workaround when reading broken video frames

Some videos are being read with additional fake/broken frames that may
cause an unexpected early exit. This attempts to skip over these broken
frames if more were expected, based on the initally acquired frame
count.

According to other reports upstream, the number of broken frames read
varies. To avoid getting stuck in an endless loop, e.g. when the video
is actually broken, this gives up after 1000 reads.

see ultralytics#2064
see opencv/opencv#15352

utils/datasets.py

@@ -199,6 +199,16 @@ def __next__(self):
             # Read video
             self.mode = 'video'
             ret_val, img0 = self.cap.read()
+
+            if not ret_val and self.frame < self.frames - 1:
+                fake_frame_skip = 1000
+                while not ret_val and fake_frame_skip > 0:
+                    fake_frame_skip -= 1
+                    ret_val, img0 = self.cap.read()
+                if not ret_val:
+                    raise Exception(
+                        f'Video stream ended unexpectedly. See this issue for details: https://github.com/ultralytics/yolov5/issues/2064')
+
             while not ret_val:
                 self.count += 1
                 self.cap.release()