code: func NewBufferStore(r Retriever, cap int) *BufferStore {
  	if cap <= 0 {
  		cap = DefaultTxnMembufCap
  	}
  	return &BufferStore{
  		r:         r,
  		MemBuffer: &lazyMemBuffer{cap: cap},
  	}
  }
docs: // NewBufferStore creates a BufferStore using r for read.