func sendBodySplitIntoChunk(w io.Writer, r *bufio.Reader) (err error) {
	// debug.Printf("sendBodySplitIntoChunk called\n")
	var b []byte
	for {
		b, err = r.ReadNext()
		// debug.Println("split into chunk n =", n, "err =", err)
		if err != nil {
			if err == io.EOF {
				// EOF is expected here as the server is closing connection.
				// debug.Println("end chunked encoding")
				_, err = w.Write([]byte(chunkEnd))
				if err != nil {
					debug.Println("write chunk end 0", err)
				}
				return
			}
			debug.Println("read error in sendBodySplitIntoChunk", err)
			return
		}

		chunkSize := []byte(fmt.Sprintf("%x\r\n", len(b)))
		if _, err = w.Write(chunkSize); err != nil {
			debug.Printf("write chunk size %v\n", err)
			return
		}
		if _, err = w.Write(b); err != nil {
			debug.Println("write chunk data:", err)
			return
		}
		if _, err = w.Write([]byte(CRLF)); err != nil {
			debug.Println("write chunk ending CRLF:", err)
			return
		}
	}
}

// copyN copys N bytes from src to dst, reading at most rdSize for each read.
// rdSize should <= buffer size of the buffered reader.
// Returns any encountered error.
func copyN(dst io.Writer, src *bufio.Reader, n, rdSize int) (err error) {
	// Most of the copy is copied from io.Copy
	for n > 0 {
		var b []byte
		var er error
		if n > rdSize {
			b, er = src.ReadN(rdSize)
		} else {
			b, er = src.ReadN(n)
		}
		nr := len(b)
		n -= nr
		if nr > 0 {
			nw, ew := dst.Write(b)
			if ew != nil {
				err = ew
				break
			}
			if nr != nw {
				err = io.ErrShortWrite
				break
			}
		}
		if er == io.EOF {
			break
		}
		if er != nil {
			err = er
			break
		}
	}
	return err
}

func skipCRLF(r *bufio.Reader) error {
	// There maybe servers using single '\n' for line ending
	if _, err := r.ReadSlice('\n'); err != nil {
		errl.Println("Error reading CRLF:", err)
		return err
	}
	return nil
}

// Send response body if header specifies chunked encoding. rdSize specifies
// the size of each read on Reader, it should be set to be the buffer size of
// the Reader, this parameter is added for testing.
func sendBodyChunked(w io.Writer, r *bufio.Reader, rdSize int) (err error) {
	// debug.Println("Sending chunked body")
	for {
		var s []byte
		// Read chunk size line, ignore chunk extension if any.
		if s, err = r.PeekSlice('\n'); err != nil {
			errl.Println("peek chunk size:", err)
			return
		}
		smid := bytes.IndexByte(s, ';')
		if smid == -1 {
			smid = len(s)
		} else {
			// use error log to find usage of chunk extension
			errl.Printf("got chunk extension: %s\n", s)
		}
		var size int64
		if size, err = ParseIntFromBytes(TrimSpace(s[:smid]), 16); err != nil {
			errl.Println("chunk size invalid:", err)
			return
		}
		/*
			if debug {
				// To debug getting malformed response status line with "0\r\n".
				if c, ok := w.(*clientConn); ok {
					debug.Printf("cli(%s) chunk size %d %#v\n", c.RemoteAddr(), size, string(s))
				}
			}
		*/
		if size == 0 {
			r.Skip(len(s))
			if err = skipCRLF(r); err != nil {
				return
			}
			if _, err = w.Write([]byte(chunkEnd)); err != nil {
				debug.Println("send chunk ending:", err)
			}
			return
		}
		// RFC 2616 19.3 only suggest tolerating single LF for
		// headers, not for chunked encoding. So assume the server will send
		// CRLF. If not, the following parse int may find errors.
		total := len(s) + int(size) + 2 // total data size for this chunk, including ending CRLF
		// PeekSlice will not advance reader, so we can just copy total sized data.
		if err = copyN(w, r, total, rdSize); err != nil {
			debug.Println("copy chunked data:", err)
			return
		}
	}
}

// Only add headers that are of interest for a proxy into request/response's header map.
func (h *Header) parseHeader(reader *bufio.Reader, raw *bytes.Buffer, url *URL) (err error) {
	h.ContLen = -1
	dummyLastLine := []byte{}
	// Read request header and body
	var s, name, val, lastLine []byte
	for {
		if s, err = reader.ReadSlice('\n'); err != nil {
			return
		}
		// There are servers that use \n for line ending, so trim first before check ending.
		// For example, the 404 page for http://plan9.bell-labs.com/magic/man2html/1/2l
		trimmed := TrimSpace(s)
		if len(trimmed) == 0 { // end of headers
			return
		}
		if (s[0] == ' ' || s[0] == '\t') && lastLine != nil { // multi-line header
			// I've never seen multi-line header used in headers that's of interest.
			// Disable multi-line support to avoid copy for now.
			errl.Printf("Multi-line support disabled: %v %s", url, s)
			return errNotSupported
			// combine previous line with current line
			// trimmed = bytes.Join([][]byte{lastLine, []byte{' '}, trimmed}, nil)
		}
		if name, val, err = splitHeader(trimmed); err != nil {
			return
		}
		// Wait Go to solve/provide the string<->[]byte optimization
		kn := string(name)
		if parseFunc, ok := headerParser[kn]; ok {
			// lastLine = append([]byte(nil), trimmed...) // copy to avoid next read invalidating the trimmed line
			lastLine = dummyLastLine
			val = TrimSpace(val)
			if len(val) == 0 {
				continue
			}
			parseFunc(h, ASCIIToLower(val), raw)
		} else {
			// mark this header as not of interest to proxy
			lastLine = nil
		}
		if hopByHopHeader[kn] {
			continue
		}
		raw.Write(s)
		// debug.Printf("len %d %s", len(s), s)
	}
	return
}

func skipSpace(r *bufio.Reader) int {
	n := 0
	for {
		c, err := r.ReadByte()
		if err != nil {
			// Bufio will keep err until next read.
			break
		}
		if c != ' ' && c != '\t' {
			r.UnreadByte()
			break
		}
		n++
	}
	return n
}

// ReadLineBytes read till '\n' is found or encounter error. The returned line
// does not include ending '\r\n' or '\n'. Returns err != nil if and only if
// len(line) == 0. Note the returned byte should not be used for append and
// maybe overwritten by next I/O operation. Copied code of readLineSlice from
// $GOROOT/src/pkg/net/textproto/reader.go
func ReadLineSlice(r *bufio.Reader) (line []byte, err error) {
	for {
		l, more, err := r.ReadLine()
		if err != nil {
			return nil, err
		}
		// Avoid the copy if the first call produced a full line.
		if line == nil && !more {
			return l, nil
		}
		line = append(line, l...)
		if !more {
			break
		}
	}
	return line, nil
}

// Use this function until we find Trailer headers actually in use.
func skipTrailer(r *bufio.Reader) error {
	// It's possible to get trailer headers, but the body will always end with
	// a line with just CRLF.
	for {
		s, err := r.ReadSlice('\n')
		if err != nil {
			errl.Println("skip trailer:", err)
			return err
		}
		if len(s) == 2 && s[0] == '\r' && s[1] == '\n' {
			return nil
		}
		errl.Printf("skip trailer: %#v", string(s))
		if len(s) == 1 || len(s) == 2 {
			return fmt.Errorf("malformed chunk body end: %#v", string(s))
		}
	}
}

// Learned from net.textproto. One difference is that this one keeps the
// ending '\n' in the returned line. Buf if there's only CRLF in the line,
// return nil for the line.
func readContinuedLineSlice(r *bufio.Reader) ([]byte, error) {
	// feedly.com request headers contains things like:
	// "$Authorization.feedly: $FeedlyAuth\r\n", so we must test for only
	// continuation spaces.
	isspace := func(b byte) bool {
		return b == ' ' || b == '\t'
	}

	// Read the first line.
	line, err := r.ReadSlice('\n')
	if err != nil {
		return nil, err
	}

	// There are servers that use \n for line ending, so trim first before check ending.
	// For example, the 404 page for http://plan9.bell-labs.com/magic/man2html/1/2l
	trimmed := TrimSpace(line)
	if len(trimmed) == 0 {
		if len(line) > 2 {
			return nil, fmt.Errorf("malformed end of headers, len: %d, %#v", len(line), string(line))
		}
		return nil, nil
	}

	if isspace(line[0]) {
		return nil, fmt.Errorf("malformed header, start with space: %#v", string(line))
	}

	// Optimistically assume that we have started to buffer the next line
	// and it starts with an ASCII letter (the next header key), so we can
	// avoid copying that buffered data around in memory and skipping over
	// non-existent whitespace.
	if r.Buffered() > 0 {
		peek, err := r.Peek(1)
		if err == nil && !isspace(peek[0]) {
			return line, nil
		}
	}

	var buf []byte
	buf = append(buf, trimmed...)

	// Read continuation lines.
	for skipSpace(r) > 0 {
		line, err := r.ReadSlice('\n')
		if err != nil {
			break
		}
		buf = append(buf, ' ')
		buf = append(buf, TrimTrailingSpace(line)...)
	}
	buf = append(buf, '\r', '\n')
	return buf, nil
}

// Send response body if header specifies chunked encoding. rdSize specifies
// the size of each read on Reader, it should be set to be the buffer size of
// the Reader, this parameter is added for testing.
func sendBodyChunked(r *bufio.Reader, w io.Writer, rdSize int) (err error) {
	// debug.Println("Sending chunked body")
	for {
		var s []byte
		// Read chunk size line, ignore chunk extension if any.
		if s, err = r.PeekSlice('\n'); err != nil {
			errl.Println("peeking chunk size:", err)
			return
		}
		// debug.Printf("Chunk size line %s\n", s)
		smid := bytes.IndexByte(s, ';')
		if smid == -1 {
			smid = len(s)
		}
		var size int64
		if size, err = ParseIntFromBytes(TrimSpace(s[:smid]), 16); err != nil {
			errl.Println("chunk size invalid:", err)
			return
		}
		// end of chunked data. As we remove trailer header in request sending
		// to server, there should be no trailer in response.
		// TODO: Is it possible for client request body to have trailers in it?
		if size == 0 {
			r.Skip(len(s))
			skipCRLF(r)
			if _, err = w.Write([]byte(chunkEnd)); err != nil {
				debug.Println("sending chunk ending:", err)
			}
			return
		}
		// The spec section 19.3 only suggest toleranting single LF for
		// headers, not for chunked encoding. So assume the server will send
		// CRLF. If not, the following parse int may find errors.
		total := len(s) + int(size) + 2 // total data size for this chunk, including ending CRLF
		// PeekSlice will not advance reader, so we can just copy total sized data.
		if err = copyN(w, r, total, rdSize); err != nil {
			debug.Println("copying chunked data:", err)
			return
		}
	}
	return
}