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  	"errors"
    12  	"fmt"
    13  	"io"
    14  	"log"
    15  	"mime"
    16  	"net"
    17  	"net/http"
    18  	"net/http/httptrace"
    19  	"net/http/internal/ascii"
    20  	"net/textproto"
    21  	"net/url"
    22  	"strings"
    23  	"sync"
    24  	"time"
    25  
    26  	"golang.org/x/net/http/httpguts"
    27  )
    28  
    29  // A ProxyRequest contains a request to be rewritten by a [ReverseProxy].
    30  type ProxyRequest struct {
    31  	// In is the request received by the proxy.
    32  	// The Rewrite function must not modify In.
    33  	In *http.Request
    34  
    35  	// Out is the request which will be sent by the proxy.
    36  	// The Rewrite function may modify or replace this request.
    37  	// Hop-by-hop headers are removed from this request
    38  	// before Rewrite is called.
    39  	Out *http.Request
    40  }
    41  
    42  // SetURL routes the outbound request to the scheme, host, and base path
    43  // provided in target. If the target's path is "/base" and the incoming
    44  // request was for "/dir", the target request will be for "/base/dir".
    45  //
    46  // SetURL rewrites the outbound Host header to match the target's host.
    47  // To preserve the inbound request's Host header (the default behavior
    48  // of [NewSingleHostReverseProxy]):
    49  //
    50  //	rewriteFunc := func(r *httputil.ProxyRequest) {
    51  //		r.SetURL(url)
    52  //		r.Out.Host = r.In.Host
    53  //	}
    54  func (r *ProxyRequest) SetURL(target *url.URL) {
    55  	rewriteRequestURL(r.Out, target)
    56  	r.Out.Host = ""
    57  }
    58  
    59  // SetXForwarded sets the X-Forwarded-For, X-Forwarded-Host, and
    60  // X-Forwarded-Proto headers of the outbound request.
    61  //
    62  //   - The X-Forwarded-For header is set to the client IP address.
    63  //   - The X-Forwarded-Host header is set to the host name requested
    64  //     by the client.
    65  //   - The X-Forwarded-Proto header is set to "http" or "https", depending
    66  //     on whether the inbound request was made on a TLS-enabled connection.
    67  //
    68  // If the outbound request contains an existing X-Forwarded-For header,
    69  // SetXForwarded appends the client IP address to it. To append to the
    70  // inbound request's X-Forwarded-For header (the default behavior of
    71  // [ReverseProxy] when using a Director function), copy the header
    72  // from the inbound request before calling SetXForwarded:
    73  //
    74  //	rewriteFunc := func(r *httputil.ProxyRequest) {
    75  //		r.Out.Header["X-Forwarded-For"] = r.In.Header["X-Forwarded-For"]
    76  //		r.SetXForwarded()
    77  //	}
    78  func (r *ProxyRequest) SetXForwarded() {
    79  	clientIP, _, err := net.SplitHostPort(r.In.RemoteAddr)
    80  	if err == nil {
    81  		prior := r.Out.Header["X-Forwarded-For"]
    82  		if len(prior) > 0 {
    83  			clientIP = strings.Join(prior, ", ") + ", " + clientIP
    84  		}
    85  		r.Out.Header.Set("X-Forwarded-For", clientIP)
    86  	} else {
    87  		r.Out.Header.Del("X-Forwarded-For")
    88  	}
    89  	r.Out.Header.Set("X-Forwarded-Host", r.In.Host)
    90  	if r.In.TLS == nil {
    91  		r.Out.Header.Set("X-Forwarded-Proto", "http")
    92  	} else {
    93  		r.Out.Header.Set("X-Forwarded-Proto", "https")
    94  	}
    95  }
    96  
    97  // ReverseProxy is an HTTP Handler that takes an incoming request and
    98  // sends it to another server, proxying the response back to the
    99  // client.
   100  //
   101  // 1xx responses are forwarded to the client if the underlying
   102  // transport supports ClientTrace.Got1xxResponse.
   103  type ReverseProxy struct {
   104  	// Rewrite must be a function which modifies
   105  	// the request into a new request to be sent
   106  	// using Transport. Its response is then copied
   107  	// back to the original client unmodified.
   108  	// Rewrite must not access the provided ProxyRequest
   109  	// or its contents after returning.
   110  	//
   111  	// The Forwarded, X-Forwarded, X-Forwarded-Host,
   112  	// and X-Forwarded-Proto headers are removed from the
   113  	// outbound request before Rewrite is called. See also
   114  	// the ProxyRequest.SetXForwarded method.
   115  	//
   116  	// Unparsable query parameters are removed from the
   117  	// outbound request before Rewrite is called.
   118  	// The Rewrite function may copy the inbound URL's
   119  	// RawQuery to the outbound URL to preserve the original
   120  	// parameter string. Note that this can lead to security
   121  	// issues if the proxy's interpretation of query parameters
   122  	// does not match that of the downstream server.
   123  	//
   124  	// At most one of Rewrite or Director may be set.
   125  	Rewrite func(*ProxyRequest)
   126  
   127  	// Director is a function which modifies
   128  	// the request into a new request to be sent
   129  	// using Transport. Its response is then copied
   130  	// back to the original client unmodified.
   131  	// Director must not access the provided Request
   132  	// after returning.
   133  	//
   134  	// By default, the X-Forwarded-For header is set to the
   135  	// value of the client IP address. If an X-Forwarded-For
   136  	// header already exists, the client IP is appended to the
   137  	// existing values. As a special case, if the header
   138  	// exists in the Request.Header map but has a nil value
   139  	// (such as when set by the Director func), the X-Forwarded-For
   140  	// header is not modified.
   141  	//
   142  	// To prevent IP spoofing, be sure to delete any pre-existing
   143  	// X-Forwarded-For header coming from the client or
   144  	// an untrusted proxy.
   145  	//
   146  	// Hop-by-hop headers are removed from the request after
   147  	// Director returns, which can remove headers added by
   148  	// Director. Use a Rewrite function instead to ensure
   149  	// modifications to the request are preserved.
   150  	//
   151  	// Unparsable query parameters are removed from the outbound
   152  	// request if Request.Form is set after Director returns.
   153  	//
   154  	// At most one of Rewrite or Director may be set.
   155  	Director func(*http.Request)
   156  
   157  	// The transport used to perform proxy requests.
   158  	// If nil, http.DefaultTransport is used.
   159  	Transport http.RoundTripper
   160  
   161  	// FlushInterval specifies the flush interval
   162  	// to flush to the client while copying the
   163  	// response body.
   164  	// If zero, no periodic flushing is done.
   165  	// A negative value means to flush immediately
   166  	// after each write to the client.
   167  	// The FlushInterval is ignored when ReverseProxy
   168  	// recognizes a response as a streaming response, or
   169  	// if its ContentLength is -1; for such responses, writes
   170  	// are flushed to the client immediately.
   171  	FlushInterval time.Duration
   172  
   173  	// ErrorLog specifies an optional logger for errors
   174  	// that occur when attempting to proxy the request.
   175  	// If nil, logging is done via the log package's standard logger.
   176  	ErrorLog *log.Logger
   177  
   178  	// BufferPool optionally specifies a buffer pool to
   179  	// get byte slices for use by io.CopyBuffer when
   180  	// copying HTTP response bodies.
   181  	BufferPool BufferPool
   182  
   183  	// ModifyResponse is an optional function that modifies the
   184  	// Response from the backend. It is called if the backend
   185  	// returns a response at all, with any HTTP status code.
   186  	// If the backend is unreachable, the optional ErrorHandler is
   187  	// called without any call to ModifyResponse.
   188  	//
   189  	// If ModifyResponse returns an error, ErrorHandler is called
   190  	// with its error value. If ErrorHandler is nil, its default
   191  	// implementation is used.
   192  	ModifyResponse func(*http.Response) error
   193  
   194  	// ErrorHandler is an optional function that handles errors
   195  	// reaching the backend or errors from ModifyResponse.
   196  	//
   197  	// If nil, the default is to log the provided error and return
   198  	// a 502 Status Bad Gateway response.
   199  	ErrorHandler func(http.ResponseWriter, *http.Request, error)
   200  }
   201  
   202  // A BufferPool is an interface for getting and returning temporary
   203  // byte slices for use by [io.CopyBuffer].
   204  type BufferPool interface {
   205  	Get() []byte
   206  	Put([]byte)
   207  }
   208  
   209  func singleJoiningSlash(a, b string) string {
   210  	aslash := strings.HasSuffix(a, "/")
   211  	bslash := strings.HasPrefix(b, "/")
   212  	switch {
   213  	case aslash && bslash:
   214  		return a + b[1:]
   215  	case !aslash && !bslash:
   216  		return a + "/" + b
   217  	}
   218  	return a + b
   219  }
   220  
   221  func joinURLPath(a, b *url.URL) (path, rawpath string) {
   222  	if a.RawPath == "" && b.RawPath == "" {
   223  		return singleJoiningSlash(a.Path, b.Path), ""
   224  	}
   225  	// Same as singleJoiningSlash, but uses EscapedPath to determine
   226  	// whether a slash should be added
   227  	apath := a.EscapedPath()
   228  	bpath := b.EscapedPath()
   229  
   230  	aslash := strings.HasSuffix(apath, "/")
   231  	bslash := strings.HasPrefix(bpath, "/")
   232  
   233  	switch {
   234  	case aslash && bslash:
   235  		return a.Path + b.Path[1:], apath + bpath[1:]
   236  	case !aslash && !bslash:
   237  		return a.Path + "/" + b.Path, apath + "/" + bpath
   238  	}
   239  	return a.Path + b.Path, apath + bpath
   240  }
   241  
   242  // NewSingleHostReverseProxy returns a new [ReverseProxy] that routes
   243  // URLs to the scheme, host, and base path provided in target. If the
   244  // target's path is "/base" and the incoming request was for "/dir",
   245  // the target request will be for /base/dir.
   246  //
   247  // NewSingleHostReverseProxy does not rewrite the Host header.
   248  //
   249  // To customize the ReverseProxy behavior beyond what
   250  // NewSingleHostReverseProxy provides, use ReverseProxy directly
   251  // with a Rewrite function. The ProxyRequest SetURL method
   252  // may be used to route the outbound request. (Note that SetURL,
   253  // unlike NewSingleHostReverseProxy, rewrites the Host header
   254  // of the outbound request by default.)
   255  //
   256  //	proxy := &ReverseProxy{
   257  //		Rewrite: func(r *ProxyRequest) {
   258  //			r.SetURL(target)
   259  //			r.Out.Host = r.In.Host // if desired
   260  //		},
   261  //	}
   262  func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
   263  	director := func(req *http.Request) {
   264  		rewriteRequestURL(req, target)
   265  	}
   266  	return &ReverseProxy{Director: director}
   267  }
   268  
   269  func rewriteRequestURL(req *http.Request, target *url.URL) {
   270  	targetQuery := target.RawQuery
   271  	req.URL.Scheme = target.Scheme
   272  	req.URL.Host = target.Host
   273  	req.URL.Path, req.URL.RawPath = joinURLPath(target, req.URL)
   274  	if targetQuery == "" || req.URL.RawQuery == "" {
   275  		req.URL.RawQuery = targetQuery + req.URL.RawQuery
   276  	} else {
   277  		req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
   278  	}
   279  }
   280  
   281  func copyHeader(dst, src http.Header) {
   282  	for k, vv := range src {
   283  		for _, v := range vv {
   284  			dst.Add(k, v)
   285  		}
   286  	}
   287  }
   288  
   289  // Hop-by-hop headers. These are removed when sent to the backend.
   290  // As of RFC 7230, hop-by-hop headers are required to appear in the
   291  // Connection header field. These are the headers defined by the
   292  // obsoleted RFC 2616 (section 13.5.1) and are used for backward
   293  // compatibility.
   294  var hopHeaders = []string{
   295  	"Connection",
   296  	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
   297  	"Keep-Alive",
   298  	"Proxy-Authenticate",
   299  	"Proxy-Authorization",
   300  	"Te",      // canonicalized version of "TE"
   301  	"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
   302  	"Transfer-Encoding",
   303  	"Upgrade",
   304  }
   305  
   306  func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
   307  	p.logf("http: proxy error: %v", err)
   308  	rw.WriteHeader(http.StatusBadGateway)
   309  }
   310  
   311  func (p *ReverseProxy) getErrorHandler() func(http.ResponseWriter, *http.Request, error) {
   312  	if p.ErrorHandler != nil {
   313  		return p.ErrorHandler
   314  	}
   315  	return p.defaultErrorHandler
   316  }
   317  
   318  // modifyResponse conditionally runs the optional ModifyResponse hook
   319  // and reports whether the request should proceed.
   320  func (p *ReverseProxy) modifyResponse(rw http.ResponseWriter, res *http.Response, req *http.Request) bool {
   321  	if p.ModifyResponse == nil {
   322  		return true
   323  	}
   324  	if err := p.ModifyResponse(res); err != nil {
   325  		res.Body.Close()
   326  		p.getErrorHandler()(rw, req, err)
   327  		return false
   328  	}
   329  	return true
   330  }
   331  
   332  func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
   333  	transport := p.Transport
   334  	if transport == nil {
   335  		transport = http.DefaultTransport
   336  	}
   337  
   338  	ctx := req.Context()
   339  	if ctx.Done() != nil {
   340  		// CloseNotifier predates context.Context, and has been
   341  		// entirely superseded by it. If the request contains
   342  		// a Context that carries a cancellation signal, don't
   343  		// bother spinning up a goroutine to watch the CloseNotify
   344  		// channel (if any).
   345  		//
   346  		// If the request Context has a nil Done channel (which
   347  		// means it is either context.Background, or a custom
   348  		// Context implementation with no cancellation signal),
   349  		// then consult the CloseNotifier if available.
   350  	} else if cn, ok := rw.(http.CloseNotifier); ok {
   351  		var cancel context.CancelFunc
   352  		ctx, cancel = context.WithCancel(ctx)
   353  		defer cancel()
   354  		notifyChan := cn.CloseNotify()
   355  		go func() {
   356  			select {
   357  			case <-notifyChan:
   358  				cancel()
   359  			case <-ctx.Done():
   360  			}
   361  		}()
   362  	}
   363  
   364  	outreq := req.Clone(ctx)
   365  	if req.ContentLength == 0 {
   366  		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
   367  	}
   368  	if outreq.Body != nil {
   369  		// Reading from the request body after returning from a handler is not
   370  		// allowed, and the RoundTrip goroutine that reads the Body can outlive
   371  		// this handler. This can lead to a crash if the handler panics (see
   372  		// Issue 46866). Although calling Close doesn't guarantee there isn't
   373  		// any Read in flight after the handle returns, in practice it's safe to
   374  		// read after closing it.
   375  		defer outreq.Body.Close()
   376  	}
   377  	if outreq.Header == nil {
   378  		outreq.Header = make(http.Header) // Issue 33142: historical behavior was to always allocate
   379  	}
   380  
   381  	if (p.Director != nil) == (p.Rewrite != nil) {
   382  		p.getErrorHandler()(rw, req, errors.New("ReverseProxy must have exactly one of Director or Rewrite set"))
   383  		return
   384  	}
   385  
   386  	if p.Director != nil {
   387  		p.Director(outreq)
   388  		if outreq.Form != nil {
   389  			outreq.URL.RawQuery = cleanQueryParams(outreq.URL.RawQuery)
   390  		}
   391  	}
   392  	outreq.Close = false
   393  
   394  	reqUpType := upgradeType(outreq.Header)
   395  	if !ascii.IsPrint(reqUpType) {
   396  		p.getErrorHandler()(rw, req, fmt.Errorf("client tried to switch to invalid protocol %q", reqUpType))
   397  		return
   398  	}
   399  	removeHopByHopHeaders(outreq.Header)
   400  
   401  	// Issue 21096: tell backend applications that care about trailer support
   402  	// that we support trailers. (We do, but we don't go out of our way to
   403  	// advertise that unless the incoming client request thought it was worth
   404  	// mentioning.) Note that we look at req.Header, not outreq.Header, since
   405  	// the latter has passed through removeHopByHopHeaders.
   406  	if httpguts.HeaderValuesContainsToken(req.Header["Te"], "trailers") {
   407  		outreq.Header.Set("Te", "trailers")
   408  	}
   409  
   410  	// After stripping all the hop-by-hop connection headers above, add back any
   411  	// necessary for protocol upgrades, such as for websockets.
   412  	if reqUpType != "" {
   413  		outreq.Header.Set("Connection", "Upgrade")
   414  		outreq.Header.Set("Upgrade", reqUpType)
   415  	}
   416  
   417  	if p.Rewrite != nil {
   418  		// Strip client-provided forwarding headers.
   419  		// The Rewrite func may use SetXForwarded to set new values
   420  		// for these or copy the previous values from the inbound request.
   421  		outreq.Header.Del("Forwarded")
   422  		outreq.Header.Del("X-Forwarded-For")
   423  		outreq.Header.Del("X-Forwarded-Host")
   424  		outreq.Header.Del("X-Forwarded-Proto")
   425  
   426  		// Remove unparsable query parameters from the outbound request.
   427  		outreq.URL.RawQuery = cleanQueryParams(outreq.URL.RawQuery)
   428  
   429  		pr := &ProxyRequest{
   430  			In:  req,
   431  			Out: outreq,
   432  		}
   433  		p.Rewrite(pr)
   434  		outreq = pr.Out
   435  	} else {
   436  		if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
   437  			// If we aren't the first proxy retain prior
   438  			// X-Forwarded-For information as a comma+space
   439  			// separated list and fold multiple headers into one.
   440  			prior, ok := outreq.Header["X-Forwarded-For"]
   441  			omit := ok && prior == nil // Issue 38079: nil now means don't populate the header
   442  			if len(prior) > 0 {
   443  				clientIP = strings.Join(prior, ", ") + ", " + clientIP
   444  			}
   445  			if !omit {
   446  				outreq.Header.Set("X-Forwarded-For", clientIP)
   447  			}
   448  		}
   449  	}
   450  
   451  	if _, ok := outreq.Header["User-Agent"]; !ok {
   452  		// If the outbound request doesn't have a User-Agent header set,
   453  		// don't send the default Go HTTP client User-Agent.
   454  		outreq.Header.Set("User-Agent", "")
   455  	}
   456  
   457  	trace := &httptrace.ClientTrace{
   458  		Got1xxResponse: func(code int, header textproto.MIMEHeader) error {
   459  			h := rw.Header()
   460  			copyHeader(h, http.Header(header))
   461  			rw.WriteHeader(code)
   462  
   463  			// Clear headers, it's not automatically done by ResponseWriter.WriteHeader() for 1xx responses
   464  			clear(h)
   465  			return nil
   466  		},
   467  	}
   468  	outreq = outreq.WithContext(httptrace.WithClientTrace(outreq.Context(), trace))
   469  
   470  	res, err := transport.RoundTrip(outreq)
   471  	if err != nil {
   472  		p.getErrorHandler()(rw, outreq, err)
   473  		return
   474  	}
   475  
   476  	// Deal with 101 Switching Protocols responses: (WebSocket, h2c, etc)
   477  	if res.StatusCode == http.StatusSwitchingProtocols {
   478  		if !p.modifyResponse(rw, res, outreq) {
   479  			return
   480  		}
   481  		p.handleUpgradeResponse(rw, outreq, res)
   482  		return
   483  	}
   484  
   485  	removeHopByHopHeaders(res.Header)
   486  
   487  	if !p.modifyResponse(rw, res, outreq) {
   488  		return
   489  	}
   490  
   491  	copyHeader(rw.Header(), res.Header)
   492  
   493  	// The "Trailer" header isn't included in the Transport's response,
   494  	// at least for *http.Transport. Build it up from Trailer.
   495  	announcedTrailers := len(res.Trailer)
   496  	if announcedTrailers > 0 {
   497  		trailerKeys := make([]string, 0, len(res.Trailer))
   498  		for k := range res.Trailer {
   499  			trailerKeys = append(trailerKeys, k)
   500  		}
   501  		rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
   502  	}
   503  
   504  	rw.WriteHeader(res.StatusCode)
   505  
   506  	err = p.copyResponse(rw, res.Body, p.flushInterval(res))
   507  	if err != nil {
   508  		defer res.Body.Close()
   509  		// Since we're streaming the response, if we run into an error all we can do
   510  		// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
   511  		// on read error while copying body.
   512  		if !shouldPanicOnCopyError(req) {
   513  			p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
   514  			return
   515  		}
   516  		panic(http.ErrAbortHandler)
   517  	}
   518  	res.Body.Close() // close now, instead of defer, to populate res.Trailer
   519  
   520  	if len(res.Trailer) > 0 {
   521  		// Force chunking if we saw a response trailer.
   522  		// This prevents net/http from calculating the length for short
   523  		// bodies and adding a Content-Length.
   524  		http.NewResponseController(rw).Flush()
   525  	}
   526  
   527  	if len(res.Trailer) == announcedTrailers {
   528  		copyHeader(rw.Header(), res.Trailer)
   529  		return
   530  	}
   531  
   532  	for k, vv := range res.Trailer {
   533  		k = http.TrailerPrefix + k
   534  		for _, v := range vv {
   535  			rw.Header().Add(k, v)
   536  		}
   537  	}
   538  }
   539  
   540  var inOurTests bool // whether we're in our own tests
   541  
   542  // shouldPanicOnCopyError reports whether the reverse proxy should
   543  // panic with http.ErrAbortHandler. This is the right thing to do by
   544  // default, but Go 1.10 and earlier did not, so existing unit tests
   545  // weren't expecting panics. Only panic in our own tests, or when
   546  // running under the HTTP server.
   547  func shouldPanicOnCopyError(req *http.Request) bool {
   548  	if inOurTests {
   549  		// Our tests know to handle this panic.
   550  		return true
   551  	}
   552  	if req.Context().Value(http.ServerContextKey) != nil {
   553  		// We seem to be running under an HTTP server, so
   554  		// it'll recover the panic.
   555  		return true
   556  	}
   557  	// Otherwise act like Go 1.10 and earlier to not break
   558  	// existing tests.
   559  	return false
   560  }
   561  
   562  // removeHopByHopHeaders removes hop-by-hop headers.
   563  func removeHopByHopHeaders(h http.Header) {
   564  	// RFC 7230, section 6.1: Remove headers listed in the "Connection" header.
   565  	for _, f := range h["Connection"] {
   566  		for _, sf := range strings.Split(f, ",") {
   567  			if sf = textproto.TrimString(sf); sf != "" {
   568  				h.Del(sf)
   569  			}
   570  		}
   571  	}
   572  	// RFC 2616, section 13.5.1: Remove a set of known hop-by-hop headers.
   573  	// This behavior is superseded by the RFC 7230 Connection header, but
   574  	// preserve it for backwards compatibility.
   575  	for _, f := range hopHeaders {
   576  		h.Del(f)
   577  	}
   578  }
   579  
   580  // flushInterval returns the p.FlushInterval value, conditionally
   581  // overriding its value for a specific request/response.
   582  func (p *ReverseProxy) flushInterval(res *http.Response) time.Duration {
   583  	resCT := res.Header.Get("Content-Type")
   584  
   585  	// For Server-Sent Events responses, flush immediately.
   586  	// The MIME type is defined in https://www.w3.org/TR/eventsource/#text-event-stream
   587  	if baseCT, _, _ := mime.ParseMediaType(resCT); baseCT == "text/event-stream" {
   588  		return -1 // negative means immediately
   589  	}
   590  
   591  	// We might have the case of streaming for which Content-Length might be unset.
   592  	if res.ContentLength == -1 {
   593  		return -1
   594  	}
   595  
   596  	return p.FlushInterval
   597  }
   598  
   599  func (p *ReverseProxy) copyResponse(dst http.ResponseWriter, src io.Reader, flushInterval time.Duration) error {
   600  	var w io.Writer = dst
   601  
   602  	if flushInterval != 0 {
   603  		mlw := &maxLatencyWriter{
   604  			dst:     dst,
   605  			flush:   http.NewResponseController(dst).Flush,
   606  			latency: flushInterval,
   607  		}
   608  		defer mlw.stop()
   609  
   610  		// set up initial timer so headers get flushed even if body writes are delayed
   611  		mlw.flushPending = true
   612  		mlw.t = time.AfterFunc(flushInterval, mlw.delayedFlush)
   613  
   614  		w = mlw
   615  	}
   616  
   617  	var buf []byte
   618  	if p.BufferPool != nil {
   619  		buf = p.BufferPool.Get()
   620  		defer p.BufferPool.Put(buf)
   621  	}
   622  	_, err := p.copyBuffer(w, src, buf)
   623  	return err
   624  }
   625  
   626  // copyBuffer returns any write errors or non-EOF read errors, and the amount
   627  // of bytes written.
   628  func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
   629  	if len(buf) == 0 {
   630  		buf = make([]byte, 32*1024)
   631  	}
   632  	var written int64
   633  	for {
   634  		nr, rerr := src.Read(buf)
   635  		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
   636  			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
   637  		}
   638  		if nr > 0 {
   639  			nw, werr := dst.Write(buf[:nr])
   640  			if nw > 0 {
   641  				written += int64(nw)
   642  			}
   643  			if werr != nil {
   644  				return written, werr
   645  			}
   646  			if nr != nw {
   647  				return written, io.ErrShortWrite
   648  			}
   649  		}
   650  		if rerr != nil {
   651  			if rerr == io.EOF {
   652  				rerr = nil
   653  			}
   654  			return written, rerr
   655  		}
   656  	}
   657  }
   658  
   659  func (p *ReverseProxy) logf(format string, args ...any) {
   660  	if p.ErrorLog != nil {
   661  		p.ErrorLog.Printf(format, args...)
   662  	} else {
   663  		log.Printf(format, args...)
   664  	}
   665  }
   666  
   667  type maxLatencyWriter struct {
   668  	dst     io.Writer
   669  	flush   func() error
   670  	latency time.Duration // non-zero; negative means to flush immediately
   671  
   672  	mu           sync.Mutex // protects t, flushPending, and dst.Flush
   673  	t            *time.Timer
   674  	flushPending bool
   675  }
   676  
   677  func (m *maxLatencyWriter) Write(p []byte) (n int, err error) {
   678  	m.mu.Lock()
   679  	defer m.mu.Unlock()
   680  	n, err = m.dst.Write(p)
   681  	if m.latency < 0 {
   682  		m.flush()
   683  		return
   684  	}
   685  	if m.flushPending {
   686  		return
   687  	}
   688  	if m.t == nil {
   689  		m.t = time.AfterFunc(m.latency, m.delayedFlush)
   690  	} else {
   691  		m.t.Reset(m.latency)
   692  	}
   693  	m.flushPending = true
   694  	return
   695  }
   696  
   697  func (m *maxLatencyWriter) delayedFlush() {
   698  	m.mu.Lock()
   699  	defer m.mu.Unlock()
   700  	if !m.flushPending { // if stop was called but AfterFunc already started this goroutine
   701  		return
   702  	}
   703  	m.flush()
   704  	m.flushPending = false
   705  }
   706  
   707  func (m *maxLatencyWriter) stop() {
   708  	m.mu.Lock()
   709  	defer m.mu.Unlock()
   710  	m.flushPending = false
   711  	if m.t != nil {
   712  		m.t.Stop()
   713  	}
   714  }
   715  
   716  func upgradeType(h http.Header) string {
   717  	if !httpguts.HeaderValuesContainsToken(h["Connection"], "Upgrade") {
   718  		return ""
   719  	}
   720  	return h.Get("Upgrade")
   721  }
   722  
   723  func (p *ReverseProxy) handleUpgradeResponse(rw http.ResponseWriter, req *http.Request, res *http.Response) {
   724  	reqUpType := upgradeType(req.Header)
   725  	resUpType := upgradeType(res.Header)
   726  	if !ascii.IsPrint(resUpType) { // We know reqUpType is ASCII, it's checked by the caller.
   727  		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch to invalid protocol %q", resUpType))
   728  	}
   729  	if !ascii.EqualFold(reqUpType, resUpType) {
   730  		p.getErrorHandler()(rw, req, fmt.Errorf("backend tried to switch protocol %q when %q was requested", resUpType, reqUpType))
   731  		return
   732  	}
   733  
   734  	backConn, ok := res.Body.(io.ReadWriteCloser)
   735  	if !ok {
   736  		p.getErrorHandler()(rw, req, fmt.Errorf("internal error: 101 switching protocols response with non-writable body"))
   737  		return
   738  	}
   739  
   740  	rc := http.NewResponseController(rw)
   741  	conn, brw, hijackErr := rc.Hijack()
   742  	if errors.Is(hijackErr, http.ErrNotSupported) {
   743  		p.getErrorHandler()(rw, req, fmt.Errorf("can't switch protocols using non-Hijacker ResponseWriter type %T", rw))
   744  		return
   745  	}
   746  
   747  	backConnCloseCh := make(chan bool)
   748  	go func() {
   749  		// Ensure that the cancellation of a request closes the backend.
   750  		// See issue https://golang.org/issue/35559.
   751  		select {
   752  		case <-req.Context().Done():
   753  		case <-backConnCloseCh:
   754  		}
   755  		backConn.Close()
   756  	}()
   757  	defer close(backConnCloseCh)
   758  
   759  	if hijackErr != nil {
   760  		p.getErrorHandler()(rw, req, fmt.Errorf("Hijack failed on protocol switch: %v", hijackErr))
   761  		return
   762  	}
   763  	defer conn.Close()
   764  
   765  	copyHeader(rw.Header(), res.Header)
   766  
   767  	res.Header = rw.Header()
   768  	res.Body = nil // so res.Write only writes the headers; we have res.Body in backConn above
   769  	if err := res.Write(brw); err != nil {
   770  		p.getErrorHandler()(rw, req, fmt.Errorf("response write: %v", err))
   771  		return
   772  	}
   773  	if err := brw.Flush(); err != nil {
   774  		p.getErrorHandler()(rw, req, fmt.Errorf("response flush: %v", err))
   775  		return
   776  	}
   777  	errc := make(chan error, 1)
   778  	spc := switchProtocolCopier{user: conn, backend: backConn}
   779  	go spc.copyToBackend(errc)
   780  	go spc.copyFromBackend(errc)
   781  	<-errc
   782  }
   783  
   784  // switchProtocolCopier exists so goroutines proxying data back and
   785  // forth have nice names in stacks.
   786  type switchProtocolCopier struct {
   787  	user, backend io.ReadWriter
   788  }
   789  
   790  func (c switchProtocolCopier) copyFromBackend(errc chan<- error) {
   791  	_, err := io.Copy(c.user, c.backend)
   792  	errc <- err
   793  }
   794  
   795  func (c switchProtocolCopier) copyToBackend(errc chan<- error) {
   796  	_, err := io.Copy(c.backend, c.user)
   797  	errc <- err
   798  }
   799  
   800  func cleanQueryParams(s string) string {
   801  	reencode := func(s string) string {
   802  		v, _ := url.ParseQuery(s)
   803  		return v.Encode()
   804  	}
   805  	for i := 0; i < len(s); {
   806  		switch s[i] {
   807  		case ';':
   808  			return reencode(s)
   809  		case '%':
   810  			if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
   811  				return reencode(s)
   812  			}
   813  			i += 3
   814  		default:
   815  			i++
   816  		}
   817  	}
   818  	return s
   819  }
   820  
   821  func ishex(c byte) bool {
   822  	switch {
   823  	case '0' <= c && c <= '9':
   824  		return true
   825  	case 'a' <= c && c <= 'f':
   826  		return true
   827  	case 'A' <= c && c <= 'F':
   828  		return true
   829  	}
   830  	return false
   831  }
   832
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