Seeking bug in case (bufferToKeep > bufferTimeAtTopQuality)

[windbella]

Environment

• Link to playable MPD file: https://dash.akamaized.net/akamai/bbb_30fps/bbb_30fps.mpd
• Dash.js version: 5.0.0

Steps to reproduce

1. set option

var config = {
    streaming: {
        buffer: {
            stableBufferTime: 10,
            bufferTimeDefault: 10,
            bufferTimeAtTopQuality: 10,
            bufferTimeAtTopQualityLongForm: 10,
            longFormContentDurationThreshold: 6000,
            bufferToKeep: 120,
            bufferPruningInterval: 10,
        },
    }
};

2. Load and play video
3. Pause (0s ~ 1s)
4. Wait for the buffer to fill up
5. Seek to 12 ~ 118 sec
6. It will no longer be downloading segments.
7. Video doesn't play when you call play.

Observed behavior

// ScheduleConstroller.js, 222L
function _shouldBuffer() {
    const currentRepresentation = representationController.getCurrentRepresentation();
    if (!type || !currentRepresentation) {
        return true;
    }
    const bufferLevel = dashMetrics.getCurrentBufferLevel(type);
    return bufferLevel < getBufferTarget();
}

When this happens, the bufferLevel is large and the _shouldBuffer method is constantly false.

// StreamProcessor.js, 296L
bufferController.prepareForPlaybackSeek()
    .then(() => {
        // Clear the buffer. We need to prune everything which is not in the target interval.
        const clearRanges = bufferController.getAllRangesWithSafetyFactor(e.seekTime); // empty
        // When everything has been pruned go on
        return bufferController.clearBuffers(clearRanges);
    })

// BufferController.js, 601L
const behindPruningRange = _getRangeBehindForPruning(seekTime, ranges); // empty
const aheadPruningRange = _getRangeAheadForPruning(seekTime, ranges); // empty

For the same reason as the code above, clearRanges will be empty.

// BufferController.js, L926
function clearBuffers(ranges) {
    return new Promise((resolve, reject) => {
        if (!ranges || !sourceBufferSink || ranges.length === 0) {
            resolve();
            return;
        }

        const promises = [];
        ranges.forEach((range) => {
            promises.push(_addClearRangeWithPromise(range));
        });


        if (!isPruningInProgress) {
            clearNextRange();
        }

        Promise.all(promises)
            .then(() => {
                resolve();
            })
            .catch((e) => {
                reject(e);
            });
    });
}

If ranges is empty, clearNextRange is not called.
Then the internal _onRemoved will not be called, nor will updateBufferLevel.
Because the bufferLevel hasn't been updated, _shouldBuffer becomes false, which means we won't receive the next buffer.

// BUfferController.js, L961
function clearBuffers(ranges) {
    return new Promise((resolve, reject) => {
        if (!ranges || !sourceBufferSink || ranges.length === 0) {
            _updateBufferLevel(); // Update bufferLevel
            resolve();
            return;
        }

I think changing the code as above should fix it.
However, there may be side effects, so I'd like you to review them.

Console output

Debug.js:169 [12734][PlaybackController] Requesting seek to time: 51 
Debug.js:169 [12739][PlaybackController] Seeking to: 51 
Debug.js:169 [12739][FragmentModel][video] abort requests 
Debug.js:169 [12740][FragmentModel][audio] abort requests 
Debug.js:169 [12740][SourceBufferSink][video] Updated append window for video. Set start to 0 and end to 634.576 
Debug.js:169 [12740][SourceBufferSink][audio] Updated append window for audio. Set start to 0 and end to 634.576 

Expected behavior

I want it to work well in situations like the above.