// 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"
	"slices"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"time"

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

// 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.
//
// [Handler.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 as 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) {
	buf := getCopyBuf()
	defer putCopyBuf(buf)

	// 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
)

const copyBufPoolSize = 32 * 1024

var copyBufPool = sync.Pool{New: func() any { return new([copyBufPoolSize]byte) }}

func getCopyBuf() []byte {
	return copyBufPool.Get().(*[copyBufPoolSize]byte)[:]
}
func putCopyBuf(b []byte) {
	if len(b) != copyBufPoolSize {
		panic("trying to put back buffer of the wrong size in the copyBufPool")
	}
	copyBufPool.Put((*[copyBufPoolSize]byte)(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, 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.
			var reason string
			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()
				reason = "client sent an HTTP request to an HTTPS server"
			} else {
				reason = err.Error()
			}
			c.server.logf("http: TLS handshake error from %s: %v", c.rwc.RemoteAddr(), reason)
			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.
//
// Error deletes the Content-Length and Content-Encoding headers,
// sets Content-Type to “text/plain; charset=utf-8”,
// and sets X-Content-Type-Options to “nosniff”.
// This configures the header properly for the error message,
// in case the caller had set it up expecting a successful output.
func Error(w ResponseWriter, error string, code int) {
	h := w.Header()
	// We delete headers which might be valid for some other content,
	// but not anymore for the error content.
	h.Del("Content-Length")
	h.Del("Content-Encoding")

	// There might be content type already set, but we reset it to
	// text/plain for the error message.
	h.Set("Content-Type", "text/plain; charset=utf-8")
	h.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}
}

// 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
//
// Patterns can match the method, host and path of a request.
// Some examples:
//
//   - "/index.html" matches the path "/index.html" for any host and method.
//   - "GET /static/" matches a GET request whose path begins with "/static/".
//   - "example.com/" matches any request to the host "example.com".
//   - "example.com/{$}" matches requests with host "example.com" and path "/".
//   - "/b/{bucket}/o/{objectname...}" matches paths whose first segment is "b"
//     and whose third segment is "o". The name "bucket" denotes the second
//     segment and "objectname" denotes the remainder of the path.
//
// In general, a pattern looks like
//
//	[METHOD ][HOST]/[PATH]
//
// All three parts are optional; "/" is a valid pattern.
// If METHOD is present, it must be followed by at least one space or tab.
//
// Literal (that is, non-wildcard) parts of a pattern match
// the corresponding parts of a request case-sensitively.
//
// A pattern with no method matches every method. A pattern
// with the method GET matches both GET and HEAD requests.
// Otherwise, the method must match exactly.
//
// A pattern with no host matches every host.
// A pattern with a host matches URLs on that host only.
//
// A path can include wildcard segments of the form {NAME} or {NAME...}.
// For example, "/b/{bucket}/o/{objectname...}".
// The wildcard name must be a valid Go identifier.
// Wildcards must be full path segments: they must be preceded by a slash and followed by
// either a slash or the end of the string.
// For example, "/b_{bucket}" is not a valid pattern.
//
// Normally a wildcard matches only a single path segment,
// ending at the next literal slash (not %2F) in the request URL.
// But if the "..." is present, then the wildcard matches the remainder of the URL path, including slashes.
// (Therefore it is invalid for a "..." wildcard to appear anywhere but at the end of a pattern.)
// The match for a wildcard can be obtained by calling [Request.PathValue] with the wildcard's name.
// A trailing slash in a path acts as an anonymous "..." wildcard.
//
// The special wildcard {$} matches only the end of the URL.
// For example, the pattern "/{$}" matches only the path "/",
// whereas the pattern "/" matches every path.
//
// For matching, both pattern paths and incoming request paths are unescaped segment by segment.
// So, for example, the path "/a%2Fb/100%25" is treated as having two segments, "a/b" and "100%".
// The pattern "/a%2fb/" matches it, but the pattern "/a/b/" does not.
//
// # Precedence
//
// If two or more patterns match a request, then the most specific pattern takes precedence.
// A pattern P1 is more specific than P2 if P1 matches a strict subset of P2’s requests;
// that is, if P2 matches all the requests of P1 and more.
// If neither is more specific, then the patterns conflict.
// There is one exception to this rule, for backwards compatibility:
// if two patterns would otherwise conflict and one has a host while the other does not,
// then the pattern with the host takes precedence.
// If a pattern passed to [ServeMux.Handle] or [ServeMux.HandleFunc] conflicts with
// another pattern that is already registered, those functions panic.
//
// As an example of the general rule, "/images/thumbnails/" is more specific than "/images/",
// so both can be registered.
// The former matches paths beginning with "/images/thumbnails/"
// and the latter will match any other path in the "/images/" subtree.
//
// As another example, consider the patterns "GET /" and "/index.html":
// both match a GET request for "/index.html", but the former pattern
// matches all other GET and HEAD requests, while the latter matches any
// request for "/index.html" that uses a different method.
// The patterns conflict.
//
// # Trailing-slash redirection
//
// Consider a [ServeMux] with a handler for a subtree, registered using a trailing slash or "..." wildcard.
// If the ServeMux receives a request for the subtree root without a trailing slash,
// it redirects the request by adding the trailing slash.
// This behavior can be overridden with a separate registration for the path without
// the trailing slash or "..." wildcard. For example, registering "/images/" causes ServeMux
// to redirect a request for "/images" to "/images/", unless "/images" has
// been registered separately.
//
// # Request sanitizing
//
// ServeMux also takes care of sanitizing the URL request path and the Host
// header, stripping the port number and redirecting any request containing . or
// .. segments or repeated slashes to an equivalent, cleaner URL.
//
// # Compatibility
//
// The pattern syntax and matching behavior of ServeMux changed significantly
// in Go 1.22. To restore the old behavior, set the GODEBUG environment variable
// to "httpmuxgo121=1". This setting is read once, at program startup; changes
// during execution will be ignored.
//
// The backwards-incompatible changes include:
//   - Wildcards are just ordinary literal path segments in 1.21.
//     For example, the pattern "/{x}" will match only that path in 1.21,
//     but will match any one-segment path in 1.22.
//   - In 1.21, no pattern was rejected, unless it was empty or conflicted with an existing pattern.
//     In 1.22, syntactically invalid patterns will cause [ServeMux.Handle] and [ServeMux.HandleFunc] to panic.
//     For example, in 1.21, the patterns "/{"  and "/a{x}" match themselves,
//     but in 1.22 they are invalid and will cause a panic when registered.
//   - In 1.22, each segment of a pattern is unescaped; this was not done in 1.21.
//     For example, in 1.22 the pattern "/%61" matches the path "/a" ("%61" being the URL escape sequence for "a"),
//     but in 1.21 it would match only the path "/%2561" (where "%25" is the escape for the percent sign).
//   - When matching patterns to paths, in 1.22 each segment of the path is unescaped; in 1.21, the entire path is unescaped.
//     This change mostly affects how paths with %2F escapes adjacent to slashes are treated.
//     See https://go.dev/issue/21955 for details.
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,
	// or the URL already has a trailing slash, then we're done.
	if !exactMatch(n, path) && u != nil && !strings.HasSuffix(path, "/") {
		// 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)
	slices.Sort(methods)
	return methods
}

// TODO(jba): 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 ServeMux.register so that callerLocation
// always refers to user code.

// Handle registers the handler for the given pattern.
// If the given pattern conflicts, with one that is already registered, Handle
// panics.
func (mux *ServeMux) Handle(pattern string, handler Handler) {
	if use121 {
		mux.mux121.handle(pattern, handler)
	} else {
		mux.register(pattern, handler)
	}
}

// HandleFunc registers the handler function for the given pattern.
// If the given pattern conflicts, with one that is already registered, HandleFunc
// panics.
func (mux *ServeMux) HandleFunc(pattern string, handler func(ResponseWriter, *Request)) {
	if use121 {
		mux.mux121.handleFunc(pattern, handler)
	} else {
		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)
	} else {
		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)
	} else {
		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 zero, the value of
	// ReadTimeout is used. If negative, or if zero and ReadTimeout
	// is zero or negative, 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 zero, the value
	// of ReadTimeout is used. If negative, or if zero and ReadTimeout
	// is zero or negative, 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 [Server.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 [Server.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 [Server.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 [Server.Shutdown] or [Server.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 slices.Contains(srv.TLSConfig.NextProtos, http2NextProtoTLS)
}

// ErrServerClosed is returned by the [Server.Serve], [ServeTLS], [ListenAndServe],
// and [ListenAndServeTLS] methods after a call to [Server.Shutdown] or [Server.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 [Server.Shutdown] or [Server.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 [Server.Shutdown] or [Server.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 !slices.Contains(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 [Server.Shutdown] or
// [Server.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
}

// 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)
	})
}