Avoid generating a Context() object every second (#38085)

Every second we were calling the getrandom() syscall to generate a uuid4
for a context that will never be looked:

 * In most setups there are no more time_changed listeners

 * The ones that do exist never care about context

 * time_changed events are never saved in the database

homeassistant/core.py

@@ -1539,6 +1539,7 @@ class Config:
 def _async_create_timer(hass: HomeAssistant) -> None:
     """Create a timer that will start on HOMEASSISTANT_START."""
     handle = None
+    timer_context = Context()
 
     def schedule_tick(now: datetime.datetime) -> None:
         """Schedule a timer tick when the next second rolls around."""
@@ -1553,12 +1554,14 @@ def _async_create_timer(hass: HomeAssistant) -> None:
         """Fire next time event."""
         now = dt_util.utcnow()
 
-        hass.bus.async_fire(EVENT_TIME_CHANGED, {ATTR_NOW: now})
+        hass.bus.async_fire(EVENT_TIME_CHANGED, {ATTR_NOW: now}, context=timer_context)
 
         # If we are more than a second late, a tick was missed
         late = monotonic() - target
         if late > 1:
-            hass.bus.async_fire(EVENT_TIMER_OUT_OF_SYNC, {ATTR_SECONDS: late})
+            hass.bus.async_fire(
+                EVENT_TIMER_OUT_OF_SYNC, {ATTR_SECONDS: late}, context=timer_context
+            )
 
         schedule_tick(now)
 

tests/test_core.py

@@ -1090,9 +1090,20 @@ def test_timer_out_of_sync(mock_monotonic, loop):
     ):
         callback(target)
 
-        event_type, event_data = hass.bus.async_fire.mock_calls[1][1]
-        assert event_type == EVENT_TIMER_OUT_OF_SYNC
-        assert abs(event_data[ATTR_SECONDS] - 2.433333) < 0.001
+        _, event_0_args, event_0_kwargs = hass.bus.async_fire.mock_calls[0]
+        event_context_0 = event_0_kwargs["context"]
+
+        event_type_0, _ = event_0_args
+        assert event_type_0 == EVENT_TIME_CHANGED
+
+        _, event_1_args, event_1_kwargs = hass.bus.async_fire.mock_calls[1]
+        event_type_1, event_data_1 = event_1_args
+        event_context_1 = event_1_kwargs["context"]
+
+        assert event_type_1 == EVENT_TIMER_OUT_OF_SYNC
+        assert abs(event_data_1[ATTR_SECONDS] - 2.433333) < 0.001
+
+        assert event_context_0 == event_context_1
 
         assert len(funcs) == 2
         fire_time_event, _ = funcs