Method sendChunkTask

internal/net/request.go:217–291

(newConcurrency bool)

}

func (d *downloader) sendChunkTask(newConcurrency bool) (err error) {
	d.mu.Lock()
	defer d.mu.Unlock()
	if d.pos >= d.maxPos {
		return nil
	}
	if newConcurrency {
		if d.concurrency <= 0 {
			return nil
		}
		if d.nextChunk > 0 { // 第一个不检查，因为已经检查过了
			if err := d.cfg.ConcurrencyLimit.Acquire(); err != nil {
				return err
			}
			defer func() {
				if err != nil {
					d.cfg.ConcurrencyLimit.Release()
				}
			}()
		}
	}

	br := d.bufMap[d.nextChunk]
	if br == nil {
		var b buffer.Block
		b, err = d.hc.NextBlock()
		if err != nil {
			return err
		}
		br = buffer.NewPipeBuffer(d.ctx, b)
		d.bufMap[d.nextChunk] = br
	}

	finalSize := int64(d.cfg.PartSize)
	switch d.nextChunk {
	case 0:
		// 最小分片在前面有助视频播放？
		firstSize := d.params.Range.Length % finalSize
		if firstSize > 0 {
			minSize := finalSize / 2
			// 最小分片太小就调整到一半
			finalSize = max(firstSize, minSize)
		}
	case 1:
		firstSize := d.params.Range.Length % finalSize
		minSize := finalSize / 2
		if firstSize > 0 && firstSize < minSize {
			finalSize += firstSize - minSize
		}
	}
	err = br.Reset(int(finalSize))
	if err != nil {
		return err // 分片算法错误或者下载中断
	}
	if newConcurrency {
		go d.downloadPart()
		d.concurrency--
	}