reverseproxy.go

Documentation: net/http/httputil

		 1  // Copyright 2011 The Go Authors. All rights reserved.
		 2  // Use of this source code is governed by a BSD-style
		 3  // license that can be found in the LICENSE file.
		 4  
		 5  // HTTP reverse proxy handler
		 6  
		 7  package httputil
		 8  
		 9  import (
		10  	"context"
		11  	"fmt"
		12  	"io"
		13  	"log"
		14  	"net"
		15  	"net/http"
		16  	"net/http/internal/ascii"
		17  	"net/textproto"
		18  	"net/url"
		19  	"strings"
		20  	"sync"
		21  	"time"
		22  
		23  	"golang.org/x/net/http/httpguts"
		24  )
		25  
		26  // ReverseProxy is an HTTP Handler that takes an incoming request and
		27  // sends it to another server, proxying the response back to the
		28  // client.
		29  //
		30  // ReverseProxy by default sets the client IP as the value of the
		31  // X-Forwarded-For header.
		32  //
		33  // If an X-Forwarded-For header already exists, the client IP is
		34  // appended to the existing values. As a special case, if the header
		35  // exists in the Request.Header map but has a nil value (such as when
		36  // set by the Director func), the X-Forwarded-For header is
		37  // not modified.
		38  //
		39  // To prevent IP spoofing, be sure to delete any pre-existing
		40  // X-Forwarded-For header coming from the client or
		41  // an untrusted proxy.
		42  type ReverseProxy struct {
		43  	// Director must be a function which modifies
		44  	// the request into a new request to be sent
		45  	// using Transport. Its response is then copied
		46  	// back to the original client unmodified.
		47  	// Director must not access the provided Request
		48  	// after returning.
		49  	Director func(*http.Request)
		50  
		51  	// The transport used to perform proxy requests.
		52  	// If nil, http.DefaultTransport is used.
		53  	Transport http.RoundTripper
		54  
		55  	// FlushInterval specifies the flush interval
		56  	// to flush to the client while copying the
		57  	// response body.
		58  	// If zero, no periodic flushing is done.
		59  	// A negative value means to flush immediately
		60  	// after each write to the client.
		61  	// The FlushInterval is ignored when ReverseProxy
		62  	// recognizes a response as a streaming response, or
		63  	// if its ContentLength is -1; for such responses, writes
		64  	// are flushed to the client immediately.
		65  	FlushInterval time.Duration
		66  
		67  	// ErrorLog specifies an optional logger for errors
		68  	// that occur when attempting to proxy the request.
		69  	// If nil, logging is done via the log package's standard logger.
		70  	ErrorLog *log.Logger
		71  
		72  	// BufferPool optionally specifies a buffer pool to
		73  	// get byte slices for use by io.CopyBuffer when
		74  	// copying HTTP response bodies.
		75  	BufferPool BufferPool
		76  
		77  	// ModifyResponse is an optional function that modifies the
		78  	// Response from the backend. It is called if the backend
		79  	// returns a response at all, with any HTTP status code.
		80  	// If the backend is unreachable, the optional ErrorHandler is
		81  	// called without any call to ModifyResponse.
		82  	//
		83  	// If ModifyResponse returns an error, ErrorHandler is called
		84  	// with its error value. If ErrorHandler is nil, its default
		85  	// implementation is used.
		86  	ModifyResponse func(*http.Response) error
		87  
		88  	// ErrorHandler is an optional function that handles errors
		89  	// reaching the backend or errors from ModifyResponse.
		90  	//
		91  	// If nil, the default is to log the provided error and return
		92  	// a 502 Status Bad Gateway response.
		93  	ErrorHandler func(http.ResponseWriter, *http.Request, error)
		94  }
		95  
		96  // A BufferPool is an interface for getting and returning temporary
		97  // byte slices for use by io.CopyBuffer.
		98  type BufferPool interface {
		99  	Get() []byte
	 100  	Put([]byte)
	 101  }
	 102  
	 103  func singleJoiningSlash(a, b string) string {
	 104  	aslash := strings.HasSuffix(a, "/")
	 105  	bslash := strings.HasPrefix(b, "/")
	 106  	switch {
	 107  	case aslash && bslash:
	 108  		return a + b[1:]
	 109  	case !aslash && !bslash:
	 110  		return a + "/" + b
	 111  	}
	 112  	return a + b
	 113  }
	 114  
	 115  func joinURLPath(a, b *url.URL) (path, rawpath string) {
	 116  	if a.RawPath == "" && b.RawPath == "" {
	 117  		return singleJoiningSlash(a.Path, b.Path), ""
	 118  	}
	 119  	// Same as singleJoiningSlash, but uses EscapedPath to determine
	 120  	// whether a slash should be added
	 121  	apath := a.EscapedPath()
	 122  	bpath := b.EscapedPath()
	 123  
	 124  	aslash := strings.HasSuffix(apath, "/")
	 125  	bslash := strings.HasPrefix(bpath, "/")
	 126  
	 127  	switch {
	 128  	case aslash && bslash:
	 129  		return a.Path + b.Path[1:], apath + bpath[1:]
	 130  	case !aslash && !bslash:
	 131  		return a.Path + "/" + b.Path, apath + "/" + bpath
	 132  	}
	 133  	return a.Path + b.Path, apath + bpath
	 134  }
	 135  
	 136  // NewSingleHostReverseProxy returns a new ReverseProxy that routes
	 137  // URLs to the scheme, host, and base path provided in target. If the
	 138  // target's path is "/base" and the incoming request was for "/dir",
	 139  // the target request will be for /base/dir.
	 140  // NewSingleHostReverseProxy does not rewrite the Host header.
	 141  // To rewrite Host headers, use ReverseProxy directly with a custom
	 142  // Director policy.
	 143  func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
	 144  	targetQuery := target.RawQuery
	 145  	director := func(req *http.Request) {
	 146  		req.URL.Scheme = target.Scheme
	 147  		req.URL.Host = target.Host
	 148  		req.URL.Path, req.URL.RawPath = joinURLPath(target, req.URL)
	 149  		if targetQuery == "" || req.URL.RawQuery == "" {
	 150  			req.URL.RawQuery = targetQuery + req.URL.RawQuery
	 151  		} else {
	 152  			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
	 153  		}
	 154  		if _, ok := req.Header["User-Agent"]; !ok {
	 155  			// explicitly disable User-Agent so it's not set to default value
	 156  			req.Header.Set("User-Agent", "")
	 157  		}
	 158  	}
	 159  	return &ReverseProxy{Director: director}
	 160  }
	 161  
	 162  func copyHeader(dst, src http.Header) {
	 163  	for k, vv := range src {
	 164  		for _, v := range vv {
	 165  			dst.Add(k, v)
	 166  		}
	 167  	}
	 168  }
	 169  
	 170  // Hop-by-hop headers. These are removed when sent to the backend.
	 171  // As of RFC 7230, hop-by-hop headers are required to appear in the
	 172  // Connection header field. These are the headers defined by the
	 173  // obsoleted RFC 2616 (section 13.5.1) and are used for backward
	 174  // compatibility.
	 175  var hopHeaders = []string{
	 176  	"Connection",
	 177  	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
	 178  	"Keep-Alive",
	 179  	"Proxy-Authenticate",
	 180  	"Proxy-Authorization",
	 181  	"Te",			// canonicalized version of "TE"
	 182  	"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
	 183  	"Transfer-Encoding",
	 184  	"Upgrade",
	 185  }
	 186  
	 187  func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
	 188  	p.logf("http: proxy error: %v", err)
	 189  	rw.WriteHeader(http.StatusBadGateway)
	 190  }
	 191  
	 192  func (p *ReverseProxy) getErrorHandler() func(http.ResponseWriter, *http.Request, error) {
	 193  	if p.ErrorHandler != nil {
	 194  		return p.ErrorHandler
	 195  	}
	 196  	return p.defaultErrorHandler
	 197  }
	 198  
	 199  // modifyResponse conditionally runs the optional ModifyResponse hook
	 200  // and reports whether the request should proceed.
	 201  func (p *ReverseProxy) modifyResponse(rw http.ResponseWriter, res *http.Response, req *http.Request) bool {
	 202  	if p.ModifyResponse == nil {
	 203  		return true
	 204  	}
	 205  	if err := p.ModifyResponse(res); err != nil {
	 206  		res.Body.Close()
	 207  		p.getErrorHandler()(rw, req, err)
	 208  		return false
	 209  	}
	 210  	return true
	 211  }
	 212  
	 213  func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
	 214  	transport := p.Transport
	 215  	if transport == nil {
	 216  		transport = http.DefaultTransport
	 217  	}
	 218  
	 219  	ctx := req.Context()
	 220  	if cn, ok := rw.(http.CloseNotifier); ok {
	 221  		var cancel context.CancelFunc
	 222  		ctx, cancel = context.WithCancel(ctx)
	 223  		defer cancel()
	 224  		notifyChan := cn.CloseNotify()
	 225  		go func() {
	 226  			select {
	 227  			case <-notifyChan:
	 228  				cancel()
	 229  			case <-ctx.Done():
	 230  			}
	 231  		}()
	 232  	}
	 233  
	 234  	outreq := req.Clone(ctx)
	 235  	if req.ContentLength == 0 {
	 236  		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
	 237  	}
	 238  	if outreq.Body != nil {
	 239  		// Reading from the request body after returning from a handler is not
	 240  		// allowed, and the RoundTrip goroutine that reads the Body can outlive
	 241  		// this handler. This can lead to a crash if the handler panics (see
	 242  		// Issue 46866). Although calling Close doesn't guarantee there isn't
	 243  		// any Read in flight after the handle returns, in practice it's safe to
	 244  		// read after closing it.
	 245  		defer outreq.Body.Close()
	 246  	}
	 247  	if outreq.Header == nil {
	 248  		outreq.Header = make(http.Header) // Issue 33142: historical behavior was to always allocate
	 249  	}
	 250  
	 251  	p.Director(outreq)
	 252  	outreq.Close = false
	 253  
	 254  	reqUpType := upgradeType(outreq.Header)
	 255  	if !ascii.IsPrint(reqUpType) {
	 256  		p.getErrorHandler()(rw, req, fmt.Errorf("client tried to switch to invalid protocol %q", reqUpType))
	 257  		return
	 258  	}
	 259  	removeConnectionHeaders(outreq.Header)
	 260  
	 261  	// Remove hop-by-hop headers to the backend. Especially
	 262  	// important is "Connection" because we want a persistent
	 263  	// connection, regardless of what the client sent to us.
	 264  	for _, h := range hopHeaders {
	 265  		outreq.Header.Del(h)
	 266  	}
	 267  
	 268  	// Issue 21096: tell backend applications that care about trailer support
	 269  	// that we support trailers. (We do, but we don't go out of our way to
	 270  	// advertise that unless the incoming client request thought it was worth
	 271  	// mentioning.) Note that we look at req.Header, not outreq.Header, since
	 272  	// the latter has passed through removeConnectionHeaders.
	 273  	if httpguts.HeaderValuesContainsToken(req.Header["Te"], "trailers") {
	 274  		outreq.Header.Set("Te", "trailers")
	 275  	}
	 276  
	 277  	// After stripping all the hop-by-hop connection headers above, add back any
	 278  	// necessary for protocol upgrades, such as for websockets.
	 279  	if reqUpType != "" {
	 280  		outreq.Header.Set("Connection", "Upgrade")
	 281  		outreq.Header.Set("Upgrade", reqUpType)
	 282  	}
	 283  
	 284  	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
	 285  		// If we aren't the first proxy retain prior
	 286  		// X-Forwarded-For information as a comma+space
	 287  		// separated list and fold multiple headers into one.
	 288  		prior, ok := outreq.Header["X-Forwarded-For"]
	 289  		omit := ok && prior == nil // Issue 38079: nil now means don't populate the header
	 290  		if len(prior) > 0 {
	 291  			clientIP = strings.Join(prior, ", ") + ", " + clientIP
	 292  		}
	 293  		if !omit {
	 294  			outreq.Header.Set("X-Forwarded-For", clientIP)
	 295  		}
	 296  	}
	 297  
	 298  	res, err := transport.RoundTrip(outreq)
	 299  	if err != nil {
	 300  		p.getErrorHandler()(rw, outreq, err)
	 301  		return
	 302  	}
	 303  
	 304  	// Deal with 101 Switching Protocols responses: (WebSocket, h2c, etc)
	 305  	if res.StatusCode == http.StatusSwitchingProtocols {
	 306  		if !p.modifyResponse(rw, res, outreq) {
	 307  			return
	 308  		}
	 309  		p.handleUpgradeResponse(rw, outreq, res)
	 310  		return
	 311  	}
	 312  
	 313  	removeConnectionHeaders(res.Header)
	 314  
	 315  	for _, h := range hopHeaders {
	 316  		res.Header.Del(h)
	 317  	}
	 318  
	 319  	if !p.modifyResponse(rw, res, outreq) {
	 320  		return
	 321  	}
	 322  
	 323  	copyHeader(rw.Header(), res.Header)
	 324  
	 325  	// The "Trailer" header isn't included in the Transport's response,
	 326  	// at least for *http.Transport. Build it up from Trailer.
	 327  	announcedTrailers := len(res.Trailer)
	 328  	if announcedTrailers > 0 {
	 329  		trailerKeys := make([]string, 0, len(res.Trailer))
	 330  		for k := range res.Trailer {
	 331  			trailerKeys = append(trailerKeys, k)
	 332  		}
	 333  		rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
	 334  	}
	 335  
	 336  	rw.WriteHeader(res.StatusCode)
	 337  
	 338  	err = p.copyResponse(rw, res.Body, p.flushInterval(res))
	 339  	if err != nil {
	 340  		defer res.Body.Close()
	 341  		// Since we're streaming the response, if we run into an error all we can do
	 342  		// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
	 343  		// on read error while copying body.
	 344  		if !shouldPanicOnCopyError(req) {
	 345  			p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
	 346  			return
	 347  		}
	 348  		panic(http.ErrAbortHandler)
	 349  	}
	 350  	res.Body.Close() // close now, instead of defer, to populate res.Trailer
	 351  
	 352  	if len(res.Trailer) > 0 {
	 353  		// Force chunking if we saw a response trailer.
	 354  		// This prevents net/http from calculating the length for short
	 355  		// bodies and adding a Content-Length.
	 356  		if fl, ok := rw.(http.Flusher); ok {
	 357  			fl.Flush()
	 358  		}
	 359  	}
	 360  
	 361  	if len(res.Trailer) == announcedTrailers {
	 362  		copyHeader(rw.Header(), res.Trailer)
	 363  		return
	 364  	}
	 365  
	 366  	for k, vv := range res.Trailer {
	 367  		k = http.TrailerPrefix + k
	 368  		for _, v := range vv {
	 369  			rw.Header().Add(k, v)
	 370  		}
	 371  	}
	 372  }
	 373  
	 374  var inOurTests bool // whether we're in our own tests
	 375  
	 376  // shouldPanicOnCopyError reports whether the reverse proxy should
	 377  // panic with http.ErrAbortHandler. This is the right thing to do by
	 378  // default, but Go 1.10 and earlier did not, so existing unit tests
	 379  // weren't expecting panics. Only panic in our own tests, or when
	 380  // running under the HTTP server.
	 381  func shouldPanicOnCopyError(req *http.Request) bool {
	 382  	if inOurTests {
	 383  		// Our tests know to handle this panic.
	 384  		return true
	 385  	}
	 386  	if req.Context().Value(http.ServerContextKey) != nil {
	 387  		// We seem to be running under an HTTP server, so
	 388  		// it'll recover the panic.
	 389  		return true
	 390  	}
	 391  	// Otherwise act like Go 1.10 and earlier to not break
	 392  	// existing tests.
	 393  	return false
	 394  }
	 395  
	 396  // removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
	 397  // See RFC 7230, section 6.1
	 398  func removeConnectionHeaders(h http.Header) {
	 399  	for _, f := range h["Connection"] {
	 400  		for _, sf := range strings.Split(f, ",") {
	 401  			if sf = textproto.TrimString(sf); sf != "" {
	 402  				h.Del(sf)
	 403  			}
	 404  		}
	 405  	}
	 406  }
	 407  
	 408  // flushInterval returns the p.FlushInterval value, conditionally
	 409  // overriding its value for a specific request/response.
	 410  func (p *ReverseProxy) flushInterval(res *http.Response) time.Duration {
	 411  	resCT := res.Header.Get("Content-Type")
	 412  
	 413  	// For Server-Sent Events responses, flush immediately.
	 414  	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
	 415  	if resCT == "text/event-stream" {
	 416  		return -1 // negative means immediately
	 417  	}
	 418  
	 419  	// We might have the case of streaming for which Content-Length might be unset.
	 420  	if res.ContentLength == -1 {
	 421  		return -1
	 422  	}
	 423  
	 424  	return p.FlushInterval
	 425  }
	 426  
	 427  func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader, flushInterval time.Duration) error {
	 428  	if flushInterval != 0 {
	 429  		if wf, ok := dst.(writeFlusher); ok {
	 430  			mlw := &maxLatencyWriter{
	 431  				dst:		 wf,
	 432  				latency: flushInterval,
	 433  			}
	 434  			defer mlw.stop()
	 435  
	 436  			// set up initial timer so headers get flushed even if body writes are delayed
	 437  			mlw.flushPending = true
	 438  			mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)
	 439  
	 440  			dst = mlw
	 441  		}
	 442  	}
	 443  
	 444  	var buf []byte
	 445  	if p.BufferPool != nil {
	 446  		buf = p.BufferPool.Get()
	 447  		defer p.BufferPool.Put(buf)
	 448  	}
	 449  	_, err := p.copyBuffer(dst, src, buf)
	 450  	return err
	 451  }
	 452  
	 453  // copyBuffer returns any write errors or non-EOF read errors, and the amount
	 454  // of bytes written.
	 455  func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
	 456  	if len(buf) == 0 {
	 457  		buf = make([]byte, 32*1024)
	 458  	}
	 459  	var written int64
	 460  	for {
	 461  		nr, rerr := src.Read(buf)
	 462  		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
	 463  			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
	 464  		}
	 465  		if nr > 0 {
	 466  			nw, werr := dst.Write(buf[:nr])
	 467  			if nw > 0 {
	 468  				written += int64(nw)
	 469  			}
	 470  			if werr != nil {
	 471  				return written, werr
	 472  			}
	 473  			if nr != nw {
	 474  				return written, io.ErrShortWrite
	 475  			}
	 476  		}
	 477  		if rerr != nil {
	 478  			if rerr == io.EOF {
	 479  				rerr = nil
	 480  			}
	 481  			return written, rerr
	 482  		}
	 483  	}
	 484  }
	 485  
	 486  func (p *ReverseProxy) logf(format string, args ...interface{}) {
	 487  	if p.ErrorLog != nil {
	 488  		p.ErrorLog.Printf(format, args...)
	 489  	} else {
	 490  		log.Printf(format, args...)
	 491  	}
	 492  }
	 493  
	 494  type writeFlusher interface {
	 495  	io.Writer
	 496  	http.Flusher
	 497  }
	 498  
	 499  type maxLatencyWriter struct {
	 500  	dst		 writeFlusher
	 501  	latency time.Duration // non-zero; negative means to flush immediately
	 502  
	 503  	mu					 sync.Mutex // protects t, flushPending, and dst.Flush
	 504  	t						*time.Timer
	 505  	flushPending bool
	 506  }
	 507  
	 508  func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
	 509  	m.mu.Lock()
	 510  	defer m.mu.Unlock()
	 511  	n, err = m.dst.Write(p)
	 512  	if m.latency < 0 {
	 513  		m.dst.Flush()
	 514  		return
	 515  	}
	 516  	if m.flushPending {
	 517  		return
	 518  	}
	 519  	if m.t == nil {
	 520  		m.t = time.AfterFunc(m.latency, m.delayedFlush)
	 521  	} else {
	 522  		m.t.Reset(m.latency)
	 523  	}
	 524  	m.flushPending = true
	 525  	return
	 526  }
	 527  
	 528  func (m *maxLatencyWriter) delayedFlush() {
	 529  	m.mu.Lock()
	 530  	defer m.mu.Unlock()
	 531  	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
	 532  		return
	 533  	}
	 534  	m.dst.Flush()
	 535  	m.flushPending = false
	 536  }
	 537  
	 538  func (m *maxLatencyWriter) stop() {
	 539  	m.mu.Lock()
	 540  	defer m.mu.Unlock()
	 541  	m.flushPending = false
	 542  	if m.t != nil {
	 543  		m.t.Stop()
	 544  	}
	 545  }
	 546  
	 547  func upgradeType(h http.Header) string {
	 548  	if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
	 549  		return ""
	 550  	}
	 551  	return h.Get("Upgrade")
	 552  }
	 553  
	 554  func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.Request, res *http.Response) {
	 555  	reqUpType := upgradeType(req.Header)
	 556  	resUpType := upgradeType(res.Header)
	 557  	if !ascii.IsPrint(resUpType) { // We know reqUpType is ASCII, it's checked by the caller.
	 558  		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch to invalid protocol %q", resUpType))
	 559  	}
	 560  	if !ascii.EqualFold(reqUpType, resUpType) {
	 561  		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
	 562  		return
	 563  	}
	 564  
	 565  	hj, ok := rw.(http.Hijacker)
	 566  	if !ok {
	 567  		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
	 568  		return
	 569  	}
	 570  	backConn, ok := res.Body.(io.ReadWriteCloser)
	 571  	if !ok {
	 572  		p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
	 573  		return
	 574  	}
	 575  
	 576  	backConnCloseCh := make(chan bool)
	 577  	go func() {
	 578  		// Ensure that the cancellation of a request closes the backend.
	 579  		// See issue https://golang.org/issue/35559.
	 580  		select {
	 581  		case <-req.Context().Done():
	 582  		case <-backConnCloseCh:
	 583  		}
	 584  		backConn.Close()
	 585  	}()
	 586  
	 587  	defer close(backConnCloseCh)
	 588  
	 589  	conn, brw, err := hj.Hijack()
	 590  	if err != nil {
	 591  		p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", err))
	 592  		return
	 593  	}
	 594  	defer conn.Close()
	 595  
	 596  	copyHeader(rw.Header(), res.Header)
	 597  
	 598  	res.Header = rw.Header()
	 599  	res.Body = nil // so res.Write only writes the headers; we have res.Body in backConn above
	 600  	if err := res.Write(brw); err != nil {
	 601  		p.getErrorHandler()(rw, req, fmt.Errorf("response write: %v", err))
	 602  		return
	 603  	}
	 604  	if err := brw.Flush(); err != nil {
	 605  		p.getErrorHandler()(rw, req, fmt.Errorf("response flush: %v", err))
	 606  		return
	 607  	}
	 608  	errc := make(chan error, 1)
	 609  	spc := switchProtocolCopier{user: conn, backend: backConn}
	 610  	go spc.copyToBackend(errc)
	 611  	go spc.copyFromBackend(errc)
	 612  	<-errc
	 613  	return
	 614  }
	 615  
	 616  // switchProtocolCopier exists so goroutines proxying data back and
	 617  // forth have nice names in stacks.
	 618  type switchProtocolCopier struct {
	 619  	user, backend io.ReadWriter
	 620  }
	 621  
	 622  func (c switchProtocolCopier) copyFromBackend(errc chan<- error) {
	 623  	_, err := io.Copy(c.user, c.backend)
	 624  	errc <- err
	 625  }
	 626  
	 627  func (c switchProtocolCopier) copyToBackend(errc chan<- error) {
	 628  	_, err := io.Copy(c.backend, c.user)
	 629  	errc <- err
	 630  }
	 631
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