fix(api): clear ChangeNotifier's internal event immediately after wai…

…ting

After the event flag is set(), the waiter no longer waits. If the event flag is not cleared
immediately, this creates a potential scenario in which the notify() needs to set the event flag
again before the waiter gets around to clearing it. This causes some events to be dropped. Simply
clear the event flag immediately after waiting.

api/src/opentrons/util/change_notifier.py

@@ -15,8 +15,8 @@ def notify(self) -> None:
 
     async def wait(self) -> None:
         """Wait until the next state change notification."""
-        self._event.clear()
         await self._event.wait()
+        self._event.clear()
 
 
 class ChangeNotifier_ts(ChangeNotifier):

api/tests/opentrons/util/test_change_notifier.py

@@ -54,3 +54,30 @@ async def _do_task_3() -> None:
     await asyncio.gather(task_1, task_2, task_3)
 
     assert results == [1, 2, 3]
+
+
+async def test_notify_while_busy() -> None:
+    """Test that waiters process a new notify() after they are done being busy."""
+    subject = ChangeNotifier()
+    results = []
+
+    async def _do_task() -> None:
+        results.append("TEST")
+        await asyncio.sleep(0.2)  # Simulate being busy
+
+    async def do_task() -> None:
+        while True:
+            await subject.wait()
+            await _do_task()
+
+    task = asyncio.create_task(do_task())
+
+    subject.notify()
+    await asyncio.sleep(0.0)
+
+    subject.notify()
+    await asyncio.sleep(0.5)
+
+    assert results == ["TEST", "TEST"]
+
+    task.cancel()