net/http: add GODEBUG setting for old ServeMux behavior

[gostls13.git] / src / net / http / server.go

// 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 7230 through 7235.

package http

import (
        "bufio"
        "bytes"
        "context"
        "crypto/tls"
        "errors"
        "fmt"
        "internal/godebug"
        "io"
        "log"
        "math/rand"
        "net"
        "net/textproto"
        "net/url"
        urlpkg "net/url"
        "path"
        "runtime"
        "sort"
        "strconv"
        "strings"
        "sync"
        "sync/atomic"
        "time"

        "golang.org/x/net/http/httpguts"
)

// TODO(jba): test

// 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 returned by
        // anything in the net/http package. Callers should not
        // compare errors against this variable.
        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 either closes the network connection or sends an HTTP/2
// RST_STREAM, depending on the HTTP protocol. 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 [Handler.ServeHTTP] has returned.
type ResponseWriter interface {
        // Header returns the header map that will be sent by
        // [ResponseWriter.WriteHeader]. The [Header] map also is the mechanism with which
        // [Handler] implementations can set HTTP trailers.
        //
        // Changing the header map after a call to [ResponseWriter.WriteHeader] (or
        // [ResponseWriter.Write]) has no effect unless the HTTP status code was of the
        // 1xx class or 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 [ResponseWriter.Write],
        // is to prefix the [Header] map keys with the [TrailerPrefix]
        // constant value.
        //
        // To suppress automatic 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 [ResponseWriter.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]. Additionally, if the total size of all written
        // data is under a few KB and there are no Flush calls, the
        // Content-Length header is added automatically.
        //
        // 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 the provided
        // 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 or 1xx informational responses.
        //
        // The provided code must be a valid HTTP 1xx-5xx status code.
        // Any number of 1xx headers may be written, followed by at most
        // one 2xx-5xx header. 1xx headers are sent immediately, but 2xx-5xx
        // headers may be buffered. Use the Flusher interface to send
        // buffered data. The header map is cleared when 2xx-5xx headers are
        // sent, but not with 1xx headers.
        //
        // The server will automatically send a 100 (Continue) header
        // on the first read from the request body if the request has
        // an "Expect: 100-continue" header.
        WriteHeader(statusCode 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.
        //
        // After a call to Hijack, the original Request.Body must not
        // be used. The original Request's Context remains valid and
        // is not canceled until the Request's ServeHTTP method
        // returns.
        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.
//
// Deprecated: the CloseNotifier interface predates Go's context package.
// New code should use Request.Context instead.
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.Value 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.Value to access 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.Pointer[response] // (which has a Request in it)

        curState atomic.Uint64 // packed (unixtime<<8|uint8(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, runHooks)
        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.w 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() error {
        if !cw.wroteHeader {
                cw.writeHeader(nil)
        }
        return 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               // a non-1xx 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"

        // canWriteContinue is an atomic boolean that says whether or
        // not a 100 Continue header can be written to the
        // connection.
        // writeContinueMu must be held while writing the header.
        // These two fields together synchronize the body reader (the
        // expectContinueReader, which wants to write 100 Continue)
        // against the main writer.
        canWriteContinue atomic.Bool
        writeContinueMu  sync.Mutex

        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

        // When fullDuplex is false (the default), we consume any remaining
        // request body before starting to write a response.
        fullDuplex 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 atomic.Bool // set true when the handler exits

        // Buffers for Date, Content-Length, and status code
        dateBuf   [len(TimeFormat)]byte
        clenBuf   [10]byte
        statusBuf [3]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 atomic.Bool // atomic (only false->true winner should send)
}

func (c *response) SetReadDeadline(deadline time.Time) error {
        return c.conn.rwc.SetReadDeadline(deadline)
}

func (c *response) SetWriteDeadline(deadline time.Time) error {
        return c.conn.rwc.SetWriteDeadline(deadline)
}

func (c *response) EnableFullDuplex() error {
        c.fullDuplex = true
        return nil
}

// 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://pkg.go.dev/net/http#ResponseWriter
//      https://pkg.go.dev/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 kk, found := strings.CutPrefix(k, TrailerPrefix); found {
                        if t == nil {
                                t = make(Header)
                        }
                        t[kk] = vv
                }
        }
        for _, k := range w.trailers {
                if t == nil {
                        t = make(Header)
                }
                for _, v := range w.handlerHeader[k] {
                        t.Add(k, v)
                }
        }
        return t
}

// 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)
        if !httpguts.ValidTrailerHeader(k) {
                // Forbidden by RFC 7230, section 4.1.2
                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")
        }
}

// writerOnly hides an io.Writer value's optional ReadFrom method
// from io.Copy.
type writerOnly struct {
        io.Writer
}

// ReadFrom is here to optimize copying from an *os.File regular file
// to a *net.TCPConn with sendfile, or from a supported src type such
// as a *net.TCPConn on Linux with splice.
func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
        bufp := copyBufPool.Get().(*[]byte)
        buf := *bufp
        defer copyBufPool.Put(bufp)

        // Our underlying w.conn.rwc is usually a *TCPConn (with its
        // own ReadFrom method). If not, just fall back to the normal
        // copy method.
        rf, ok := w.conn.rwc.(io.ReaderFrom)
        if !ok {
                return io.CopyBuffer(writerOnly{w}, src, buf)
        }

        // Copy the first sniffLen bytes before switching to ReadFrom.
        // This ensures we don't start writing the response before the
        // source is available (see golang.org/issue/5660) and provides
        // enough bytes to perform Content-Type sniffing when required.
        if !w.cw.wroteHeader {
                n0, err := io.CopyBuffer(writerOnly{w}, io.LimitReader(src, sniffLen), buf)
                n += n0
                if err != nil || n0 < sniffLen {
                        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.CopyBuffer(writerOnly{w}, src, buf)
        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 {
        _   incomparable
        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
        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 past the end of the previous request's body already
                // (since we wouldn't be in a background read otherwise), so
                // this is a pipelined HTTP request. Prior to Go 1.11 we used to
                // send on the CloseNotify channel and cancel the context here,
                // but the behavior was documented as only "may", and we only
                // did that because that's how CloseNotify accidentally behaved
                // in very early Go releases prior to context support. Once we
                // added context support, people used a Handler's
                // Request.Context() and passed it along. Having that context
                // cancel on pipelined HTTP requests caused problems.
                // Fortunately, almost nothing uses HTTP/1.x pipelining.
                // Unfortunately, apt-get does, or sometimes does.
                // New Go 1.11 behavior: don't fire CloseNotify or cancel
                // contexts on pipelined requests. Shouldn't affect people, but
                // fixes cases like Issue 23921. This does mean that a client
                // closing their TCP connection after sending a pipelined
                // request won't cancel the context, but we'll catch that on any
                // write failure (in checkConnErrorWriter.Write).
                // If the server never writes, yes, there are still contrived
                // server & client behaviors where this fails to ever cancel the
                // context, but that's kinda why HTTP/1.x pipelining died
                // anyway.
        }
        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 }

// handleReadError is called whenever a Read from the client returns a
// non-nil error.
//
// The provided non-nil err is almost always io.EOF or a "use of
// closed network connection". In any case, the error is not
// particularly interesting, except perhaps for debugging during
// development. Any error means the connection is dead and we should
// down its context.
//
// It may be called from multiple goroutines.
func (cr *connReader) handleReadError(_ error) {
        cr.conn.cancelCtx()
        cr.closeNotify()
}

// may be called from multiple goroutines.
func (cr *connReader) closeNotify() {
        res := cr.conn.curReq.Load()
        if res != nil && !res.didCloseNotify.Swap(true) {
                res.closeNotifyCh <- true
        }
}

func (cr *connReader) Read(p []byte) (n int, err error) {
        cr.lock()
        if cr.inRead {
                cr.unlock()
                if cr.conn.hijacked() {
                        panic("invalid Body.Read call. After hijacked, the original Request must not be used")
                }
                panic("invalid concurrent Body.Read call")
        }
        if cr.hitReadLimit() {
                cr.unlock()
                return 0, io.EOF
        }
        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() any {
                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
}

// tlsHandshakeTimeout returns the time limit permitted for the TLS
// handshake, or zero for unlimited.
//
// It returns the minimum of any positive ReadHeaderTimeout,
// ReadTimeout, or WriteTimeout.
func (srv *Server) tlsHandshakeTimeout() time.Duration {
        var ret time.Duration
        for _, v := range [...]time.Duration{
                srv.ReadHeaderTimeout,
                srv.ReadTimeout,
                srv.WriteTimeout,
        } {
                if v <= 0 {
                        continue
                }
                if ret == 0 || v < ret {
                        ret = v
                }
        }
        return ret
}

// wrapper around io.ReadCloser which on first read, sends an
// HTTP/1.1 100 Continue header
type expectContinueReader struct {
        resp       *response
        readCloser io.ReadCloser
        closed     atomic.Bool
        sawEOF     atomic.Bool
}

func (ecr *expectContinueReader) Read(p []byte) (n int, err error) {
        if ecr.closed.Load() {
                return 0, ErrBodyReadAfterClose
        }
        w := ecr.resp
        if !w.wroteContinue && w.canWriteContinue.Load() && !w.conn.hijacked() {
                w.wroteContinue = true
                w.writeContinueMu.Lock()
                if w.canWriteContinue.Load() {
                        w.conn.bufw.WriteString("HTTP/1.1 100 Continue\r\n\r\n")
                        w.conn.bufw.Flush()
                        w.canWriteContinue.Store(false)
                }
                w.writeContinueMu.Unlock()
        }
        n, err = ecr.readCloser.Read(p)
        if err == io.EOF {
                ecr.sawEOF.Store(true)
        }
        return
}

func (ecr *expectContinueReader) Close() error {
        ecr.closed.Store(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 7230 section 3 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)
        if err != nil {
                if c.r.hitReadLimit() {
                        return nil, errTooLarge
                }
                return nil, err
        }

        if !http1ServerSupportsRequest(req) {
                return nil, statusError{StatusHTTPVersionNotSupported, "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 && req.Method != "CONNECT" {
                return nil, badRequestError("missing required Host header")
        }
        if len(hosts) == 1 && !httpguts.ValidHostHeader(hosts[0]) {
                return nil, badRequestError("malformed Host header")
        }
        for k, vv := range req.Header {
                if !httpguts.ValidHeaderFieldName(k) {
                        return nil, badRequestError("invalid header name")
                }
                for _, v := range vv {
                        if !httpguts.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 checkWriteHeaderCode(code int) {
        // Issue 22880: require valid WriteHeader status codes.
        // For now we only enforce that it's three digits.
        // In the future we might block things over 599 (600 and above aren't defined
        // at https://httpwg.org/specs/rfc7231.html#status.codes).
        // But for now any three digits.
        //
        // We used to send "HTTP/1.1 000 0" on the wire in responses but there's
        // no equivalent bogus thing we can realistically send in HTTP/2,
        // so we'll consistently panic instead and help people find their bugs
        // early. (We can't return an error from WriteHeader even if we wanted to.)
        if code < 100 || code > 999 {
                panic(fmt.Sprintf("invalid WriteHeader code %v", code))
        }
}

// relevantCaller searches the call stack for the first function outside of net/http.
// The purpose of this function is to provide more helpful error messages.
func relevantCaller() runtime.Frame {
        pc := make([]uintptr, 16)
        n := runtime.Callers(1, pc)
        frames := runtime.CallersFrames(pc[:n])
        var frame runtime.Frame
        for {
                frame, more := frames.Next()
                if !strings.HasPrefix(frame.Function, "net/http.") {
                        return frame
                }
                if !more {
                        break
                }
        }
        return frame
}

func (w *response) WriteHeader(code int) {
        if w.conn.hijacked() {
                caller := relevantCaller()
                w.conn.server.logf("http: response.WriteHeader on hijacked connection from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
                return
        }
        if w.wroteHeader {
                caller := relevantCaller()
                w.conn.server.logf("http: superfluous response.WriteHeader call from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
                return
        }
        checkWriteHeaderCode(code)

        // Handle informational headers.
        //
        // We shouldn't send any further headers after 101 Switching Protocols,
        // so it takes the non-informational path.
        if code >= 100 && code <= 199 && code != StatusSwitchingProtocols {
                // Prevent a potential race with an automatically-sent 100 Continue triggered by Request.Body.Read()
                if code == 100 && w.canWriteContinue.Load() {
                        w.writeContinueMu.Lock()
                        w.canWriteContinue.Store(false)
                        w.writeContinueMu.Unlock()
                }

                writeStatusLine(w.conn.bufw, w.req.ProtoAtLeast(1, 1), code, w.statusBuf[:])

                // Per RFC 8297 we must not clear the current header map
                w.handlerHeader.WriteSubset(w.conn.bufw, excludedHeadersNoBody)
                w.conn.bufw.Write(crlf)
                w.conn.bufw.Flush()

                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.Load() && !trailers && !hasTE && bodyAllowedForStatus(w.status) && !header.has("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 an 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.Load() {
                w.closeAfterReply = true
        }

        // We do this by default because there are a number of clients that
        // send a full request before starting to read the response, and they
        // can deadlock if we start writing the response with unconsumed body
        // remaining. See Issue 15527 for some history.
        //
        // If full duplex mode has been enabled with ResponseController.EnableFullDuplex,
        // then leave the request body alone.
        if w.req.ContentLength != 0 && !w.closeAfterReply && !w.fullDuplex {
                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(io.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 the Content-Encoding was set and is non-blank,
                // we shouldn't sniff the body. See Issue 31753.
                ce := header.Get("Content-Encoding")
                hasCE := len(ce) > 0
                if !hasCE && !haveType && !hasTE && len(p) > 0 {
                        setHeader.contentType = DetectContentType(p)
                }
        } else {
                for _, k := range suppressedHeaders(code) {
                        delHeader(k)
                }
        }

        if !header.has("Date") {
                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) || code == StatusNoContent {
                // Response has no body.
                delHeader("Transfer-Encoding")
        } else if hasCL {
                // Content-Length has been provided, so no chunking is to be done.
                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
                        delHeader("Transfer-Encoding")
                } 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
        }

        // Only override the Connection header if it is not a successful
        // protocol switch response and if KeepAlives are not enabled.
        // See https://golang.org/issue/36381.
        delConnectionHeader := w.closeAfterReply &&
                (!keepAlivesEnabled || !hasToken(cw.header.get("Connection"), "close")) &&
                !isProtocolSwitchResponse(w.status, header)
        if delConnectionHeader {
                delHeader("Connection")
                if w.req.ProtoAtLeast(1, 1) {
                        setHeader.connection = "close"
                }
        }

        writeStatusLine(w.conn.bufw, w.req.ProtoAtLeast(1, 1), code, w.statusBuf[:])
        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 7230 section 7 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)
                }
        }
}

// writeStatusLine writes an HTTP/1.x Status-Line (RFC 7230 Section 3.1.2)
// to bw. is11 is whether the HTTP request is HTTP/1.1. false means HTTP/1.0.
// code is the response status code.
// scratch is an optional scratch buffer. If it has at least capacity 3, it's used.
func writeStatusLine(bw *bufio.Writer, is11 bool, code int, scratch []byte) {
        if is11 {
                bw.WriteString("HTTP/1.1 ")
        } else {
                bw.WriteString("HTTP/1.0 ")
        }
        if text := StatusText(code); text != "" {
                bw.Write(strconv.AppendInt(scratch[:0], int64(code), 10))
                bw.WriteByte(' ')
                bw.WriteString(text)
                bw.WriteString("\r\n")
        } else {
                // don't worry about performance
                fmt.Fprintf(bw, "%03d status code %d\r\n", code, code)
        }
}

// 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.bufw, 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 {
                        caller := relevantCaller()
                        w.conn.server.logf("http: response.Write on hijacked connection from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
                }
                return 0, ErrHijacked
        }

        if w.canWriteContinue.Load() {
                // Body reader wants to write 100 Continue but hasn't yet.
                // Tell it not to. The store must be done while holding the lock
                // because the lock makes sure that there is not an active write
                // this very moment.
                w.writeContinueMu.Lock()
                w.canWriteContinue.Store(false)
                w.writeContinueMu.Unlock()
        }

        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.Store(true)

        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() {
        w.FlushError()
}

func (w *response) FlushError() error {
        if !w.wroteHeader {
                w.WriteHeader(StatusOK)
        }
        err := w.w.Flush()
        e2 := w.cw.flush()
        if err == nil {
                err = e2
        }
        return err
}

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),
// and may be modified by tests.
//
// TODO(bcmills): This should arguably be a server configuration parameter,
// not a hard-coded value.
var rstAvoidanceDelay = 500 * time.Millisecond

type closeWriter interface {
        CloseWrite() error
}

var _ closeWriter = (*net.TCPConn)(nil)

// closeWriteAndWait flushes any outstanding data and sends a FIN packet (if
// client is connected via TCP), signaling 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()
        }

        // When we return from closeWriteAndWait, the caller will fully close the
        // connection. If client is still writing to the connection, this will cause
        // the write to fail with ECONNRESET or similar. Unfortunately, many TCP
        // implementations will also drop unread packets from the client's read buffer
        // when a write fails, causing our final response to be truncated away too.
        //
        // As a result, https://www.rfc-editor.org/rfc/rfc7230#section-6.6 recommends
        // that “[t]he server … continues to read from the connection until it
        // receives a corresponding close by the client, or until the server is
        // reasonably certain that its own TCP stack has received the client's
        // acknowledgement of the packet(s) containing the server's last response.”
        //
        // Unfortunately, we have no straightforward way to be “reasonably certain”
        // that we have received the client's ACK, and at any rate we don't want to
        // allow a misbehaving client to soak up server connections indefinitely by
        // withholding an ACK, nor do we want to go through the complexity or overhead
        // of using low-level APIs to figure out when a TCP round-trip has completed.
        //
        // Instead, we declare that we are “reasonably certain” that we received the
        // ACK if maxRSTAvoidanceDelay has elapsed.
        time.Sleep(rstAvoidanceDelay)
}

// validNextProto reports whether the proto is a valid ALPN protocol name.
// Everything is valid except the empty string and built-in protocol types,
// so that those can't be overridden with alternate implementations.
func validNextProto(proto string) bool {
        switch proto {
        case "", "http/1.1", "http/1.0":
                return false
        }
        return true
}

const (
        runHooks  = true
        skipHooks = false
)

func (c *conn) setState(nc net.Conn, state ConnState, runHook bool) {
        srv := c.server
        switch state {
        case StateNew:
                srv.trackConn(c, true)
        case StateHijacked, StateClosed:
                srv.trackConn(c, false)
        }
        if state > 0xff || state < 0 {
                panic("internal error")
        }
        packedState := uint64(time.Now().Unix()<<8) | uint64(state)
        c.curState.Store(packedState)
        if !runHook {
                return
        }
        if hook := srv.ConnState; hook != nil {
                hook(nc, state)
        }
}

func (c *conn) getState() (state ConnState, unixSec int64) {
        packedState := c.curState.Load()
        return ConnState(packedState & 0xff), int64(packedState >> 8)
}

// 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.
func badRequestError(e string) error { return statusError{StatusBadRequest, e} }

// statusError is an error used to respond to a request with an HTTP status.
// The text should be plain text without user info or other embedded errors.
type statusError struct {
        code int
        text string
}

func (e statusError) Error() string { return StatusText(e.code) + ": " + e.text }

// 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) {
        if ra := c.rwc.RemoteAddr(); ra != nil {
                c.remoteAddr = ra.String()
        }
        ctx = context.WithValue(ctx, LocalAddrContextKey, c.rwc.LocalAddr())
        var inFlightResponse *response
        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 inFlightResponse != nil {
                        inFlightResponse.cancelCtx()
                }
                if !c.hijacked() {
                        if inFlightResponse != nil {
                                inFlightResponse.conn.r.abortPendingRead()
                                inFlightResponse.reqBody.Close()
                        }
                        c.close()
                        c.setState(c.rwc, StateClosed, runHooks)
                }
        }()

        if tlsConn, ok := c.rwc.(*tls.Conn); ok {
                tlsTO := c.server.tlsHandshakeTimeout()
                if tlsTO > 0 {
                        dl := time.Now().Add(tlsTO)
                        c.rwc.SetReadDeadline(dl)
                        c.rwc.SetWriteDeadline(dl)
                }
                if err := tlsConn.HandshakeContext(ctx); err != nil {
                        // If the handshake failed due to the client not speaking
                        // TLS, assume they're speaking plaintext HTTP and write a
                        // 400 response on the TLS conn's underlying net.Conn.
                        if re, ok := err.(tls.RecordHeaderError); ok && re.Conn != nil && tlsRecordHeaderLooksLikeHTTP(re.RecordHeader) {
                                io.WriteString(re.Conn, "HTTP/1.0 400 Bad Request\r\n\r\nClient sent an HTTP request to an HTTPS server.\n")
                                re.Conn.Close()
                                return
                        }
                        c.server.logf("http: TLS handshake error from %s: %v", c.rwc.RemoteAddr(), err)
                        return
                }
                // Restore Conn-level deadlines.
                if tlsTO > 0 {
                        c.rwc.SetReadDeadline(time.Time{})
                        c.rwc.SetWriteDeadline(time.Time{})
                }
                c.tlsState = new(tls.ConnectionState)
                *c.tlsState = tlsConn.ConnectionState()
                if proto := c.tlsState.NegotiatedProtocol; validNextProto(proto) {
                        if fn := c.server.TLSNextProto[proto]; fn != nil {
                                h := initALPNRequest{ctx, tlsConn, serverHandler{c.server}}
                                // Mark freshly created HTTP/2 as active and prevent any server state hooks
                                // from being run on these connections. This prevents closeIdleConns from
                                // closing such connections. See issue https://golang.org/issue/39776.
                                c.setState(c.rwc, StateActive, skipHooks)
                                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, runHooks)
                }
                if err != nil {
                        const errorHeaders = "\r\nContent-Type: text/plain; charset=utf-8\r\nConnection: close\r\n\r\n"

                        switch {
                        case 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

                        case isUnsupportedTEError(err):
                                // Respond as per RFC 7230 Section 3.3.1 which says,
                                //      A server that receives a request message with a
                                //      transfer coding it does not understand SHOULD
                                //      respond with 501 (Unimplemented).
                                code := StatusNotImplemented

                                // We purposefully aren't echoing back the transfer-encoding's value,
                                // so as to mitigate the risk of cross side scripting by an attacker.
                                fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s%sUnsupported transfer encoding", code, StatusText(code), errorHeaders)
                                return

                        case isCommonNetReadError(err):
                                return // don't reply

                        default:
                                if v, ok := err.(statusError); ok {
                                        fmt.Fprintf(c.rwc, "HTTP/1.1 %d %s: %s%s%d %s: %s", v.code, StatusText(v.code), v.text, errorHeaders, v.code, StatusText(v.code), v.text)
                                        return
                                }
                                const publicErr = "400 Bad Request"
                                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}
                                w.canWriteContinue.Store(true)
                        }
                } 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 {
                        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.
                inFlightResponse = w
                serverHandler{c.server}.ServeHTTP(w, w.req)
                inFlightResponse = nil
                w.cancelCtx()
                if c.hijacked() {
                        return
                }
                w.finishRequest()
                c.rwc.SetWriteDeadline(time.Time{})
                if !w.shouldReuseConnection() {
                        if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
                                c.closeWriteAndWait()
                        }
                        return
                }
                c.setState(c.rwc, StateIdle, runHooks)
                c.curReq.Store(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))
                } else {
                        c.rwc.SetReadDeadline(time.Time{})
                }

                // Wait for the connection to become readable again before trying to
                // read the next request. This prevents a ReadHeaderTimeout or
                // ReadTimeout from starting until the first bytes of the next request
                // have been received.
                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 7231 5.1.1 which says
        // "A server that receives an Expect field-value other
        // than 100-continue MAY respond with a 417 (Expectation
        // Failed) status code to indicate that the unexpected
        // expectation cannot be met."
        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.Load() {
                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.Load() {
                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 RawPath if set) 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. The prefix must
// match exactly: if the prefix in the request contains escaped characters
// the reply is also an HTTP 404 not found error.
func StripPrefix(prefix string, h Handler) Handler {
        if prefix == "" {
                return h
        }
        return HandlerFunc(func(w ResponseWriter, r *Request) {
                p := strings.TrimPrefix(r.URL.Path, prefix)
                rp := strings.TrimPrefix(r.URL.RawPath, prefix)
                if len(p) < len(r.URL.Path) && (r.URL.RawPath == "" || len(rp) < len(r.URL.RawPath)) {
                        r2 := new(Request)
                        *r2 = *r
                        r2.URL = new(url.URL)
                        *r2.URL = *r.URL
                        r2.URL.Path = p
                        r2.URL.RawPath = rp
                        h.ServeHTTP(w, r2)
                } 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.
//
// If the Content-Type header has not been set, Redirect sets it
// to "text/html; charset=utf-8" and writes a small HTML body.
// Setting the Content-Type header to any value, including nil,
// disables that behavior.
func Redirect(w ResponseWriter, r *Request, url string, code int) {
        if u, err := urlpkg.Parse(url); err == nil {
                // If url was relative, make its path absolute by
                // combining with request path.
                // The client would probably do this for us,
                // but doing it ourselves is more reliable.
                // See RFC 7231, section 7.1.2
                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 url == "" || url[0] != '/' {
                                // make relative path absolute
                                olddir, _ := path.Split(oldpath)
                                url = olddir + url
                        }

                        var query string
                        if i := strings.Index(url, "?"); i != -1 {
                                url, query = url[:i], url[i:]
                        }

                        // clean up but preserve trailing slash
                        trailing := strings.HasSuffix(url, "/")
                        url = path.Clean(url)
                        if trailing && !strings.HasSuffix(url, "/") {
                                url += "/"
                        }
                        url += query
                }
        }

        h := w.Header()

        // RFC 7231 notes that a short HTML body is usually included in
        // the response because older user agents may not understand 301/307.
        // Do it only if the request didn't already have a Content-Type header.
        _, hadCT := h["Content-Type"]

        h.Set("Location", hexEscapeNonASCII(url))
        if !hadCT && (r.Method == "GET" || r.Method == "HEAD") {
                h.Set("Content-Type", "text/html; charset=utf-8")
        }
        w.WriteHeader(code)

        // Shouldn't send the body for POST or HEAD; that leaves GET.
        if !hadCT && r.Method == "GET" {
                body := "<a href=\"" + htmlEscape(url) + "\">" + StatusText(code) + "</a>.\n"
                fmt.Fprintln(w, body)
        }
}

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}
}

// TODO(jba): rewrite the following doc for enhanced patterns (proposal
// https://go.dev/issue/61410).

// 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 with "/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 and the Host
// header, stripping the port number and redirecting any request containing . or
// .. elements or repeated slashes to an equivalent, cleaner URL.
type ServeMux struct {
        mu       sync.RWMutex
        tree     routingNode
        index    routingIndex
        patterns []*pattern  // TODO(jba): remove if possible
        mux121   serveMux121 // used only when GODEBUG=httpmuxgo121=1
}

// NewServeMux allocates and returns a new ServeMux.
func NewServeMux() *ServeMux {
        return &ServeMux{}
}

// DefaultServeMux is the default ServeMux used by Serve.
var DefaultServeMux = &defaultServeMux

var defaultServeMux ServeMux

// cleanPath returns 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 != "/" {
                // Fast path for common case of p being the string we want:
                if len(p) == len(np)+1 && strings.HasPrefix(p, np) {
                        np = p
                } else {
                        np += "/"
                }
        }
        return np
}

// stripHostPort returns h without any trailing ":<port>".
func stripHostPort(h string) string {
        // If no port on host, return unchanged
        if !strings.Contains(h, ":") {
                return h
        }
        host, _, err := net.SplitHostPort(h)
        if err != nil {
                return h // on error, return unchanged
        }
        return host
}

// 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. If the host contains a port, it is ignored
// when matching handlers.
//
// The path and host are used unchanged for CONNECT requests.
//
// Handler also returns the registered pattern that matches the
// request or, in the case of internally-generated redirects,
// the path 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 use121 {
                return mux.mux121.findHandler(r)
        }
        h, p, _, _ := mux.findHandler(r)
        return h, p
}

// findHandler finds a handler for a request.
// If there is a matching handler, it returns it and the pattern that matched.
// Otherwise it returns a Redirect or NotFound handler with the path that would match
// after the redirect.
func (mux *ServeMux) findHandler(r *Request) (h Handler, patStr string, _ *pattern, matches []string) {
        var n *routingNode
        host := r.URL.Host
        escapedPath := r.URL.EscapedPath()
        path := escapedPath
        // CONNECT requests are not canonicalized.
        if r.Method == "CONNECT" {
                // If r.URL.Path is /tree and its handler is not registered,
                // the /tree -> /tree/ redirect applies to CONNECT requests
                // but the path canonicalization does not.
                _, _, u := mux.matchOrRedirect(host, r.Method, path, r.URL)
                if u != nil {
                        return RedirectHandler(u.String(), StatusMovedPermanently), u.Path, nil, nil
                }
                // Redo the match, this time with r.Host instead of r.URL.Host.
                // Pass a nil URL to skip the trailing-slash redirect logic.
                n, matches, _ = mux.matchOrRedirect(r.Host, r.Method, path, nil)
        } else {
                // All other requests have any port stripped and path cleaned
                // before passing to mux.handler.
                host = stripHostPort(r.Host)
                path = cleanPath(path)

                // If the given path is /tree and its handler is not registered,
                // redirect for /tree/.
                var u *url.URL
                n, matches, u = mux.matchOrRedirect(host, r.Method, path, r.URL)
                if u != nil {
                        return RedirectHandler(u.String(), StatusMovedPermanently), u.Path, nil, nil
                }
                if path != escapedPath {
                        // Redirect to cleaned path.
                        patStr := ""
                        if n != nil {
                                patStr = n.pattern.String()
                        }
                        u := &url.URL{Path: path, RawQuery: r.URL.RawQuery}
                        return RedirectHandler(u.String(), StatusMovedPermanently), patStr, nil, nil
                }
        }
        if n == nil {
                // We didn't find a match with the request method. To distinguish between
                // Not Found and Method Not Allowed, see if there is another pattern that
                // matches except for the method.
                allowedMethods := mux.matchingMethods(host, path)
                if len(allowedMethods) > 0 {
                        return HandlerFunc(func(w ResponseWriter, r *Request) {
                                w.Header().Set("Allow", strings.Join(allowedMethods, ", "))
                                Error(w, StatusText(StatusMethodNotAllowed), StatusMethodNotAllowed)
                        }), "", nil, nil
                }
                return NotFoundHandler(), "", nil, nil
        }
        return n.handler, n.pattern.String(), n.pattern, matches
}

// matchOrRedirect looks up a node in the tree that matches the host, method and path.
//
// If the url argument is non-nil, handler also deals with trailing-slash
// redirection: when a path doesn't match exactly, the match is tried again
// after appending "/" to the path. If that second match succeeds, the last
// return value is the URL to redirect to.
func (mux *ServeMux) matchOrRedirect(host, method, path string, u *url.URL) (_ *routingNode, matches []string, redirectTo *url.URL) {
        mux.mu.RLock()
        defer mux.mu.RUnlock()

        n, matches := mux.tree.match(host, method, path)
        // If we have an exact match, or we were asked not to try trailing-slash redirection,
        // then we're done.
        if !exactMatch(n, path) && u != nil {
                // If there is an exact match with a trailing slash, then redirect.
                path += "/"
                n2, _ := mux.tree.match(host, method, path)
                if exactMatch(n2, path) {
                        return nil, nil, &url.URL{Path: cleanPath(u.Path) + "/", RawQuery: u.RawQuery}
                }
        }
        return n, matches, nil
}

// exactMatch reports whether the node's pattern exactly matches the path.
// As a special case, if the node is nil, exactMatch return false.
//
// Before wildcards were introduced, it was clear that an exact match meant
// that the pattern and path were the same string. The only other possibility
// was that a trailing-slash pattern, like "/", matched a path longer than
// it, like "/a".
//
// With wildcards, we define an inexact match as any one where a multi wildcard
// matches a non-empty string. All other matches are exact.
// For example, these are all exact matches:
//
//      pattern   path
//      /a        /a
//      /{x}      /a
//      /a/{$}    /a/
//      /a/       /a/
//
// The last case has a multi wildcard (implicitly), but the match is exact because
// the wildcard matches the empty string.
//
// Examples of matches that are not exact:
//
//      pattern   path
//      /         /a
//      /a/{x...} /a/b
func exactMatch(n *routingNode, path string) bool {
        if n == nil {
                return false
        }
        // We can't directly implement the definition (empty match for multi
        // wildcard) because we don't record a match for anonymous multis.

        // If there is no multi, the match is exact.
        if !n.pattern.lastSegment().multi {
                return true
        }

        // If the path doesn't end in a trailing slash, then the multi match
        // is non-empty.
        if len(path) > 0 && path[len(path)-1] != '/' {
                return false
        }
        // Only patterns ending in {$} or a multi wildcard can
        // match a path with a trailing slash.
        // For the match to be exact, the number of pattern
        // segments should be the same as the number of slashes in the path.
        // E.g. "/a/b/{$}" and "/a/b/{...}" exactly match "/a/b/", but "/a/" does not.
        return len(n.pattern.segments) == strings.Count(path, "/")
}

// matchingMethods return a sorted list of all methods that would match with the given host and path.
func (mux *ServeMux) matchingMethods(host, path string) []string {
        // Hold the read lock for the entire method so that the two matches are done
        // on the same set of registered patterns.
        mux.mu.RLock()
        defer mux.mu.RUnlock()
        ms := map[string]bool{}
        mux.tree.matchingMethods(host, path, ms)
        // matchOrRedirect will try appending a trailing slash if there is no match.
        mux.tree.matchingMethods(host, path+"/", ms)
        methods := mapKeys(ms)
        sort.Strings(methods)
        return methods
}

// TODO: replace with maps.Keys when it is defined.
func mapKeys[K comparable, V any](m map[K]V) []K {
        var ks []K
        for k := range m {
                ks = append(ks, k)
        }
        return ks
}

// 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
        }
        var h Handler
        if use121 {
                h, _ = mux.mux121.findHandler(r)
        } else {
                h, _, r.pat, r.matches = mux.findHandler(r)
        }
        h.ServeHTTP(w, r)
}

// The four functions below all call register so that callerLocation
// always refers to user code.

// 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) {
        if use121 {
                mux.mux121.handle(pattern, handler)
        }
        mux.register(pattern, handler)
}

// HandleFunc registers the handler function for the given pattern.
func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
        if use121 {
                mux.mux121.handleFunc(pattern, handler)
        }
        mux.register(pattern, HandlerFunc(handler))
}

// Handle registers the handler for the given pattern in [DefaultServeMux].
// The documentation for [ServeMux] explains how patterns are matched.
func Handle(pattern string, handler Handler) {
        if use121 {
                DefaultServeMux.mux121.handle(pattern, handler)
        }
        DefaultServeMux.register(pattern, handler)
}

// HandleFunc registers the handler function for the given pattern in [DefaultServeMux].
// The documentation for [ServeMux] explains how patterns are matched.
func HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
        if use121 {
                DefaultServeMux.mux121.handleFunc(pattern, handler)
        }
        DefaultServeMux.register(pattern, HandlerFunc(handler))
}

func (mux *ServeMux) register(pattern string, handler Handler) {
        if err := mux.registerErr(pattern, handler); err != nil {
                panic(err)
        }
}

func (mux *ServeMux) registerErr(patstr string, handler Handler) error {
        if patstr == "" {
                return errors.New("http: invalid pattern")
        }
        if handler == nil {
                return errors.New("http: nil handler")
        }
        if f, ok := handler.(HandlerFunc); ok && f == nil {
                return errors.New("http: nil handler")
        }

        pat, err := parsePattern(patstr)
        if err != nil {
                return fmt.Errorf("parsing %q: %w", patstr, err)
        }

        // Get the caller's location, for better conflict error messages.
        // Skip register and whatever calls it.
        _, file, line, ok := runtime.Caller(3)
        if !ok {
                pat.loc = "unknown location"
        } else {
                pat.loc = fmt.Sprintf("%s:%d", file, line)
        }

        mux.mu.Lock()
        defer mux.mu.Unlock()
        // Check for conflict.
        if err := mux.index.possiblyConflictingPatterns(pat, func(pat2 *pattern) error {
                if pat.conflictsWith(pat2) {
                        d := describeConflict(pat, pat2)
                        return fmt.Errorf("pattern %q (registered at %s) conflicts with pattern %q (registered at %s):\n%s",
                                pat, pat.loc, pat2, pat2.loc, d)
                }
                return nil
        }); err != nil {
                return err
        }
        mux.tree.addPattern(pat, handler)
        mux.index.addPattern(pat)
        mux.patterns = append(mux.patterns, pat)
        return nil
}

// 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.
//
// The handler is typically nil, in which case [DefaultServeMux] is used.
//
// HTTP/2 support is only enabled if the Listener returns *tls.Conn
// connections and they were configured with "h2" in the TLS
// Config.NextProtos.
//
// Serve always returns a non-nil error.
func Serve(l net.Listener, handler Handler) error {
        srv := &Server{Handler: handler}
        return srv.Serve(l)
}

// ServeTLS accepts incoming HTTPS 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.
//
// The handler is typically nil, in which case [DefaultServeMux] is used.
//
// 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.
//
// ServeTLS always returns a non-nil error.
func ServeTLS(l net.Listener, handler Handler, certFile, keyFile string) error {
        srv := &Server{Handler: handler}
        return srv.ServeTLS(l, certFile, keyFile)
}

// A Server defines parameters for running an HTTP server.
// The zero value for Server is a valid configuration.
type Server struct {
        // Addr optionally specifies the TCP address for the server to listen on,
        // in the form "host:port". If empty, ":http" (port 80) is used.
        // The service names are defined in RFC 6335 and assigned by IANA.
        // See net.Dial for details of the address format.
        Addr string

        Handler Handler // handler to invoke, http.DefaultServeMux if nil

        // DisableGeneralOptionsHandler, if true, passes "OPTIONS *" requests to the Handler,
        // otherwise responds with 200 OK and Content-Length: 0.
        DisableGeneralOptionsHandler bool

        // TLSConfig optionally provides a TLS configuration for use
        // by ServeTLS and ListenAndServeTLS. Note that this value is
        // cloned by ServeTLS and ListenAndServeTLS, so it's not
        // possible to modify the configuration with methods like
        // tls.Config.SetSessionTicketKeys. To use
        // SetSessionTicketKeys, use Server.Serve with a TLS Listener
        // instead.
        TLSConfig *tls.Config

        // ReadTimeout is the maximum duration for reading the entire
        // request, including the body. A zero or negative value means
        // there will be no timeout.
        //
        // 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. If ReadHeaderTimeout
        // is zero, the value of ReadTimeout is used. If both are
        // zero, there is no timeout.
        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.
        // A zero or negative value means there will be no timeout.
        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 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, unexpected behavior from handlers, and
        // underlying FileSystem errors.
        // If nil, logging is done via the log package's standard logger.
        ErrorLog *log.Logger

        // BaseContext optionally specifies a function that returns
        // the base context for incoming requests on this server.
        // The provided Listener is the specific Listener that's
        // about to start accepting requests.
        // If BaseContext is nil, the default is context.Background().
        // If non-nil, it must return a non-nil context.
        BaseContext func(net.Listener) context.Context

        // ConnContext optionally specifies a function that modifies
        // the context used for a new connection c. The provided ctx
        // is derived from the base context and has a ServerContextKey
        // value.
        ConnContext func(ctx context.Context, c net.Conn) context.Context

        inShutdown atomic.Bool // true when server is in shutdown

        disableKeepAlives atomic.Bool
        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{}
        onShutdown []func()

        listenerGroup sync.WaitGroup
}

// 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.inShutdown.Store(true)
        srv.mu.Lock()
        defer srv.mu.Unlock()
        err := srv.closeListenersLocked()

        // Unlock srv.mu while waiting for listenerGroup.
        // The group Add and Done calls are made with srv.mu held,
        // to avoid adding a new listener in the window between
        // us setting inShutdown above and waiting here.
        srv.mu.Unlock()
        srv.listenerGroup.Wait()
        srv.mu.Lock()

        for c := range srv.activeConn {
                c.rwc.Close()
                delete(srv.activeConn, c)
        }
        return err
}

// shutdownPollIntervalMax is the max polling interval when checking
// quiescence during Server.Shutdown. Polling starts with a small
// interval and backs off to the max.
// 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.
const shutdownPollIntervalMax = 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,
// Shutdown returns the context's error, otherwise it returns any
// error returned from closing the Server's underlying Listener(s).
//
// When Shutdown is called, Serve, ListenAndServe, and
// ListenAndServeTLS immediately return ErrServerClosed. Make sure the
// program doesn't exit and waits instead for Shutdown to return.
//
// 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. See RegisterOnShutdown for a way to
// register shutdown notification functions.
//
// Once Shutdown has been called on a server, it may not be reused;
// future calls to methods such as Serve will return ErrServerClosed.
func (srv *Server) Shutdown(ctx context.Context) error {
        srv.inShutdown.Store(true)

        srv.mu.Lock()
        lnerr := srv.closeListenersLocked()
        for _, f := range srv.onShutdown {
                go f()
        }
        srv.mu.Unlock()
        srv.listenerGroup.Wait()

        pollIntervalBase := time.Millisecond
        nextPollInterval := func() time.Duration {
                // Add 10% jitter.
                interval := pollIntervalBase + time.Duration(rand.Intn(int(pollIntervalBase/10)))
                // Double and clamp for next time.
                pollIntervalBase *= 2
                if pollIntervalBase > shutdownPollIntervalMax {
                        pollIntervalBase = shutdownPollIntervalMax
                }
                return interval
        }

        timer := time.NewTimer(nextPollInterval())
        defer timer.Stop()
        for {
                if srv.closeIdleConns() {
                        return lnerr
                }
                select {
                case <-ctx.Done():
                        return ctx.Err()
                case <-timer.C:
                        timer.Reset(nextPollInterval())
                }
        }
}

// RegisterOnShutdown registers a function to call on Shutdown.
// This can be used to gracefully shutdown connections that have
// undergone ALPN protocol upgrade or that have been hijacked.
// This function should start protocol-specific graceful shutdown,
// but should not wait for shutdown to complete.
func (srv *Server) RegisterOnShutdown(f func()) {
        srv.mu.Lock()
        srv.onShutdown = append(srv.onShutdown, f)
        srv.mu.Unlock()
}

// 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, unixSec := c.getState()
                // Issue 22682: treat StateNew connections as if
                // they're idle if we haven't read the first request's
                // header in over 5 seconds.
                if st == StateNew && unixSec < time.Now().Unix()-5 {
                        st = StateIdle
                }
                if st != StateIdle || unixSec == 0 {
                        // Assume unixSec == 0 means it's a very new
                        // connection, without state set yet.
                        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
                }
        }
        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 !sh.srv.DisableGeneralOptionsHandler && req.RequestURI == "*" && req.Method == "OPTIONS" {
                handler = globalOptionsHandler{}
        }

        handler.ServeHTTP(rw, req)
}

// AllowQuerySemicolons returns a handler that serves requests by converting any
// unescaped semicolons in the URL query to ampersands, and invoking the handler h.
//
// This restores the pre-Go 1.17 behavior of splitting query parameters on both
// semicolons and ampersands. (See golang.org/issue/25192). Note that this
// behavior doesn't match that of many proxies, and the mismatch can lead to
// security issues.
//
// AllowQuerySemicolons should be invoked before Request.ParseForm is called.
func AllowQuerySemicolons(h Handler) Handler {
        return HandlerFunc(func(w ResponseWriter, r *Request) {
                if strings.Contains(r.URL.RawQuery, ";") {
                        r2 := new(Request)
                        *r2 = *r
                        r2.URL = new(url.URL)
                        *r2.URL = *r.URL
                        r2.URL.RawQuery = strings.ReplaceAll(r.URL.RawQuery, ";", "&")
                        h.ServeHTTP(w, r2)
                } else {
                        h.ServeHTTP(w, r)
                }
        })
}

// 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. After Shutdown or Close,
// the returned error is ErrServerClosed.
func (srv *Server) ListenAndServe() error {
        if srv.shuttingDown() {
                return ErrServerClosed
        }
        addr := srv.Addr
        if addr == "" {
                addr = ":http"
        }
        ln, err := net.Listen("tcp", addr)
        if err != nil {
                return err
        }
        return srv.Serve(ln)
}

var testHookServerServe func(*Server, net.Listener) // used if non-nil

// shouldConfigureHTTP2ForServe 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)
}

// ErrServerClosed is returned by the Server's Serve, ServeTLS, ListenAndServe,
// and ListenAndServeTLS methods after a call to Shutdown or Close.
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.
//
// HTTP/2 support is only enabled if the Listener returns *tls.Conn
// connections and they were configured with "h2" in the TLS
// Config.NextProtos.
//
// Serve always returns a non-nil error and closes l.
// After Shutdown or Close, the returned error is ErrServerClosed.
func (srv *Server) Serve(l net.Listener) error {
        if fn := testHookServerServe; fn != nil {
                fn(srv, l) // call hook with unwrapped listener
        }

        origListener := l
        l = &onceCloseListener{Listener: l}
        defer l.Close()

        if err := srv.setupHTTP2_Serve(); err != nil {
                return err
        }

        if !srv.trackListener(&l, true) {
                return ErrServerClosed
        }
        defer srv.trackListener(&l, false)

        baseCtx := context.Background()
        if srv.BaseContext != nil {
                baseCtx = srv.BaseContext(origListener)
                if baseCtx == nil {
                        panic("BaseContext returned a nil context")
                }
        }

        var tempDelay time.Duration // how long to sleep on accept failure

        ctx := context.WithValue(baseCtx, ServerContextKey, srv)
        for {
                rw, err := l.Accept()
                if err != nil {
                        if srv.shuttingDown() {
                                return ErrServerClosed
                        }
                        if ne, ok := err.(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", err, tempDelay)
                                time.Sleep(tempDelay)
                                continue
                        }
                        return err
                }
                connCtx := ctx
                if cc := srv.ConnContext; cc != nil {
                        connCtx = cc(connCtx, rw)
                        if connCtx == nil {
                                panic("ConnContext returned nil")
                        }
                }
                tempDelay = 0
                c := srv.newConn(rw)
                c.setState(c.rwc, StateNew, runHooks) // before Serve can return
                go c.serve(connCtx)
        }
}

// ServeTLS accepts incoming connections on the Listener l, creating a
// new service goroutine for each. The service goroutines perform TLS
// setup and then read requests, calling srv.Handler to reply to them.
//
// Files 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.
//
// ServeTLS always returns a non-nil error. After Shutdown or Close, the
// returned error is ErrServerClosed.
func (srv *Server) ServeTLS(l net.Listener, certFile, keyFile string) error {
        // Setup HTTP/2 before srv.Serve, to initialize srv.TLSConfig
        // before we clone it and create the TLS Listener.
        if err := srv.setupHTTP2_ServeTLS(); 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
                }
        }

        tlsListener := tls.NewListener(l, config)
        return srv.Serve(tlsListener)
}

// trackListener adds or removes a net.Listener to the set of tracked
// listeners.
//
// We store a pointer to interface in the map set, in case the
// net.Listener is not comparable. This is safe because we only call
// trackListener via Serve and can track+defer untrack the same
// pointer to local variable there. We never need to compare a
// Listener from another caller.
//
// It reports whether the server is still up (not Shutdown or Closed).
func (s *Server) trackListener(ln *net.Listener, add bool) bool {
        s.mu.Lock()
        defer s.mu.Unlock()
        if s.listeners == nil {
                s.listeners = make(map[*net.Listener]struct{})
        }
        if add {
                if s.shuttingDown() {
                        return false
                }
                s.listeners[ln] = struct{}{}
                s.listenerGroup.Add(1)
        } else {
                delete(s.listeners, ln)
                s.listenerGroup.Done()
        }
        return true
}

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 !s.disableKeepAlives.Load() && !s.shuttingDown()
}

func (s *Server) shuttingDown() bool {
        return s.inShutdown.Load()
}

// 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 {
                srv.disableKeepAlives.Store(false)
                return
        }
        srv.disableKeepAlives.Store(true)

        // Close idle HTTP/1 conns:
        srv.closeIdleConns()

        // TODO: Issue 26303: close HTTP/2 conns as soon as they become idle.
}

func (s *Server) logf(format string, args ...any) {
        if s.ErrorLog != nil {
                s.ErrorLog.Printf(format, args...)
        } else {
                log.Printf(format, args...)
        }
}

// logf prints to the ErrorLog of the *Server associated with request r
// via ServerContextKey. If there's no associated server, or if ErrorLog
// is nil, logging is done via the log package's standard logger.
func logf(r *Request, format string, args ...any) {
        s, _ := r.Context().Value(ServerContextKey).(*Server)
        if s != nil && 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.
//
// The handler is typically nil, in which case [DefaultServeMux] is used.
//
// 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.
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 ServeTLS 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. After Shutdown or
// Close, the returned error is ErrServerClosed.
func (srv *Server) ListenAndServeTLS(certFile, keyFile string) error {
        if srv.shuttingDown() {
                return ErrServerClosed
        }
        addr := srv.Addr
        if addr == "" {
                addr = ":https"
        }

        ln, err := net.Listen("tcp", addr)
        if err != nil {
                return err
        }

        defer ln.Close()

        return srv.ServeTLS(ln, certFile, keyFile)
}

// setupHTTP2_ServeTLS conditionally configures HTTP/2 on
// srv and reports whether there was an error setting it up. If it is
// not configured for policy reasons, nil is returned.
func (srv *Server) setupHTTP2_ServeTLS() 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_ServeTLS because Serve is called after tls.Listen,
// and may be called concurrently. See shouldConfigureHTTP2ForServe.
//
// 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()
        }
}

var http2server = godebug.New("http2server")

// 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 omitBundledHTTP2 {
                return
        }
        if http2server.Value() == "0" {
                http2server.IncNonDefault()
                return
        }
        // Enable HTTP/2 by default if the user hasn't otherwise
        // configured their TLSNextProto map.
        if srv.TLSNextProto == nil {
                conf := &http2Server{
                        NewWriteScheduler: func() http2WriteScheduler { return http2NewPriorityWriteScheduler(nil) },
                }
                srv.nextProtoErr = http2ConfigureServer(srv, conf)
        }
}

// 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 supports the Pusher interface but 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 context will be created and this context will
        // be used instead.
        testContext context.Context
}

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) {
        ctx := h.testContext
        if ctx == nil {
                var cancelCtx context.CancelFunc
                ctx, cancelCtx = context.WithTimeout(r.Context(), h.dt)
                defer cancelCtx()
        }
        r = r.WithContext(ctx)
        done := make(chan struct{})
        tw := &timeoutWriter{
                w:   w,
                h:   make(Header),
                req: r,
        }
        panicChan := make(chan any, 1)
        go func() {
                defer func() {
                        if p := recover(); p != nil {
                                panicChan <- p
                        }
                }()
                h.handler.ServeHTTP(tw, r)
                close(done)
        }()
        select {
        case p := <-panicChan:
                panic(p)
        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())
        case <-ctx.Done():
                tw.mu.Lock()
                defer tw.mu.Unlock()
                switch err := ctx.Err(); err {
                case context.DeadlineExceeded:
                        w.WriteHeader(StatusServiceUnavailable)
                        io.WriteString(w, h.errorBody())
                        tw.err = ErrHandlerTimeout
                default:
                        w.WriteHeader(StatusServiceUnavailable)
                        tw.err = err
                }
        }
}

type timeoutWriter struct {
        w    ResponseWriter
        h    Header
        wbuf bytes.Buffer
        req  *Request

        mu          sync.Mutex
        err         error
        wroteHeader bool
        code        int
}

var _ Pusher = (*timeoutWriter)(nil)

// Push implements the Pusher interface.
func (tw *timeoutWriter) Push(target string, opts *PushOptions) error {
        if pusher, ok := tw.w.(Pusher); ok {
                return pusher.Push(target, opts)
        }
        return ErrNotSupported
}

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.err != nil {
                return 0, tw.err
        }
        if !tw.wroteHeader {
                tw.writeHeaderLocked(StatusOK)
        }
        return tw.wbuf.Write(p)
}

func (tw *timeoutWriter) writeHeaderLocked(code int) {
        checkWriteHeaderCode(code)

        switch {
        case tw.err != nil:
                return
        case tw.wroteHeader:
                if tw.req != nil {
                        caller := relevantCaller()
                        logf(tw.req, "http: superfluous response.WriteHeader call from %s (%s:%d)", caller.Function, path.Base(caller.File), caller.Line)
                }
        default:
                tw.wroteHeader = true
                tw.code = code
        }
}

func (tw *timeoutWriter) WriteHeader(code int) {
        tw.mu.Lock()
        defer tw.mu.Unlock()
        tw.writeHeaderLocked(code)
}

// onceCloseListener wraps a net.Listener, protecting it from
// multiple Close calls.
type onceCloseListener struct {
        net.Listener
        once     sync.Once
        closeErr error
}

func (oc *onceCloseListener) Close() error {
        oc.once.Do(oc.close)
        return oc.closeErr
}

func (oc *onceCloseListener) close() { oc.closeErr = oc.Listener.Close() }

// 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(io.Discard, mb)
        }
}

// initALPNRequest is an HTTP handler that initializes certain
// uninitialized fields in its *Request. Such partially-initialized
// Requests come from ALPN protocol handlers.
type initALPNRequest struct {
        ctx context.Context
        c   *tls.Conn
        h   serverHandler
}

// BaseContext is an exported but unadvertised http.Handler method
// recognized by x/net/http2 to pass down a context; the TLSNextProto
// API predates context support so we shoehorn through the only
// interface we have available.
func (h initALPNRequest) BaseContext() context.Context { return h.ctx }

func (h initALPNRequest) 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
}

// tlsRecordHeaderLooksLikeHTTP reports whether a TLS record header
// looks like it might've been a misdirected plaintext HTTP request.
func tlsRecordHeaderLooksLikeHTTP(hdr [5]byte) bool {
        switch string(hdr[:]) {
        case "GET /", "HEAD ", "POST ", "PUT /", "OPTIO":
                return true
        }
        return false
}

// MaxBytesHandler returns a Handler that runs h with its ResponseWriter and Request.Body wrapped by a MaxBytesReader.
func MaxBytesHandler(h Handler, n int64) Handler {
        return HandlerFunc(func(w ResponseWriter, r *Request) {
                r2 := *r
                r2.Body = MaxBytesReader(w, r.Body, n)
                h.ServeHTTP(w, &r2)
        })
}