clear the c->buffer in the comm_point_send_reply does resolve the "can't fit qbuffer in c->buffer" issue, but it breaks the mesh reply list function that need to reuse the answer. because the c->buffer is cleared in the comm_point_send_reply, it cannot be resued again. it means that it is not inappropriate to clear c->buffer in the comm_point_send_reply.
After some investigation, i found it is appropriate to clear c->buffer before use in the http2_query_read_done.