code: func (c *Context) InitTxnWithStartTS(startTS uint64) error {
  	if c.txn.Valid() {
  		return nil
  	}
  	if c.Store != nil {
  		membufCap := kv.DefaultTxnMembufCap
  		if c.sessionVars.LightningMode {
  			membufCap = kv.ImportingTxnMembufCap
  		}
  		txn, err := c.Store.BeginWithStartTS(startTS)
  		if err != nil {
  			return errors.Trace(err)
  		}
  		txn.SetCap(membufCap)
  		c.txn.Transaction = txn
  	}
  	return nil
  }
docs: // InitTxnWithStartTS implements the sessionctx.Context interface with startTS.