log when a frame slot was overwritten before it was read

A lapped read and a missing segment ended up as the same debug line at every caller, so there was no way to tell a slow reader from a missing frame.

frigate/test/test_shared_memory_frame_manager.py

@@ -217,13 +217,19 @@ class TestSharedMemoryFrameManagerFrameTimes(unittest.TestCase):
         segments = _segments(cam_frame0=2_500, cam_frame_times=8 * 2)
         np.frombuffer(segments["cam_frame_times"].buf, dtype=np.float64)[0] = 200.0
 
-        with patch(
-            "frigate.util.image.UntrackedSharedMemory",
-            side_effect=_open_from(segments),
+        with (
+            patch(
+                "frigate.util.image.UntrackedSharedMemory",
+                side_effect=_open_from(segments),
+            ),
+            self.assertLogs("frigate.util.image", "DEBUG") as logs,
         ):
             arr = manager.get("cam_frame0", (50, 50), frame_time=100.25)
 
         self.assertIsNone(arr)
+        self.assertIn(
+            "cam_frame0 no longer holds the frame from 100.25", logs.output[0]
+        )
 
     def test_get_returns_none_when_slot_index_is_beyond_ledger(self) -> None:
         manager = SharedMemoryFrameManager()

frigate/util/image.py

@@ -1108,6 +1108,7 @@ class SharedMemoryFrameManager(FrameManager):
             self.close(f"{name.rpartition('_frame')[0]}_frame_times")
 
             if not self._holds(name, frame_time):
+                logger.debug("%s no longer holds the frame from %s", name, frame_time)
                 return None
 
         try: