VideoDecoder output guarantees

My use case requires me to decode multiple video streams at the same time. The source files can be of extremely high bitrate, 4K videos, so I would like to be mindful of how I use the GPU.

My root issue is that I cannot find any guarantee of when I might receive a frame or when I need to send more data to the decoder. If I investigate the packets and check the P or B frames, I see no reason why a packet would not be sent to the output. When I call flush I receive frames in the output perfectly.

Based on that experience, I thought I could decode GOPs, then call flush. In terms of decoding the bare minimum it's far from optimal, but at least it gives me some guarantee of when I can receive a frame. However, this is not trivial to do either since I work with libav under the hood, what I consider a keyframe might not be accepted by WebCodecs. Same issue as described here. The idea of flush without invalidating whatever inner state it has seemed like a good idea to me.

The answer to my question could be the optimizeForLatency, as in the specification it seems to do exactly what I want. However, as I read the threads here, I am now confused on what it does and what it doesn't. As @dalecurtis mentioned here, in Chromium, this flag does not affect flush directly, just omits using threading which can hold back frames. According to another thread though, I got the impression this only applies to software decoders, not hardware-accelerated ones. Does this have an effect in hw-accelerated decoding? If so, what are the performance drawbacks to consider?

Very different in nature but what I love about FFmpeg/libav is that this is very trivial to do:

int ret = avcodec_receive_frame(decoder->codec_ctx, decoder->frame);
if (ret == AVERROR(EAGAIN)) {
  // I know the decoder needs more frames before it can give me something
}

To summarize, my question is how can I have a guarantee that I passed enough packets (or how can I know for sure that I didn't ) to the VideoDecoder to surely have an output? Ideally, I want to decode the smallest amount possible for a guaranteed output.

Start by reading the VideoDecoder specification sections covering output, flush, and optimizeForLatency. Compare their intended behavior with the linked discussions in issues 650, 698, and 206, including the hardware-acceleration question. Done means the issue documents whether callers can know when more packets are needed, how latency optimization affects output, and any relevant performance tradeoffs.