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Source file src/net/http/server.go

Documentation: net/http

  // Copyright 2009 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // HTTP server. See RFC 2616.
  
  package http
  
  import (
  	"bufio"
  	"bytes"
  	"context"
  	"crypto/tls"
  	"errors"
  	"fmt"
  	"io"
  	"io/ioutil"
  	"log"
  	"net"
  	"net/textproto"
  	"net/url"
  	"os"
  	"path"
  	"runtime"
  	"strconv"
  	"strings"
  	"sync"
  	"sync/atomic"
  	"time"
  
  	"golang_org/x/net/lex/httplex"
  )
  
  // Errors used by the HTTP server.
  var (
  	// ErrBodyNotAllowed is returned by ResponseWriter.Write calls
  	// when the HTTP method or response code does not permit a
  	// body.
  	ErrBodyNotAllowed = errors.New("http: request method or response status code does not allow body")
  
  	// ErrHijacked is returned by ResponseWriter.Write calls when
  	// the underlying connection has been hijacked using the
  	// Hijacker interface. A zero-byte write on a hijacked
  	// connection will return ErrHijacked without any other side
  	// effects.
  	ErrHijacked = errors.New("http: connection has been hijacked")
  
  	// ErrContentLength is returned by ResponseWriter.Write calls
  	// when a Handler set a Content-Length response header with a
  	// declared size and then attempted to write more bytes than
  	// declared.
  	ErrContentLength = errors.New("http: wrote more than the declared Content-Length")
  
  	// Deprecated: ErrWriteAfterFlush is no longer used.
  	ErrWriteAfterFlush = errors.New("unused")
  )
  
  // A Handler responds to an HTTP request.
  //
  // ServeHTTP should write reply headers and data to the ResponseWriter
  // and then return. Returning signals that the request is finished; it
  // is not valid to use the ResponseWriter or read from the
  // Request.Body after or concurrently with the completion of the
  // ServeHTTP call.
  //
  // Depending on the HTTP client software, HTTP protocol version, and
  // any intermediaries between the client and the Go server, it may not
  // be possible to read from the Request.Body after writing to the
  // ResponseWriter. Cautious handlers should read the Request.Body
  // first, and then reply.
  //
  // Except for reading the body, handlers should not modify the
  // provided Request.
  //
  // If ServeHTTP panics, the server (the caller of ServeHTTP) assumes
  // that the effect of the panic was isolated to the active request.
  // It recovers the panic, logs a stack trace to the server error log,
  // and hangs up the connection. To abort a handler so the client sees
  // an interrupted response but the server doesn't log an error, panic
  // with the value ErrAbortHandler.
  type Handler interface {
  	ServeHTTP(ResponseWriter, *Request)
  }
  
  // A ResponseWriter interface is used by an HTTP handler to
  // construct an HTTP response.
  //
  // A ResponseWriter may not be used after the Handler.ServeHTTP method
  // has returned.
  type ResponseWriter interface {
  	// Header returns the header map that will be sent by
  	// WriteHeader. The Header map also is the mechanism with which
  	// Handlers can set HTTP trailers.
  	//
  	// Changing the header map after a call to WriteHeader (or
  	// Write) has no effect unless the modified headers are
  	// trailers.
  	//
  	// There are two ways to set Trailers. The preferred way is to
  	// predeclare in the headers which trailers you will later
  	// send by setting the "Trailer" header to the names of the
  	// trailer keys which will come later. In this case, those
  	// keys of the Header map are treated as if they were
  	// trailers. See the example. The second way, for trailer
  	// keys not known to the Handler until after the first Write,
  	// is to prefix the Header map keys with the TrailerPrefix
  	// constant value. See TrailerPrefix.
  	//
  	// To suppress implicit response headers (such as "Date"), set
  	// their value to nil.
  	Header() Header
  
  	// Write writes the data to the connection as part of an HTTP reply.
  	//
  	// If WriteHeader has not yet been called, Write calls
  	// WriteHeader(http.StatusOK) before writing the data. If the Header
  	// does not contain a Content-Type line, Write adds a Content-Type set
  	// to the result of passing the initial 512 bytes of written data to
  	// DetectContentType.
  	//
  	// Depending on the HTTP protocol version and the client, calling
  	// Write or WriteHeader may prevent future reads on the
  	// Request.Body. For HTTP/1.x requests, handlers should read any
  	// needed request body data before writing the response. Once the
  	// headers have been flushed (due to either an explicit Flusher.Flush
  	// call or writing enough data to trigger a flush), the request body
  	// may be unavailable. For HTTP/2 requests, the Go HTTP server permits
  	// handlers to continue to read the request body while concurrently
  	// writing the response. However, such behavior may not be supported
  	// by all HTTP/2 clients. Handlers should read before writing if
  	// possible to maximize compatibility.
  	Write([]byte) (int, error)
  
  	// WriteHeader sends an HTTP response header with status code.
  	// If WriteHeader is not called explicitly, the first call to Write
  	// will trigger an implicit WriteHeader(http.StatusOK).
  	// Thus explicit calls to WriteHeader are mainly used to
  	// send error codes.
  	WriteHeader(int)
  }
  
  // The Flusher interface is implemented by ResponseWriters that allow
  // an HTTP handler to flush buffered data to the client.
  //
  // The default HTTP/1.x and HTTP/2 ResponseWriter implementations
  // support Flusher, but ResponseWriter wrappers may not. Handlers
  // should always test for this ability at runtime.
  //
  // Note that even for ResponseWriters that support Flush,
  // if the client is connected through an HTTP proxy,
  // the buffered data may not reach the client until the response
  // completes.
  type Flusher interface {
  	// Flush sends any buffered data to the client.
  	Flush()
  }
  
  // The Hijacker interface is implemented by ResponseWriters that allow
  // an HTTP handler to take over the connection.
  //
  // The default ResponseWriter for HTTP/1.x connections supports
  // Hijacker, but HTTP/2 connections intentionally do not.
  // ResponseWriter wrappers may also not support Hijacker. Handlers
  // should always test for this ability at runtime.
  type Hijacker interface {
  	// Hijack lets the caller take over the connection.
  	// After a call to Hijack the HTTP server library
  	// will not do anything else with the connection.
  	//
  	// It becomes the caller's responsibility to manage
  	// and close the connection.
  	//
  	// The returned net.Conn may have read or write deadlines
  	// already set, depending on the configuration of the
  	// Server. It is the caller's responsibility to set
  	// or clear those deadlines as needed.
  	//
  	// The returned bufio.Reader may contain unprocessed buffered
  	// data from the client.
  	Hijack() (net.Conn, *bufio.ReadWriter, error)
  }
  
  // The CloseNotifier interface is implemented by ResponseWriters which
  // allow detecting when the underlying connection has gone away.
  //
  // This mechanism can be used to cancel long operations on the server
  // if the client has disconnected before the response is ready.
  type CloseNotifier interface {
  	// CloseNotify returns a channel that receives at most a
  	// single value (true) when the client connection has gone
  	// away.
  	//
  	// CloseNotify may wait to notify until Request.Body has been
  	// fully read.
  	//
  	// After the Handler has returned, there is no guarantee
  	// that the channel receives a value.
  	//
  	// If the protocol is HTTP/1.1 and CloseNotify is called while
  	// processing an idempotent request (such a GET) while
  	// HTTP/1.1 pipelining is in use, the arrival of a subsequent
  	// pipelined request may cause a value to be sent on the
  	// returned channel. In practice HTTP/1.1 pipelining is not
  	// enabled in browsers and not seen often in the wild. If this
  	// is a problem, use HTTP/2 or only use CloseNotify on methods
  	// such as POST.
  	CloseNotify() <-chan bool
  }
  
  var (
  	// ServerContextKey is a context key. It can be used in HTTP
  	// handlers with context.WithValue to access the server that
  	// started the handler. The associated value will be of
  	// type *Server.
  	ServerContextKey = &contextKey{"http-server"}
  
  	// LocalAddrContextKey is a context key. It can be used in
  	// HTTP handlers with context.WithValue to access the address
  	// the local address the connection arrived on.
  	// The associated value will be of type net.Addr.
  	LocalAddrContextKey = &contextKey{"local-addr"}
  )
  
  // A conn represents the server side of an HTTP connection.
  type conn struct {
  	// server is the server on which the connection arrived.
  	// Immutable; never nil.
  	server *Server
  
  	// cancelCtx cancels the connection-level context.
  	cancelCtx context.CancelFunc
  
  	// rwc is the underlying network connection.
  	// This is never wrapped by other types and is the value given out
  	// to CloseNotifier callers. It is usually of type *net.TCPConn or
  	// *tls.Conn.
  	rwc net.Conn
  
  	// remoteAddr is rwc.RemoteAddr().String(). It is not populated synchronously
  	// inside the Listener's Accept goroutine, as some implementations block.
  	// It is populated immediately inside the (*conn).serve goroutine.
  	// This is the value of a Handler's (*Request).RemoteAddr.
  	remoteAddr string
  
  	// tlsState is the TLS connection state when using TLS.
  	// nil means not TLS.
  	tlsState *tls.ConnectionState
  
  	// werr is set to the first write error to rwc.
  	// It is set via checkConnErrorWriter{w}, where bufw writes.
  	werr error
  
  	// r is bufr's read source. It's a wrapper around rwc that provides
  	// io.LimitedReader-style limiting (while reading request headers)
  	// and functionality to support CloseNotifier. See *connReader docs.
  	r *connReader
  
  	// bufr reads from r.
  	bufr *bufio.Reader
  
  	// bufw writes to checkConnErrorWriter{c}, which populates werr on error.
  	bufw *bufio.Writer
  
  	// lastMethod is the method of the most recent request
  	// on this connection, if any.
  	lastMethod string
  
  	curReq atomic.Value // of *response (which has a Request in it)
  
  	curState atomic.Value // of ConnState
  
  	// mu guards hijackedv
  	mu sync.Mutex
  
  	// hijackedv is whether this connection has been hijacked
  	// by a Handler with the Hijacker interface.
  	// It is guarded by mu.
  	hijackedv bool
  }
  
  func (c *conn) hijacked() bool {
  	c.mu.Lock()
  	defer c.mu.Unlock()
  	return c.hijackedv
  }
  
  // c.mu must be held.
  func (c *conn) hijackLocked() (rwc net.Conn, buf *bufio.ReadWriter, err error) {
  	if c.hijackedv {
  		return nil, nil, ErrHijacked
  	}
  	c.r.abortPendingRead()
  
  	c.hijackedv = true
  	rwc = c.rwc
  	rwc.SetDeadline(time.Time{})
  
  	buf = bufio.NewReadWriter(c.bufr, bufio.NewWriter(rwc))
  	if c.r.hasByte {
  		if _, err := c.bufr.Peek(c.bufr.Buffered() + 1); err != nil {
  			return nil, nil, fmt.Errorf("unexpected Peek failure reading buffered byte: %v", err)
  		}
  	}
  	c.setState(rwc, StateHijacked)
  	return
  }
  
  // This should be >= 512 bytes for DetectContentType,
  // but otherwise it's somewhat arbitrary.
  const bufferBeforeChunkingSize = 2048
  
  // chunkWriter writes to a response's conn buffer, and is the writer
  // wrapped by the response.bufw buffered writer.
  //
  // chunkWriter also is responsible for finalizing the Header, including
  // conditionally setting the Content-Type and setting a Content-Length
  // in cases where the handler's final output is smaller than the buffer
  // size. It also conditionally adds chunk headers, when in chunking mode.
  //
  // See the comment above (*response).Write for the entire write flow.
  type chunkWriter struct {
  	res *response
  
  	// header is either nil or a deep clone of res.handlerHeader
  	// at the time of res.WriteHeader, if res.WriteHeader is
  	// called and extra buffering is being done to calculate
  	// Content-Type and/or Content-Length.
  	header Header
  
  	// wroteHeader tells whether the header's been written to "the
  	// wire" (or rather: w.conn.buf). this is unlike
  	// (*response).wroteHeader, which tells only whether it was
  	// logically written.
  	wroteHeader bool
  
  	// set by the writeHeader method:
  	chunking bool // using chunked transfer encoding for reply body
  }
  
  var (
  	crlf       = []byte("\r\n")
  	colonSpace = []byte(": ")
  )
  
  func (cw *chunkWriter) Write(p []byte) (n int, err error) {
  	if !cw.wroteHeader {
  		cw.writeHeader(p)
  	}
  	if cw.res.req.Method == "HEAD" {
  		// Eat writes.
  		return len(p), nil
  	}
  	if cw.chunking {
  		_, err = fmt.Fprintf(cw.res.conn.bufw, "%x\r\n", len(p))
  		if err != nil {
  			cw.res.conn.rwc.Close()
  			return
  		}
  	}
  	n, err = cw.res.conn.bufw.Write(p)
  	if cw.chunking && err == nil {
  		_, err = cw.res.conn.bufw.Write(crlf)
  	}
  	if err != nil {
  		cw.res.conn.rwc.Close()
  	}
  	return
  }
  
  func (cw *chunkWriter) flush() {
  	if !cw.wroteHeader {
  		cw.writeHeader(nil)
  	}
  	cw.res.conn.bufw.Flush()
  }
  
  func (cw *chunkWriter) close() {
  	if !cw.wroteHeader {
  		cw.writeHeader(nil)
  	}
  	if cw.chunking {
  		bw := cw.res.conn.bufw // conn's bufio writer
  		// zero chunk to mark EOF
  		bw.WriteString("0\r\n")
  		if trailers := cw.res.finalTrailers(); trailers != nil {
  			trailers.Write(bw) // the writer handles noting errors
  		}
  		// final blank line after the trailers (whether
  		// present or not)
  		bw.WriteString("\r\n")
  	}
  }
  
  // A response represents the server side of an HTTP response.
  type response struct {
  	conn             *conn
  	req              *Request // request for this response
  	reqBody          io.ReadCloser
  	cancelCtx        context.CancelFunc // when ServeHTTP exits
  	wroteHeader      bool               // reply header has been (logically) written
  	wroteContinue    bool               // 100 Continue response was written
  	wants10KeepAlive bool               // HTTP/1.0 w/ Connection "keep-alive"
  	wantsClose       bool               // HTTP request has Connection "close"
  
  	w  *bufio.Writer // buffers output in chunks to chunkWriter
  	cw chunkWriter
  
  	// handlerHeader is the Header that Handlers get access to,
  	// which may be retained and mutated even after WriteHeader.
  	// handlerHeader is copied into cw.header at WriteHeader
  	// time, and privately mutated thereafter.
  	handlerHeader Header
  	calledHeader  bool // handler accessed handlerHeader via Header
  
  	written       int64 // number of bytes written in body
  	contentLength int64 // explicitly-declared Content-Length; or -1
  	status        int   // status code passed to WriteHeader
  
  	// close connection after this reply.  set on request and
  	// updated after response from handler if there's a
  	// "Connection: keep-alive" response header and a
  	// Content-Length.
  	closeAfterReply bool
  
  	// requestBodyLimitHit is set by requestTooLarge when
  	// maxBytesReader hits its max size. It is checked in
  	// WriteHeader, to make sure we don't consume the
  	// remaining request body to try to advance to the next HTTP
  	// request. Instead, when this is set, we stop reading
  	// subsequent requests on this connection and stop reading
  	// input from it.
  	requestBodyLimitHit bool
  
  	// trailers are the headers to be sent after the handler
  	// finishes writing the body. This field is initialized from
  	// the Trailer response header when the response header is
  	// written.
  	trailers []string
  
  	handlerDone atomicBool // set true when the handler exits
  
  	// Buffers for Date and Content-Length
  	dateBuf [len(TimeFormat)]byte
  	clenBuf [10]byte
  
  	// closeNotifyCh is the channel returned by CloseNotify.
  	// TODO(bradfitz): this is currently (for Go 1.8) always
  	// non-nil. Make this lazily-created again as it used to be?
  	closeNotifyCh  chan bool
  	didCloseNotify int32 // atomic (only 0->1 winner should send)
  }
  
  // TrailerPrefix is a magic prefix for ResponseWriter.Header map keys
  // that, if present, signals that the map entry is actually for
  // the response trailers, and not the response headers. The prefix
  // is stripped after the ServeHTTP call finishes and the values are
  // sent in the trailers.
  //
  // This mechanism is intended only for trailers that are not known
  // prior to the headers being written. If the set of trailers is fixed
  // or known before the header is written, the normal Go trailers mechanism
  // is preferred:
  //    https://golang.org/pkg/net/http/#ResponseWriter
  //    https://golang.org/pkg/net/http/#example_ResponseWriter_trailers
  const TrailerPrefix = "Trailer:"
  
  // finalTrailers is called after the Handler exits and returns a non-nil
  // value if the Handler set any trailers.
  func (w *response) finalTrailers() Header {
  	var t Header
  	for k, vv := range w.handlerHeader {
  		if strings.HasPrefix(k, TrailerPrefix) {
  			if t == nil {
  				t = make(Header)
  			}
  			t[strings.TrimPrefix(k, TrailerPrefix)] = vv
  		}
  	}
  	for _, k := range w.trailers {
  		if t == nil {
  			t = make(Header)
  		}
  		for _, v := range w.handlerHeader[k] {
  			t.Add(k, v)
  		}
  	}
  	return t
  }
  
  type atomicBool int32
  
  func (b *atomicBool) isSet() bool { return atomic.LoadInt32((*int32)(b)) != 0 }
  func (b *atomicBool) setTrue()    { atomic.StoreInt32((*int32)(b), 1) }
  
  // declareTrailer is called for each Trailer header when the
  // response header is written. It notes that a header will need to be
  // written in the trailers at the end of the response.
  func (w *response) declareTrailer(k string) {
  	k = CanonicalHeaderKey(k)
  	switch k {
  	case "Transfer-Encoding", "Content-Length", "Trailer":
  		// Forbidden by RFC 2616 14.40.
  		return
  	}
  	w.trailers = append(w.trailers, k)
  }
  
  // requestTooLarge is called by maxBytesReader when too much input has
  // been read from the client.
  func (w *response) requestTooLarge() {
  	w.closeAfterReply = true
  	w.requestBodyLimitHit = true
  	if !w.wroteHeader {
  		w.Header().Set("Connection", "close")
  	}
  }
  
  // needsSniff reports whether a Content-Type still needs to be sniffed.
  func (w *response) needsSniff() bool {
  	_, haveType := w.handlerHeader["Content-Type"]
  	return !w.cw.wroteHeader && !haveType && w.written < sniffLen
  }
  
  // writerOnly hides an io.Writer value's optional ReadFrom method
  // from io.Copy.
  type writerOnly struct {
  	io.Writer
  }
  
  func srcIsRegularFile(src io.Reader) (isRegular bool, err error) {
  	switch v := src.(type) {
  	case *os.File:
  		fi, err := v.Stat()
  		if err != nil {
  			return false, err
  		}
  		return fi.Mode().IsRegular(), nil
  	case *io.LimitedReader:
  		return srcIsRegularFile(v.R)
  	default:
  		return
  	}
  }
  
  // ReadFrom is here to optimize copying from an *os.File regular file
  // to a *net.TCPConn with sendfile.
  func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
  	// Our underlying w.conn.rwc is usually a *TCPConn (with its
  	// own ReadFrom method). If not, or if our src isn't a regular
  	// file, just fall back to the normal copy method.
  	rf, ok := w.conn.rwc.(io.ReaderFrom)
  	regFile, err := srcIsRegularFile(src)
  	if err != nil {
  		return 0, err
  	}
  	if !ok || !regFile {
  		bufp := copyBufPool.Get().(*[]byte)
  		defer copyBufPool.Put(bufp)
  		return io.CopyBuffer(writerOnly{w}, src, *bufp)
  	}
  
  	// sendfile path:
  
  	if !w.wroteHeader {
  		w.WriteHeader(StatusOK)
  	}
  
  	if w.needsSniff() {
  		n0, err := io.Copy(writerOnly{w}, io.LimitReader(src, sniffLen))
  		n += n0
  		if err != nil {
  			return n, err
  		}
  	}
  
  	w.w.Flush()  // get rid of any previous writes
  	w.cw.flush() // make sure Header is written; flush data to rwc
  
  	// Now that cw has been flushed, its chunking field is guaranteed initialized.
  	if !w.cw.chunking && w.bodyAllowed() {
  		n0, err := rf.ReadFrom(src)
  		n += n0
  		w.written += n0
  		return n, err
  	}
  
  	n0, err := io.Copy(writerOnly{w}, src)
  	n += n0
  	return n, err
  }
  
  // debugServerConnections controls whether all server connections are wrapped
  // with a verbose logging wrapper.
  const debugServerConnections = false
  
  // Create new connection from rwc.
  func (srv *Server) newConn(rwc net.Conn) *conn {
  	c := &conn{
  		server: srv,
  		rwc:    rwc,
  	}
  	if debugServerConnections {
  		c.rwc = newLoggingConn("server", c.rwc)
  	}
  	return c
  }
  
  type readResult struct {
  	n   int
  	err error
  	b   byte // byte read, if n == 1
  }
  
  // connReader is the io.Reader wrapper used by *conn. It combines a
  // selectively-activated io.LimitedReader (to bound request header
  // read sizes) with support for selectively keeping an io.Reader.Read
  // call blocked in a background goroutine to wait for activity and
  // trigger a CloseNotifier channel.
  type connReader struct {
  	conn *conn
  
  	mu      sync.Mutex // guards following
  	hasByte bool
  	byteBuf [1]byte
  	bgErr   error // non-nil means error happened on background read
  	cond    *sync.Cond
  	inRead  bool
  	aborted bool  // set true before conn.rwc deadline is set to past
  	remain  int64 // bytes remaining
  }
  
  func (cr *connReader) lock() {
  	cr.mu.Lock()
  	if cr.cond == nil {
  		cr.cond = sync.NewCond(&cr.mu)
  	}
  }
  
  func (cr *connReader) unlock() { cr.mu.Unlock() }
  
  func (cr *connReader) startBackgroundRead() {
  	cr.lock()
  	defer cr.unlock()
  	if cr.inRead {
  		panic("invalid concurrent Body.Read call")
  	}
  	if cr.hasByte {
  		return
  	}
  	cr.inRead = true
  	cr.conn.rwc.SetReadDeadline(time.Time{})
  	go cr.backgroundRead()
  }
  
  func (cr *connReader) backgroundRead() {
  	n, err := cr.conn.rwc.Read(cr.byteBuf[:])
  	cr.lock()
  	if n == 1 {
  		cr.hasByte = true
  		// We were at EOF already (since we wouldn't be in a
  		// background read otherwise), so this is a pipelined
  		// HTTP request.
  		cr.closeNotifyFromPipelinedRequest()
  	}
  	if ne, ok := err.(net.Error); ok && cr.aborted && ne.Timeout() {
  		// Ignore this error. It's the expected error from
  		// another goroutine calling abortPendingRead.
  	} else if err != nil {
  		cr.handleReadError(err)
  	}
  	cr.aborted = false
  	cr.inRead = false
  	cr.unlock()
  	cr.cond.Broadcast()
  }
  
  func (cr *connReader) abortPendingRead() {
  	cr.lock()
  	defer cr.unlock()
  	if !cr.inRead {
  		return
  	}
  	cr.aborted = true
  	cr.conn.rwc.SetReadDeadline(aLongTimeAgo)
  	for cr.inRead {
  		cr.cond.Wait()
  	}
  	cr.conn.rwc.SetReadDeadline(time.Time{})
  }
  
  func (cr *connReader) setReadLimit(remain int64) { cr.remain = remain }
  func (cr *connReader) setInfiniteReadLimit()     { cr.remain = maxInt64 }
  func (cr *connReader) hitReadLimit() bool        { return cr.remain <= 0 }
  
  // may be called from multiple goroutines.
  func (cr *connReader) handleReadError(err error) {
  	cr.conn.cancelCtx()
  	cr.closeNotify()
  }
  
  // closeNotifyFromPipelinedRequest simply calls closeNotify.
  //
  // This method wrapper is here for documentation. The callers are the
  // cases where we send on the closenotify channel because of a
  // pipelined HTTP request, per the previous Go behavior and
  // documentation (that this "MAY" happen).
  //
  // TODO: consider changing this behavior and making context
  // cancelation and closenotify work the same.
  func (cr *connReader) closeNotifyFromPipelinedRequest() {
  	cr.closeNotify()
  }
  
  // may be called from multiple goroutines.
  func (cr *connReader) closeNotify() {
  	res, _ := cr.conn.curReq.Load().(*response)
  	if res != nil {
  		if atomic.CompareAndSwapInt32(&res.didCloseNotify, 0, 1) {
  			res.closeNotifyCh <- true
  		}
  	}
  }
  
  func (cr *connReader) Read(p []byte) (n int, err error) {
  	cr.lock()
  	if cr.inRead {
  		cr.unlock()
  		panic("invalid concurrent Body.Read call")
  	}
  	if cr.hitReadLimit() {
  		cr.unlock()
  		return 0, io.EOF
  	}
  	if cr.bgErr != nil {
  		err = cr.bgErr
  		cr.unlock()
  		return 0, err
  	}
  	if len(p) == 0 {
  		cr.unlock()
  		return 0, nil
  	}
  	if int64(len(p)) > cr.remain {
  		p = p[:cr.remain]
  	}
  	if cr.hasByte {
  		p[0] = cr.byteBuf[0]
  		cr.hasByte = false
  		cr.unlock()
  		return 1, nil
  	}
  	cr.inRead = true
  	cr.unlock()
  	n, err = cr.conn.rwc.Read(p)
  
  	cr.lock()
  	cr.inRead = false
  	if err != nil {
  		cr.handleReadError(err)
  	}
  	cr.remain -= int64(n)
  	cr.unlock()
  
  	cr.cond.Broadcast()
  	return n, err
  }
  
  var (
  	bufioReaderPool   sync.Pool
  	bufioWriter2kPool sync.Pool
  	bufioWriter4kPool sync.Pool
  )
  
  var copyBufPool = sync.Pool{
  	New: func() interface{} {
  		b := make([]byte, 32*1024)
  		return &b
  	},
  }
  
  func bufioWriterPool(size int) *sync.Pool {
  	switch size {
  	case 2 << 10:
  		return &bufioWriter2kPool
  	case 4 << 10:
  		return &bufioWriter4kPool
  	}
  	return nil
  }
  
  func newBufioReader(r io.Reader) *bufio.Reader {
  	if v := bufioReaderPool.Get(); v != nil {
  		br := v.(*bufio.Reader)
  		br.Reset(r)
  		return br
  	}
  	// Note: if this reader size is ever changed, update
  	// TestHandlerBodyClose's assumptions.
  	return bufio.NewReader(r)
  }
  
  func putBufioReader(br *bufio.Reader) {
  	br.Reset(nil)
  	bufioReaderPool.Put(br)
  }
  
  func newBufioWriterSize(w io.Writer, size int) *bufio.Writer {
  	pool := bufioWriterPool(size)
  	if pool != nil {
  		if v := pool.Get(); v != nil {
  			bw := v.(*bufio.Writer)
  			bw.Reset(w)
  			return bw
  		}
  	}
  	return bufio.NewWriterSize(w, size)
  }
  
  func putBufioWriter(bw *bufio.Writer) {
  	bw.Reset(nil)
  	if pool := bufioWriterPool(bw.Available()); pool != nil {
  		pool.Put(bw)
  	}
  }
  
  // DefaultMaxHeaderBytes is the maximum permitted size of the headers
  // in an HTTP request.
  // This can be overridden by setting Server.MaxHeaderBytes.
  const DefaultMaxHeaderBytes = 1 << 20 // 1 MB
  
  func (srv *Server) maxHeaderBytes() int {
  	if srv.MaxHeaderBytes > 0 {
  		return srv.MaxHeaderBytes
  	}
  	return DefaultMaxHeaderBytes
  }
  
  func (srv *Server) initialReadLimitSize() int64 {
  	return int64(srv.maxHeaderBytes()) + 4096 // bufio slop
  }
  
  // wrapper around io.ReaderCloser which on first read, sends an
  // HTTP/1.1 100 Continue header
  type expectContinueReader struct {
  	resp       *response
  	readCloser io.ReadCloser
  	closed     bool
  	sawEOF     bool
  }
  
  func (ecr *expectContinueReader) Read(p []byte) (n int, err error) {
  	if ecr.closed {
  		return 0, ErrBodyReadAfterClose
  	}
  	if !ecr.resp.wroteContinue && !ecr.resp.conn.hijacked() {
  		ecr.resp.wroteContinue = true
  		ecr.resp.conn.bufw.WriteString("HTTP/1.1 100 Continue\r\n\r\n")
  		ecr.resp.conn.bufw.Flush()
  	}
  	n, err = ecr.readCloser.Read(p)
  	if err == io.EOF {
  		ecr.sawEOF = true
  	}
  	return
  }
  
  func (ecr *expectContinueReader) Close() error {
  	ecr.closed = true
  	return ecr.readCloser.Close()
  }
  
  // TimeFormat is the time format to use when generating times in HTTP
  // headers. It is like time.RFC1123 but hard-codes GMT as the time
  // zone. The time being formatted must be in UTC for Format to
  // generate the correct format.
  //
  // For parsing this time format, see ParseTime.
  const TimeFormat = "Mon, 02 Jan 2006 15:04:05 GMT"
  
  // appendTime is a non-allocating version of []byte(t.UTC().Format(TimeFormat))
  func appendTime(b []byte, t time.Time) []byte {
  	const days = "SunMonTueWedThuFriSat"
  	const months = "JanFebMarAprMayJunJulAugSepOctNovDec"
  
  	t = t.UTC()
  	yy, mm, dd := t.Date()
  	hh, mn, ss := t.Clock()
  	day := days[3*t.Weekday():]
  	mon := months[3*(mm-1):]
  
  	return append(b,
  		day[0], day[1], day[2], ',', ' ',
  		byte('0'+dd/10), byte('0'+dd%10), ' ',
  		mon[0], mon[1], mon[2], ' ',
  		byte('0'+yy/1000), byte('0'+(yy/100)%10), byte('0'+(yy/10)%10), byte('0'+yy%10), ' ',
  		byte('0'+hh/10), byte('0'+hh%10), ':',
  		byte('0'+mn/10), byte('0'+mn%10), ':',
  		byte('0'+ss/10), byte('0'+ss%10), ' ',
  		'G', 'M', 'T')
  }
  
  var errTooLarge = errors.New("http: request too large")
  
  // Read next request from connection.
  func (c *conn) readRequest(ctx context.Context) (w *response, err error) {
  	if c.hijacked() {
  		return nil, ErrHijacked
  	}
  
  	var (
  		wholeReqDeadline time.Time // or zero if none
  		hdrDeadline      time.Time // or zero if none
  	)
  	t0 := time.Now()
  	if d := c.server.readHeaderTimeout(); d != 0 {
  		hdrDeadline = t0.Add(d)
  	}
  	if d := c.server.ReadTimeout; d != 0 {
  		wholeReqDeadline = t0.Add(d)
  	}
  	c.rwc.SetReadDeadline(hdrDeadline)
  	if d := c.server.WriteTimeout; d != 0 {
  		defer func() {
  			c.rwc.SetWriteDeadline(time.Now().Add(d))
  		}()
  	}
  
  	c.r.setReadLimit(c.server.initialReadLimitSize())
  	if c.lastMethod == "POST" {
  		// RFC 2616 section 4.1 tolerance for old buggy clients.
  		peek, _ := c.bufr.Peek(4) // ReadRequest will get err below
  		c.bufr.Discard(numLeadingCRorLF(peek))
  	}
  	req, err := readRequest(c.bufr, keepHostHeader)
  	if err != nil {
  		if c.r.hitReadLimit() {
  			return nil, errTooLarge
  		}
  		return nil, err
  	}
  
  	if !http1ServerSupportsRequest(req) {
  		return nil, badRequestError("unsupported protocol version")
  	}
  
  	c.lastMethod = req.Method
  	c.r.setInfiniteReadLimit()
  
  	hosts, haveHost := req.Header["Host"]
  	isH2Upgrade := req.isH2Upgrade()
  	if req.ProtoAtLeast(1, 1) && (!haveHost || len(hosts) == 0) && !isH2Upgrade {
  		return nil, badRequestError("missing required Host header")
  	}
  	if len(hosts) > 1 {
  		return nil, badRequestError("too many Host headers")
  	}
  	if len(hosts) == 1 && !httplex.ValidHostHeader(hosts[0]) {
  		return nil, badRequestError("malformed Host header")
  	}
  	for k, vv := range req.Header {
  		if !httplex.ValidHeaderFieldName(k) {
  			return nil, badRequestError("invalid header name")
  		}
  		for _, v := range vv {
  			if !httplex.ValidHeaderFieldValue(v) {
  				return nil, badRequestError("invalid header value")
  			}
  		}
  	}
  	delete(req.Header, "Host")
  
  	ctx, cancelCtx := context.WithCancel(ctx)
  	req.ctx = ctx
  	req.RemoteAddr = c.remoteAddr
  	req.TLS = c.tlsState
  	if body, ok := req.Body.(*body); ok {
  		body.doEarlyClose = true
  	}
  
  	// Adjust the read deadline if necessary.
  	if !hdrDeadline.Equal(wholeReqDeadline) {
  		c.rwc.SetReadDeadline(wholeReqDeadline)
  	}
  
  	w = &response{
  		conn:          c,
  		cancelCtx:     cancelCtx,
  		req:           req,
  		reqBody:       req.Body,
  		handlerHeader: make(Header),
  		contentLength: -1,
  		closeNotifyCh: make(chan bool, 1),
  
  		// We populate these ahead of time so we're not
  		// reading from req.Header after their Handler starts
  		// and maybe mutates it (Issue 14940)
  		wants10KeepAlive: req.wantsHttp10KeepAlive(),
  		wantsClose:       req.wantsClose(),
  	}
  	if isH2Upgrade {
  		w.closeAfterReply = true
  	}
  	w.cw.res = w
  	w.w = newBufioWriterSize(&w.cw, bufferBeforeChunkingSize)
  	return w, nil
  }
  
  // http1ServerSupportsRequest reports whether Go's HTTP/1.x server
  // supports the given request.
  func http1ServerSupportsRequest(req *Request) bool {
  	if req.ProtoMajor == 1 {
  		return true
  	}
  	// Accept "PRI * HTTP/2.0" upgrade requests, so Handlers can
  	// wire up their own HTTP/2 upgrades.
  	if req.ProtoMajor == 2 && req.ProtoMinor == 0 &&
  		req.Method == "PRI" && req.RequestURI == "*" {
  		return true
  	}
  	// Reject HTTP/0.x, and all other HTTP/2+ requests (which
  	// aren't encoded in ASCII anyway).
  	return false
  }
  
  func (w *response) Header() Header {
  	if w.cw.header == nil && w.wroteHeader && !w.cw.wroteHeader {
  		// Accessing the header between logically writing it
  		// and physically writing it means we need to allocate
  		// a clone to snapshot the logically written state.
  		w.cw.header = w.handlerHeader.clone()
  	}
  	w.calledHeader = true
  	return w.handlerHeader
  }
  
  // maxPostHandlerReadBytes is the max number of Request.Body bytes not
  // consumed by a handler that the server will read from the client
  // in order to keep a connection alive. If there are more bytes than
  // this then the server to be paranoid instead sends a "Connection:
  // close" response.
  //
  // This number is approximately what a typical machine's TCP buffer
  // size is anyway.  (if we have the bytes on the machine, we might as
  // well read them)
  const maxPostHandlerReadBytes = 256 << 10
  
  func (w *response) WriteHeader(code int) {
  	if w.conn.hijacked() {
  		w.conn.server.logf("http: response.WriteHeader on hijacked connection")
  		return
  	}
  	if w.wroteHeader {
  		w.conn.server.logf("http: multiple response.WriteHeader calls")
  		return
  	}
  	w.wroteHeader = true
  	w.status = code
  
  	if w.calledHeader && w.cw.header == nil {
  		w.cw.header = w.handlerHeader.clone()
  	}
  
  	if cl := w.handlerHeader.get("Content-Length"); cl != "" {
  		v, err := strconv.ParseInt(cl, 10, 64)
  		if err == nil && v >= 0 {
  			w.contentLength = v
  		} else {
  			w.conn.server.logf("http: invalid Content-Length of %q", cl)
  			w.handlerHeader.Del("Content-Length")
  		}
  	}
  }
  
  // extraHeader is the set of headers sometimes added by chunkWriter.writeHeader.
  // This type is used to avoid extra allocations from cloning and/or populating
  // the response Header map and all its 1-element slices.
  type extraHeader struct {
  	contentType      string
  	connection       string
  	transferEncoding string
  	date             []byte // written if not nil
  	contentLength    []byte // written if not nil
  }
  
  // Sorted the same as extraHeader.Write's loop.
  var extraHeaderKeys = [][]byte{
  	[]byte("Content-Type"),
  	[]byte("Connection"),
  	[]byte("Transfer-Encoding"),
  }
  
  var (
  	headerContentLength = []byte("Content-Length: ")
  	headerDate          = []byte("Date: ")
  )
  
  // Write writes the headers described in h to w.
  //
  // This method has a value receiver, despite the somewhat large size
  // of h, because it prevents an allocation. The escape analysis isn't
  // smart enough to realize this function doesn't mutate h.
  func (h extraHeader) Write(w *bufio.Writer) {
  	if h.date != nil {
  		w.Write(headerDate)
  		w.Write(h.date)
  		w.Write(crlf)
  	}
  	if h.contentLength != nil {
  		w.Write(headerContentLength)
  		w.Write(h.contentLength)
  		w.Write(crlf)
  	}
  	for i, v := range []string{h.contentType, h.connection, h.transferEncoding} {
  		if v != "" {
  			w.Write(extraHeaderKeys[i])
  			w.Write(colonSpace)
  			w.WriteString(v)
  			w.Write(crlf)
  		}
  	}
  }
  
  // writeHeader finalizes the header sent to the client and writes it
  // to cw.res.conn.bufw.
  //
  // p is not written by writeHeader, but is the first chunk of the body
  // that will be written. It is sniffed for a Content-Type if none is
  // set explicitly. It's also used to set the Content-Length, if the
  // total body size was small and the handler has already finished
  // running.
  func (cw *chunkWriter) writeHeader(p []byte) {
  	if cw.wroteHeader {
  		return
  	}
  	cw.wroteHeader = true
  
  	w := cw.res
  	keepAlivesEnabled := w.conn.server.doKeepAlives()
  	isHEAD := w.req.Method == "HEAD"
  
  	// header is written out to w.conn.buf below. Depending on the
  	// state of the handler, we either own the map or not. If we
  	// don't own it, the exclude map is created lazily for
  	// WriteSubset to remove headers. The setHeader struct holds
  	// headers we need to add.
  	header := cw.header
  	owned := header != nil
  	if !owned {
  		header = w.handlerHeader
  	}
  	var excludeHeader map[string]bool
  	delHeader := func(key string) {
  		if owned {
  			header.Del(key)
  			return
  		}
  		if _, ok := header[key]; !ok {
  			return
  		}
  		if excludeHeader == nil {
  			excludeHeader = make(map[string]bool)
  		}
  		excludeHeader[key] = true
  	}
  	var setHeader extraHeader
  
  	// Don't write out the fake "Trailer:foo" keys. See TrailerPrefix.
  	trailers := false
  	for k := range cw.header {
  		if strings.HasPrefix(k, TrailerPrefix) {
  			if excludeHeader == nil {
  				excludeHeader = make(map[string]bool)
  			}
  			excludeHeader[k] = true
  			trailers = true
  		}
  	}
  	for _, v := range cw.header["Trailer"] {
  		trailers = true
  		foreachHeaderElement(v, cw.res.declareTrailer)
  	}
  
  	te := header.get("Transfer-Encoding")
  	hasTE := te != ""
  
  	// If the handler is done but never sent a Content-Length
  	// response header and this is our first (and last) write, set
  	// it, even to zero. This helps HTTP/1.0 clients keep their
  	// "keep-alive" connections alive.
  	// Exceptions: 304/204/1xx responses never get Content-Length, and if
  	// it was a HEAD request, we don't know the difference between
  	// 0 actual bytes and 0 bytes because the handler noticed it
  	// was a HEAD request and chose not to write anything. So for
  	// HEAD, the handler should either write the Content-Length or
  	// write non-zero bytes. If it's actually 0 bytes and the
  	// handler never looked at the Request.Method, we just don't
  	// send a Content-Length header.
  	// Further, we don't send an automatic Content-Length if they
  	// set a Transfer-Encoding, because they're generally incompatible.
  	if w.handlerDone.isSet() && !trailers && !hasTE && bodyAllowedForStatus(w.status) && header.get("Content-Length") == "" && (!isHEAD || len(p) > 0) {
  		w.contentLength = int64(len(p))
  		setHeader.contentLength = strconv.AppendInt(cw.res.clenBuf[:0], int64(len(p)), 10)
  	}
  
  	// If this was an HTTP/1.0 request with keep-alive and we sent a
  	// Content-Length back, we can make this a keep-alive response ...
  	if w.wants10KeepAlive && keepAlivesEnabled {
  		sentLength := header.get("Content-Length") != ""
  		if sentLength && header.get("Connection") == "keep-alive" {
  			w.closeAfterReply = false
  		}
  	}
  
  	// Check for a explicit (and valid) Content-Length header.
  	hasCL := w.contentLength != -1
  
  	if w.wants10KeepAlive && (isHEAD || hasCL || !bodyAllowedForStatus(w.status)) {
  		_, connectionHeaderSet := header["Connection"]
  		if !connectionHeaderSet {
  			setHeader.connection = "keep-alive"
  		}
  	} else if !w.req.ProtoAtLeast(1, 1) || w.wantsClose {
  		w.closeAfterReply = true
  	}
  
  	if header.get("Connection") == "close" || !keepAlivesEnabled {
  		w.closeAfterReply = true
  	}
  
  	// If the client wanted a 100-continue but we never sent it to
  	// them (or, more strictly: we never finished reading their
  	// request body), don't reuse this connection because it's now
  	// in an unknown state: we might be sending this response at
  	// the same time the client is now sending its request body
  	// after a timeout.  (Some HTTP clients send Expect:
  	// 100-continue but knowing that some servers don't support
  	// it, the clients set a timer and send the body later anyway)
  	// If we haven't seen EOF, we can't skip over the unread body
  	// because we don't know if the next bytes on the wire will be
  	// the body-following-the-timer or the subsequent request.
  	// See Issue 11549.
  	if ecr, ok := w.req.Body.(*expectContinueReader); ok && !ecr.sawEOF {
  		w.closeAfterReply = true
  	}
  
  	// Per RFC 2616, we should consume the request body before
  	// replying, if the handler hasn't already done so. But we
  	// don't want to do an unbounded amount of reading here for
  	// DoS reasons, so we only try up to a threshold.
  	// TODO(bradfitz): where does RFC 2616 say that? See Issue 15527
  	// about HTTP/1.x Handlers concurrently reading and writing, like
  	// HTTP/2 handlers can do. Maybe this code should be relaxed?
  	if w.req.ContentLength != 0 && !w.closeAfterReply {
  		var discard, tooBig bool
  
  		switch bdy := w.req.Body.(type) {
  		case *expectContinueReader:
  			if bdy.resp.wroteContinue {
  				discard = true
  			}
  		case *body:
  			bdy.mu.Lock()
  			switch {
  			case bdy.closed:
  				if !bdy.sawEOF {
  					// Body was closed in handler with non-EOF error.
  					w.closeAfterReply = true
  				}
  			case bdy.unreadDataSizeLocked() >= maxPostHandlerReadBytes:
  				tooBig = true
  			default:
  				discard = true
  			}
  			bdy.mu.Unlock()
  		default:
  			discard = true
  		}
  
  		if discard {
  			_, err := io.CopyN(ioutil.Discard, w.reqBody, maxPostHandlerReadBytes+1)
  			switch err {
  			case nil:
  				// There must be even more data left over.
  				tooBig = true
  			case ErrBodyReadAfterClose:
  				// Body was already consumed and closed.
  			case io.EOF:
  				// The remaining body was just consumed, close it.
  				err = w.reqBody.Close()
  				if err != nil {
  					w.closeAfterReply = true
  				}
  			default:
  				// Some other kind of error occurred, like a read timeout, or
  				// corrupt chunked encoding. In any case, whatever remains
  				// on the wire must not be parsed as another HTTP request.
  				w.closeAfterReply = true
  			}
  		}
  
  		if tooBig {
  			w.requestTooLarge()
  			delHeader("Connection")
  			setHeader.connection = "close"
  		}
  	}
  
  	code := w.status
  	if bodyAllowedForStatus(code) {
  		// If no content type, apply sniffing algorithm to body.
  		_, haveType := header["Content-Type"]
  		if !haveType && !hasTE {
  			setHeader.contentType = DetectContentType(p)
  		}
  	} else {
  		for _, k := range suppressedHeaders(code) {
  			delHeader(k)
  		}
  	}
  
  	if _, ok := header["Date"]; !ok {
  		setHeader.date = appendTime(cw.res.dateBuf[:0], time.Now())
  	}
  
  	if hasCL && hasTE && te != "identity" {
  		// TODO: return an error if WriteHeader gets a return parameter
  		// For now just ignore the Content-Length.
  		w.conn.server.logf("http: WriteHeader called with both Transfer-Encoding of %q and a Content-Length of %d",
  			te, w.contentLength)
  		delHeader("Content-Length")
  		hasCL = false
  	}
  
  	if w.req.Method == "HEAD" || !bodyAllowedForStatus(code) {
  		// do nothing
  	} else if code == StatusNoContent {
  		delHeader("Transfer-Encoding")
  	} else if hasCL {
  		delHeader("Transfer-Encoding")
  	} else if w.req.ProtoAtLeast(1, 1) {
  		// HTTP/1.1 or greater: Transfer-Encoding has been set to identity,  and no
  		// content-length has been provided. The connection must be closed after the
  		// reply is written, and no chunking is to be done. This is the setup
  		// recommended in the Server-Sent Events candidate recommendation 11,
  		// section 8.
  		if hasTE && te == "identity" {
  			cw.chunking = false
  			w.closeAfterReply = true
  		} else {
  			// HTTP/1.1 or greater: use chunked transfer encoding
  			// to avoid closing the connection at EOF.
  			cw.chunking = true
  			setHeader.transferEncoding = "chunked"
  			if hasTE && te == "chunked" {
  				// We will send the chunked Transfer-Encoding header later.
  				delHeader("Transfer-Encoding")
  			}
  		}
  	} else {
  		// HTTP version < 1.1: cannot do chunked transfer
  		// encoding and we don't know the Content-Length so
  		// signal EOF by closing connection.
  		w.closeAfterReply = true
  		delHeader("Transfer-Encoding") // in case already set
  	}
  
  	// Cannot use Content-Length with non-identity Transfer-Encoding.
  	if cw.chunking {
  		delHeader("Content-Length")
  	}
  	if !w.req.ProtoAtLeast(1, 0) {
  		return
  	}
  
  	if w.closeAfterReply && (!keepAlivesEnabled || !hasToken(cw.header.get("Connection"), "close")) {
  		delHeader("Connection")
  		if w.req.ProtoAtLeast(1, 1) {
  			setHeader.connection = "close"
  		}
  	}
  
  	w.conn.bufw.WriteString(statusLine(w.req, code))
  	cw.header.WriteSubset(w.conn.bufw, excludeHeader)
  	setHeader.Write(w.conn.bufw)
  	w.conn.bufw.Write(crlf)
  }
  
  // foreachHeaderElement splits v according to the "#rule" construction
  // in RFC 2616 section 2.1 and calls fn for each non-empty element.
  func foreachHeaderElement(v string, fn func(string)) {
  	v = textproto.TrimString(v)
  	if v == "" {
  		return
  	}
  	if !strings.Contains(v, ",") {
  		fn(v)
  		return
  	}
  	for _, f := range strings.Split(v, ",") {
  		if f = textproto.TrimString(f); f != "" {
  			fn(f)
  		}
  	}
  }
  
  // statusLines is a cache of Status-Line strings, keyed by code (for
  // HTTP/1.1) or negative code (for HTTP/1.0). This is faster than a
  // map keyed by struct of two fields. This map's max size is bounded
  // by 2*len(statusText), two protocol types for each known official
  // status code in the statusText map.
  var (
  	statusMu    sync.RWMutex
  	statusLines = make(map[int]string)
  )
  
  // statusLine returns a response Status-Line (RFC 2616 Section 6.1)
  // for the given request and response status code.
  func statusLine(req *Request, code int) string {
  	// Fast path:
  	key := code
  	proto11 := req.ProtoAtLeast(1, 1)
  	if !proto11 {
  		key = -key
  	}
  	statusMu.RLock()
  	line, ok := statusLines[key]
  	statusMu.RUnlock()
  	if ok {
  		return line
  	}
  
  	// Slow path:
  	proto := "HTTP/1.0"
  	if proto11 {
  		proto = "HTTP/1.1"
  	}
  	codestring := fmt.Sprintf("%03d", code)
  	text, ok := statusText[code]
  	if !ok {
  		text = "status code " + codestring
  	}
  	line = proto + " " + codestring + " " + text + "\r\n"
  	if ok {
  		statusMu.Lock()
  		defer statusMu.Unlock()
  		statusLines[key] = line
  	}
  	return line
  }
  
  // bodyAllowed reports whether a Write is allowed for this response type.
  // It's illegal to call this before the header has been flushed.
  func (w *response) bodyAllowed() bool {
  	if !w.wroteHeader {
  		panic("")
  	}
  	return bodyAllowedForStatus(w.status)
  }
  
  // The Life Of A Write is like this:
  //
  // Handler starts. No header has been sent. The handler can either
  // write a header, or just start writing. Writing before sending a header
  // sends an implicitly empty 200 OK header.
  //
  // If the handler didn't declare a Content-Length up front, we either
  // go into chunking mode or, if the handler finishes running before
  // the chunking buffer size, we compute a Content-Length and send that
  // in the header instead.
  //
  // Likewise, if the handler didn't set a Content-Type, we sniff that
  // from the initial chunk of output.
  //
  // The Writers are wired together like:
  //
  // 1. *response (the ResponseWriter) ->
  // 2. (*response).w, a *bufio.Writer of bufferBeforeChunkingSize bytes
  // 3. chunkWriter.Writer (whose writeHeader finalizes Content-Length/Type)
  //    and which writes the chunk headers, if needed.
  // 4. conn.buf, a bufio.Writer of default (4kB) bytes, writing to ->
  // 5. checkConnErrorWriter{c}, which notes any non-nil error on Write
  //    and populates c.werr with it if so. but otherwise writes to:
  // 6. the rwc, the net.Conn.
  //
  // TODO(bradfitz): short-circuit some of the buffering when the
  // initial header contains both a Content-Type and Content-Length.
  // Also short-circuit in (1) when the header's been sent and not in
  // chunking mode, writing directly to (4) instead, if (2) has no
  // buffered data. More generally, we could short-circuit from (1) to
  // (3) even in chunking mode if the write size from (1) is over some
  // threshold and nothing is in (2).  The answer might be mostly making
  // bufferBeforeChunkingSize smaller and having bufio's fast-paths deal
  // with this instead.
  func (w *response) Write(data []byte) (n int, err error) {
  	return w.write(len(data), data, "")
  }
  
  func (w *response) WriteString(data string) (n int, err error) {
  	return w.write(len(data), nil, data)
  }
  
  // either dataB or dataS is non-zero.
  func (w *response) write(lenData int, dataB []byte, dataS string) (n int, err error) {
  	if w.conn.hijacked() {
  		if lenData > 0 {
  			w.conn.server.logf("http: response.Write on hijacked connection")
  		}
  		return 0, ErrHijacked
  	}
  	if !w.wroteHeader {
  		w.WriteHeader(StatusOK)
  	}
  	if lenData == 0 {
  		return 0, nil
  	}
  	if !w.bodyAllowed() {
  		return 0, ErrBodyNotAllowed
  	}
  
  	w.written += int64(lenData) // ignoring errors, for errorKludge
  	if w.contentLength != -1 && w.written > w.contentLength {
  		return 0, ErrContentLength
  	}
  	if dataB != nil {
  		return w.w.Write(dataB)
  	} else {
  		return w.w.WriteString(dataS)
  	}
  }
  
  func (w *response) finishRequest() {
  	w.handlerDone.setTrue()
  
  	if !w.wroteHeader {
  		w.WriteHeader(StatusOK)
  	}
  
  	w.w.Flush()
  	putBufioWriter(w.w)
  	w.cw.close()
  	w.conn.bufw.Flush()
  
  	w.conn.r.abortPendingRead()
  
  	// Close the body (regardless of w.closeAfterReply) so we can
  	// re-use its bufio.Reader later safely.
  	w.reqBody.Close()
  
  	if w.req.MultipartForm != nil {
  		w.req.MultipartForm.RemoveAll()
  	}
  }
  
  // shouldReuseConnection reports whether the underlying TCP connection can be reused.
  // It must only be called after the handler is done executing.
  func (w *response) shouldReuseConnection() bool {
  	if w.closeAfterReply {
  		// The request or something set while executing the
  		// handler indicated we shouldn't reuse this
  		// connection.
  		return false
  	}
  
  	if w.req.Method != "HEAD" && w.contentLength != -1 && w.bodyAllowed() && w.contentLength != w.written {
  		// Did not write enough. Avoid getting out of sync.
  		return false
  	}
  
  	// There was some error writing to the underlying connection
  	// during the request, so don't re-use this conn.
  	if w.conn.werr != nil {
  		return false
  	}
  
  	if w.closedRequestBodyEarly() {
  		return false
  	}
  
  	return true
  }
  
  func (w *response) closedRequestBodyEarly() bool {
  	body, ok := w.req.Body.(*body)
  	return ok && body.didEarlyClose()
  }
  
  func (w *response) Flush() {
  	if !w.wroteHeader {
  		w.WriteHeader(StatusOK)
  	}
  	w.w.Flush()
  	w.cw.flush()
  }
  
  func (c *conn) finalFlush() {
  	if c.bufr != nil {
  		// Steal the bufio.Reader (~4KB worth of memory) and its associated
  		// reader for a future connection.
  		putBufioReader(c.bufr)
  		c.bufr = nil
  	}
  
  	if c.bufw != nil {
  		c.bufw.Flush()
  		// Steal the bufio.Writer (~4KB worth of memory) and its associated
  		// writer for a future connection.
  		putBufioWriter(c.bufw)
  		c.bufw = nil
  	}
  }
  
  // Close the connection.
  func (c *conn) close() {
  	c.finalFlush()
  	c.rwc.Close()
  }
  
  // rstAvoidanceDelay is the amount of time we sleep after closing the
  // write side of a TCP connection before closing the entire socket.
  // By sleeping, we increase the chances that the client sees our FIN
  // and processes its final data before they process the subsequent RST
  // from closing a connection with known unread data.
  // This RST seems to occur mostly on BSD systems. (And Windows?)
  // This timeout is somewhat arbitrary (~latency around the planet).
  const rstAvoidanceDelay = 500 * time.Millisecond
  
  type closeWriter interface {
  	CloseWrite() error
  }
  
  var _ closeWriter = (*net.TCPConn)(nil)
  
  // closeWrite flushes any outstanding data and sends a FIN packet (if
  // client is connected via TCP), signalling that we're done. We then
  // pause for a bit, hoping the client processes it before any
  // subsequent RST.
  //
  // See https://golang.org/issue/3595
  func (c *conn) closeWriteAndWait() {
  	c.finalFlush()
  	if tcp, ok := c.rwc.(closeWriter); ok {
  		tcp.CloseWrite()
  	}
  	time.Sleep(rstAvoidanceDelay)
  }
  
  // validNPN reports whether the proto is not a blacklisted Next
  // Protocol Negotiation protocol. Empty and built-in protocol types
  // are blacklisted and can't be overridden with alternate
  // implementations.
  func validNPN(proto string) bool {
  	switch proto {
  	case "", "http/1.1", "http/1.0":
  		return false
  	}
  	return true
  }
  
  func (c *conn) setState(nc net.Conn, state ConnState) {
  	srv := c.server
  	switch state {
  	case StateNew:
  		srv.trackConn(c, true)
  	case StateHijacked, StateClosed:
  		srv.trackConn(c, false)
  	}
  	c.curState.Store(connStateInterface[state])
  	if hook := srv.ConnState; hook != nil {
  		hook(nc, state)
  	}
  }
  
  // connStateInterface is an array of the interface{} versions of
  // ConnState values, so we can use them in atomic.Values later without
  // paying the cost of shoving their integers in an interface{}.
  var connStateInterface = [...]interface{}{
  	StateNew:      StateNew,
  	StateActive:   StateActive,
  	StateIdle:     StateIdle,
  	StateHijacked: StateHijacked,
  	StateClosed:   StateClosed,
  }
  
  // badRequestError is a literal string (used by in the server in HTML,
  // unescaped) to tell the user why their request was bad. It should
  // be plain text without user info or other embedded errors.
  type badRequestError string
  
  func (e badRequestError) Error() string { return "Bad Request: " + string(e) }
  
  // ErrAbortHandler is a sentinel panic value to abort a handler.
  // While any panic from ServeHTTP aborts the response to the client,
  // panicking with ErrAbortHandler also suppresses logging of a stack
  // trace to the server's error log.
  var ErrAbortHandler = errors.New("net/http: abort Handler")
  
  // isCommonNetReadError reports whether err is a common error
  // encountered during reading a request off the network when the
  // client has gone away or had its read fail somehow. This is used to
  // determine which logs are interesting enough to log about.
  func isCommonNetReadError(err error) bool {
  	if err == io.EOF {
  		return true
  	}
  	if neterr, ok := err.(net.Error); ok && neterr.Timeout() {
  		return true
  	}
  	if oe, ok := err.(*net.OpError); ok && oe.Op == "read" {
  		return true
  	}
  	return false
  }
  
  // Serve a new connection.
  func (c *conn) serve(ctx context.Context) {
  	c.remoteAddr = c.rwc.RemoteAddr().String()
  	defer func() {
  		if err := recover(); err != nil && err != ErrAbortHandler {
  			const size = 64 << 10
  			buf := make([]byte, size)
  			buf = buf[:runtime.Stack(buf, false)]
  			c.server.logf("http: panic serving %v: %v\n%s", c.remoteAddr, err, buf)
  		}
  		if !c.hijacked() {
  			c.close()
  			c.setState(c.rwc, StateClosed)
  		}
  	}()
  
  	if tlsConn, ok := c.rwc.(*tls.Conn); ok {
  		if d := c.server.ReadTimeout; d != 0 {
  			c.rwc.SetReadDeadline(time.Now().Add(d))
  		}
  		if d := c.server.WriteTimeout; d != 0 {
  			c.rwc.SetWriteDeadline(time.Now().Add(d))
  		}
  		if err := tlsConn.Handshake(); err != nil {
  			c.server.logf("http: TLS handshake error from %s: %v", c.rwc.RemoteAddr(), err)
  			return
  		}
  		c.tlsState = new(tls.ConnectionState)
  		*c.tlsState = tlsConn.ConnectionState()
  		if proto := c.tlsState.NegotiatedProtocol; validNPN(proto) {
  			if fn := c.server.TLSNextProto[proto]; fn != nil {
  				h := initNPNRequest{tlsConn, serverHandler{c.server}}
  				fn(c.server, tlsConn, h)
  			}
  			return
  		}
  	}
  
  	// HTTP/1.x from here on.
  
  	ctx, cancelCtx := context.WithCancel(ctx)
  	c.cancelCtx = cancelCtx
  	defer cancelCtx()
  
  	c.r = &connReader{conn: c}
  	c.bufr = newBufioReader(c.r)
  	c.bufw = newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)
  
  	for {
  		w, err := c.readRequest(ctx)
  		if c.r.remain != c.server.initialReadLimitSize() {
  			// If we read any bytes off the wire, we're active.
  			c.setState(c.rwc, StateActive)
  		}
  		if err != nil {
  			const errorHeaders = "\r\nContent-Type: text/plain; charset=utf-8\r\nConnection: close\r\n\r\n"
  
  			if err == errTooLarge {
  				// Their HTTP client may or may not be
  				// able to read this if we're
  				// responding to them and hanging up
  				// while they're still writing their
  				// request. Undefined behavior.
  				const publicErr = "431 Request Header Fields Too Large"
  				fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
  				c.closeWriteAndWait()
  				return
  			}
  			if isCommonNetReadError(err) {
  				return // don't reply
  			}
  
  			publicErr := "400 Bad Request"
  			if v, ok := err.(badRequestError); ok {
  				publicErr = publicErr + ": " + string(v)
  			}
  
  			fmt.Fprintf(c.rwc, "HTTP/1.1 "+publicErr+errorHeaders+publicErr)
  			return
  		}
  
  		// Expect 100 Continue support
  		req := w.req
  		if req.expectsContinue() {
  			if req.ProtoAtLeast(1, 1) && req.ContentLength != 0 {
  				// Wrap the Body reader with one that replies on the connection
  				req.Body = &expectContinueReader{readCloser: req.Body, resp: w}
  			}
  		} else if req.Header.get("Expect") != "" {
  			w.sendExpectationFailed()
  			return
  		}
  
  		c.curReq.Store(w)
  
  		if requestBodyRemains(req.Body) {
  			registerOnHitEOF(req.Body, w.conn.r.startBackgroundRead)
  		} else {
  			if w.conn.bufr.Buffered() > 0 {
  				w.conn.r.closeNotifyFromPipelinedRequest()
  			}
  			w.conn.r.startBackgroundRead()
  		}
  
  		// HTTP cannot have multiple simultaneous active requests.[*]
  		// Until the server replies to this request, it can't read another,
  		// so we might as well run the handler in this goroutine.
  		// [*] Not strictly true: HTTP pipelining. We could let them all process
  		// in parallel even if their responses need to be serialized.
  		// But we're not going to implement HTTP pipelining because it
  		// was never deployed in the wild and the answer is HTTP/2.
  		serverHandler{c.server}.ServeHTTP(w, w.req)
  		w.cancelCtx()
  		if c.hijacked() {
  			return
  		}
  		w.finishRequest()
  		if !w.shouldReuseConnection() {
  			if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
  				c.closeWriteAndWait()
  			}
  			return
  		}
  		c.setState(c.rwc, StateIdle)
  		c.curReq.Store((*response)(nil))
  
  		if !w.conn.server.doKeepAlives() {
  			// We're in shutdown mode. We might've replied
  			// to the user without "Connection: close" and
  			// they might think they can send another
  			// request, but such is life with HTTP/1.1.
  			return
  		}
  
  		if d := c.server.idleTimeout(); d != 0 {
  			c.rwc.SetReadDeadline(time.Now().Add(d))
  			if _, err := c.bufr.Peek(4); err != nil {
  				return
  			}
  		}
  		c.rwc.SetReadDeadline(time.Time{})
  	}
  }
  
  func (w *response) sendExpectationFailed() {
  	// TODO(bradfitz): let ServeHTTP handlers handle
  	// requests with non-standard expectation[s]? Seems
  	// theoretical at best, and doesn't fit into the
  	// current ServeHTTP model anyway. We'd need to
  	// make the ResponseWriter an optional
  	// "ExpectReplier" interface or something.
  	//
  	// For now we'll just obey RFC 2616 14.20 which says
  	// "If a server receives a request containing an
  	// Expect field that includes an expectation-
  	// extension that it does not support, it MUST
  	// respond with a 417 (Expectation Failed) status."
  	w.Header().Set("Connection", "close")
  	w.WriteHeader(StatusExpectationFailed)
  	w.finishRequest()
  }
  
  // Hijack implements the Hijacker.Hijack method. Our response is both a ResponseWriter
  // and a Hijacker.
  func (w *response) Hijack() (rwc net.Conn, buf *bufio.ReadWriter, err error) {
  	if w.handlerDone.isSet() {
  		panic("net/http: Hijack called after ServeHTTP finished")
  	}
  	if w.wroteHeader {
  		w.cw.flush()
  	}
  
  	c := w.conn
  	c.mu.Lock()
  	defer c.mu.Unlock()
  
  	// Release the bufioWriter that writes to the chunk writer, it is not
  	// used after a connection has been hijacked.
  	rwc, buf, err = c.hijackLocked()
  	if err == nil {
  		putBufioWriter(w.w)
  		w.w = nil
  	}
  	return rwc, buf, err
  }
  
  func (w *response) CloseNotify() <-chan bool {
  	if w.handlerDone.isSet() {
  		panic("net/http: CloseNotify called after ServeHTTP finished")
  	}
  	return w.closeNotifyCh
  }
  
  func registerOnHitEOF(rc io.ReadCloser, fn func()) {
  	switch v := rc.(type) {
  	case *expectContinueReader:
  		registerOnHitEOF(v.readCloser, fn)
  	case *body:
  		v.registerOnHitEOF(fn)
  	default:
  		panic("unexpected type " + fmt.Sprintf("%T", rc))
  	}
  }
  
  // requestBodyRemains reports whether future calls to Read
  // on rc might yield more data.
  func requestBodyRemains(rc io.ReadCloser) bool {
  	if rc == NoBody {
  		return false
  	}
  	switch v := rc.(type) {
  	case *expectContinueReader:
  		return requestBodyRemains(v.readCloser)
  	case *body:
  		return v.bodyRemains()
  	default:
  		panic("unexpected type " + fmt.Sprintf("%T", rc))
  	}
  }
  
  // The HandlerFunc type is an adapter to allow the use of
  // ordinary functions as HTTP handlers. If f is a function
  // with the appropriate signature, HandlerFunc(f) is a
  // Handler that calls f.
  type HandlerFunc func(ResponseWriter, *Request)
  
  // ServeHTTP calls f(w, r).
  func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
  	f(w, r)
  }
  
  // Helper handlers
  
  // Error replies to the request with the specified error message and HTTP code.
  // It does not otherwise end the request; the caller should ensure no further
  // writes are done to w.
  // The error message should be plain text.
  func Error(w ResponseWriter, error string, code int) {
  	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
  	w.Header().Set("X-Content-Type-Options", "nosniff")
  	w.WriteHeader(code)
  	fmt.Fprintln(w, error)
  }
  
  // NotFound replies to the request with an HTTP 404 not found error.
  func NotFound(w ResponseWriter, r *Request) { Error(w, "404 page not found", StatusNotFound) }
  
  // NotFoundHandler returns a simple request handler
  // that replies to each request with a ``404 page not found'' reply.
  func NotFoundHandler() Handler { return HandlerFunc(NotFound) }
  
  // StripPrefix returns a handler that serves HTTP requests
  // by removing the given prefix from the request URL's Path
  // and invoking the handler h. StripPrefix handles a
  // request for a path that doesn't begin with prefix by
  // replying with an HTTP 404 not found error.
  func StripPrefix(prefix string, h Handler) Handler {
  	if prefix == "" {
  		return h
  	}
  	return HandlerFunc(func(w ResponseWriter, r *Request) {
  		if p := strings.TrimPrefix(r.URL.Path, prefix); len(p) < len(r.URL.Path) {
  			r.URL.Path = p
  			h.ServeHTTP(w, r)
  		} else {
  			NotFound(w, r)
  		}
  	})
  }
  
  // Redirect replies to the request with a redirect to url,
  // which may be a path relative to the request path.
  //
  // The provided code should be in the 3xx range and is usually
  // StatusMovedPermanently, StatusFound or StatusSeeOther.
  func Redirect(w ResponseWriter, r *Request, urlStr string, code int) {
  	if u, err := url.Parse(urlStr); err == nil {
  		// If url was relative, make absolute by
  		// combining with request path.
  		// The browser would probably do this for us,
  		// but doing it ourselves is more reliable.
  
  		// NOTE(rsc): RFC 2616 says that the Location
  		// line must be an absolute URI, like
  		// "http://www.google.com/redirect/",
  		// not a path like "/redirect/".
  		// Unfortunately, we don't know what to
  		// put in the host name section to get the
  		// client to connect to us again, so we can't
  		// know the right absolute URI to send back.
  		// Because of this problem, no one pays attention
  		// to the RFC; they all send back just a new path.
  		// So do we.
  		if u.Scheme == "" && u.Host == "" {
  			oldpath := r.URL.Path
  			if oldpath == "" { // should not happen, but avoid a crash if it does
  				oldpath = "/"
  			}
  
  			// no leading http://server
  			if urlStr == "" || urlStr[0] != '/' {
  				// make relative path absolute
  				olddir, _ := path.Split(oldpath)
  				urlStr = olddir + urlStr
  			}
  
  			var query string
  			if i := strings.Index(urlStr, "?"); i != -1 {
  				urlStr, query = urlStr[:i], urlStr[i:]
  			}
  
  			// clean up but preserve trailing slash
  			trailing := strings.HasSuffix(urlStr, "/")
  			urlStr = path.Clean(urlStr)
  			if trailing && !strings.HasSuffix(urlStr, "/") {
  				urlStr += "/"
  			}
  			urlStr += query
  		}
  	}
  
  	w.Header().Set("Location", hexEscapeNonASCII(urlStr))
  	w.WriteHeader(code)
  
  	// RFC 2616 recommends that a short note "SHOULD" be included in the
  	// response because older user agents may not understand 301/307.
  	// Shouldn't send the response for POST or HEAD; that leaves GET.
  	if r.Method == "GET" {
  		note := "<a href=\"" + htmlEscape(urlStr) + "\">" + statusText[code] + "</a>.\n"
  		fmt.Fprintln(w, note)
  	}
  }
  
  var htmlReplacer = strings.NewReplacer(
  	"&", "&amp;",
  	"<", "&lt;",
  	">", "&gt;",
  	// "&#34;" is shorter than "&quot;".
  	`"`, "&#34;",
  	// "&#39;" is shorter than "&apos;" and apos was not in HTML until HTML5.
  	"'", "&#39;",
  )
  
  func htmlEscape(s string) string {
  	return htmlReplacer.Replace(s)
  }
  
  // Redirect to a fixed URL
  type redirectHandler struct {
  	url  string
  	code int
  }
  
  func (rh *redirectHandler) ServeHTTP(w ResponseWriter, r *Request) {
  	Redirect(w, r, rh.url, rh.code)
  }
  
  // RedirectHandler returns a request handler that redirects
  // each request it receives to the given url using the given
  // status code.
  //
  // The provided code should be in the 3xx range and is usually
  // StatusMovedPermanently, StatusFound or StatusSeeOther.
  func RedirectHandler(url string, code int) Handler {
  	return &redirectHandler{url, code}
  }
  
  // ServeMux is an HTTP request multiplexer.
  // It matches the URL of each incoming request against a list of registered
  // patterns and calls the handler for the pattern that
  // most closely matches the URL.
  //
  // Patterns name fixed, rooted paths, like "/favicon.ico",
  // or rooted subtrees, like "/images/" (note the trailing slash).
  // Longer patterns take precedence over shorter ones, so that
  // if there are handlers registered for both "/images/"
  // and "/images/thumbnails/", the latter handler will be
  // called for paths beginning "/images/thumbnails/" and the
  // former will receive requests for any other paths in the
  // "/images/" subtree.
  //
  // Note that since a pattern ending in a slash names a rooted subtree,
  // the pattern "/" matches all paths not matched by other registered
  // patterns, not just the URL with Path == "/".
  //
  // If a subtree has been registered and a request is received naming the
  // subtree root without its trailing slash, ServeMux redirects that
  // request to the subtree root (adding the trailing slash). This behavior can
  // be overridden with a separate registration for the path without
  // the trailing slash. For example, registering "/images/" causes ServeMux
  // to redirect a request for "/images" to "/images/", unless "/images" has
  // been registered separately.
  //
  // Patterns may optionally begin with a host name, restricting matches to
  // URLs on that host only. Host-specific patterns take precedence over
  // general patterns, so that a handler might register for the two patterns
  // "/codesearch" and "codesearch.google.com/" without also taking over
  // requests for "http://www.google.com/".
  //
  // ServeMux also takes care of sanitizing the URL request path,
  // redirecting any request containing . or .. elements or repeated slashes
  // to an equivalent, cleaner URL.
  type ServeMux struct {
  	mu    sync.RWMutex
  	m     map[string]muxEntry
  	hosts bool // whether any patterns contain hostnames
  }
  
  type muxEntry struct {
  	explicit bool
  	h        Handler
  	pattern  string
  }
  
  // NewServeMux allocates and returns a new ServeMux.
  func NewServeMux() *ServeMux { return new(ServeMux) }
  
  // DefaultServeMux is the default ServeMux used by Serve.
  var DefaultServeMux = &defaultServeMux
  
  var defaultServeMux ServeMux
  
  // Does path match pattern?
  func pathMatch(pattern, path string) bool {
  	if len(pattern) == 0 {
  		// should not happen
  		return false
  	}
  	n := len(pattern)
  	if pattern[n-1] != '/' {
  		return pattern == path
  	}
  	return len(path) >= n && path[0:n] == pattern
  }
  
  // Return the canonical path for p, eliminating . and .. elements.
  func cleanPath(p string) string {
  	if p == "" {
  		return "/"
  	}
  	if p[0] != '/' {
  		p = "/" + p
  	}
  	np := path.Clean(p)
  	// path.Clean removes trailing slash except for root;
  	// put the trailing slash back if necessary.
  	if p[len(p)-1] == '/' && np != "/" {
  		np += "/"
  	}
  	return np
  }
  
  // Find a handler on a handler map given a path string
  // Most-specific (longest) pattern wins
  func (mux *ServeMux) match(path string) (h Handler, pattern string) {
  	var n = 0
  	for k, v := range mux.m {
  		if !pathMatch(k, path) {
  			continue
  		}
  		if h == nil || len(k) > n {
  			n = len(k)
  			h = v.h
  			pattern = v.pattern
  		}
  	}
  	return
  }
  
  // Handler returns the handler to use for the given request,
  // consulting r.Method, r.Host, and r.URL.Path. It always returns
  // a non-nil handler. If the path is not in its canonical form, the
  // handler will be an internally-generated handler that redirects
  // to the canonical path.
  //
  // Handler also returns the registered pattern that matches the
  // request or, in the case of internally-generated redirects,
  // the pattern that will match after following the redirect.
  //
  // If there is no registered handler that applies to the request,
  // Handler returns a ``page not found'' handler and an empty pattern.
  func (mux *ServeMux) Handler(r *Request) (h Handler, pattern string) {
  	if r.Method != "CONNECT" {
  		if p := cleanPath(r.URL.Path); p != r.URL.Path {
  			_, pattern = mux.handler(r.Host, p)
  			url := *r.URL
  			url.Path = p
  			return RedirectHandler(url.String(), StatusMovedPermanently), pattern
  		}
  	}
  
  	return mux.handler(r.Host, r.URL.Path)
  }
  
  // handler is the main implementation of Handler.
  // The path is known to be in canonical form, except for CONNECT methods.
  func (mux *ServeMux) handler(host, path string) (h Handler, pattern string) {
  	mux.mu.RLock()
  	defer mux.mu.RUnlock()
  
  	// Host-specific pattern takes precedence over generic ones
  	if mux.hosts {
  		h, pattern = mux.match(host + path)
  	}
  	if h == nil {
  		h, pattern = mux.match(path)
  	}
  	if h == nil {
  		h, pattern = NotFoundHandler(), ""
  	}
  	return
  }
  
  // ServeHTTP dispatches the request to the handler whose
  // pattern most closely matches the request URL.
  func (mux *ServeMux) ServeHTTP(w ResponseWriter, r *Request) {
  	if r.RequestURI == "*" {
  		if r.ProtoAtLeast(1, 1) {
  			w.Header().Set("Connection", "close")
  		}
  		w.WriteHeader(StatusBadRequest)
  		return
  	}
  	h, _ := mux.Handler(r)
  	h.ServeHTTP(w, r)
  }
  
  // Handle registers the handler for the given pattern.
  // If a handler already exists for pattern, Handle panics.
  func (mux *ServeMux) Handle(pattern string, handler Handler) {
  	mux.mu.Lock()
  	defer mux.mu.Unlock()
  
  	if pattern == "" {
  		panic("http: invalid pattern " + pattern)
  	}
  	if handler == nil {
  		panic("http: nil handler")
  	}
  	if mux.m[pattern].explicit {
  		panic("http: multiple registrations for " + pattern)
  	}
  
  	if mux.m == nil {
  		mux.m = make(map[string]muxEntry)
  	}
  	mux.m[pattern] = muxEntry{explicit: true, h: handler, pattern: pattern}
  
  	if pattern[0] != '/' {
  		mux.hosts = true
  	}
  
  	// Helpful behavior:
  	// If pattern is /tree/, insert an implicit permanent redirect for /tree.
  	// It can be overridden by an explicit registration.
  	n := len(pattern)
  	if n > 0 && pattern[n-1] == '/' && !mux.m[pattern[0:n-1]].explicit {
  		// If pattern contains a host name, strip it and use remaining
  		// path for redirect.
  		path := pattern
  		if pattern[0] != '/' {
  			// In pattern, at least the last character is a '/', so
  			// strings.Index can't be -1.
  			path = pattern[strings.Index(pattern, "/"):]
  		}
  		url := &url.URL{Path: path}
  		mux.m[pattern[0:n-1]] = muxEntry{h: RedirectHandler(url.String(), StatusMovedPermanently), pattern: pattern}
  	}
  }
  
  // HandleFunc registers the handler function for the given pattern.
  func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
  	mux.Handle(pattern, HandlerFunc(handler))
  }
  
  // Handle registers the handler for the given pattern
  // in the DefaultServeMux.
  // The documentation for ServeMux explains how patterns are matched.
  func Handle(pattern string, handler Handler) { DefaultServeMux.Handle(pattern, handler) }
  
  // HandleFunc registers the handler function for the given pattern
  // in the DefaultServeMux.
  // The documentation for ServeMux explains how patterns are matched.
  func HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
  	DefaultServeMux.HandleFunc(pattern, handler)
  }
  
  // Serve accepts incoming HTTP connections on the listener l,
  // creating a new service goroutine for each. The service goroutines
  // read requests and then call handler to reply to them.
  // Handler is typically nil, in which case the DefaultServeMux is used.
  func Serve(l net.Listener, handler Handler) error {
  	srv := &Server{Handler: handler}
  	return srv.Serve(l)
  }
  
  // A Server defines parameters for running an HTTP server.
  // The zero value for Server is a valid configuration.
  type Server struct {
  	Addr      string      // TCP address to listen on, ":http" if empty
  	Handler   Handler     // handler to invoke, http.DefaultServeMux if nil
  	TLSConfig *tls.Config // optional TLS config, used by ListenAndServeTLS
  
  	// ReadTimeout is the maximum duration for reading the entire
  	// request, including the body.
  	//
  	// Because ReadTimeout does not let Handlers make per-request
  	// decisions on each request body's acceptable deadline or
  	// upload rate, most users will prefer to use
  	// ReadHeaderTimeout. It is valid to use them both.
  	ReadTimeout time.Duration
  
  	// ReadHeaderTimeout is the amount of time allowed to read
  	// request headers. The connection's read deadline is reset
  	// after reading the headers and the Handler can decide what
  	// is considered too slow for the body.
  	ReadHeaderTimeout time.Duration
  
  	// WriteTimeout is the maximum duration before timing out
  	// writes of the response. It is reset whenever a new
  	// request's header is read. Like ReadTimeout, it does not
  	// let Handlers make decisions on a per-request basis.
  	WriteTimeout time.Duration
  
  	// IdleTimeout is the maximum amount of time to wait for the
  	// next request when keep-alives are enabled. If IdleTimeout
  	// is zero, the value of ReadTimeout is used. If both are
  	// zero, there is no timeout.
  	IdleTimeout time.Duration
  
  	// MaxHeaderBytes controls the maximum number of bytes the
  	// server will read parsing the request header's keys and
  	// values, including the request line. It does not limit the
  	// size of the request body.
  	// If zero, DefaultMaxHeaderBytes is used.
  	MaxHeaderBytes int
  
  	// TLSNextProto optionally specifies a function to take over
  	// ownership of the provided TLS connection when an NPN/ALPN
  	// protocol upgrade has occurred. The map key is the protocol
  	// name negotiated. The Handler argument should be used to
  	// handle HTTP requests and will initialize the Request's TLS
  	// and RemoteAddr if not already set. The connection is
  	// automatically closed when the function returns.
  	// If TLSNextProto is not nil, HTTP/2 support is not enabled
  	// automatically.
  	TLSNextProto map[string]func(*Server, *tls.Conn, Handler)
  
  	// ConnState specifies an optional callback function that is
  	// called when a client connection changes state. See the
  	// ConnState type and associated constants for details.
  	ConnState func(net.Conn, ConnState)
  
  	// ErrorLog specifies an optional logger for errors accepting
  	// connections and unexpected behavior from handlers.
  	// If nil, logging goes to os.Stderr via the log package's
  	// standard logger.
  	ErrorLog *log.Logger
  
  	disableKeepAlives int32     // accessed atomically.
  	inShutdown        int32     // accessed atomically (non-zero means we're in Shutdown)
  	nextProtoOnce     sync.Once // guards setupHTTP2_* init
  	nextProtoErr      error     // result of http2.ConfigureServer if used
  
  	mu         sync.Mutex
  	listeners  map[net.Listener]struct{}
  	activeConn map[*conn]struct{}
  	doneChan   chan struct{}
  }
  
  func (s *Server) getDoneChan() <-chan struct{} {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	return s.getDoneChanLocked()
  }
  
  func (s *Server) getDoneChanLocked() chan struct{} {
  	if s.doneChan == nil {
  		s.doneChan = make(chan struct{})
  	}
  	return s.doneChan
  }
  
  func (s *Server) closeDoneChanLocked() {
  	ch := s.getDoneChanLocked()
  	select {
  	case <-ch:
  		// Already closed. Don't close again.
  	default:
  		// Safe to close here. We're the only closer, guarded
  		// by s.mu.
  		close(ch)
  	}
  }
  
  // Close immediately closes all active net.Listeners and any
  // connections in state StateNew, StateActive, or StateIdle. For a
  // graceful shutdown, use Shutdown.
  //
  // Close does not attempt to close (and does not even know about)
  // any hijacked connections, such as WebSockets.
  //
  // Close returns any error returned from closing the Server's
  // underlying Listener(s).
  func (srv *Server) Close() error {
  	srv.mu.Lock()
  	defer srv.mu.Unlock()
  	srv.closeDoneChanLocked()
  	err := srv.closeListenersLocked()
  	for c := range srv.activeConn {
  		c.rwc.Close()
  		delete(srv.activeConn, c)
  	}
  	return err
  }
  
  // shutdownPollInterval is how often we poll for quiescence
  // during Server.Shutdown. This is lower during tests, to
  // speed up tests.
  // Ideally we could find a solution that doesn't involve polling,
  // but which also doesn't have a high runtime cost (and doesn't
  // involve any contentious mutexes), but that is left as an
  // exercise for the reader.
  var shutdownPollInterval = 500 * time.Millisecond
  
  // Shutdown gracefully shuts down the server without interrupting any
  // active connections. Shutdown works by first closing all open
  // listeners, then closing all idle connections, and then waiting
  // indefinitely for connections to return to idle and then shut down.
  // If the provided context expires before the shutdown is complete,
  // then the context's error is returned.
  //
  // Shutdown does not attempt to close nor wait for hijacked
  // connections such as WebSockets. The caller of Shutdown should
  // separately notify such long-lived connections of shutdown and wait
  // for them to close, if desired.
  func (srv *Server) Shutdown(ctx context.Context) error {
  	atomic.AddInt32(&srv.inShutdown, 1)
  	defer atomic.AddInt32(&srv.inShutdown, -1)
  
  	srv.mu.Lock()
  	lnerr := srv.closeListenersLocked()
  	srv.closeDoneChanLocked()
  	srv.mu.Unlock()
  
  	ticker := time.NewTicker(shutdownPollInterval)
  	defer ticker.Stop()
  	for {
  		if srv.closeIdleConns() {
  			return lnerr
  		}
  		select {
  		case <-ctx.Done():
  			return ctx.Err()
  		case <-ticker.C:
  		}
  	}
  }
  
  // closeIdleConns closes all idle connections and reports whether the
  // server is quiescent.
  func (s *Server) closeIdleConns() bool {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	quiescent := true
  	for c := range s.activeConn {
  		st, ok := c.curState.Load().(ConnState)
  		if !ok || st != StateIdle {
  			quiescent = false
  			continue
  		}
  		c.rwc.Close()
  		delete(s.activeConn, c)
  	}
  	return quiescent
  }
  
  func (s *Server) closeListenersLocked() error {
  	var err error
  	for ln := range s.listeners {
  		if cerr := ln.Close(); cerr != nil && err == nil {
  			err = cerr
  		}
  		delete(s.listeners, ln)
  	}
  	return err
  }
  
  // A ConnState represents the state of a client connection to a server.
  // It's used by the optional Server.ConnState hook.
  type ConnState int
  
  const (
  	// StateNew represents a new connection that is expected to
  	// send a request immediately. Connections begin at this
  	// state and then transition to either StateActive or
  	// StateClosed.
  	StateNew ConnState = iota
  
  	// StateActive represents a connection that has read 1 or more
  	// bytes of a request. The Server.ConnState hook for
  	// StateActive fires before the request has entered a handler
  	// and doesn't fire again until the request has been
  	// handled. After the request is handled, the state
  	// transitions to StateClosed, StateHijacked, or StateIdle.
  	// For HTTP/2, StateActive fires on the transition from zero
  	// to one active request, and only transitions away once all
  	// active requests are complete. That means that ConnState
  	// cannot be used to do per-request work; ConnState only notes
  	// the overall state of the connection.
  	StateActive
  
  	// StateIdle represents a connection that has finished
  	// handling a request and is in the keep-alive state, waiting
  	// for a new request. Connections transition from StateIdle
  	// to either StateActive or StateClosed.
  	StateIdle
  
  	// StateHijacked represents a hijacked connection.
  	// This is a terminal state. It does not transition to StateClosed.
  	StateHijacked
  
  	// StateClosed represents a closed connection.
  	// This is a terminal state. Hijacked connections do not
  	// transition to StateClosed.
  	StateClosed
  )
  
  var stateName = map[ConnState]string{
  	StateNew:      "new",
  	StateActive:   "active",
  	StateIdle:     "idle",
  	StateHijacked: "hijacked",
  	StateClosed:   "closed",
  }
  
  func (c ConnState) String() string {
  	return stateName[c]
  }
  
  // serverHandler delegates to either the server's Handler or
  // DefaultServeMux and also handles "OPTIONS *" requests.
  type serverHandler struct {
  	srv *Server
  }
  
  func (sh serverHandler) ServeHTTP(rw ResponseWriter, req *Request) {
  	handler := sh.srv.Handler
  	if handler == nil {
  		handler = DefaultServeMux
  	}
  	if req.RequestURI == "*" && req.Method == "OPTIONS" {
  		handler = globalOptionsHandler{}
  	}
  	handler.ServeHTTP(rw, req)
  }
  
  // ListenAndServe listens on the TCP network address srv.Addr and then
  // calls Serve to handle requests on incoming connections.
  // Accepted connections are configured to enable TCP keep-alives.
  // If srv.Addr is blank, ":http" is used.
  // ListenAndServe always returns a non-nil error.
  func (srv *Server) ListenAndServe() error {
  	addr := srv.Addr
  	if addr == "" {
  		addr = ":http"
  	}
  	ln, err := net.Listen("tcp", addr)
  	if err != nil {
  		return err
  	}
  	return srv.Serve(tcpKeepAliveListener{ln.(*net.TCPListener)})
  }
  
  var testHookServerServe func(*Server, net.Listener) // used if non-nil
  
  // shouldDoServeHTTP2 reports whether Server.Serve should configure
  // automatic HTTP/2. (which sets up the srv.TLSNextProto map)
  func (srv *Server) shouldConfigureHTTP2ForServe() bool {
  	if srv.TLSConfig == nil {
  		// Compatibility with Go 1.6:
  		// If there's no TLSConfig, it's possible that the user just
  		// didn't set it on the http.Server, but did pass it to
  		// tls.NewListener and passed that listener to Serve.
  		// So we should configure HTTP/2 (to set up srv.TLSNextProto)
  		// in case the listener returns an "h2" *tls.Conn.
  		return true
  	}
  	// The user specified a TLSConfig on their http.Server.
  	// In this, case, only configure HTTP/2 if their tls.Config
  	// explicitly mentions "h2". Otherwise http2.ConfigureServer
  	// would modify the tls.Config to add it, but they probably already
  	// passed this tls.Config to tls.NewListener. And if they did,
  	// it's too late anyway to fix it. It would only be potentially racy.
  	// See Issue 15908.
  	return strSliceContains(srv.TLSConfig.NextProtos, http2NextProtoTLS)
  }
  
  var ErrServerClosed = errors.New("http: Server closed")
  
  // Serve accepts incoming connections on the Listener l, creating a
  // new service goroutine for each. The service goroutines read requests and
  // then call srv.Handler to reply to them.
  //
  // For HTTP/2 support, srv.TLSConfig should be initialized to the
  // provided listener's TLS Config before calling Serve. If
  // srv.TLSConfig is non-nil and doesn't include the string "h2" in
  // Config.NextProtos, HTTP/2 support is not enabled.
  //
  // Serve always returns a non-nil error. After Shutdown or Close, the
  // returned error is ErrServerClosed.
  func (srv *Server) Serve(l net.Listener) error {
  	defer l.Close()
  	if fn := testHookServerServe; fn != nil {
  		fn(srv, l)
  	}
  	var tempDelay time.Duration // how long to sleep on accept failure
  
  	if err := srv.setupHTTP2_Serve(); err != nil {
  		return err
  	}
  
  	srv.trackListener(l, true)
  	defer srv.trackListener(l, false)
  
  	baseCtx := context.Background() // base is always background, per Issue 16220
  	ctx := context.WithValue(baseCtx, ServerContextKey, srv)
  	ctx = context.WithValue(ctx, LocalAddrContextKey, l.Addr())
  	for {
  		rw, e := l.Accept()
  		if e != nil {
  			select {
  			case <-srv.getDoneChan():
  				return ErrServerClosed
  			default:
  			}
  			if ne, ok := e.(net.Error); ok && ne.Temporary() {
  				if tempDelay == 0 {
  					tempDelay = 5 * time.Millisecond
  				} else {
  					tempDelay *= 2
  				}
  				if max := 1 * time.Second; tempDelay > max {
  					tempDelay = max
  				}
  				srv.logf("http: Accept error: %v; retrying in %v", e, tempDelay)
  				time.Sleep(tempDelay)
  				continue
  			}
  			return e
  		}
  		tempDelay = 0
  		c := srv.newConn(rw)
  		c.setState(c.rwc, StateNew) // before Serve can return
  		go c.serve(ctx)
  	}
  }
  
  func (s *Server) trackListener(ln net.Listener, add bool) {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	if s.listeners == nil {
  		s.listeners = make(map[net.Listener]struct{})
  	}
  	if add {
  		// If the *Server is being reused after a previous
  		// Close or Shutdown, reset its doneChan:
  		if len(s.listeners) == 0 && len(s.activeConn) == 0 {
  			s.doneChan = nil
  		}
  		s.listeners[ln] = struct{}{}
  	} else {
  		delete(s.listeners, ln)
  	}
  }
  
  func (s *Server) trackConn(c *conn, add bool) {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	if s.activeConn == nil {
  		s.activeConn = make(map[*conn]struct{})
  	}
  	if add {
  		s.activeConn[c] = struct{}{}
  	} else {
  		delete(s.activeConn, c)
  	}
  }
  
  func (s *Server) idleTimeout() time.Duration {
  	if s.IdleTimeout != 0 {
  		return s.IdleTimeout
  	}
  	return s.ReadTimeout
  }
  
  func (s *Server) readHeaderTimeout() time.Duration {
  	if s.ReadHeaderTimeout != 0 {
  		return s.ReadHeaderTimeout
  	}
  	return s.ReadTimeout
  }
  
  func (s *Server) doKeepAlives() bool {
  	return atomic.LoadInt32(&s.disableKeepAlives) == 0 && !s.shuttingDown()
  }
  
  func (s *Server) shuttingDown() bool {
  	return atomic.LoadInt32(&s.inShutdown) != 0
  }
  
  // SetKeepAlivesEnabled controls whether HTTP keep-alives are enabled.
  // By default, keep-alives are always enabled. Only very
  // resource-constrained environments or servers in the process of
  // shutting down should disable them.
  func (srv *Server) SetKeepAlivesEnabled(v bool) {
  	if v {
  		atomic.StoreInt32(&srv.disableKeepAlives, 0)
  		return
  	}
  	atomic.StoreInt32(&srv.disableKeepAlives, 1)
  
  	// Close idle HTTP/1 conns:
  	srv.closeIdleConns()
  
  	// Close HTTP/2 conns, as soon as they become idle, but reset
  	// the chan so future conns (if the listener is still active)
  	// still work and don't get a GOAWAY immediately, before their
  	// first request:
  	srv.mu.Lock()
  	defer srv.mu.Unlock()
  	srv.closeDoneChanLocked() // closes http2 conns
  	srv.doneChan = nil
  }
  
  func (s *Server) logf(format string, args ...interface{}) {
  	if s.ErrorLog != nil {
  		s.ErrorLog.Printf(format, args...)
  	} else {
  		log.Printf(format, args...)
  	}
  }
  
  // ListenAndServe listens on the TCP network address addr
  // and then calls Serve with handler to handle requests
  // on incoming connections.
  // Accepted connections are configured to enable TCP keep-alives.
  // Handler is typically nil, in which case the DefaultServeMux is
  // used.
  //
  // A trivial example server is:
  //
  //	package main
  //
  //	import (
  //		"io"
  //		"net/http"
  //		"log"
  //	)
  //
  //	// hello world, the web server
  //	func HelloServer(w http.ResponseWriter, req *http.Request) {
  //		io.WriteString(w, "hello, world!\n")
  //	}
  //
  //	func main() {
  //		http.HandleFunc("/hello", HelloServer)
  //		log.Fatal(http.ListenAndServe(":12345", nil))
  //	}
  //
  // ListenAndServe always returns a non-nil error.
  func ListenAndServe(addr string, handler Handler) error {
  	server := &Server{Addr: addr, Handler: handler}
  	return server.ListenAndServe()
  }
  
  // ListenAndServeTLS acts identically to ListenAndServe, except that it
  // expects HTTPS connections. Additionally, files containing a certificate and
  // matching private key for the server must be provided. If the certificate
  // is signed by a certificate authority, the certFile should be the concatenation
  // of the server's certificate, any intermediates, and the CA's certificate.
  //
  // A trivial example server is:
  //
  //	import (
  //		"log"
  //		"net/http"
  //	)
  //
  //	func handler(w http.ResponseWriter, req *http.Request) {
  //		w.Header().Set("Content-Type", "text/plain")
  //		w.Write([]byte("This is an example server.\n"))
  //	}
  //
  //	func main() {
  //		http.HandleFunc("/", handler)
  //		log.Printf("About to listen on 10443. Go to https://127.0.0.1:10443/")
  //		err := http.ListenAndServeTLS(":10443", "cert.pem", "key.pem", nil)
  //		log.Fatal(err)
  //	}
  //
  // One can use generate_cert.go in crypto/tls to generate cert.pem and key.pem.
  //
  // ListenAndServeTLS always returns a non-nil error.
  func ListenAndServeTLS(addr, certFile, keyFile string, handler Handler) error {
  	server := &Server{Addr: addr, Handler: handler}
  	return server.ListenAndServeTLS(certFile, keyFile)
  }
  
  // ListenAndServeTLS listens on the TCP network address srv.Addr and
  // then calls Serve to handle requests on incoming TLS connections.
  // Accepted connections are configured to enable TCP keep-alives.
  //
  // Filenames containing a certificate and matching private key for the
  // server must be provided if neither the Server's TLSConfig.Certificates
  // nor TLSConfig.GetCertificate are populated. If the certificate is
  // signed by a certificate authority, the certFile should be the
  // concatenation of the server's certificate, any intermediates, and
  // the CA's certificate.
  //
  // If srv.Addr is blank, ":https" is used.
  //
  // ListenAndServeTLS always returns a non-nil error.
  func (srv *Server) ListenAndServeTLS(certFile, keyFile string) error {
  	addr := srv.Addr
  	if addr == "" {
  		addr = ":https"
  	}
  
  	// Setup HTTP/2 before srv.Serve, to initialize srv.TLSConfig
  	// before we clone it and create the TLS Listener.
  	if err := srv.setupHTTP2_ListenAndServeTLS(); err != nil {
  		return err
  	}
  
  	config := cloneTLSConfig(srv.TLSConfig)
  	if !strSliceContains(config.NextProtos, "http/1.1") {
  		config.NextProtos = append(config.NextProtos, "http/1.1")
  	}
  
  	configHasCert := len(config.Certificates) > 0 || config.GetCertificate != nil
  	if !configHasCert || certFile != "" || keyFile != "" {
  		var err error
  		config.Certificates = make([]tls.Certificate, 1)
  		config.Certificates[0], err = tls.LoadX509KeyPair(certFile, keyFile)
  		if err != nil {
  			return err
  		}
  	}
  
  	ln, err := net.Listen("tcp", addr)
  	if err != nil {
  		return err
  	}
  
  	tlsListener := tls.NewListener(tcpKeepAliveListener{ln.(*net.TCPListener)}, config)
  	return srv.Serve(tlsListener)
  }
  
  // setupHTTP2_ListenAndServeTLS conditionally configures HTTP/2 on
  // srv and returns whether there was an error setting it up. If it is
  // not configured for policy reasons, nil is returned.
  func (srv *Server) setupHTTP2_ListenAndServeTLS() error {
  	srv.nextProtoOnce.Do(srv.onceSetNextProtoDefaults)
  	return srv.nextProtoErr
  }
  
  // setupHTTP2_Serve is called from (*Server).Serve and conditionally
  // configures HTTP/2 on srv using a more conservative policy than
  // setupHTTP2_ListenAndServeTLS because Serve may be called
  // concurrently.
  //
  // The tests named TestTransportAutomaticHTTP2* and
  // TestConcurrentServerServe in server_test.go demonstrate some
  // of the supported use cases and motivations.
  func (srv *Server) setupHTTP2_Serve() error {
  	srv.nextProtoOnce.Do(srv.onceSetNextProtoDefaults_Serve)
  	return srv.nextProtoErr
  }
  
  func (srv *Server) onceSetNextProtoDefaults_Serve() {
  	if srv.shouldConfigureHTTP2ForServe() {
  		srv.onceSetNextProtoDefaults()
  	}
  }
  
  // onceSetNextProtoDefaults configures HTTP/2, if the user hasn't
  // configured otherwise. (by setting srv.TLSNextProto non-nil)
  // It must only be called via srv.nextProtoOnce (use srv.setupHTTP2_*).
  func (srv *Server) onceSetNextProtoDefaults() {
  	if strings.Contains(os.Getenv("GODEBUG"), "http2server=0") {
  		return
  	}
  	// Enable HTTP/2 by default if the user hasn't otherwise
  	// configured their TLSNextProto map.
  	if srv.TLSNextProto == nil {
  		srv.nextProtoErr = http2ConfigureServer(srv, nil)
  	}
  }
  
  // TimeoutHandler returns a Handler that runs h with the given time limit.
  //
  // The new Handler calls h.ServeHTTP to handle each request, but if a
  // call runs for longer than its time limit, the handler responds with
  // a 503 Service Unavailable error and the given message in its body.
  // (If msg is empty, a suitable default message will be sent.)
  // After such a timeout, writes by h to its ResponseWriter will return
  // ErrHandlerTimeout.
  //
  // TimeoutHandler buffers all Handler writes to memory and does not
  // support the Hijacker or Flusher interfaces.
  func TimeoutHandler(h Handler, dt time.Duration, msg string) Handler {
  	return &timeoutHandler{
  		handler: h,
  		body:    msg,
  		dt:      dt,
  	}
  }
  
  // ErrHandlerTimeout is returned on ResponseWriter Write calls
  // in handlers which have timed out.
  var ErrHandlerTimeout = errors.New("http: Handler timeout")
  
  type timeoutHandler struct {
  	handler Handler
  	body    string
  	dt      time.Duration
  
  	// When set, no timer will be created and this channel will
  	// be used instead.
  	testTimeout <-chan time.Time
  }
  
  func (h *timeoutHandler) errorBody() string {
  	if h.body != "" {
  		return h.body
  	}
  	return "<html><head><title>Timeout</title></head><body><h1>Timeout</h1></body></html>"
  }
  
  func (h *timeoutHandler) ServeHTTP(w ResponseWriter, r *Request) {
  	var t *time.Timer
  	timeout := h.testTimeout
  	if timeout == nil {
  		t = time.NewTimer(h.dt)
  		timeout = t.C
  	}
  	done := make(chan struct{})
  	tw := &timeoutWriter{
  		w: w,
  		h: make(Header),
  	}
  	go func() {
  		h.handler.ServeHTTP(tw, r)
  		close(done)
  	}()
  	select {
  	case <-done:
  		tw.mu.Lock()
  		defer tw.mu.Unlock()
  		dst := w.Header()
  		for k, vv := range tw.h {
  			dst[k] = vv
  		}
  		if !tw.wroteHeader {
  			tw.code = StatusOK
  		}
  		w.WriteHeader(tw.code)
  		w.Write(tw.wbuf.Bytes())
  		if t != nil {
  			t.Stop()
  		}
  	case <-timeout:
  		tw.mu.Lock()
  		defer tw.mu.Unlock()
  		w.WriteHeader(StatusServiceUnavailable)
  		io.WriteString(w, h.errorBody())
  		tw.timedOut = true
  		return
  	}
  }
  
  type timeoutWriter struct {
  	w    ResponseWriter
  	h    Header
  	wbuf bytes.Buffer
  
  	mu          sync.Mutex
  	timedOut    bool
  	wroteHeader bool
  	code        int
  }
  
  func (tw *timeoutWriter) Header() Header { return tw.h }
  
  func (tw *timeoutWriter) Write(p []byte) (int, error) {
  	tw.mu.Lock()
  	defer tw.mu.Unlock()
  	if tw.timedOut {
  		return 0, ErrHandlerTimeout
  	}
  	if !tw.wroteHeader {
  		tw.writeHeader(StatusOK)
  	}
  	return tw.wbuf.Write(p)
  }
  
  func (tw *timeoutWriter) WriteHeader(code int) {
  	tw.mu.Lock()
  	defer tw.mu.Unlock()
  	if tw.timedOut || tw.wroteHeader {
  		return
  	}
  	tw.writeHeader(code)
  }
  
  func (tw *timeoutWriter) writeHeader(code int) {
  	tw.wroteHeader = true
  	tw.code = code
  }
  
  // tcpKeepAliveListener sets TCP keep-alive timeouts on accepted
  // connections. It's used by ListenAndServe and ListenAndServeTLS so
  // dead TCP connections (e.g. closing laptop mid-download) eventually
  // go away.
  type tcpKeepAliveListener struct {
  	*net.TCPListener
  }
  
  func (ln tcpKeepAliveListener) Accept() (c net.Conn, err error) {
  	tc, err := ln.AcceptTCP()
  	if err != nil {
  		return
  	}
  	tc.SetKeepAlive(true)
  	tc.SetKeepAlivePeriod(3 * time.Minute)
  	return tc, nil
  }
  
  // globalOptionsHandler responds to "OPTIONS *" requests.
  type globalOptionsHandler struct{}
  
  func (globalOptionsHandler) ServeHTTP(w ResponseWriter, r *Request) {
  	w.Header().Set("Content-Length", "0")
  	if r.ContentLength != 0 {
  		// Read up to 4KB of OPTIONS body (as mentioned in the
  		// spec as being reserved for future use), but anything
  		// over that is considered a waste of server resources
  		// (or an attack) and we abort and close the connection,
  		// courtesy of MaxBytesReader's EOF behavior.
  		mb := MaxBytesReader(w, r.Body, 4<<10)
  		io.Copy(ioutil.Discard, mb)
  	}
  }
  
  // initNPNRequest is an HTTP handler that initializes certain
  // uninitialized fields in its *Request. Such partially-initialized
  // Requests come from NPN protocol handlers.
  type initNPNRequest struct {
  	c *tls.Conn
  	h serverHandler
  }
  
  func (h initNPNRequest) ServeHTTP(rw ResponseWriter, req *Request) {
  	if req.TLS == nil {
  		req.TLS = &tls.ConnectionState{}
  		*req.TLS = h.c.ConnectionState()
  	}
  	if req.Body == nil {
  		req.Body = NoBody
  	}
  	if req.RemoteAddr == "" {
  		req.RemoteAddr = h.c.RemoteAddr().String()
  	}
  	h.h.ServeHTTP(rw, req)
  }
  
  // loggingConn is used for debugging.
  type loggingConn struct {
  	name string
  	net.Conn
  }
  
  var (
  	uniqNameMu   sync.Mutex
  	uniqNameNext = make(map[string]int)
  )
  
  func newLoggingConn(baseName string, c net.Conn) net.Conn {
  	uniqNameMu.Lock()
  	defer uniqNameMu.Unlock()
  	uniqNameNext[baseName]++
  	return &loggingConn{
  		name: fmt.Sprintf("%s-%d", baseName, uniqNameNext[baseName]),
  		Conn: c,
  	}
  }
  
  func (c *loggingConn) Write(p []byte) (n int, err error) {
  	log.Printf("%s.Write(%d) = ....", c.name, len(p))
  	n, err = c.Conn.Write(p)
  	log.Printf("%s.Write(%d) = %d, %v", c.name, len(p), n, err)
  	return
  }
  
  func (c *loggingConn) Read(p []byte) (n int, err error) {
  	log.Printf("%s.Read(%d) = ....", c.name, len(p))
  	n, err = c.Conn.Read(p)
  	log.Printf("%s.Read(%d) = %d, %v", c.name, len(p), n, err)
  	return
  }
  
  func (c *loggingConn) Close() (err error) {
  	log.Printf("%s.Close() = ...", c.name)
  	err = c.Conn.Close()
  	log.Printf("%s.Close() = %v", c.name, err)
  	return
  }
  
  // checkConnErrorWriter writes to c.rwc and records any write errors to c.werr.
  // It only contains one field (and a pointer field at that), so it
  // fits in an interface value without an extra allocation.
  type checkConnErrorWriter struct {
  	c *conn
  }
  
  func (w checkConnErrorWriter) Write(p []byte) (n int, err error) {
  	n, err = w.c.rwc.Write(p)
  	if err != nil && w.c.werr == nil {
  		w.c.werr = err
  		w.c.cancelCtx()
  	}
  	return
  }
  
  func numLeadingCRorLF(v []byte) (n int) {
  	for _, b := range v {
  		if b == '\r' || b == '\n' {
  			n++
  			continue
  		}
  		break
  	}
  	return
  
  }
  
  func strSliceContains(ss []string, s string) bool {
  	for _, v := range ss {
  		if v == s {
  			return true
  		}
  	}
  	return false
  }
  

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