Extraction of the last frame from a video raises av.error.EOFError

[MhhhxX]

Problem description:

Create a test video with ffmpeg. I use version 6.1.1-3ubuntu5:

ffmpeg -f lavfi -i testsrc -t 30 testsrc.mp4

Check the total number of frames with pyav version 14.0.1:

import av

with av.open('/path/to/testsrc.mp4') as container:
    stream = container.streams.video[0]
    print(stream.frames)

It should print 750.

Try to extract the last frame in that video which should be at index 749:

import av

frame_num = 749

with av.open('/path/to/testsrc.mp4') as container:
    stream = container.streams.video[0]
    for i in range(0, frame_num + 1):
        try:
            frame = next(container.decode(video=0))
        except EOFError as e:
            print(f'frame: {i}', e)
            raise

image = frame.to_image()
image.show()

Expected

It should open the last frame in the video with my standard image viewer.

Actual

Raises an av.error.EOFError and crashes

Does anyone know how to fix that problem? Ffmpeg extracts the last frame without problems if you run ffmpeg -i testsrc.mp4 -vf "select=eq(n\,749)" -vframes 1 testsrc.png

Update

The problem is caused by bframes in the video. Turning them off fixes the index in the for loop:

ffmpeg -f lavfi -i testsrc -t 30 -bf 0 testsrc.mp4