copIterator获取数据通过调用 copIterator

Copyfunc (it *copIterator) Next(ctx context.Context) (kv.ResultSubset, error) {
	var (
		resp   *copResponse
		ok     bool
		closed bool
	)
	...
	// 如果数据不需要排序，那么直接从 respChan 中获取数据
	if it.respChan != nil {
		// Get next fetched resp from chan
		resp, ok, closed = it.recvFromRespCh(ctx, it.respChan)
		if !ok || closed {
			it.actionOnExceed.close()
			return nil, nil
		}
		// 表示读到 respChan 最后一个数据
		if resp == finCopResp {
			it.actionOnExceed.destroyTokenIfNeeded(func() {
				it.sendRate.PutToken()
			})
			return it.Next(ctx)
		}
	} else {
		for {
			if it.curr >= len(it.tasks) {
				// Resp will be nil if iterator is finishCh.
				it.actionOnExceed.close()
				return nil, nil
			}
			// 如果数据是有序的，那么从 task 的 respChan 获取数据
			task := it.tasks[it.curr]
			resp, ok, closed = it.recvFromRespCh(ctx, task.respChan)
			if closed { 
				return nil, nil
			}
			if ok {
				break
			}
			it.actionOnExceed.destroyTokenIfNeeded(func() {
				it.sendRate.PutToken()
			}) 
			it.tasks[it.curr] = nil
			it.curr++
		}
	}

	if resp.err != nil {
		return nil, errors.Trace(resp.err)
	}

	err := it.store.CheckVisibility(it.req.StartTs)
	if err != nil {
		return nil, errors.Trace(err)
	}
	return resp, nil
}