func (rws *responseWriterState) writeChunk(p []byte) (n int, err error) {
	if !rws.wroteHeader {
		rws.writeHeader(200)
	}

	isHeadResp := rws.req.Method == "HEAD"
	if !rws.sentHeader {
		rws.sentHeader = true
		var ctype, clen string
		if clen = rws.snapHeader.Get("Content-Length"); clen != "" {
			rws.snapHeader.Del("Content-Length")
			clen64, err := strconv.ParseInt(clen, 10, 64)
			if err == nil && clen64 >= 0 {
				rws.sentContentLen = clen64
			} else {
				clen = ""
			}
		}
		if clen == "" && rws.handlerDone && bodyAllowedForStatus(rws.status) && (len(p) > 0 || !isHeadResp) {
			clen = strconv.Itoa(len(p))
		}
		_, hasContentType := rws.snapHeader["Content-Type"]
		// If the Content-Encoding is non-blank, we shouldn't
		// sniff the body. See Issue golang.org/issue/31753.
		ce := rws.snapHeader.Get("Content-Encoding")
		hasCE := len(ce) > 0
		if !hasCE && !hasContentType && bodyAllowedForStatus(rws.status) && len(p) > 0 {
			ctype = http.DetectContentType(p)
		}
		var date string
		if _, ok := rws.snapHeader["Date"]; !ok {
			// TODO(bradfitz): be faster here, like net/http? measure.
			date = time.Now().UTC().Format(http.TimeFormat)
		}

		for _, v := range rws.snapHeader["Trailer"] {
			foreachHeaderElement(v, rws.declareTrailer)
		}

		// "Connection" headers aren't allowed in HTTP/2 (RFC 7540, 8.1.2.2),
		// but respect "Connection" == "close" to mean sending a GOAWAY and tearing
		// down the TCP connection when idle, like we do for HTTP/1.
		// TODO: remove more Connection-specific header fields here, in addition
		// to "Connection".
		if _, ok := rws.snapHeader["Connection"]; ok {
			v := rws.snapHeader.Get("Connection")
			delete(rws.snapHeader, "Connection")
			if v == "close" {
				rws.conn.startGracefulShutdown()
			}
		}

		endStream := (rws.handlerDone && !rws.hasTrailers() && len(p) == 0) || isHeadResp
		err = rws.conn.writeHeaders(rws.stream, &writeResHeaders{
			streamID:      rws.stream.id,
			httpResCode:   rws.status,
			h:             rws.snapHeader,
			endStream:     endStream,
			contentType:   ctype,
			contentLength: clen,
			date:          date,
		})
		if err != nil {
			rws.dirty = true
			return 0, err
		}
		if endStream {
			return 0, nil
		}
	}
	if isHeadResp {
		return len(p), nil
	}
	if len(p) == 0 && !rws.handlerDone {
		return 0, nil
	}

	if rws.handlerDone {
		rws.promoteUndeclaredTrailers()
	}

	// only send trailers if they have actually been defined by the
	// server handler.
	hasNonemptyTrailers := rws.hasNonemptyTrailers()
	endStream := rws.handlerDone && !hasNonemptyTrailers
	if len(p) > 0 || endStream {
		// only send a 0 byte DATA frame if we're ending the stream.
		if err := rws.conn.writeDataFromHandler(rws.stream, p, endStream); err != nil {
			rws.dirty = true
			return 0, err
		}
	}

	if rws.handlerDone && hasNonemptyTrailers {
		err = rws.conn.writeHeaders(rws.stream, &writeResHeaders{
			streamID:  rws.stream.id,
			h:         rws.handlerHeader,
			trailers:  rws.trailers,
			endStream: true,
		})
		if err != nil {
			rws.dirty = true
		}
		return len(p), err
	}
	return len(p), nil
}