update reverseproxy from std libraries

go.mod

@@ -26,6 +26,8 @@ require (
 	github.com/templexxx/xor v0.0.0-20170926022130-0af8e873c554 // indirect
 	github.com/tjfoc/gmsm v0.0.0-20171124023159-98aa888b79d8 // indirect
 	github.com/vaughan0/go-ini v0.0.0-20130923145212-a98ad7ee00ec
+	github.com/xtaci/lossyconn v0.0.0-20190602105132-8df528c0c9ae // indirect
 	golang.org/x/crypto v0.0.0-20180505025534-4ec37c66abab // indirect
 	golang.org/x/net v0.0.0-20180524181706-dfa909b99c79
+	golang.org/x/text v0.3.2 // indirect
 )

go.sum

@@ -46,7 +46,12 @@ github.com/tjfoc/gmsm v0.0.0-20171124023159-98aa888b79d8 h1:6CNSDqI1wiE+JqyOy5Qt
 github.com/tjfoc/gmsm v0.0.0-20171124023159-98aa888b79d8/go.mod h1:XxO4hdhhrzAd+G4CjDqaOkd0hUzmtPR/d3EiBBMn/wc=
 github.com/vaughan0/go-ini v0.0.0-20130923145212-a98ad7ee00ec h1:DGmKwyZwEB8dI7tbLt/I/gQuP559o/0FrAkHKlQM/Ks=
 github.com/vaughan0/go-ini v0.0.0-20130923145212-a98ad7ee00ec/go.mod h1:owBmyHYMLkxyrugmfwE/DLJyW8Ro9mkphwuVErQ0iUw=
+github.com/xtaci/lossyconn v0.0.0-20190602105132-8df528c0c9ae h1:J0GxkO96kL4WF+AIT3M4mfUVinOCPgf2uUWYFUzN0sM=
+github.com/xtaci/lossyconn v0.0.0-20190602105132-8df528c0c9ae/go.mod h1:gXtu8J62kEgmN++bm9BVICuT/e8yiLI2KFobd/TRFsE=
 golang.org/x/crypto v0.0.0-20180505025534-4ec37c66abab h1:w4c/LoOA2vE8SYwh8wEEQVRUwpph7TtcjH7AtZvOjy0=
 golang.org/x/crypto v0.0.0-20180505025534-4ec37c66abab/go.mod h1:6SG95UA2DQfeDnfUPMdvaQW0Q7yPrPDi9nlGo2tz2b4=
 golang.org/x/net v0.0.0-20180524181706-dfa909b99c79 h1:1FDlG4HI84rVePw1/0E/crL5tt2N+1blLJpY6UZ6krs=
 golang.org/x/net v0.0.0-20180524181706-dfa909b99c79/go.mod h1:mL1N/T3taQHkDXs73rZJwtUhF3w3ftmwwsq0BUmARs4=
+golang.org/x/text v0.3.2 h1:tW2bmiBqwgJj/UpqtC8EpXEZVYOwU0yG4iWbprSVAcs=
+golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
+golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=

utils/vhost/http.go

@@ -89,14 +89,18 @@ func NewHttpReverseProxy(option HttpReverseProxyOptions, vhostRouter *VhostRoute
 				return rp.CreateConnection(host, url, remote)
 			},
 		},
-		WebSocketDialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
-			url := ctx.Value("url").(string)
-			host := getHostFromAddr(ctx.Value("host").(string))
-			remote := ctx.Value("remote").(string)
-			return rp.CreateConnection(host, url, remote)
-		},
+		//WebSocketDialContext: func(ctx context.Context, network, addr string) (net.Conn, error) {
+		//url := ctx.Value("url").(string)
+		//host := getHostFromAddr(ctx.Value("host").(string))
+		//remote := ctx.Value("remote").(string)
+		//return rp.CreateConnection(host, url, remote)
+		//},
 		BufferPool: newWrapPool(),
 		ErrorLog:   log.New(newWrapLogger(), "", 0),
+		ErrorHandler: func(rw http.ResponseWriter, req *http.Request, err error) {
+			rw.WriteHeader(http.StatusNotFound)
+			rw.Write(getNotFoundPageContent())
+		},
 	}
 	rp.proxy = proxy
 	return rp

utils/vhost/reverseproxy.go

@@ -8,6 +8,7 @@ package vhost
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"log"
 	"net"
@@ -17,13 +18,9 @@ import (
 	"sync"
 	"time"
 
-	frpIo "github.com/fatedier/golib/io"
+	"golang.org/x/net/http/httpguts"
 )
 
-// onExitFlushLoop is a callback set by tests to detect the state of the
-// flushLoop() goroutine.
-var onExitFlushLoop func()
-
 // ReverseProxy is an HTTP Handler that takes an incoming request and
 // sends it to another server, proxying the response back to the
 // client.
@@ -44,12 +41,17 @@ type ReverseProxy struct {
 	// to flush to the client while copying the
 	// response body.
 	// If zero, no periodic flushing is done.
+	// A negative value means to flush immediately
+	// after each write to the client.
+	// The FlushInterval is ignored when ReverseProxy
+	// recognizes a response as a streaming response;
+	// for such responses, writes are flushed to the client
+	// immediately.
 	FlushInterval time.Duration
 
 	// ErrorLog specifies an optional logger for errors
 	// that occur when attempting to proxy the request.
-	// If nil, logging goes to os.Stderr via the log package's
-	// standard logger.
+	// If nil, logging is done via the log package's standard logger.
 	ErrorLog *log.Logger
 
 	// BufferPool optionally specifies a buffer pool to
@@ -57,12 +59,23 @@ type ReverseProxy struct {
 	// copying HTTP response bodies.
 	BufferPool BufferPool
 
-	// ModifyResponse is an optional function that
-	// modifies the Response from the backend.
-	// If it returns an error, the proxy returns a StatusBadGateway error.
+	// ModifyResponse is an optional function that modifies the
+	// Response from the backend. It is called if the backend
+	// returns a response at all, with any HTTP status code.
+	// If the backend is unreachable, the optional ErrorHandler is
+	// called without any call to ModifyResponse.
+	//
+	// If ModifyResponse returns an error, ErrorHandler is called
+	// with its error value. If ErrorHandler is nil, its default
+	// implementation is used.
 	ModifyResponse func(*http.Response) error
 
-	WebSocketDialContext func(ctx context.Context, network, addr string) (net.Conn, error)
+	// ErrorHandler is an optional function that handles errors
+	// reaching the backend or errors from ModifyResponse.
+	//
+	// If nil, the default is to log the provided error and return
+	// a 502 Status Bad Gateway response.
+	ErrorHandler func(http.ResponseWriter, *http.Request, error)
 }
 
 // A BufferPool is an interface for getting and returning temporary
@@ -118,18 +131,11 @@ func copyHeader(dst, src http.Header) {
 	}
 }
 
-func cloneHeader(h http.Header) http.Header {
-	h2 := make(http.Header, len(h))
-	for k, vv := range h {
-		vv2 := make([]string, len(vv))
-		copy(vv2, vv)
-		h2[k] = vv2
-	}
-	return h2
-}
-
 // Hop-by-hop headers. These are removed when sent to the backend.
-// http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html
+// As of RFC 7230, hop-by-hop headers are required to appear in the
+// Connection header field. These are the headers defined by the
+// obsoleted RFC 2616 (section 13.5.1) and are used for backward
+// compatibility.
 var hopHeaders = []string{
 	"Connection",
 	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
@@ -137,55 +143,38 @@ var hopHeaders = []string{
 	"Proxy-Authenticate",
 	"Proxy-Authorization",
 	"Te",      // canonicalized version of "TE"
-	"Trailer", // not Trailers per URL above; http://www.rfc-editor.org/errata_search.php?eid=4522
+	"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
 	"Transfer-Encoding",
 	"Upgrade",
 }
 
+func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
+	p.logf("http: proxy error: %v", err)
+	rw.WriteHeader(http.StatusBadGateway)
+}
+
+func (p *ReverseProxy) getErrorHandler() func(http.ResponseWriter, *http.Request, error) {
+	if p.ErrorHandler != nil {
+		return p.ErrorHandler
+	}
+	return p.defaultErrorHandler
+}
+
+// modifyResponse conditionally runs the optional ModifyResponse hook
+// and reports whether the request should proceed.
+func (p *ReverseProxy) modifyResponse(rw http.ResponseWriter, res *http.Response, req *http.Request) bool {
+	if p.ModifyResponse == nil {
+		return true
+	}
+	if err := p.ModifyResponse(res); err != nil {
+		res.Body.Close()
+		p.getErrorHandler()(rw, req, err)
+		return false
+	}
+	return true
+}
+
 func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
-	if IsWebsocketRequest(req) {
-		p.serveWebSocket(rw, req)
-	} else {
-		p.serveHTTP(rw, req)
-	}
-}
-
-func (p *ReverseProxy) serveWebSocket(rw http.ResponseWriter, req *http.Request) {
-	if p.WebSocketDialContext == nil {
-		rw.WriteHeader(500)
-		return
-	}
-
-	req = req.WithContext(context.WithValue(req.Context(), "url", req.URL.Path))
-	req = req.WithContext(context.WithValue(req.Context(), "host", req.Host))
-	req = req.WithContext(context.WithValue(req.Context(), "remote", req.RemoteAddr))
-
-	targetConn, err := p.WebSocketDialContext(req.Context(), "tcp", "")
-	if err != nil {
-		rw.WriteHeader(501)
-		return
-	}
-	defer targetConn.Close()
-
-	p.Director(req)
-
-	hijacker, ok := rw.(http.Hijacker)
-	if !ok {
-		rw.WriteHeader(500)
-		return
-	}
-	conn, _, errHijack := hijacker.Hijack()
-	if errHijack != nil {
-		rw.WriteHeader(500)
-		return
-	}
-	defer conn.Close()
-
-	req.Write(targetConn)
-	frpIo.Join(conn, targetConn)
-}
-
-func (p *ReverseProxy) serveHTTP(rw http.ResponseWriter, req *http.Request) {
 	transport := p.Transport
 	if transport == nil {
 		transport = http.DefaultTransport
@@ -206,38 +195,49 @@ func (p *ReverseProxy) serveHTTP(rw http.ResponseWriter, req *http.Request) {
 		}()
 	}
 
-	outreq := req.WithContext(ctx) // includes shallow copies of maps, but okay
+	outreq := req.Clone(ctx)
 	if req.ContentLength == 0 {
 		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
 	}
 
-	outreq.Header = cloneHeader(req.Header)
-
-	// Modify for frp
+	// =============================
+	// Modified for frp
 	outreq = outreq.WithContext(context.WithValue(outreq.Context(), "url", req.URL.Path))
 	outreq = outreq.WithContext(context.WithValue(outreq.Context(), "host", req.Host))
 	outreq = outreq.WithContext(context.WithValue(outreq.Context(), "remote", req.RemoteAddr))
+	// =============================
 
 	p.Director(outreq)
 	outreq.Close = false
 
-	// Remove hop-by-hop headers listed in the "Connection" header.
-	// See RFC 2616, section 14.10.
-	if c := outreq.Header.Get("Connection"); c != "" {
-		for _, f := range strings.Split(c, ",") {
-			if f = strings.TrimSpace(f); f != "" {
-				outreq.Header.Del(f)
-			}
-		}
-	}
+	reqUpType := upgradeType(outreq.Header)
+	removeConnectionHeaders(outreq.Header)
 
 	// Remove hop-by-hop headers to the backend. Especially
 	// important is "Connection" because we want a persistent
 	// connection, regardless of what the client sent to us.
 	for _, h := range hopHeaders {
-		if outreq.Header.Get(h) != "" {
-			outreq.Header.Del(h)
+		hv := outreq.Header.Get(h)
+		if hv == "" {
+			continue
 		}
+		if h == "Te" && hv == "trailers" {
+			// Issue 21096: tell backend applications that
+			// care about trailer support that we support
+			// trailers. (We do, but we don't go out of
+			// our way to advertise that unless the
+			// incoming client request thought it was
+			// worth mentioning)
+			continue
+		}
+		outreq.Header.Del(h)
+	}
+
+	// After stripping all the hop-by-hop connection headers above, add back any
+	// necessary for protocol upgrades, such as for websockets.
+	if reqUpType != "" {
+		outreq.Header.Set("Connection", "Upgrade")
+		outreq.Header.Set("Upgrade", reqUpType)
 	}
 
 	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
@@ -252,33 +252,27 @@ func (p *ReverseProxy) serveHTTP(rw http.ResponseWriter, req *http.Request) {
 
 	res, err := transport.RoundTrip(outreq)
 	if err != nil {
-		p.logf("http: proxy error: %v", err)
-		rw.WriteHeader(http.StatusNotFound)
-
-		rw.Write(getNotFoundPageContent())
+		p.getErrorHandler()(rw, outreq, err)
 		return
 	}
 
-	// Remove hop-by-hop headers listed in the
-	// "Connection" header of the response.
-	if c := res.Header.Get("Connection"); c != "" {
-		for _, f := range strings.Split(c, ",") {
-			if f = strings.TrimSpace(f); f != "" {
-				res.Header.Del(f)
-			}
+	// Deal with 101 Switching Protocols responses: (WebSocket, h2c, etc)
+	if res.StatusCode == http.StatusSwitchingProtocols {
+		if !p.modifyResponse(rw, res, outreq) {
+			return
 		}
+		p.handleUpgradeResponse(rw, outreq, res)
+		return
 	}
 
+	removeConnectionHeaders(res.Header)
+
 	for _, h := range hopHeaders {
 		res.Header.Del(h)
 	}
 
-	if p.ModifyResponse != nil {
-		if err := p.ModifyResponse(res); err != nil {
-			p.logf("http: proxy error: %v", err)
-			rw.WriteHeader(http.StatusBadGateway)
-			return
-		}
+	if !p.modifyResponse(rw, res, outreq) {
+		return
 	}
 
 	copyHeader(rw.Header(), res.Header)
@@ -295,6 +289,21 @@ func (p *ReverseProxy) serveHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 
 	rw.WriteHeader(res.StatusCode)
+
+	err = p.copyResponse(rw, res.Body, p.flushInterval(req, res))
+	if err != nil {
+		defer res.Body.Close()
+		// Since we're streaming the response, if we run into an error all we can do
+		// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
+		// on read error while copying body.
+		if !shouldPanicOnCopyError(req) {
+			p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
+			return
+		}
+		panic(http.ErrAbortHandler)
+	}
+	res.Body.Close() // close now, instead of defer, to populate res.Trailer
+
 	if len(res.Trailer) > 0 {
 		// Force chunking if we saw a response trailer.
 		// This prevents net/http from calculating the length for short
@@ -303,8 +312,6 @@ func (p *ReverseProxy) serveHTTP(rw http.ResponseWriter, req *http.Request) {
 			fl.Flush()
 		}
 	}
-	p.copyResponse(rw, res.Body)
-	res.Body.Close() // close now, instead of defer, to populate res.Trailer
 
 	if len(res.Trailer) == announcedTrailers {
 		copyHeader(rw.Header(), res.Trailer)
@@ -319,16 +326,68 @@ func (p *ReverseProxy) serveHTTP(rw http.ResponseWriter, req *http.Request) {
 	}
 }
 
-func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
-	if p.FlushInterval != 0 {
+var inOurTests bool // whether we're in our own tests
+
+// shouldPanicOnCopyError reports whether the reverse proxy should
+// panic with http.ErrAbortHandler. This is the right thing to do by
+// default, but Go 1.10 and earlier did not, so existing unit tests
+// weren't expecting panics. Only panic in our own tests, or when
+// running under the HTTP server.
+func shouldPanicOnCopyError(req *http.Request) bool {
+	if inOurTests {
+		// Our tests know to handle this panic.
+		return true
+	}
+	if req.Context().Value(http.ServerContextKey) != nil {
+		// We seem to be running under an HTTP server, so
+		// it'll recover the panic.
+		return true
+	}
+	// Otherwise act like Go 1.10 and earlier to not break
+	// existing tests.
+	return false
+}
+
+// removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
+// See RFC 7230, section 6.1
+func removeConnectionHeaders(h http.Header) {
+	for _, f := range h["Connection"] {
+		for _, sf := range strings.Split(f, ",") {
+			if sf = strings.TrimSpace(sf); sf != "" {
+				h.Del(sf)
+			}
+		}
+	}
+}
+
+// flushInterval returns the p.FlushInterval value, conditionally
+// overriding its value for a specific request/response.
+func (p *ReverseProxy) flushInterval(req *http.Request, res *http.Response) time.Duration {
+	resCT := res.Header.Get("Content-Type")
+
+	// For Server-Sent Events responses, flush immediately.
+	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
+	if resCT == "text/event-stream" {
+		return -1 // negative means immediately
+	}
+
+	// TODO: more specific cases? e.g. res.ContentLength == -1?
+	return p.FlushInterval
+}
+
+func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
+	if flushInterval != 0 {
 		if wf, ok := dst.(writeFlusher); ok {
 			mlw := &maxLatencyWriter{
 				dst:     wf,
-				latency: p.FlushInterval,
-				done:    make(chan bool),
+				latency: flushInterval,
 			}
-			go mlw.flushLoop()
 			defer mlw.stop()
+
+			// set up initial timer so headers get flushed even if body writes are delayed
+			mlw.flushPending = true
+			mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)
+
 			dst = mlw
 		}
 	}
@@ -336,13 +395,14 @@ func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) {
 	var buf []byte
 	if p.BufferPool != nil {
 		buf = p.BufferPool.Get()
+		defer p.BufferPool.Put(buf)
 	}
-	p.copyBuffer(dst, src, buf)
-	if p.BufferPool != nil {
-		p.BufferPool.Put(buf)
-	}
+	_, err := p.copyBuffer(dst, src, buf)
+	return err
 }
 
+// copyBuffer returns any write errors or non-EOF read errors, and the amount
+// of bytes written.
 func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
 	if len(buf) == 0 {
 		buf = make([]byte, 32*1024)
@@ -366,6 +426,9 @@ func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int
 			}
 		}
 		if rerr != nil {
+			if rerr == io.EOF {
+				rerr = nil
+			}
 			return written, rerr
 		}
 	}
@@ -386,47 +449,115 @@ type writeFlusher interface {
 
 type maxLatencyWriter struct {
 	dst     writeFlusher
-	latency time.Duration
+	latency time.Duration // non-zero; negative means to flush immediately
 
-	mu   sync.Mutex // protects Write + Flush
-	done chan bool
+	mu           sync.Mutex // protects t, flushPending, and dst.Flush
+	t            *time.Timer
+	flushPending bool
 }
 
-func (m *maxLatencyWriter) Write(p []byte) (int, error) {
+func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
-	return m.dst.Write(p)
+	n, err = m.dst.Write(p)
+	if m.latency < 0 {
+		m.dst.Flush()
+		return
+	}
+	if m.flushPending {
+		return
+	}
+	if m.t == nil {
+		m.t = time.AfterFunc(m.latency, m.delayedFlush)
+	} else {
+		m.t.Reset(m.latency)
+	}
+	m.flushPending = true
+	return
 }
 
-func (m *maxLatencyWriter) flushLoop() {
-	t := time.NewTicker(m.latency)
-	defer t.Stop()
-	for {
-		select {
-		case <-m.done:
-			if onExitFlushLoop != nil {
-				onExitFlushLoop()
-			}
-			return
-		case <-t.C:
-			m.mu.Lock()
-			m.dst.Flush()
-			m.mu.Unlock()
-		}
+func (m *maxLatencyWriter) delayedFlush() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
+		return
+	}
+	m.dst.Flush()
+	m.flushPending = false
+}
+
+func (m *maxLatencyWriter) stop() {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	m.flushPending = false
+	if m.t != nil {
+		m.t.Stop()
 	}
 }
 
-func (m *maxLatencyWriter) stop() { m.done <- true }
-
-func IsWebsocketRequest(req *http.Request) bool {
-	containsHeader := func(name, value string) bool {
-		items := strings.Split(req.Header.Get(name), ",")
-		for _, item := range items {
-			if value == strings.ToLower(strings.TrimSpace(item)) {
-				return true
-			}
-		}
-		return false
+func upgradeType(h http.Header) string {
+	if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
+		return ""
 	}
-	return containsHeader("Connection", "upgrade") && containsHeader("Upgrade", "websocket")
+	return strings.ToLower(h.Get("Upgrade"))
+}
+
+func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.Request, res *http.Response) {
+	reqUpType := upgradeType(req.Header)
+	resUpType := upgradeType(res.Header)
+	if reqUpType != resUpType {
+		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
+		return
+	}
+
+	copyHeader(res.Header, rw.Header())
+
+	hj, ok := rw.(http.Hijacker)
+	if !ok {
+		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
+		return
+	}
+	backConn, ok := res.Body.(io.ReadWriteCloser)
+	if !ok {
+		p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
+		return
+	}
+	defer backConn.Close()
+	conn, brw, err := hj.Hijack()
+	if err != nil {
+		p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", err))
+		return
+	}
+	defer conn.Close()
+	res.Body = nil // so res.Write only writes the headers; we have res.Body in backConn above
+	if err := res.Write(brw); err != nil {
+		p.getErrorHandler()(rw, req, fmt.Errorf("response write: %v", err))
+		return
+	}
+	if err := brw.Flush(); err != nil {
+		p.getErrorHandler()(rw, req, fmt.Errorf("response flush: %v", err))
+		return
+	}
+	errc := make(chan error, 1)
+	spc := switchProtocolCopier{user: conn, backend: backConn}
+	go spc.copyToBackend(errc)
+	go spc.copyFromBackend(errc)
+	<-errc
+	return
+}
+
+// switchProtocolCopier exists so goroutines proxying data back and
+// forth have nice names in stacks.
+type switchProtocolCopier struct {
+	user, backend io.ReadWriter
+}
+
+func (c switchProtocolCopier) copyFromBackend(errc chan<- error) {
+	_, err := io.Copy(c.user, c.backend)
+	errc <- err
+}
+
+func (c switchProtocolCopier) copyToBackend(errc chan<- error) {
+	_, err := io.Copy(c.backend, c.user)
+	errc <- err
 }