Rate events over at least one second (#24455)

* Rate events over at least one second

EventsPerSecond.eps() divided the event count by the time since start(),
which can be a few milliseconds right after a restart. Frames buffered
during an ffmpeg restart then report as 100+ fps, and the same happens to
the detector fps. Use a window of at least one second.

* Keep sub-second windows consistent

Floor the divisor at the window length when the window is shorter than a
second, so a caller with a sub-second window still gets its true rate.

frigate/test/test_builtin.py

@@ -36,6 +36,31 @@ class TestEventsPerSecond(unittest.TestCase):
             clock[0] += 100.0
             self.assertEqual(eps.eps(), 0.0)
 
+    def test_burst_after_start_is_not_divided_by_a_tiny_window(self) -> None:
+        eps = EventsPerSecond(last_n_seconds=10)
+        clock = [1000.0]
+        with patch("frigate.util.builtin.time.monotonic", side_effect=lambda: clock[0]):
+            eps.start()
+            # eleven buffered frames arrive within 100 ms of starting
+            for _ in range(11):
+                clock[0] += 0.01
+                eps.update()
+            # 11 events over less than a second is at most 11 per second
+            self.assertLessEqual(eps.eps(), 11.0)
+
+    def test_subsecond_window_keeps_its_rate(self) -> None:
+        eps = EventsPerSecond(last_n_seconds=0.5)
+        clock = [1000.0]
+        with patch("frigate.util.builtin.time.monotonic", side_effect=lambda: clock[0]):
+            eps.start()
+            # twenty events per second for two seconds
+            for _ in range(40):
+                clock[0] += 0.05
+                eps.update()
+            # read between events, so none sits exactly on the window edge
+            clock[0] += 0.01
+            self.assertAlmostEqual(eps.eps(), 20.0)
+
 
 if __name__ == "__main__":
     unittest.main()

frigate/util/builtin.py

@@ -56,10 +56,13 @@ class EventsPerSecond:
             self._start = now
         # compute the (approximate) events in the last n seconds
         self.expire_timestamps(now)
-        seconds = min(now - self._start, self._last_n_seconds)
+        # rate over at least one second (or the whole window, if shorter),
-        # avoid divide by zero
+        # so a burst of events right after start() is not divided by a
-        if seconds == 0:
+        # tiny window
-            seconds = 1
+        seconds = max(
+            min(now - self._start, self._last_n_seconds),
+            min(1.0, self._last_n_seconds),
+        )
         return len(self._timestamps) / seconds
 
     # remove aged out timestamps