netramesh

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// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// HTTP reverse proxy handler
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package httputil
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import (
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	"context"
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	"io"
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	"log"
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	"net"
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	"net/http"
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	"net/url"
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	"strings"
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	"sync"
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	"time"
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)
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// onExitFlushLoop is a callback set by tests to detect the state of the
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// flushLoop() goroutine.
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var onExitFlushLoop func()
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// ReverseProxy is an HTTP Handler that takes an incoming request and
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// sends it to another server, proxying the response back to the
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// client.
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type ReverseProxy struct {
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	// Director must be a function which modifies
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	// the request into a new request to be sent
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	// using Transport. Its response is then copied
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	// back to the original client unmodified.
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	// Director must not access the provided Request
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	// after returning.
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	Director func(*http.Request)
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	// The transport used to perform proxy requests.
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	// If nil, http.DefaultTransport is used.
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	Transport http.RoundTripper
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	// FlushInterval specifies the flush interval
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	// to flush to the client while copying the
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	// response body.
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	// If zero, no periodic flushing is done.
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	FlushInterval time.Duration
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	// ErrorLog specifies an optional logger for errors
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	// that occur when attempting to proxy the request.
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	// If nil, logging goes to os.Stderr via the log package's
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	// standard logger.
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	ErrorLog *log.Logger
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	// BufferPool optionally specifies a buffer pool to
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	// get byte slices for use by io.CopyBuffer when
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	// copying HTTP response bodies.
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	BufferPool BufferPool
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	// ModifyResponse is an optional function that modifies the
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	// Response from the backend. It is called if the backend
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	// returns a response at all, with any HTTP status code.
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	// If the backend is unreachable, the optional ErrorHandler is
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	// called without any call to ModifyResponse.
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	//
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	// If ModifyResponse returns an error, ErrorHandler is called
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	// with its error value. If ErrorHandler is nil, its default
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	// implementation is used.
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	ModifyResponse func(*http.Response) error
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	// ErrorHandler is an optional function that handles errors
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	// reaching the backend or errors from ModifyResponse.
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	//
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	// If nil, the default is to log the provided error and return
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	// a 502 Status Bad Gateway response.
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	ErrorHandler func(http.ResponseWriter, *http.Request, error)
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}
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// A BufferPool is an interface for getting and returning temporary
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// byte slices for use by io.CopyBuffer.
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type BufferPool interface {
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	Get() []byte
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	Put([]byte)
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}
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func singleJoiningSlash(a, b string) string {
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	aslash := strings.HasSuffix(a, "/")
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	bslash := strings.HasPrefix(b, "/")
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	switch {
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	case aslash && bslash:
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		return a + b[1:]
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	case !aslash && !bslash:
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		return a + "/" + b
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	}
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	return a + b
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}
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// NewSingleHostReverseProxy returns a new ReverseProxy that routes
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// URLs to the scheme, host, and base path provided in target. If the
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// target's path is "/base" and the incoming request was for "/dir",
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// the target request will be for /base/dir.
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// NewSingleHostReverseProxy does not rewrite the Host header.
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// To rewrite Host headers, use ReverseProxy directly with a custom
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// Director policy.
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func NewSingleHostReverseProxy(target *url.URL) *ReverseProxy {
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	targetQuery := target.RawQuery
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	director := func(req *http.Request) {
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		req.URL.Scheme = target.Scheme
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		req.URL.Host = target.Host
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		req.URL.Path = singleJoiningSlash(target.Path, req.URL.Path)
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		if targetQuery == "" || req.URL.RawQuery == "" {
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			req.URL.RawQuery = targetQuery + req.URL.RawQuery
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		} else {
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			req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
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		}
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		if _, ok := req.Header["User-Agent"]; !ok {
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			// explicitly disable User-Agent so it's not set to default value
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			req.Header.Set("User-Agent", "")
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		}
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	}
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	return &ReverseProxy{Director: director}
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}
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func copyHeader(dst, src http.Header) {
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	for k, vv := range src {
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		for _, v := range vv {
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			dst.Add(k, v)
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		}
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	}
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}
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func cloneHeader(h http.Header) http.Header {
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	h2 := make(http.Header, len(h))
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	for k, vv := range h {
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		vv2 := make([]string, len(vv))
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		copy(vv2, vv)
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		h2[k] = vv2
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	}
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	return h2
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}
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// Hop-by-hop headers. These are removed when sent to the backend.
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// As of RFC 7230, hop-by-hop headers are required to appear in the
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// Connection header field. These are the headers defined by the
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// obsoleted RFC 2616 (section 13.5.1) and are used for backward
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// compatibility.
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var hopHeaders = []string{
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	"Connection",
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	"Proxy-Connection", // non-standard but still sent by libcurl and rejected by e.g. google
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	"Keep-Alive",
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	"Proxy-Authenticate",
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	"Proxy-Authorization",
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	"Te",      // canonicalized version of "TE"
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	"Trailer", // not Trailers per URL above; https://www.rfc-editor.org/errata_search.php?eid=4522
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	"Transfer-Encoding",
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	"Upgrade",
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}
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func (p *ReverseProxy) defaultErrorHandler(rw http.ResponseWriter, req *http.Request, err error) {
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	p.logf("http: proxy error: %v", err)
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	rw.WriteHeader(http.StatusBadGateway)
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}
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func (p *ReverseProxy) getErrorHandler() func(http.ResponseWriter, *http.Request, error) {
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	if p.ErrorHandler != nil {
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		return p.ErrorHandler
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	}
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	return p.defaultErrorHandler
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}
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func (p *ReverseProxy) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
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	transport := p.Transport
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	if transport == nil {
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		transport = http.DefaultTransport
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	}
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	ctx := req.Context()
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	if cn, ok := rw.(http.CloseNotifier); ok {
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		var cancel context.CancelFunc
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		ctx, cancel = context.WithCancel(ctx)
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		defer cancel()
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		notifyChan := cn.CloseNotify()
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		go func() {
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			select {
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			case <-notifyChan:
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				cancel()
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			case <-ctx.Done():
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			}
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		}()
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	}
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	outreq := req.WithContext(ctx) // includes shallow copies of maps, but okay
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	if req.ContentLength == 0 {
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		outreq.Body = nil // Issue 16036: nil Body for http.Transport retries
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	}
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	outreq.Header = cloneHeader(req.Header)
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	p.Director(outreq)
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	outreq.Close = false
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	removeConnectionHeaders(outreq.Header)
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	// Remove hop-by-hop headers to the backend. Especially
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	// important is "Connection" because we want a persistent
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	// connection, regardless of what the client sent to us.
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	for _, h := range hopHeaders {
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		hv := outreq.Header.Get(h)
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		if hv == "" {
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			continue
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		}
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		if h == "Te" && hv == "trailers" {
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			// Issue 21096: tell backend applications that
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			// care about trailer support that we support
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			// trailers. (We do, but we don't go out of
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			// our way to advertise that unless the
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			// incoming client request thought it was
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			// worth mentioning)
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			continue
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		}
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		outreq.Header.Del(h)
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	}
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	if clientIP, _, err := net.SplitHostPort(req.RemoteAddr); err == nil {
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		// If we aren't the first proxy retain prior
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		// X-Forwarded-For information as a comma+space
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		// separated list and fold multiple headers into one.
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		if prior, ok := outreq.Header["X-Forwarded-For"]; ok {
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			clientIP = strings.Join(prior, ", ") + ", " + clientIP
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		}
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		outreq.Header.Set("X-Forwarded-For", clientIP)
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	}
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	res, err := transport.RoundTrip(outreq)
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	if err != nil {
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		p.getErrorHandler()(rw, outreq, err)
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		return
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	}
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	removeConnectionHeaders(res.Header)
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	for _, h := range hopHeaders {
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		res.Header.Del(h)
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	}
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	if p.ModifyResponse != nil {
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		if err := p.ModifyResponse(res); err != nil {
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			res.Body.Close()
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			p.getErrorHandler()(rw, outreq, err)
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			return
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		}
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	}
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	copyHeader(rw.Header(), res.Header)
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	// The "Trailer" header isn't included in the Transport's response,
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	// at least for *http.Transport. Build it up from Trailer.
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	announcedTrailers := len(res.Trailer)
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	if announcedTrailers > 0 {
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		trailerKeys := make([]string, 0, len(res.Trailer))
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		for k := range res.Trailer {
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			trailerKeys = append(trailerKeys, k)
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		}
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		rw.Header().Add("Trailer", strings.Join(trailerKeys, ", "))
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	}
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	rw.WriteHeader(res.StatusCode)
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	if len(res.Trailer) > 0 {
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		// Force chunking if we saw a response trailer.
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		// This prevents net/http from calculating the length for short
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		// bodies and adding a Content-Length.
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		if fl, ok := rw.(http.Flusher); ok {
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			fl.Flush()
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		}
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	}
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	err = p.copyResponse(rw, res.Body)
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	if err != nil {
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		defer res.Body.Close()
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		// Since we're streaming the response, if we run into an error all we can do
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		// is abort the request. Issue 23643: ReverseProxy should use ErrAbortHandler
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		// on read error while copying body.
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		if !shouldPanicOnCopyError(req) {
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			p.logf("suppressing panic for copyResponse error in test; copy error: %v", err)
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			return
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		}
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		panic(http.ErrAbortHandler)
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	}
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	res.Body.Close() // close now, instead of defer, to populate res.Trailer
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	if len(res.Trailer) == announcedTrailers {
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		copyHeader(rw.Header(), res.Trailer)
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		return
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	}
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	for k, vv := range res.Trailer {
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		k = http.TrailerPrefix + k
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		for _, v := range vv {
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			rw.Header().Add(k, v)
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		}
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	}
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}
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var inOurTests bool // whether we're in our own tests
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// shouldPanicOnCopyError reports whether the reverse proxy should
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// panic with http.ErrAbortHandler. This is the right thing to do by
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// default, but Go 1.10 and earlier did not, so existing unit tests
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// weren't expecting panics. Only panic in our own tests, or when
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// running under the HTTP server.
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func shouldPanicOnCopyError(req *http.Request) bool {
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	if inOurTests {
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		// Our tests know to handle this panic.
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		return true
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	}
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	if req.Context().Value(http.ServerContextKey) != nil {
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		// We seem to be running under an HTTP server, so
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		// it'll recover the panic.
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		return true
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	}
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	// Otherwise act like Go 1.10 and earlier to not break
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	// existing tests.
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	return false
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}
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// removeConnectionHeaders removes hop-by-hop headers listed in the "Connection" header of h.
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// See RFC 7230, section 6.1
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func removeConnectionHeaders(h http.Header) {
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	if c := h.Get("Connection"); c != "" {
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		for _, f := range strings.Split(c, ",") {
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			if f = strings.TrimSpace(f); f != "" {
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				h.Del(f)
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			}
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		}
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	}
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}
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func (p *ReverseProxy) copyResponse(dst io.Writer, src io.Reader) error {
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	if p.FlushInterval != 0 {
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		if wf, ok := dst.(writeFlusher); ok {
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			mlw := &maxLatencyWriter{
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				dst:     wf,
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				latency: p.FlushInterval,
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				done:    make(chan bool),
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			}
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			go mlw.flushLoop()
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			defer mlw.stop()
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			dst = mlw
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		}
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	}
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	var buf []byte
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	if p.BufferPool != nil {
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		buf = p.BufferPool.Get()
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		defer p.BufferPool.Put(buf)
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	}
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	_, err := p.copyBuffer(dst, src, buf)
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	return err
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}
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// copyBuffer returns any write errors or non-EOF read errors, and the amount
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// of bytes written.
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func (p *ReverseProxy) copyBuffer(dst io.Writer, src io.Reader, buf []byte) (int64, error) {
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	if len(buf) == 0 {
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		buf = make([]byte, 32*1024)
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	}
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	var written int64
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	for {
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		nr, rerr := src.Read(buf)
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		if rerr != nil && rerr != io.EOF && rerr != context.Canceled {
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			p.logf("httputil: ReverseProxy read error during body copy: %v", rerr)
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		}
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		if nr > 0 {
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			nw, werr := dst.Write(buf[:nr])
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			if nw > 0 {
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				written += int64(nw)
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			}
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			if werr != nil {
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				return written, werr
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			}
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			if nr != nw {
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				return written, io.ErrShortWrite
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			}
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		}
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		if rerr != nil {
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			if rerr == io.EOF {
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				rerr = nil
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			}
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			return written, rerr
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		}
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	}
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}
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func (p *ReverseProxy) logf(format string, args ...interface{}) {
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	if p.ErrorLog != nil {
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		p.ErrorLog.Printf(format, args...)
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	} else {
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		log.Printf(format, args...)
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	}
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}
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type writeFlusher interface {
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	io.Writer
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	http.Flusher
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}
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type maxLatencyWriter struct {
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	dst     writeFlusher
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	latency time.Duration
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	mu   sync.Mutex // protects Write + Flush
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	done chan bool
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}
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func (m *maxLatencyWriter) Write(p []byte) (int, error) {
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	m.mu.Lock()
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	defer m.mu.Unlock()
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	return m.dst.Write(p)
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}
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func (m *maxLatencyWriter) flushLoop() {
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	t := time.NewTicker(m.latency)
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	defer t.Stop()
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	for {
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		select {
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		case <-m.done:
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			if onExitFlushLoop != nil {
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				onExitFlushLoop()
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			}
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			return
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		case <-t.C:
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			m.mu.Lock()
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			m.dst.Flush()
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			m.mu.Unlock()
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		}
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	}
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}
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func (m *maxLatencyWriter) stop() { m.done <- true }
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