3
+    assert cache_info.misses == 2
+    assert cache_info.currsize == 2
+
+
+async def test_message_to_json(caplog):
+    """Test we can serialize websocket messages."""
+
+    json_str = message_to_json({"id": 1, "message": "xyz"})
+
+    assert json_str == '{"id": 1, "message": "xyz"}'
+
+    json_str2 = message_to_json({"id": 1, "message": _Unserializeable()})
+
+    assert (
+        json_str2
+        == '{"id": 1, "type": "result", "success": false, "error": {"code": "unknown_error", "message": "Invalid JSON in response"}}'
+    )
+    assert "Unable to serialize to JSON" in caplog.text
+
+
+class _Unserializeable:
+    """A class that cannot be serialized."""