Source File
	transport.go

Belonging Package
	net/http/internal/http2

// Copyright 2015 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.

// Transport code.

package http2

import (
	"bufio"
	"bytes"
	"compress/flate"
	"compress/gzip"
	"context"
	"crypto/rand"
	"crypto/tls"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"log"
	"math"
	"math/bits"
	mathrand "math/rand"
	"net"
	"net/http/httptrace"
	"net/http/internal"
	"net/http/internal/httpcommon"
	"net/textproto"
	"slices"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"golang.org/x/net/http/httpguts"
	"golang.org/x/net/http2/hpack"
	"golang.org/x/net/idna"
)

const (
	// transportDefaultConnFlow is how many connection-level flow control
	// tokens we give the server at start-up, past the default 64k.
	transportDefaultConnFlow = 1 << 30

	// transportDefaultStreamFlow is how many stream-level flow
	// control tokens we announce to the peer, and how many bytes
	// we buffer per stream.
	transportDefaultStreamFlow = 4 << 20

	defaultUserAgent = "Go-http-client/2.0"

	// initialMaxConcurrentStreams is a connections maxConcurrentStreams until
	// it's received servers initial SETTINGS frame, which corresponds with the
	// spec's minimum recommended value.
	initialMaxConcurrentStreams = 100

	// defaultMaxConcurrentStreams is a connections default maxConcurrentStreams
	// if the server doesn't include one in its initial SETTINGS frame.
	defaultMaxConcurrentStreams = 1000
)

// Transport is an HTTP/2 Transport.
//
// A Transport internally caches connections to servers. It is safe
// for concurrent use by multiple goroutines.
type Transport struct {
	t1       TransportConfig
	connPool noDialClientConnPool
	*transportTestHooks
}

// Hook points used for testing.
// Outside of tests, t.transportTestHooks is nil and these all have minimal implementations.
// Inside tests, see the testSyncHooks function docs.

type transportTestHooks struct {
	newclientconn func(*ClientConn)
}

func (t *Transport) maxHeaderListSize() uint32 {
	n := t.t1.MaxHeaderListSize()
	if b := t.t1.MaxResponseHeaderBytes(); b != 0 {
		n = b
		if n > 0 {
			n = adjustHTTP1MaxHeaderSize(n)
		}
	}
	if n <= 0 {
		return 10 << 20
	}
	if n >= 0xffffffff {
		return 0
	}
	return uint32(n)
}

func (t *Transport) disableCompression() bool {
	return t.t1 != nil && t.t1.DisableCompression()
}

func NewTransport(t1 TransportConfig) *Transport {
	connPool := new(clientConnPool)
	t2 := &Transport{
		connPool: noDialClientConnPool{connPool},
		t1:       t1,
	}
	connPool.t = t2
	return t2
}

func (t *Transport) AddConn(scheme, authority string, c net.Conn) error {
	addr := authorityAddr(scheme, authority)
	used, err := t.connPool.addConnIfNeeded(addr, t, c)
	if !used {
		go c.Close()
	}
	return err
}

// unencryptedTransport is a Transport with a RoundTrip method that
// always permits http:// URLs.
type unencryptedTransport Transport

func (t *unencryptedTransport) RoundTrip(req *ClientRequest) (*ClientResponse, error) {
	return (*Transport)(t).RoundTripOpt(req, RoundTripOpt{})
}

// ClientConn is the state of a single HTTP/2 client connection to an
// HTTP/2 server.
type ClientConn struct {
	t             *Transport
	tconn         net.Conn             // usually *tls.Conn, except specialized impls
	tlsState      *tls.ConnectionState // nil only for specialized impls
	atomicReused  uint32               // whether conn is being reused; atomic
	singleUse     bool                 // whether being used for a single http.Request
	getConnCalled bool                 // used by clientConnPool

	// readLoop goroutine fields:
	readerDone chan struct{} // closed on error
	readerErr  error         // set before readerDone is closed

	idleTimeout time.Duration // or 0 for never
	idleTimer   *time.Timer

	mu               sync.Mutex  // guards following
	cond             *sync.Cond  // hold mu; broadcast on flow/closed changes
	flow             connOutflow // our conn-level flow control quota (cs.outflow is per stream)
	inflow           inflow      // peer's conn-level flow control
	doNotReuse       bool        // whether conn is marked to not be reused for any future requests
	closing          bool
	closed           bool
	closedOnIdle     bool                     // true if conn was closed for idleness
	seenSettings     bool                     // true if we've seen a settings frame, false otherwise
	seenSettingsChan chan struct{}            // closed when seenSettings is true or frame reading fails
	wantSettingsAck  bool                     // we sent a SETTINGS frame and haven't heard back
	goAway           *GoAwayFrame             // if non-nil, the GoAwayFrame we received
	goAwayDebug      string                   // goAway frame's debug data, retained as a string
	streams          map[uint32]*clientStream // client-initiated
	streamsReserved  int                      // incr by ReserveNewRequest; decr on RoundTrip
	nextStreamID     uint32
	pendingRequests  int                       // requests blocked and waiting to be sent because len(streams) == maxConcurrentStreams
	pings            map[[8]byte]chan struct{} // in flight ping data to notification channel
	br               *bufio.Reader
	lastActive       time.Time
	lastIdle         time.Time // time last idle
	// Settings from peer: (also guarded by wmu)
	maxFrameSize                uint32
	maxConcurrentStreams        uint32
	peerMaxHeaderListSize       uint64
	peerMaxHeaderTableSize      uint32
	initialStreamRecvWindowSize int32
	readIdleTimeout             time.Duration
	pingTimeout                 time.Duration
	extendedConnectAllowed      bool
	strictMaxConcurrentStreams  bool

	// rstStreamPingsBlocked works around an unfortunate gRPC behavior.
	// gRPC strictly limits the number of PING frames that it will receive.
	// The default is two pings per two hours, but the limit resets every time
	// the gRPC endpoint sends a HEADERS or DATA frame. See golang/go#70575.
	//
	// rstStreamPingsBlocked is set after receiving a response to a PING frame
	// bundled with an RST_STREAM (see pendingResets below), and cleared after
	// receiving a HEADERS or DATA frame.
	rstStreamPingsBlocked bool

	// pendingResets is the number of RST_STREAM frames we have sent to the peer,
	// without confirming that the peer has received them. When we send a RST_STREAM,
	// we bundle it with a PING frame, unless a PING is already in flight. We count
	// the reset stream against the connection's concurrency limit until we get
	// a PING response. This limits the number of requests we'll try to send to a
	// completely unresponsive connection.
	pendingResets int

	// readBeforeStreamID is the smallest stream ID that has not been followed by
	// a frame read from the peer. We use this to determine when a request may
	// have been sent to a completely unresponsive connection:
	// If the request ID is less than readBeforeStreamID, then we have had some
	// indication of life on the connection since sending the request.
	readBeforeStreamID uint32

	// reqHeaderMu is a 1-element semaphore channel controlling access to sending new requests.
	// Write to reqHeaderMu to lock it, read from it to unlock.
	// Lock reqmu BEFORE mu or wmu.
	reqHeaderMu chan struct{}

	// internalStateHook reports state changes back to the net/http.ClientConn.
	// Note that this is different from the user state hook registered by
	// net/http.ClientConn.SetStateHook: The internal hook calls ClientConn,
	// which calls the user hook.
	internalStateHook func()

	// wmu is held while writing.
	// Acquire BEFORE mu when holding both, to avoid blocking mu on network writes.
	// Only acquire both at the same time when changing peer settings.
	wmu  sync.Mutex
	bw   *bufio.Writer
	fr   *Framer
	werr error        // first write error that has occurred
	hbuf bytes.Buffer // HPACK encoder writes into this
	henc *hpack.Encoder
}

// clientStream is the state for a single HTTP/2 stream. One of these
// is created for each Transport.RoundTrip call.
type clientStream struct {
	cc *ClientConn

	// Fields of Request that we may access even after the response body is closed.
	ctx       context.Context
	reqCancel <-chan struct{}

	trace         *httptrace.ClientTrace // or nil
	ID            uint32
	bufPipe       pipe // buffered pipe with the flow-controlled response payload
	requestedGzip bool
	isHead        bool

	abortOnce sync.Once
	abort     chan struct{} // closed to signal stream should end immediately
	abortErr  error         // set if abort is closed

	peerClosed chan struct{} // closed when the peer sends an END_STREAM flag
	donec      chan struct{} // closed after the stream is in the closed state
	on100      chan struct{} // buffered; written to if a 100 is received

	respHeaderRecv chan struct{}   // closed when headers are received
	res            *ClientResponse // set if respHeaderRecv is closed

	flow        outflow // guarded by cc.mu
	inflow      inflow  // guarded by cc.mu
	bytesRemain int64   // -1 means unknown; owned by transportResponseBody.Read
	readErr     error   // sticky read error; owned by transportResponseBody.Read

	reqBody              io.ReadCloser
	reqBodyContentLength int64         // -1 means unknown
	reqBodyClosed        chan struct{} // guarded by cc.mu; non-nil on Close, closed when done

	// owned by writeRequest:
	sentEndStream bool // sent an END_STREAM flag to the peer
	sentHeaders   bool

	// owned by clientConnReadLoop:
	firstByte       bool  // got the first response byte
	pastHeaders     bool  // got first MetaHeadersFrame (actual headers)
	pastTrailers    bool  // got optional second MetaHeadersFrame (trailers)
	readClosed      bool  // peer sent an END_STREAM flag
	readAborted     bool  // read loop reset the stream
	totalHeaderSize int64 // total size of 1xx headers seen

	trailer    Header  // accumulated trailers
	resTrailer *Header // client's Response.Trailer

	staticResp ClientResponse
}

var got1xxFuncForTests func(int, textproto.MIMEHeader) error

// get1xxTraceFunc returns the value of request's httptrace.ClientTrace.Got1xxResponse func,
// if any. It returns nil if not set or if the Go version is too old.
func (cs *clientStream) get1xxTraceFunc() func(int, textproto.MIMEHeader) error {
	if fn := got1xxFuncForTests; fn != nil {
		return fn
	}
	return traceGot1xxResponseFunc(cs.trace)
}

func (cs *clientStream) abortStream(err error) {
	cs.cc.mu.Lock()
	defer cs.cc.mu.Unlock()
	cs.abortStreamLocked(err)
}

func (cs *clientStream) abortStreamLocked(err error) {
	cs.abortOnce.Do(func() {
		cs.abortErr = err
		close(cs.abort)
	})
	if cs.reqBody != nil {
		cs.closeReqBodyLocked()
	}
	// TODO(dneil): Clean up tests where cs.cc.cond is nil.
	if cs.cc.cond != nil {
		// Wake up writeRequestBody if it is waiting on flow control.
		cs.cc.cond.Broadcast()
	}
}

func (cs *clientStream) abortRequestBodyWrite() {
	cc := cs.cc
	cc.mu.Lock()
	defer cc.mu.Unlock()
	if cs.reqBody != nil && cs.reqBodyClosed == nil {
		cs.closeReqBodyLocked()
		cc.cond.Broadcast()
	}
}

func (cs *clientStream) closeReqBodyLocked() {
	if cs.reqBodyClosed != nil {
		return
	}
	cs.reqBodyClosed = make(chan struct{})
	reqBodyClosed := cs.reqBodyClosed
	go func() {
		cs.reqBody.Close()
		close(reqBodyClosed)
	}()
}

type stickyErrWriter struct {
	conn    net.Conn
	timeout time.Duration
	err     *error
}

func (sew stickyErrWriter) Write(p []byte) (n int, err error) {
	if *sew.err != nil {
		return 0, *sew.err
	}
	n, err = writeWithByteTimeout(sew.conn, sew.timeout, p)
	*sew.err = err
	return n, err
}

// noCachedConnError is the concrete type of ErrNoCachedConn, which
// needs to be detected by net/http regardless of whether it's its
// bundled version (in h2_bundle.go with a rewritten type name) or
// from a user's x/net/http2. As such, as it has a unique method name
// (IsHTTP2NoCachedConnError) that net/http sniffs for via func
// isNoCachedConnError.
type noCachedConnError struct{}

func (noCachedConnError) IsHTTP2NoCachedConnError() {}
func (noCachedConnError) Error() string             { return "http2: no cached connection was available" }

// isNoCachedConnError reports whether err is of type noCachedConnError
// or its equivalent renamed type in net/http2's h2_bundle.go. Both types
// may coexist in the same running program.
func isNoCachedConnError(err error) bool {
	_, ok := err.(interface{ IsHTTP2NoCachedConnError() })
	return ok
}

var ErrNoCachedConn error = noCachedConnError{}

// RoundTripOpt are options for the Transport.RoundTripOpt method.
type RoundTripOpt struct {
	// OnlyCachedConn controls whether RoundTripOpt may
	// create a new TCP connection. If set true and
	// no cached connection is available, RoundTripOpt
	// will return ErrNoCachedConn.
	OnlyCachedConn bool
}

func (t *Transport) RoundTrip(req *ClientRequest) (*ClientResponse, error) {
	return t.RoundTripOpt(req, RoundTripOpt{})
}

// authorityAddr returns a given authority (a host/IP, or host:port / ip:port)
// and returns a host:port. The port 443 is added if needed.
func authorityAddr(scheme string, authority string) (addr string) {
	host, port, err := net.SplitHostPort(authority)
	if err != nil { // authority didn't have a port
		host = authority
		port = ""
	}
	if port == "" { // authority's port was empty
		port = "443"
		if scheme == "http" {
			port = "80"
		}
	}
	if a, err := idna.ToASCII(host); err == nil {
		host = a
	}
	// IPv6 address literal, without a port:
	if strings.HasPrefix(host, "[") && strings.HasSuffix(host, "]") {
		return host + ":" + port
	}
	return net.JoinHostPort(host, port)
}

// RoundTripOpt is like RoundTrip, but takes options.
func (t *Transport) RoundTripOpt(req *ClientRequest, opt RoundTripOpt) (*ClientResponse, error) {
	switch req.URL.Scheme {
	case "https":
	case "http":
	default:
		return nil, errors.New("http2: unsupported scheme")
	}

	addr := authorityAddr(req.URL.Scheme, req.URL.Host)
	for retry := 0; ; retry++ {
		cc, err := t.connPool.GetClientConn(req, addr)
		if err != nil {
			t.vlogf("http2: Transport failed to get client conn for %s: %v", addr, err)
			return nil, err
		}
		reused := !atomic.CompareAndSwapUint32(&cc.atomicReused, 0, 1)
		traceGotConn(req, cc, reused)
		res, err := cc.RoundTrip(req)
		if err != nil && retry <= 6 {
			roundTripErr := err
			if req, err = shouldRetryRequest(req, err); err == nil {
				// After the first retry, do exponential backoff with 10% jitter.
				if retry == 0 {
					t.vlogf("RoundTrip retrying after failure: %v", roundTripErr)
					continue
				}
				backoff := float64(uint(1) << (uint(retry) - 1))
				backoff += backoff * (0.1 * mathrand.Float64())
				d := time.Second * time.Duration(backoff)
				tm := time.NewTimer(d)
				select {
				case <-tm.C:
					t.vlogf("RoundTrip retrying after failure: %v", roundTripErr)
					continue
				case <-req.Context.Done():
					tm.Stop()
					err = req.Context.Err()
				}
			}
		}
		if err == errClientConnNotEstablished {
			// This ClientConn was created recently,
			// this is the first request to use it,
			// and the connection is closed and not usable.
			//
			// In this state, cc.idleTimer will remove the conn from the pool
			// when it fires. Stop the timer and remove it here so future requests
			// won't try to use this connection.
			//
			// If the timer has already fired and we're racing it, the redundant
			// call to MarkDead is harmless.
			if cc.idleTimer != nil {
				cc.idleTimer.Stop()
			}
			t.connPool.MarkDead(cc)
		}
		if err != nil {
			t.vlogf("RoundTrip failure: %v", err)
			return nil, err
		}
		return res, nil
	}
}

func (t *Transport) IdleConnStrsForTesting() []string {
	var ret []string
	t.connPool.mu.Lock()
	defer t.connPool.mu.Unlock()
	for k, ccs := range t.connPool.conns {
		for _, cc := range ccs {
			if cc.idleState().canTakeNewRequest {
				ret = append(ret, k)
			}
		}
	}
	slices.Sort(ret)
	return ret
}

// CloseIdleConnections closes any connections which were previously
// connected from previous requests but are now sitting idle.
// It does not interrupt any connections currently in use.
func (t *Transport) CloseIdleConnections() {
	t.connPool.closeIdleConnections()
}

var (
	errClientConnClosed         = errors.New("http2: client conn is closed")
	errClientConnUnusable       = errors.New("http2: client conn not usable")
	errClientConnNotEstablished = errors.New("http2: client conn could not be established")
	errClientConnGotGoAway      = errors.New("http2: Transport received Server's graceful shutdown GOAWAY")
	errClientConnForceClosed    = errors.New("http2: client connection force closed via ClientConn.Close")
)

// shouldRetryRequest is called by RoundTrip when a request fails to get
// response headers. It is always called with a non-nil error.
// It returns either a request to retry or an error if the request can't be replayed.
// If the request is retried, it always clones the request (since requests
// contain an unreusable clientStream).
func shouldRetryRequest(req *ClientRequest, err error) (*ClientRequest, error) {
	if !canRetryError(err) {
		return nil, err
	}
	// If the Body is nil (or http.NoBody), it's safe to reuse this request's Body.
	if req.Body == nil || req.Body == NoBody {
		return req.Clone(), nil
	}

	// If the request body can be reset back to its original
	// state via the optional req.GetBody, do that.
	if req.GetBody != nil {
		body, err := req.GetBody()
		if err != nil {
			return nil, err
		}
		newReq := req.Clone()
		newReq.Body = body
		return newReq, nil
	}

	// The Request.Body can't reset back to the beginning, but we
	// don't seem to have started to read from it yet, so reuse the body.
	if err == errClientConnUnusable {
		return req.Clone(), nil
	}

	return nil, fmt.Errorf("http2: Transport: cannot retry err [%v] after Request.Body was written; define Request.GetBody to avoid this error", err)
}

func canRetryError(err error) bool {
	if err == errClientConnUnusable || err == errClientConnGotGoAway {
		return true
	}
	if se, ok := err.(StreamError); ok {
		return se.Code == ErrCodeRefusedStream
	}
	return false
}

func (t *Transport) dialClientConn(ctx context.Context, addr string, singleUse bool) (*ClientConn, error) {
	if t.transportTestHooks != nil {
		return t.newClientConn(nil, singleUse, nil)
	}
	host, _, err := net.SplitHostPort(addr)
	if err != nil {
		return nil, err
	}
	tconn, err := t.dialTLS(ctx, "tcp", addr, t.newTLSConfig(host))
	if err != nil {
		return nil, err
	}
	return t.newClientConn(tconn, singleUse, nil)
}

func (t *Transport) newTLSConfig(host string) *tls.Config {
	cfg := new(tls.Config)
	if !slices.Contains(cfg.NextProtos, NextProtoTLS) {
		cfg.NextProtos = append([]string{NextProtoTLS}, cfg.NextProtos...)
	}
	if cfg.ServerName == "" {
		cfg.ServerName = host
	}
	return cfg
}

func (t *Transport) dialTLS(ctx context.Context, network, addr string, tlsCfg *tls.Config) (net.Conn, error) {
	tlsCn, err := t.dialTLSWithContext(ctx, network, addr, tlsCfg)
	if err != nil {
		return nil, err
	}
	state := tlsCn.ConnectionState()
	if p := state.NegotiatedProtocol; p != NextProtoTLS {
		return nil, fmt.Errorf("http2: unexpected ALPN protocol %q; want %q", p, NextProtoTLS)
	}
	if !state.NegotiatedProtocolIsMutual {
		return nil, errors.New("http2: could not negotiate protocol mutually")
	}
	return tlsCn, nil
}

// disableKeepAlives reports whether connections should be closed as
// soon as possible after handling the first request.
func (t *Transport) disableKeepAlives() bool {
	return t.t1 != nil && t.t1.DisableKeepAlives()
}

func (t *Transport) expectContinueTimeout() time.Duration {
	if t.t1 == nil {
		return 0
	}
	return t.t1.ExpectContinueTimeout()
}

func (t *Transport) NewClientConn(c net.Conn, internalStateHook func()) (NetHTTPClientConn, error) {
	cc, err := t.newClientConn(c, t.disableKeepAlives(), internalStateHook)
	if err != nil {
		return NetHTTPClientConn{}, err
	}

	// RoundTrip should block when the conn is at its concurrency limit,
	// not return an error. Setting strictMaxConcurrentStreams enables this.
	cc.strictMaxConcurrentStreams = true

	return NetHTTPClientConn{cc}, nil
}

func (t *Transport) newClientConn(c net.Conn, singleUse bool, internalStateHook func()) (*ClientConn, error) {
	conf := configFromTransport(t)
	cc := &ClientConn{
		t:                           t,
		tconn:                       c,
		readerDone:                  make(chan struct{}),
		nextStreamID:                1,
		maxFrameSize:                16 << 10, // spec default
		initialStreamRecvWindowSize: int32(conf.MaxReceiveBufferPerStream),
		maxConcurrentStreams:        initialMaxConcurrentStreams, // "infinite", per spec. Use a smaller value until we have received server settings.
		strictMaxConcurrentStreams:  conf.StrictMaxConcurrentRequests,
		peerMaxHeaderListSize:       0xffffffffffffffff, // "infinite", per spec. Use 2^64-1 instead.
		streams:                     make(map[uint32]*clientStream),
		singleUse:                   singleUse,
		seenSettingsChan:            make(chan struct{}),
		wantSettingsAck:             true,
		readIdleTimeout:             conf.SendPingTimeout,
		pingTimeout:                 conf.PingTimeout,
		pings:                       make(map[[8]byte]chan struct{}),
		reqHeaderMu:                 make(chan struct{}, 1),
		lastActive:                  time.Now(),
		internalStateHook:           internalStateHook,
	}
	if t.transportTestHooks != nil {
		t.transportTestHooks.newclientconn(cc)
		c = cc.tconn
	}
	if VerboseLogs {
		t.vlogf("http2: Transport creating client conn %p to %v", cc, c.RemoteAddr())
	}

	cc.cond = sync.NewCond(&cc.mu)
	cc.flow.init()

	// TODO: adjust this writer size to account for frame size +
	// MTU + crypto/tls record padding.
	cc.bw = bufio.NewWriter(stickyErrWriter{
		conn:    c,
		timeout: conf.WriteByteTimeout,
		err:     &cc.werr,
	})
	cc.br = bufio.NewReader(c)
	cc.fr = NewFramer(cc.bw, cc.br)
	cc.fr.SetMaxReadFrameSize(uint32(conf.MaxReadFrameSize))
	if conf.CountError != nil {
		cc.fr.countError = conf.CountError
	}
	maxHeaderTableSize := uint32(conf.MaxDecoderHeaderTableSize)
	cc.fr.ReadMetaHeaders = hpack.NewDecoder(maxHeaderTableSize, nil)
	cc.fr.MaxHeaderListSize = t.maxHeaderListSize()

	cc.henc = hpack.NewEncoder(&cc.hbuf)
	cc.henc.SetMaxDynamicTableSizeLimit(uint32(conf.MaxEncoderHeaderTableSize))
	cc.peerMaxHeaderTableSize = initialHeaderTableSize

	if cs, ok := c.(connectionStater); ok {
		state := cs.ConnectionState()
		cc.tlsState = &state
	}

	initialSettings := []Setting{
		{ID: SettingEnablePush, Val: 0},
		{ID: SettingInitialWindowSize, Val: uint32(cc.initialStreamRecvWindowSize)},
	}
	initialSettings = append(initialSettings, Setting{ID: SettingMaxFrameSize, Val: uint32(conf.MaxReadFrameSize)})
	if max := t.maxHeaderListSize(); max != 0 {
		initialSettings = append(initialSettings, Setting{ID: SettingMaxHeaderListSize, Val: max})
	}
	if maxHeaderTableSize != initialHeaderTableSize {
		initialSettings = append(initialSettings, Setting{ID: SettingHeaderTableSize, Val: maxHeaderTableSize})
	}

	cc.bw.Write(clientPreface)
	cc.fr.WriteSettings(initialSettings...)
	cc.fr.WriteWindowUpdate(0, uint32(conf.MaxReceiveBufferPerConnection))
	cc.inflow.init(int32(conf.MaxReceiveBufferPerConnection) + initialWindowSize)
	cc.bw.Flush()
	if cc.werr != nil {
		cc.Close()
		return nil, cc.werr
	}

	// Start the idle timer after the connection is fully initialized.
	if d := t.idleConnTimeout(); d != 0 {
		cc.idleTimeout = d
		cc.idleTimer = time.AfterFunc(d, cc.onIdleTimeout)
	}

	go cc.readLoop()
	return cc, nil
}

func (cc *ClientConn) healthCheck() {
	pingTimeout := cc.pingTimeout
	// We don't need to periodically ping in the health check, because the readLoop of ClientConn will
	// trigger the healthCheck again if there is no frame received.
	ctx, cancel := context.WithTimeout(context.Background(), pingTimeout)
	defer cancel()
	cc.vlogf("http2: Transport sending health check")
	err := cc.Ping(ctx)
	if err != nil {
		cc.vlogf("http2: Transport health check failure: %v", err)
		cc.closeForLostPing()
	} else {
		cc.vlogf("http2: Transport health check success")
	}
}

// SetDoNotReuse marks cc as not reusable for future HTTP requests.
func (cc *ClientConn) SetDoNotReuse() {
	cc.mu.Lock()
	defer cc.mu.Unlock()
	cc.doNotReuse = true
}

// CanTakeNewRequest reports whether the connection can take a new request,
// meaning it has not been closed or received or sent a GOAWAY.
//
// If the caller is going to immediately make a new request on this
// connection, use ReserveNewRequest instead.
func (cc *ClientConn) CanTakeNewRequest() bool {
	cc.mu.Lock()
	defer cc.mu.Unlock()
	return cc.canTakeNewRequestLocked()
}

// ReserveNewRequest is like CanTakeNewRequest but also reserves a
// concurrent stream in cc. The reservation is decremented on the
// next call to RoundTrip.
func (cc *ClientConn) ReserveNewRequest() bool {
	cc.mu.Lock()
	defer cc.mu.Unlock()
	if st := cc.idleStateLocked(); !st.canTakeNewRequest {
		return false
	}
	cc.streamsReserved++
	return true
}

// ClientConnState describes the state of a ClientConn.
type ClientConnState struct {
	// Closed is whether the connection is closed.
	Closed bool

	// Closing is whether the connection is in the process of
	// closing. It may be closing due to shutdown, being a
	// single-use connection, being marked as DoNotReuse, or
	// having received a GOAWAY frame.
	Closing bool

	// StreamsActive is how many streams are active.
	StreamsActive int

	// StreamsReserved is how many streams have been reserved via
	// ClientConn.ReserveNewRequest.
	StreamsReserved int

	// StreamsPending is how many requests have been sent in excess
	// of the peer's advertised MaxConcurrentStreams setting and
	// are waiting for other streams to complete.
	StreamsPending int

	// MaxConcurrentStreams is how many concurrent streams the
	// peer advertised as acceptable. Zero means no SETTINGS
	// frame has been received yet.
	MaxConcurrentStreams uint32

	// LastIdle, if non-zero, is when the connection last
	// transitioned to idle state.
	LastIdle time.Time
}

// State returns a snapshot of cc's state.
func (cc *ClientConn) State() ClientConnState {
	cc.wmu.Lock()
	maxConcurrent := cc.maxConcurrentStreams
	if !cc.seenSettings {
		maxConcurrent = 0
	}
	cc.wmu.Unlock()

	cc.mu.Lock()
	defer cc.mu.Unlock()
	return ClientConnState{
		Closed:               cc.closed,
		Closing:              cc.closing || cc.singleUse || cc.doNotReuse || cc.goAway != nil,
		StreamsActive:        len(cc.streams) + cc.pendingResets,
		StreamsReserved:      cc.streamsReserved,
		StreamsPending:       cc.pendingRequests,
		LastIdle:             cc.lastIdle,
		MaxConcurrentStreams: maxConcurrent,
	}
}

// clientConnIdleState describes the suitability of a client
// connection to initiate a new RoundTrip request.
type clientConnIdleState struct {
	canTakeNewRequest bool
}

func (cc *ClientConn) idleState() clientConnIdleState {
	cc.mu.Lock()
	defer cc.mu.Unlock()
	return cc.idleStateLocked()
}

func (cc *ClientConn) idleStateLocked() (st clientConnIdleState) {
	if cc.singleUse && cc.nextStreamID > 1 {
		return
	}
	var maxConcurrentOkay bool
	if cc.strictMaxConcurrentStreams {
		// We'll tell the caller we can take a new request to
		// prevent the caller from dialing a new TCP
		// connection, but then we'll block later before
		// writing it.
		maxConcurrentOkay = true
	} else {
		// We can take a new request if the total of
		//   - active streams;
		//   - reservation slots for new streams; and
		//   - streams for which we have sent a RST_STREAM and a PING,
		//     but received no subsequent frame
		// is less than the concurrency limit.
		maxConcurrentOkay = cc.currentRequestCountLocked() < int(cc.maxConcurrentStreams)
	}

	st.canTakeNewRequest = maxConcurrentOkay && cc.isUsableLocked()

	// If this connection has never been used for a request and is closed,
	// then let it take a request (which will fail).
	// If the conn was closed for idleness, we're racing the idle timer;
	// don't try to use the conn. (Issue #70515.)
	//
	// This avoids a situation where an error early in a connection's lifetime
	// goes unreported.
	if cc.nextStreamID == 1 && cc.streamsReserved == 0 && cc.closed && !cc.closedOnIdle {
		st.canTakeNewRequest = true
	}

	return
}

func (cc *ClientConn) isUsableLocked() bool {
	return cc.goAway == nil &&
		!cc.closed &&
		!cc.closing &&
		!cc.doNotReuse &&
		int64(cc.nextStreamID)+2*int64(cc.pendingRequests) < math.MaxInt32 &&
		!cc.tooIdleLocked()
}

// canReserveLocked reports whether a net/http.ClientConn can reserve a slot on this conn.
//
// This follows slightly different rules than clientConnIdleState.canTakeNewRequest.
// We only permit reservations up to the conn's concurrency limit.
// This differs from ClientConn.ReserveNewRequest, which permits reservations
// past the limit when StrictMaxConcurrentStreams is set.
func (cc *ClientConn) canReserveLocked() bool {
	if cc.currentRequestCountLocked() >= int(cc.maxConcurrentStreams) {
		return false
	}
	if !cc.isUsableLocked() {
		return false
	}
	return true
}

// currentRequestCountLocked reports the number of concurrency slots currently in use,
// including active streams, reserved slots, and reset streams waiting for acknowledgement.
func (cc *ClientConn) currentRequestCountLocked() int {
	return len(cc.streams) + cc.streamsReserved + cc.pendingResets
}

func (cc *ClientConn) canTakeNewRequestLocked() bool {
	st := cc.idleStateLocked()
	return st.canTakeNewRequest
}

// availableLocked reports the number of concurrency slots available.
func (cc *ClientConn) availableLocked() int {
	if !cc.canTakeNewRequestLocked() {
		return 0
	}
	return max(0, int(cc.maxConcurrentStreams)-cc.currentRequestCountLocked())
}

// tooIdleLocked reports whether this connection has been been sitting idle
// for too much wall time.
func (cc *ClientConn) tooIdleLocked() bool {
	// The Round(0) strips the monontonic clock reading so the
	// times are compared based on their wall time. We don't want
	// to reuse a connection that's been sitting idle during
	// VM/laptop suspend if monotonic time was also frozen.
	return cc.idleTimeout != 0 && !cc.lastIdle.IsZero() && time.Since(cc.lastIdle.Round(0)) > cc.idleTimeout
}

// onIdleTimeout is called from a time.AfterFunc goroutine. It will
// only be called when we're idle, but because we're coming from a new
// goroutine, there could be a new request coming in at the same time,
// so this simply calls the synchronized closeIfIdle to shut down this
// connection. The timer could just call closeIfIdle, but this is more
// clear.
func (cc *ClientConn) onIdleTimeout() {
	cc.closeIfIdle()
}

func (cc *ClientConn) closeConn() {
	t := time.AfterFunc(250*time.Millisecond, cc.forceCloseConn)
	defer t.Stop()
	cc.tconn.Close()
	cc.maybeCallStateHook()
}

// A tls.Conn.Close can hang for a long time if the peer is unresponsive.
// Try to shut it down more aggressively.
func (cc *ClientConn) forceCloseConn() {
	tc, ok := cc.tconn.(*tls.Conn)
	if !ok {
		return
	}
	if nc := tc.NetConn(); nc != nil {
		nc.Close()
	}
}

func (cc *ClientConn) closeIfIdle() {
	cc.mu.Lock()
	if len(cc.streams) > 0 || cc.streamsReserved > 0 {
		cc.mu.Unlock()
		return
	}
	cc.closed = true
	cc.closedOnIdle = true
	nextID := cc.nextStreamID
	// TODO: do clients send GOAWAY too? maybe? Just Close:
	cc.mu.Unlock()

	if VerboseLogs {
		cc.vlogf("http2: Transport closing idle conn %p (forSingleUse=%v, maxStream=%v)", cc, cc.singleUse, nextID-2)
	}
	cc.closeConn()
}

func (cc *ClientConn) isDoNotReuseAndIdle() bool {
	cc.mu.Lock()
	defer cc.mu.Unlock()
	return cc.doNotReuse && len(cc.streams) == 0
}

var shutdownEnterWaitStateHook = func() {}

// Shutdown gracefully closes the client connection, waiting for running streams to complete.
func (cc *ClientConn) Shutdown(ctx context.Context) error {
	if err := cc.sendGoAway(); err != nil {
		return err
	}
	// Wait for all in-flight streams to complete or connection to close
	done := make(chan struct{})
	cancelled := false // guarded by cc.mu
	go func() {
		cc.mu.Lock()
		defer cc.mu.Unlock()
		for {
			if len(cc.streams) == 0 || cc.closed {
				cc.closed = true
				close(done)
				break
			}
			if cancelled {
				break
			}
			cc.cond.Wait()
		}
	}()
	shutdownEnterWaitStateHook()
	select {
	case <-done:
		cc.closeConn()
		return nil
	case <-ctx.Done():
		cc.mu.Lock()
		// Free the goroutine above
		cancelled = true
		cc.cond.Broadcast()
		cc.mu.Unlock()
		return ctx.Err()
	}
}

func (cc *ClientConn) sendGoAway() error {
	cc.mu.Lock()
	closing := cc.closing
	cc.closing = true
	maxStreamID := cc.nextStreamID
	cc.mu.Unlock()
	if closing {
		// GOAWAY sent already
		return nil
	}

	cc.wmu.Lock()
	defer cc.wmu.Unlock()
	// Send a graceful shutdown frame to server
	if err := cc.fr.WriteGoAway(maxStreamID, ErrCodeNo, nil); err != nil {
		return err
	}
	if err := cc.bw.Flush(); err != nil {
		return err
	}
	// Prevent new requests
	return nil
}

// closes the client connection immediately. In-flight requests are interrupted.
// err is sent to streams.
func (cc *ClientConn) closeForError(err error) {
	cc.mu.Lock()
	cc.closed = true
	for _, cs := range cc.streams {
		cs.abortStreamLocked(err)
	}
	cc.cond.Broadcast()
	cc.mu.Unlock()
	cc.closeConn()
}

func (cc *ClientConn) goAwayAndClose(code ErrCode) {
	cc.mu.Lock()
	closed := cc.closed
	cc.closing = true
	cc.closed = true
	cc.mu.Unlock()
	if closed {
		return
	}
	if f := cc.fr.countError; f != nil {
		f(fmt.Sprintf("conn_close_error_%s", code.stringToken()))
	}
	done := make(chan struct{})
	go func() {
		defer close(done)
		cc.wmu.Lock()
		cc.fr.WriteGoAway(0, code, nil)
		cc.bw.Flush()
		cc.wmu.Unlock()
	}()
	select {
	case <-done:
	case <-time.After(250 * time.Millisecond):
	}
	cc.closeForError(fmt.Errorf("http2: closing connection with %v", code))
}

// Close closes the client connection immediately.
//
// In-flight requests are interrupted. For a graceful shutdown, use Shutdown instead.
func (cc *ClientConn) Close() error {
	cc.closeForError(errClientConnForceClosed)
	return nil
}

// closes the client connection immediately. In-flight requests are interrupted.
func (cc *ClientConn) closeForLostPing() {
	err := errors.New("http2: client connection lost")
	if f := cc.fr.countError; f != nil {
		f("conn_close_lost_ping")
	}
	cc.closeForError(err)
}

// errRequestCanceled is a copy of net/http's errRequestCanceled because it's not
// exported. At least they'll be DeepEqual for h1-vs-h2 comparisons tests.
var errRequestCanceled = internal.ErrRequestCanceled

func (cc *ClientConn) responseHeaderTimeout() time.Duration {
	if cc.t.t1 != nil {
		return cc.t.t1.ResponseHeaderTimeout()
	}
	// No way to do this (yet?) with just an http2.Transport. Probably
	// no need. Request.Cancel this is the new way. We only need to support
	// this for compatibility with the old http.Transport fields when
	// we're doing transparent http2.
	return 0
}

// actualContentLength returns a sanitized version of
// req.ContentLength, where 0 actually means zero (not unknown) and -1
// means unknown.
func actualContentLength(req *ClientRequest) int64 {
	if req.Body == nil || req.Body == NoBody {
		return 0
	}
	if req.ContentLength != 0 {
		return req.ContentLength
	}
	return -1
}

func (cc *ClientConn) decrStreamReservations() {
	cc.mu.Lock()
	defer cc.mu.Unlock()
	cc.decrStreamReservationsLocked()
}

func (cc *ClientConn) decrStreamReservationsLocked() {
	if cc.streamsReserved > 0 {
		cc.streamsReserved--
	}
}

func (cc *ClientConn) RoundTrip(req *ClientRequest) (*ClientResponse, error) {
	return cc.roundTrip(req, nil)
}

func (cc *ClientConn) roundTrip(req *ClientRequest, streamf func(*clientStream)) (*ClientResponse, error) {
	ctx := req.Context
	req.stream = clientStream{
		cc:                   cc,
		ctx:                  ctx,
		reqCancel:            req.Cancel,
		isHead:               req.Method == "HEAD",
		reqBody:              req.Body,
		reqBodyContentLength: actualContentLength(req),
		trace:                httptrace.ContextClientTrace(ctx),
		peerClosed:           make(chan struct{}),
		abort:                make(chan struct{}),
		respHeaderRecv:       make(chan struct{}),
		donec:                make(chan struct{}),
		resTrailer:           req.ResTrailer,
	}
	cs := &req.stream

	cs.requestedGzip = httpcommon.IsRequestGzip(req.Method, req.Header, cc.t.disableCompression())

	go cs.doRequest(req, streamf)

	waitDone := func() error {
		select {
		case <-cs.donec:
			return nil
		case <-ctx.Done():
			return ctx.Err()
		case <-cs.reqCancel:
			return errRequestCanceled
		}
	}

	handleResponseHeaders := func() (*ClientResponse, error) {
		res := cs.res
		if res.StatusCode > 299 {
			// On error or status code 3xx, 4xx, 5xx, etc abort any
			// ongoing write, assuming that the server doesn't care
			// about our request body. If the server replied with 1xx or
			// 2xx, however, then assume the server DOES potentially
			// want our body (e.g. full-duplex streaming:
			// golang.org/issue/13444). If it turns out the server
			// doesn't, they'll RST_STREAM us soon enough. This is a
			// heuristic to avoid adding knobs to Transport. Hopefully
			// we can keep it.
			cs.abortRequestBodyWrite()
		}
		res.TLS = cc.tlsState
		if res.Body == NoBody && actualContentLength(req) == 0 {
			// If there isn't a request or response body still being
			// written, then wait for the stream to be closed before
			// RoundTrip returns.
			if err := waitDone(); err != nil {
				return nil, err
			}
		}
		return res, nil
	}

	cancelRequest := func(cs *clientStream, err error) error {
		cs.cc.mu.Lock()
		bodyClosed := cs.reqBodyClosed
		cs.cc.mu.Unlock()
		// Wait for the request body to be closed.
		//
		// If nothing closed the body before now, abortStreamLocked
		// will have started a goroutine to close it.
		//
		// Closing the body before returning avoids a race condition
		// with net/http checking its readTrackingBody to see if the
		// body was read from or closed. See golang/go#60041.
		//
		// The body is closed in a separate goroutine without the
		// connection mutex held, but dropping the mutex before waiting
		// will keep us from holding it indefinitely if the body
		// close is slow for some reason.
		if bodyClosed != nil {
			<-bodyClosed
		}
		return err
	}

	for {
		select {
		case <-cs.respHeaderRecv:
			return handleResponseHeaders()
		case <-cs.abort:
			select {
			case <-cs.respHeaderRecv:
				// If both cs.respHeaderRecv and cs.abort are signaling,
				// pick respHeaderRecv. The server probably wrote the
				// response and immediately reset the stream.
				// golang.org/issue/49645
				return handleResponseHeaders()
			default:
				waitDone()
				return nil, cs.abortErr
			}
		case <-ctx.Done():
			err := ctx.Err()
			cs.abortStream(err)
			return nil, cancelRequest(cs, err)
		case <-cs.reqCancel:
			cs.abortStream(errRequestCanceled)
			return nil, cancelRequest(cs, errRequestCanceled)
		}
	}
}

// doRequest runs for the duration of the request lifetime.
//
// It sends the request and performs post-request cleanup (closing Request.Body, etc.).
func (cs *clientStream) doRequest(req *ClientRequest, streamf func(*clientStream)) {
	err := cs.writeRequest(req, streamf)
	cs.cleanupWriteRequest(err)
}

var errExtendedConnectNotSupported = errors.New("net/http: extended connect not supported by peer")

// writeRequest sends a request.
//
// It returns nil after the request is written, the response read,
// and the request stream is half-closed by the peer.
//
// It returns non-nil if the request ends otherwise.
// If the returned error is StreamError, the error Code may be used in resetting the stream.
func (cs *clientStream) writeRequest(req *ClientRequest, streamf func(*clientStream)) (err error) {
	cc := cs.cc
	ctx := cs.ctx

	// wait for setting frames to be received, a server can change this value later,
	// but we just wait for the first settings frame
	var isExtendedConnect bool
	if req.Method == "CONNECT" && req.Header.Get(":protocol") != "" {
		isExtendedConnect = true
	}

	// Acquire the new-request lock by writing to reqHeaderMu.
	// This lock guards the critical section covering allocating a new stream ID
	// (requires mu) and creating the stream (requires wmu).
	if cc.reqHeaderMu == nil {
		panic("RoundTrip on uninitialized ClientConn") // for tests
	}
	if isExtendedConnect {
		select {
		case <-cs.reqCancel:
			return errRequestCanceled
		case <-ctx.Done():
			return ctx.Err()
		case <-cc.seenSettingsChan:
			if !cc.extendedConnectAllowed {
				return errExtendedConnectNotSupported
			}
		}
	}
	select {
	case cc.reqHeaderMu <- struct{}{}:
	case <-cs.reqCancel:
		return errRequestCanceled
	case <-ctx.Done():
		return ctx.Err()
	}

	cc.mu.Lock()
	if cc.idleTimer != nil {
		cc.idleTimer.Stop()
	}
	cc.decrStreamReservationsLocked()
	if err := cc.awaitOpenSlotForStreamLocked(cs); err != nil {
		cc.mu.Unlock()
		<-cc.reqHeaderMu
		return err
	}
	cc.addStreamLocked(cs) // assigns stream ID
	if isConnectionCloseRequest(req) {
		cc.doNotReuse = true
	}
	cc.mu.Unlock()

	if streamf != nil {
		streamf(cs)
	}

	continueTimeout := cc.t.expectContinueTimeout()
	if continueTimeout != 0 {
		if !httpguts.HeaderValuesContainsToken(req.Header["Expect"], "100-continue") {
			continueTimeout = 0
		} else {
			cs.on100 = make(chan struct{}, 1)
		}
	}

	// Past this point (where we send request headers), it is possible for
	// RoundTrip to return successfully. Since the RoundTrip contract permits
	// the caller to "mutate or reuse" the Request after closing the Response's Body,
	// we must take care when referencing the Request from here on.
	err = cs.encodeAndWriteHeaders(req)
	<-cc.reqHeaderMu
	if err != nil {
		return err
	}

	hasBody := cs.reqBodyContentLength != 0
	if !hasBody {
		cs.sentEndStream = true
	} else {
		if continueTimeout != 0 {
			traceWait100Continue(cs.trace)
			timer := time.NewTimer(continueTimeout)
			select {
			case <-timer.C:
				err = nil
			case <-cs.on100:
				err = nil
			case <-cs.abort:
				err = cs.abortErr
			case <-ctx.Done():
				err = ctx.Err()
			case <-cs.reqCancel:
				err = errRequestCanceled
			}
			timer.Stop()
			if err != nil {
				traceWroteRequest(cs.trace, err)
				return err
			}
		}

		if err = cs.writeRequestBody(req); err != nil {
			if err != errStopReqBodyWrite {
				traceWroteRequest(cs.trace, err)
				return err
			}
		} else {
			cs.sentEndStream = true
		}
	}

	traceWroteRequest(cs.trace, err)

	var respHeaderTimer <-chan time.Time
	var respHeaderRecv chan struct{}
	if d := cc.responseHeaderTimeout(); d != 0 {
		timer := time.NewTimer(d)
		defer timer.Stop()
		respHeaderTimer = timer.C
		respHeaderRecv = cs.respHeaderRecv
	}
	// Wait until the peer half-closes its end of the stream,
	// or until the request is aborted (via context, error, or otherwise),
	// whichever comes first.
	for {
		select {
		case <-cs.peerClosed:
			return nil
		case <-respHeaderTimer:
			return errTimeout
		case <-respHeaderRecv:
			respHeaderRecv = nil
			respHeaderTimer = nil // keep waiting for END_STREAM
		case <-cs.abort:
			return cs.abortErr
		case <-ctx.Done():
			return ctx.Err()
		case <-cs.reqCancel:
			return errRequestCanceled
		}
	}
}

func (cs *clientStream) encodeAndWriteHeaders(req *ClientRequest) error {
	cc := cs.cc
	ctx := cs.ctx

	cc.wmu.Lock()
	defer cc.wmu.Unlock()

	// If the request was canceled while waiting for cc.mu, just quit.
	select {
	case <-cs.abort:
		return cs.abortErr
	case <-ctx.Done():
		return ctx.Err()
	case <-cs.reqCancel:
		return errRequestCanceled
	default:
	}

	// Encode headers.
	//
	// we send: HEADERS{1}, CONTINUATION{0,} + DATA{0,} (DATA is
	// sent by writeRequestBody below, along with any Trailers,
	// again in form HEADERS{1}, CONTINUATION{0,})
	cc.hbuf.Reset()
	res, err := encodeRequestHeaders(req, cs.requestedGzip, cc.peerMaxHeaderListSize, func(name, value string) {
		cc.writeHeader(name, value)
	})
	if err != nil {
		return fmt.Errorf("http2: %w", err)
	}
	hdrs := cc.hbuf.Bytes()

	// Write the request.
	endStream := !res.HasBody && !res.HasTrailers
	cs.sentHeaders = true
	err = cc.writeHeaders(cs.ID, endStream, int(cc.maxFrameSize), hdrs)
	traceWroteHeaders(cs.trace)
	return err
}

func encodeRequestHeaders(req *ClientRequest, addGzipHeader bool, peerMaxHeaderListSize uint64, headerf func(name, value string)) (httpcommon.EncodeHeadersResult, error) {
	return httpcommon.EncodeHeaders(req.Context, httpcommon.EncodeHeadersParam{
		Request: httpcommon.Request{
			Header:              req.Header,
			Trailer:             req.Trailer,
			URL:                 req.URL,
			Host:                req.Host,
			Method:              req.Method,
			ActualContentLength: actualContentLength(req),
		},
		AddGzipHeader:         addGzipHeader,
		PeerMaxHeaderListSize: peerMaxHeaderListSize,
		DefaultUserAgent:      defaultUserAgent,
	}, headerf)
}

// cleanupWriteRequest performs post-request tasks.
//
// If err (the result of writeRequest) is non-nil and the stream is not closed,
// cleanupWriteRequest will send a reset to the peer.
func (cs *clientStream) cleanupWriteRequest(err error) {
	cc := cs.cc

	if cs.ID == 0 {
		// We were canceled before creating the stream, so return our reservation.
		cc.decrStreamReservations()
	}

	// TODO: write h12Compare test showing whether
	// Request.Body is closed by the Transport,
	// and in multiple cases: server replies <=299 and >299
	// while still writing request body
	cc.mu.Lock()
	mustCloseBody := false
	if cs.reqBody != nil && cs.reqBodyClosed == nil {
		mustCloseBody = true
		cs.reqBodyClosed = make(chan struct{})
	}
	bodyClosed := cs.reqBodyClosed
	closeOnIdle := cc.singleUse || cc.doNotReuse || cc.t.disableKeepAlives() || cc.goAway != nil
	// Have we read any frames from the connection since sending this request?
	readSinceStream := cc.readBeforeStreamID > cs.ID
	cc.mu.Unlock()
	if mustCloseBody {
		cs.reqBody.Close()
		close(bodyClosed)
	}
	if bodyClosed != nil {
		<-bodyClosed
	}
	if err != nil && cs.sentEndStream {
		// If the connection is closed immediately after the response is read,
		// we may be aborted before finishing up here. If the stream was closed
		// cleanly on both sides, there is no error.
		select {
		case <-cs.peerClosed:
			err = nil
		default:
		}
	}
	if err != nil {
		cs.abortStream(err) // possibly redundant, but harmless
		if ce, ok := err.(ConnectionError); ok {
			cc.goAwayAndClose(ErrCode(ce))
		} else if cs.sentHeaders {
			if se, ok := err.(StreamError); ok {
				if se.Cause != errFromPeer {
					cc.writeStreamReset(cs.ID, se.Code, false, err)
				}
			} else {
				// We're cancelling an in-flight request.
				//
				// This could be due to the server becoming unresponsive.
				// To avoid sending too many requests on a dead connection,
				// if we haven't read any frames from the connection since
				// sending this request, we let it continue to consume
				// a concurrency slot until we can confirm the server is
				// still responding.
				// We do this by sending a PING frame along with the RST_STREAM
				// (unless a ping is already in flight).
				//
				// For simplicity, we don't bother tracking the PING payload:
				// We reset cc.pendingResets any time we receive a PING ACK.
				//
				// We skip this if the conn is going to be closed on idle,
				// because it's short lived and will probably be closed before
				// we get the ping response.
				ping := false
				if !closeOnIdle && !readSinceStream {
					cc.mu.Lock()
					// rstStreamPingsBlocked works around a gRPC behavior:
					// see comment on the field for details.
					if !cc.rstStreamPingsBlocked {
						if cc.pendingResets == 0 {
							ping = true
						}
						cc.pendingResets++
					}
					cc.mu.Unlock()
				}
				cc.writeStreamReset(cs.ID, ErrCodeCancel, ping, err)
			}
		}
		cs.bufPipe.CloseWithError(err) // no-op if already closed
	} else {
		if cs.sentHeaders && !cs.sentEndStream {
			cc.writeStreamReset(cs.ID, ErrCodeNo, false, nil)
		}
		cs.bufPipe.CloseWithError(errRequestCanceled)
	}
	if cs.ID != 0 {
		cc.forgetStreamID(cs.ID)
	}

	cc.wmu.Lock()
	werr := cc.werr
	cc.wmu.Unlock()
	if werr != nil {
		cc.Close()
	}

	close(cs.donec)
	cc.maybeCallStateHook()
}

// awaitOpenSlotForStreamLocked waits until len(streams) < maxConcurrentStreams.
// Must hold cc.mu.
func (cc *ClientConn) awaitOpenSlotForStreamLocked(cs *clientStream) error {
	for {
		if cc.closed && cc.nextStreamID == 1 && cc.streamsReserved == 0 {
			// This is the very first request sent to this connection.
			// Return a fatal error which aborts the retry loop.
			return errClientConnNotEstablished
		}
		cc.lastActive = time.Now()
		if cc.closed || !cc.canTakeNewRequestLocked() {
			return errClientConnUnusable
		}
		cc.lastIdle = time.Time{}
		if cc.currentRequestCountLocked() < int(cc.maxConcurrentStreams) {
			return nil
		}
		cc.pendingRequests++
		cc.cond.Wait()
		cc.pendingRequests--
		select {
		case <-cs.abort:
			return cs.abortErr
		default:
		}
	}
}

// requires cc.wmu be held
func (cc *ClientConn) writeHeaders(streamID uint32, endStream bool, maxFrameSize int, hdrs []byte) error {
	first := true // first frame written (HEADERS is first, then CONTINUATION)
	for len(hdrs) > 0 && cc.werr == nil {
		chunk := hdrs
		if len(chunk) > maxFrameSize {
			chunk = chunk[:maxFrameSize]
		}
		hdrs = hdrs[len(chunk):]
		endHeaders := len(hdrs) == 0
		if first {
			cc.fr.WriteHeaders(HeadersFrameParam{
				StreamID:      streamID,
				BlockFragment: chunk,
				EndStream:     endStream,
				EndHeaders:    endHeaders,
			})
			first = false
		} else {
			cc.fr.WriteContinuation(streamID, endHeaders, chunk)
		}
	}
	cc.bw.Flush()
	return cc.werr
}

// internal error values; they don't escape to callers
var (
	// abort request body write; don't send cancel
	errStopReqBodyWrite = errors.New("http2: aborting request body write")

	// abort request body write, but send stream reset of cancel.
	errStopReqBodyWriteAndCancel = errors.New("http2: canceling request")

	errReqBodyTooLong = errors.New("http2: request body larger than specified content length")
)

// frameScratchBufferLen returns the length of a buffer to use for
// outgoing request bodies to read/write to/from.
//
// It returns max(1, min(peer's advertised max frame size,
// Request.ContentLength+1, 512KB)).
func (cs *clientStream) frameScratchBufferLen(maxFrameSize int) int {
	const max = 512 << 10
	n := min(int64(maxFrameSize), max)
	if cl := cs.reqBodyContentLength; cl != -1 && cl+1 < n {
		// Add an extra byte past the declared content-length to
		// give the caller's Request.Body io.Reader a chance to
		// give us more bytes than they declared, so we can catch it
		// early.
		n = cl + 1
	}
	if n < 1 {
		return 1
	}
	return int(n) // doesn't truncate; max is 512K
}

// Seven bufPools manage different frame sizes. This helps to avoid scenarios where long-running
// streaming requests using small frame sizes occupy large buffers initially allocated for prior
// requests needing big buffers. The size ranges are as follows:
// {0 KB, 16 KB], {16 KB, 32 KB], {32 KB, 64 KB], {64 KB, 128 KB], {128 KB, 256 KB],
// {256 KB, 512 KB], {512 KB, infinity}
// In practice, the maximum scratch buffer size should not exceed 512 KB due to
// frameScratchBufferLen(maxFrameSize), thus the "infinity pool" should never be used.
// It exists mainly as a safety measure, for potential future increases in max buffer size.
var bufPools [7]sync.Pool // of *[]byte
func bufPoolIndex(size int) int {
	if size <= 16384 {
		return 0
	}
	size -= 1
	bits := bits.Len(uint(size))
	index := bits - 14
	if index >= len(bufPools) {
		return len(bufPools) - 1
	}
	return index
}

func (cs *clientStream) writeRequestBody(req *ClientRequest) (err error) {
	cc := cs.cc
	body := cs.reqBody
	sentEnd := false // whether we sent the final DATA frame w/ END_STREAM

	hasTrailers := req.Trailer != nil
	remainLen := cs.reqBodyContentLength
	hasContentLen := remainLen != -1

	cc.mu.Lock()
	maxFrameSize := int(cc.maxFrameSize)
	cc.mu.Unlock()

	// Scratch buffer for reading into & writing from.
	scratchLen := cs.frameScratchBufferLen(maxFrameSize)
	var buf []byte
	index := bufPoolIndex(scratchLen)
	if bp, ok := bufPools[index].Get().(*[]byte); ok && len(*bp) >= scratchLen {
		defer bufPools[index].Put(bp)
		buf = *bp
	} else {
		buf = make([]byte, scratchLen)
		defer bufPools[index].Put(&buf)
	}

	var sawEOF bool
	for !sawEOF {
		n, err := body.Read(buf)
		if hasContentLen {
			remainLen -= int64(n)
			if remainLen == 0 && err == nil {
				// The request body's Content-Length was predeclared and
				// we just finished reading it all, but the underlying io.Reader
				// returned the final chunk with a nil error (which is one of
				// the two valid things a Reader can do at EOF). Because we'd prefer
				// to send the END_STREAM bit early, double-check that we're actually
				// at EOF. Subsequent reads should return (0, EOF) at this point.
				// If either value is different, we return an error in one of two ways below.
				var scratch [1]byte
				var n1 int
				n1, err = body.Read(scratch[:])
				remainLen -= int64(n1)
			}
			if remainLen < 0 {
				err = errReqBodyTooLong
				return err
			}
		}
		if err != nil {
			cc.mu.Lock()
			bodyClosed := cs.reqBodyClosed != nil
			cc.mu.Unlock()
			switch {
			case bodyClosed:
				return errStopReqBodyWrite
			case err == io.EOF:
				sawEOF = true
				err = nil
			default:
				return err
			}
		}

		remain := buf[:n]
		for len(remain) > 0 && err == nil {
			var allowed int32
			allowed, err = cs.awaitFlowControl(len(remain))
			if err != nil {
				return err
			}
			cc.wmu.Lock()
			data := remain[:allowed]
			remain = remain[allowed:]
			sentEnd = sawEOF && len(remain) == 0 && !hasTrailers
			err = cc.fr.WriteData(cs.ID, sentEnd, data)
			if err == nil {
				// TODO(bradfitz): this flush is for latency, not bandwidth.
				// Most requests won't need this. Make this opt-in or
				// opt-out?  Use some heuristic on the body type? Nagel-like
				// timers?  Based on 'n'? Only last chunk of this for loop,
				// unless flow control tokens are low? For now, always.
				// If we change this, see comment below.
				err = cc.bw.Flush()
			}
			cc.wmu.Unlock()
		}
		if err != nil {
			return err
		}
	}

	if sentEnd {
		// Already sent END_STREAM (which implies we have no
		// trailers) and flushed, because currently all
		// WriteData frames above get a flush. So we're done.
		return nil
	}

	// Since the RoundTrip contract permits the caller to "mutate or reuse"
	// a request after the Response's Body is closed, verify that this hasn't
	// happened before accessing the trailers.
	cc.mu.Lock()
	trailer := req.Trailer
	err = cs.abortErr
	cc.mu.Unlock()
	if err != nil {
		return err
	}

	cc.wmu.Lock()
	defer cc.wmu.Unlock()
	var trls []byte
	if len(trailer) > 0 {
		trls, err = cc.encodeTrailers(trailer)
		if err != nil {
			return err
		}
	}

	// Two ways to send END_STREAM: either with trailers, or
	// with an empty DATA frame.
	if len(trls) > 0 {
		err = cc.writeHeaders(cs.ID, true, maxFrameSize, trls)
	} else {
		err = cc.fr.WriteData(cs.ID, true, nil)
	}
	if ferr := cc.bw.Flush(); ferr != nil && err == nil {
		err = ferr
	}
	return err
}

// awaitFlowControl waits for [1, min(maxBytes, cc.cs.maxFrameSize)] flow
// control tokens from the server.
// It returns either the non-zero number of tokens taken or an error
// if the stream is dead.
func (cs *clientStream) awaitFlowControl(maxBytes int) (taken int32, err error) {
	cc := cs.cc
	ctx := cs.ctx
	cc.mu.Lock()
	defer cc.mu.Unlock()
	for {
		if cc.closed {
			return 0, errClientConnClosed
		}
		if cs.reqBodyClosed != nil {
			return 0, errStopReqBodyWrite
		}
		select {
		case <-cs.abort:
			return 0, cs.abortErr
		case <-ctx.Done():
			return 0, ctx.Err()
		case <-cs.reqCancel:
			return 0, errRequestCanceled
		default:
		}
		avail, ok := cs.flow.available()
		if !ok {
			return 0, ConnectionError(ErrCodeFlowControl)
		}
		if avail > 0 {
			take := avail
			if int(take) > maxBytes {

				take = int32(maxBytes) // can't truncate int; take is int32
			}
			if take > int32(cc.maxFrameSize) {
				take = int32(cc.maxFrameSize)
			}
			cs.flow.take(take)
			return take, nil
		}
		cc.cond.Wait()
	}
}

// requires cc.wmu be held.
func (cc *ClientConn) encodeTrailers(trailer Header) ([]byte, error) {
	cc.hbuf.Reset()

	hlSize := uint64(0)
	for k, vv := range trailer {
		for _, v := range vv {
			hf := hpack.HeaderField{Name: k, Value: v}
			hlSize += uint64(hf.Size())
		}
	}
	if hlSize > cc.peerMaxHeaderListSize {
		return nil, errRequestHeaderListSize
	}

	for k, vv := range trailer {
		lowKey, ascii := httpcommon.LowerHeader(k)
		if !ascii {
			// Skip writing invalid headers. Per RFC 7540, Section 8.1.2, header
			// field names have to be ASCII characters (just as in HTTP/1.x).
			continue
		}
		// Transfer-Encoding, etc.. have already been filtered at the
		// start of RoundTrip
		for _, v := range vv {
			cc.writeHeader(lowKey, v)
		}
	}
	return cc.hbuf.Bytes(), nil
}

func (cc *ClientConn) writeHeader(name, value string) {
	if VerboseLogs {
		log.Printf("http2: Transport encoding header %q = %q", name, value)
	}
	cc.henc.WriteField(hpack.HeaderField{Name: name, Value: value})
}

type resAndError struct {
	_   incomparable
	res *ClientResponse
	err error
}

// requires cc.mu be held.
func (cc *ClientConn) addStreamLocked(cs *clientStream) {
	cs.flow.conn = &cc.flow
	cs.inflow.init(cc.initialStreamRecvWindowSize)
	cs.ID = cc.nextStreamID
	cc.nextStreamID += 2
	cc.streams[cs.ID] = cs
	if cs.ID == 0 {
		panic("assigned stream ID 0")
	}
}

func (cc *ClientConn) forgetStreamID(id uint32) {
	cc.mu.Lock()
	slen := len(cc.streams)
	delete(cc.streams, id)
	if len(cc.streams) != slen-1 {
		panic("forgetting unknown stream id")
	}
	cc.lastActive = time.Now()
	if len(cc.streams) == 0 && cc.idleTimer != nil {
		cc.idleTimer.Reset(cc.idleTimeout)
		cc.lastIdle = time.Now()
	}
	// Wake up writeRequestBody via clientStream.awaitFlowControl and
	// wake up RoundTrip if there is a pending request.
	cc.cond.Broadcast()

	closeOnIdle := cc.singleUse || cc.doNotReuse || cc.t.disableKeepAlives() || cc.goAway != nil
	if closeOnIdle && cc.streamsReserved == 0 && len(cc.streams) == 0 {
		if VerboseLogs {
			cc.vlogf("http2: Transport closing idle conn %p (forSingleUse=%v, maxStream=%v)", cc, cc.singleUse, cc.nextStreamID-2)
		}
		cc.closed = true
		defer cc.closeConn()
	}

	cc.mu.Unlock()
}

// clientConnReadLoop is the state owned by the clientConn's frame-reading readLoop.
type clientConnReadLoop struct {
	_  incomparable
	cc *ClientConn
}

// readLoop runs in its own goroutine and reads and dispatches frames.
func (cc *ClientConn) readLoop() {
	rl := &clientConnReadLoop{cc: cc}
	defer rl.cleanup()
	cc.readerErr = rl.run()
	if ce, ok := cc.readerErr.(ConnectionError); ok {
		cc.wmu.Lock()
		cc.fr.WriteGoAway(0, ErrCode(ce), nil)
		cc.wmu.Unlock()
	}
}

// GoAwayError is returned by the Transport when the server closes the
// TCP connection after sending a GOAWAY frame.
type GoAwayError struct {
	LastStreamID uint32
	ErrCode      ErrCode
	DebugData    string
}

func (e GoAwayError) Error() string {
	return fmt.Sprintf("http2: server sent GOAWAY and closed the connection; LastStreamID=%v, ErrCode=%v, debug=%q",
		e.LastStreamID, e.ErrCode, e.DebugData)
}

func isEOFOrNetReadError(err error) bool {
	if err == io.EOF {
		return true
	}
	ne, ok := err.(*net.OpError)
	return ok && ne.Op == "read"
}

func (rl *clientConnReadLoop) cleanup() {
	cc := rl.cc
	defer cc.closeConn()
	defer close(cc.readerDone)

	if cc.idleTimer != nil {
		cc.idleTimer.Stop()
	}

	// Close any response bodies if the server closes prematurely.
	// TODO: also do this if we've written the headers but not
	// gotten a response yet.
	err := cc.readerErr
	cc.mu.Lock()
	if cc.goAway != nil && isEOFOrNetReadError(err) {
		err = GoAwayError{
			LastStreamID: cc.goAway.LastStreamID,
			ErrCode:      cc.goAway.ErrCode,
			DebugData:    cc.goAwayDebug,
		}
	} else if err == io.EOF {
		err = io.ErrUnexpectedEOF
	}
	cc.closed = true

	// If the connection has never been used, and has been open for only a short time,
	// leave it in the connection pool for a little while.
	//
	// This avoids a situation where new connections are constantly created,
	// added to the pool, fail, and are removed from the pool, without any error
	// being surfaced to the user.
	unusedWaitTime := 5 * time.Second
	if cc.idleTimeout > 0 && unusedWaitTime > cc.idleTimeout {
		unusedWaitTime = cc.idleTimeout
	}
	idleTime := time.Now().Sub(cc.lastActive)
	if atomic.LoadUint32(&cc.atomicReused) == 0 && idleTime < unusedWaitTime && !cc.closedOnIdle {
		cc.idleTimer = time.AfterFunc(unusedWaitTime-idleTime, func() {
			cc.t.connPool.MarkDead(cc)
		})
	} else {
		cc.mu.Unlock() // avoid any deadlocks in MarkDead
		cc.t.connPool.MarkDead(cc)
		cc.mu.Lock()
	}

	for _, cs := range cc.streams {
		select {
		case <-cs.peerClosed:
			// The server closed the stream before closing the conn,
			// so no need to interrupt it.
		default:
			cs.abortStreamLocked(err)
		}
	}
	cc.cond.Broadcast()
	cc.mu.Unlock()

	if !cc.seenSettings {
		// If we have a pending request that wants extended CONNECT,
		// let it continue and fail with the connection error.
		cc.extendedConnectAllowed = true
		close(cc.seenSettingsChan)
	}
}

// countReadFrameError calls ClientConn.fr.countError with a string
// representing err.
func (cc *ClientConn) countReadFrameError(err error) {
	f := cc.fr.countError
	if f == nil || err == nil {
		return
	}
	if ce, ok := err.(ConnectionError); ok {
		errCode := ErrCode(ce)
		f(fmt.Sprintf("read_frame_conn_error_%s", errCode.stringToken()))
		return
	}
	if errors.Is(err, io.EOF) {
		f("read_frame_eof")
		return
	}
	if errors.Is(err, io.ErrUnexpectedEOF) {
		f("read_frame_unexpected_eof")
		return
	}
	if errors.Is(err, ErrFrameTooLarge) {
		f("read_frame_too_large")
		return
	}
	f("read_frame_other")
}

// errStopReadLoop is an error which cleanly exits the client's read loop.
var errStopReadLoop = errors.New("client connection is closing (BUG: this is not user visible)")

func (rl *clientConnReadLoop) run() error {
	cc := rl.cc
	gotSettings := false
	readIdleTimeout := cc.readIdleTimeout
	var t *time.Timer
	if readIdleTimeout != 0 {
		t = time.AfterFunc(readIdleTimeout, cc.healthCheck)
	}
	for {
		f, err := cc.fr.ReadFrame()
		if t != nil {
			t.Reset(readIdleTimeout)
		}
		if err != nil {
			cc.vlogf("http2: Transport readFrame error on conn %p: (%T) %v", cc, err, err)
		}
		if se, ok := err.(StreamError); ok {
			if cs := rl.streamByID(se.StreamID, notHeaderOrDataFrame); cs != nil {
				if se.Cause == nil {
					se.Cause = cc.fr.errDetail
				}
				rl.endStreamError(cs, se)
			}
			continue
		} else if err != nil {
			cc.countReadFrameError(err)
			return err
		}
		if VerboseLogs {
			cc.vlogf("http2: Transport received %s", summarizeFrame(f))
		}
		if !gotSettings {
			if _, ok := f.(*SettingsFrame); !ok {
				cc.logf("protocol error: received %T before a SETTINGS frame", f)
				return ConnectionError(ErrCodeProtocol)
			}
			gotSettings = true
		}

		switch f := f.(type) {
		case *MetaHeadersFrame:
			err = rl.processHeaders(f)
		case *DataFrame:
			err = rl.processData(f)
		case *GoAwayFrame:
			err = rl.processGoAway(f)
		case *RSTStreamFrame:
			err = rl.processResetStream(f)
		case *SettingsFrame:
			err = rl.processSettings(f)
		case *PushPromiseFrame:
			err = rl.processPushPromise(f)
		case *WindowUpdateFrame:
			err = rl.processWindowUpdate(f)
		case *PingFrame:
			err = rl.processPing(f)
		default:
			cc.logf("Transport: unhandled response frame type %T", f)
		}
		if err != nil {
			if VerboseLogs && err != errStopReadLoop {
				cc.vlogf("http2: Transport conn %p received error from processing frame %v: %v", cc, summarizeFrame(f), err)
			}
			return err
		}
	}
}

func (rl *clientConnReadLoop) processHeaders(f *MetaHeadersFrame) error {
	cs := rl.streamByID(f.StreamID, headerOrDataFrame)
	if cs == nil {
		// We'd get here if we canceled a request while the
		// server had its response still in flight. So if this
		// was just something we canceled, ignore it.
		return nil
	}
	if cs.readClosed {
		rl.endStreamError(cs, StreamError{
			StreamID: f.StreamID,
			Code:     ErrCodeProtocol,
			Cause:    errors.New("protocol error: headers after END_STREAM"),
		})
		return nil
	}
	if !cs.firstByte {
		if cs.trace != nil {
			// TODO(bradfitz): move first response byte earlier,
			// when we first read the 9 byte header, not waiting
			// until all the HEADERS+CONTINUATION frames have been
			// merged. This works for now.
			traceFirstResponseByte(cs.trace)
		}
		cs.firstByte = true
	}
	if !cs.pastHeaders {
		cs.pastHeaders = true
	} else {
		return rl.processTrailers(cs, f)
	}

	res, err := rl.handleResponse(cs, f)
	if err != nil {
		if _, ok := err.(ConnectionError); ok {
			return err
		}
		// Any other error type is a stream error.
		rl.endStreamError(cs, StreamError{
			StreamID: f.StreamID,
			Code:     ErrCodeProtocol,
			Cause:    err,
		})
		return nil // return nil from process* funcs to keep conn alive
	}
	if res == nil {
		// (nil, nil) special case. See handleResponse docs.
		return nil
	}
	cs.res = res
	close(cs.respHeaderRecv)
	if f.StreamEnded() {
		rl.endStream(cs)
	}
	return nil
}

// may return error types nil, or ConnectionError. Any other error value
// is a StreamError of type ErrCodeProtocol. The returned error in that case
// is the detail.
//
// As a special case, handleResponse may return (nil, nil) to skip the
// frame (currently only used for 1xx responses).
func (rl *clientConnReadLoop) handleResponse(cs *clientStream, f *MetaHeadersFrame) (*ClientResponse, error) {
	if f.Truncated {
		return nil, errResponseHeaderListSize
	}

	status := f.PseudoValue("status")
	if status == "" {
		return nil, errors.New("malformed response from server: missing status pseudo header")
	}
	statusCode, err := strconv.Atoi(status)
	if err != nil {
		return nil, errors.New("malformed response from server: malformed non-numeric status pseudo header")
	}

	regularFields := f.RegularFields()
	strs := make([]string, len(regularFields))
	header := make(Header, len(regularFields))
	res := &cs.staticResp
	cs.staticResp = ClientResponse{
		Header:     header,
		StatusCode: statusCode,
		Status:     status,
	}
	for _, hf := range regularFields {
		key := httpcommon.CanonicalHeader(hf.Name)
		if key == "Trailer" {
			t := res.Trailer
			if t == nil {
				t = make(Header)
				res.Trailer = t
			}
			foreachHeaderElement(hf.Value, func(v string) {
				t[httpcommon.CanonicalHeader(v)] = nil
			})
		} else {
			vv := header[key]
			if vv == nil && len(strs) > 0 {
				// More than likely this will be a single-element key.
				// Most headers aren't multi-valued.
				// Set the capacity on strs[0] to 1, so any future append
				// won't extend the slice into the other strings.
				vv, strs = strs[:1:1], strs[1:]
				vv[0] = hf.Value
				header[key] = vv
			} else {
				header[key] = append(vv, hf.Value)
			}
		}
	}

	if statusCode >= 100 && statusCode <= 199 {
		if f.StreamEnded() {
			return nil, errors.New("1xx informational response with END_STREAM flag")
		}
		if fn := cs.get1xxTraceFunc(); fn != nil {
			// If the 1xx response is being delivered to the user,
			// then they're responsible for limiting the number
			// of responses.
			if err := fn(statusCode, textproto.MIMEHeader(header)); err != nil {
				return nil, err
			}
		} else {
			// If the user didn't examine the 1xx response, then we
			// limit the size of all 1xx headers.
			//
			// This differs a bit from the HTTP/1 implementation, which
			// limits the size of all 1xx headers plus the final response.
			// Use the larger limit of MaxHeaderListSize and
			// net/http.Transport.MaxResponseHeaderBytes.
			limit := int64(cs.cc.t.maxHeaderListSize())
			if t1 := cs.cc.t.t1; t1 != nil && t1.MaxResponseHeaderBytes() > limit {
				limit = t1.MaxResponseHeaderBytes()
			}
			for _, h := range f.Fields {
				cs.totalHeaderSize += int64(h.Size())
			}
			if cs.totalHeaderSize > limit {
				if VerboseLogs {
					log.Printf("http2: 1xx informational responses too large")
				}
				return nil, errors.New("header list too large")
			}
		}
		if statusCode == 100 {
			traceGot100Continue(cs.trace)
			select {
			case cs.on100 <- struct{}{}:
			default:
			}
		}
		cs.pastHeaders = false // do it all again
		return nil, nil
	}

	// Delete various headers that might mess up framing for HTTP/1. This is
	// not a problem for HTTP/2, but someone might use HTTP/2 transport as a
	// reverse proxy which forwards the response to an HTTP/1 client. Our
	// HTTP/1 client transport will properly reject improper headers such as
	// multiple conflicting Content-Length headers, but other implementations
	// might not.
	// TODO: just reject such responses? We deleted them for compatibility
	// since this was done in a security fix (go.dev/issue/81115). However,
	// rejecting them seems entirely reasonable and relatively safe.

	// Connection-specific header fields must not appear in an HTTP/2 message,
	// and any message containing them is malformed. RFC 9113, Section 8.2.2.
	for _, k := range connHeaders {
		delete(res.Header, k)
	}
	res.ContentLength = -1
	if clens, ok := res.Header["Content-Length"]; ok {
		// Repeated Content-Length values may be collapsed into one only if
		// they are identical per RFC 9110 Section 8.6.
		// No need to trim whitespace, HTTP/2 header values must not have
		// extraneous whitespace per RFC 9113 Section 8.2.1.
		// Non-canonical headers are already rejected by our framer at this
		// point.
		conflicting := slices.ContainsFunc(clens[1:], func(clen string) bool {
			return clen != clens[0]
		})
		cl, err := strconv.ParseUint(clens[0], 10, 63)
		if conflicting || err != nil {
			delete(res.Header, "Content-Length")
		} else {
			res.Header["Content-Length"] = clens[:1]
			res.ContentLength = int64(cl)
		}
	}

	if res.ContentLength < 0 && f.StreamEnded() && !cs.isHead {
		res.ContentLength = 0
	}

	if cs.isHead {
		res.Body = NoBody
		return res, nil
	}

	if f.StreamEnded() {
		if res.ContentLength > 0 {
			res.Body = missingBody{}
		} else {
			res.Body = NoBody
		}
		return res, nil
	}

	cs.bufPipe.setBuffer(&dataBuffer{expected: res.ContentLength})
	cs.bytesRemain = res.ContentLength
	res.Body = transportResponseBody{cs}

	if cs.requestedGzip && asciiEqualFold(res.Header.Get("Content-Encoding"), "gzip") {
		res.Header.Del("Content-Encoding")
		res.Header.Del("Content-Length")
		res.ContentLength = -1
		res.Body = &gzipReader{body: res.Body}
		res.Uncompressed = true
	}
	return res, nil
}

func (rl *clientConnReadLoop) processTrailers(cs *clientStream, f *MetaHeadersFrame) error {
	if cs.pastTrailers {
		// Too many HEADERS frames for this stream.
		return ConnectionError(ErrCodeProtocol)
	}
	cs.pastTrailers = true
	if !f.StreamEnded() {
		// We expect that any headers for trailers also
		// has END_STREAM.
		return ConnectionError(ErrCodeProtocol)
	}
	if len(f.PseudoFields()) > 0 {
		// No pseudo header fields are defined for trailers.
		// TODO: ConnectionError might be overly harsh? Check.
		return ConnectionError(ErrCodeProtocol)
	}
	if f.Truncated {
		rl.endStreamError(cs, StreamError{
			StreamID: f.StreamID,
			Code:     ErrCodeProtocol,
			Cause:    errResponseHeaderListSize,
		})
		return nil
	}

	trailer := make(Header)
	for _, hf := range f.RegularFields() {
		key := httpcommon.CanonicalHeader(hf.Name)
		trailer[key] = append(trailer[key], hf.Value)
	}
	cs.trailer = trailer

	rl.endStream(cs)
	return nil
}

// transportResponseBody is the concrete type of Transport.RoundTrip's
// Response.Body. It is an io.ReadCloser.
type transportResponseBody struct {
	cs *clientStream
}

func (b transportResponseBody) Read(p []byte) (n int, err error) {
	cs := b.cs
	cc := cs.cc

	if cs.readErr != nil {
		return 0, cs.readErr
	}
	n, err = b.cs.bufPipe.Read(p)
	if cs.bytesRemain != -1 {
		if int64(n) > cs.bytesRemain {
			n = int(cs.bytesRemain)
			if err == nil {
				err = errors.New("net/http: server replied with more than declared Content-Length; truncated")
				cs.abortStream(err)
			}
			cs.readErr = err
			return int(cs.bytesRemain), err
		}
		cs.bytesRemain -= int64(n)
		if err == io.EOF && cs.bytesRemain > 0 {
			err = io.ErrUnexpectedEOF
			cs.readErr = err
			return n, err
		}
	}
	if n == 0 {
		// No flow control tokens to send back.
		return
	}

	cc.mu.Lock()
	connAdd := cc.inflow.add(n)
	var streamAdd int32
	if err == nil { // No need to refresh if the stream is over or failed.
		streamAdd = cs.inflow.add(n)
	}
	cc.mu.Unlock()

	if connAdd != 0 || streamAdd != 0 {
		cc.wmu.Lock()
		defer cc.wmu.Unlock()
		if connAdd != 0 {
			cc.fr.WriteWindowUpdate(0, mustUint31(connAdd))
		}
		if streamAdd != 0 {
			cc.fr.WriteWindowUpdate(cs.ID, mustUint31(streamAdd))
		}
		cc.bw.Flush()
	}
	return
}

var errClosedResponseBody = errors.New("http2: response body closed")

func (b transportResponseBody) Close() error {
	cs := b.cs
	cc := cs.cc

	cs.bufPipe.BreakWithError(errClosedResponseBody)
	cs.abortStream(errClosedResponseBody)

	unread := cs.bufPipe.Len()
	if unread > 0 {
		cc.mu.Lock()
		// Return connection-level flow control.
		connAdd := cc.inflow.add(unread)
		cc.mu.Unlock()

		// TODO(dneil): Acquiring this mutex can block indefinitely.
		// Move flow control return to a goroutine?
		cc.wmu.Lock()
		// Return connection-level flow control.
		if connAdd > 0 {
			cc.fr.WriteWindowUpdate(0, uint32(connAdd))
		}
		cc.bw.Flush()
		cc.wmu.Unlock()
	}

	select {
	case <-cs.donec:
	case <-cs.ctx.Done():
		// See golang/go#49366: The net/http package can cancel the
		// request context after the response body is fully read.
		// Don't treat this as an error.
		return nil
	case <-cs.reqCancel:
		return errRequestCanceled
	}
	return nil
}

func (rl *clientConnReadLoop) processData(f *DataFrame) error {
	cc := rl.cc
	cs := rl.streamByID(f.StreamID, headerOrDataFrame)
	data := f.Data()
	if cs == nil {
		cc.mu.Lock()
		neverSent := cc.nextStreamID
		cc.mu.Unlock()
		if f.StreamID >= neverSent {
			// We never asked for this.
			cc.logf("http2: Transport received unsolicited DATA frame; closing connection")
			return ConnectionError(ErrCodeProtocol)
		}
		// We probably did ask for this, but canceled. Just ignore it.
		// TODO: be stricter here? only silently ignore things which
		// we canceled, but not things which were closed normally
		// by the peer? Tough without accumulating too much state.

		// But at least return their flow control:
		if f.Length > 0 {
			cc.mu.Lock()
			ok := cc.inflow.take(f.Length)
			connAdd := cc.inflow.add(int(f.Length))
			cc.mu.Unlock()
			if !ok {
				return ConnectionError(ErrCodeFlowControl)
			}
			if connAdd > 0 {
				cc.wmu.Lock()
				cc.fr.WriteWindowUpdate(0, uint32(connAdd))
				cc.bw.Flush()
				cc.wmu.Unlock()
			}
		}
		return nil
	}
	if cs.readClosed {
		cc.logf("protocol error: received DATA after END_STREAM")
		rl.endStreamError(cs, StreamError{
			StreamID: f.StreamID,
			Code:     ErrCodeProtocol,
		})
		return nil
	}
	if !cs.pastHeaders {
		cc.logf("protocol error: received DATA before a HEADERS frame")
		rl.endStreamError(cs, StreamError{
			StreamID: f.StreamID,
			Code:     ErrCodeProtocol,
		})
		return nil
	}
	if f.Length > 0 {
		if cs.isHead && len(data) > 0 {
			cc.logf("protocol error: received DATA on a HEAD request")
			rl.endStreamError(cs, StreamError{
				StreamID: f.StreamID,
				Code:     ErrCodeProtocol,
			})
			return nil
		}
		// Check connection-level flow control.
		cc.mu.Lock()
		if !takeInflows(&cc.inflow, &cs.inflow, f.Length) {
			cc.mu.Unlock()
			return ConnectionError(ErrCodeFlowControl)
		}
		// Return any padded flow control now, since we won't
		// refund it later on body reads.
		var refund int
		if pad := int(f.Length) - len(data); pad > 0 {
			refund += pad
		}

		didReset := false
		var err error
		if len(data) > 0 {
			if _, err = cs.bufPipe.Write(data); err != nil {
				// Return len(data) now if the stream is already closed,
				// since data will never be read.
				didReset = true
				refund += len(data)
			}
		}

		sendConn := cc.inflow.add(refund)
		var sendStream int32
		if !didReset {
			sendStream = cs.inflow.add(refund)
		}
		cc.mu.Unlock()

		if sendConn > 0 || sendStream > 0 {
			cc.wmu.Lock()
			if sendConn > 0 {
				cc.fr.WriteWindowUpdate(0, uint32(sendConn))
			}
			if sendStream > 0 {
				cc.fr.WriteWindowUpdate(cs.ID, uint32(sendStream))
			}
			cc.bw.Flush()
			cc.wmu.Unlock()
		}

		if err != nil {
			rl.endStreamError(cs, err)
			return nil
		}
	}

	if f.StreamEnded() {
		rl.endStream(cs)
	}
	return nil
}

func (rl *clientConnReadLoop) endStream(cs *clientStream) {
	// TODO: check that any declared content-length matches, like
	// server.go's (*stream).endStream method.
	if !cs.readClosed {
		cs.readClosed = true
		// Close cs.bufPipe and cs.peerClosed with cc.mu held to avoid a
		// race condition: The caller can read io.EOF from Response.Body
		// and close the body before we close cs.peerClosed, causing
		// cleanupWriteRequest to send a RST_STREAM.
		rl.cc.mu.Lock()
		defer rl.cc.mu.Unlock()
		cs.bufPipe.closeWithErrorAndCode(io.EOF, cs.copyTrailers)
		close(cs.peerClosed)
	}
}

func (rl *clientConnReadLoop) endStreamError(cs *clientStream, err error) {
	cs.readAborted = true
	cs.abortStream(err)
}

func (rl *clientConnReadLoop) endStreamErrorLocked(cs *clientStream, err error) {
	cs.readAborted = true
	cs.abortStreamLocked(err)
}

// Constants passed to streamByID for documentation purposes.
const (
	headerOrDataFrame    = true
	notHeaderOrDataFrame = false
)

// streamByID returns the stream with the given id, or nil if no stream has that id.
// If headerOrData is true, it clears rst.StreamPingsBlocked.
func (rl *clientConnReadLoop) streamByID(id uint32, headerOrData bool) *clientStream {
	rl.cc.mu.Lock()
	defer rl.cc.mu.Unlock()
	return rl.streamByIDLocked(id, headerOrData)
}

func (rl *clientConnReadLoop) streamByIDLocked(id uint32, headerOrData bool) *clientStream {
	if headerOrData {
		// Work around an unfortunate gRPC behavior.
		// See comment on ClientConn.rstStreamPingsBlocked for details.
		rl.cc.rstStreamPingsBlocked = false
	}
	rl.cc.readBeforeStreamID = rl.cc.nextStreamID
	cs := rl.cc.streams[id]
	if cs != nil && !cs.readAborted {
		return cs
	}
	return nil
}

func (cs *clientStream) copyTrailers() {
	for k, vv := range cs.trailer {
		t := cs.resTrailer
		if *t == nil {
			*t = make(Header)
		}
		(*t)[k] = vv
	}
}

func (rl *clientConnReadLoop) processGoAway(f *GoAwayFrame) error {
	cc := rl.cc
	cc.t.connPool.MarkDead(cc)
	if f.ErrCode != 0 {
		// TODO: deal with GOAWAY more. particularly the error code
		cc.vlogf("transport got GOAWAY with error code = %v", f.ErrCode)
		if fn := cc.fr.countError; fn != nil {
			fn("recv_goaway_" + f.ErrCode.stringToken())
		}
	}

	cc.mu.Lock()
	defer cc.mu.Unlock()

	old := cc.goAway
	cc.goAway = f

	// Merge the previous and current GoAway error frames.
	if cc.goAwayDebug == "" {
		cc.goAwayDebug = string(f.DebugData())
	}
	if old != nil && old.ErrCode != ErrCodeNo {
		cc.goAway.ErrCode = old.ErrCode
	}
	last := f.LastStreamID
	if len(cc.streams) == 0 {
		// Received a GOAWAY and no streams active, just close the conn.
		return errStopReadLoop
	}
	for streamID, cs := range cc.streams {
		if streamID <= last {
			// The server's GOAWAY indicates that it received this stream.
			// It will either finish processing it, or close the connection
			// without doing so. Either way, leave the stream alone for now.
			continue
		}
		if streamID == 1 && cc.goAway.ErrCode != ErrCodeNo {
			// Don't retry the first stream on a connection if we get a non-NO error.
			// If the server is sending an error on a new connection,
			// retrying the request on a new one probably isn't going to work.
			cs.abortStreamLocked(fmt.Errorf("http2: Transport received GOAWAY from server ErrCode:%v", cc.goAway.ErrCode))
		} else {
			// Aborting the stream with errClentConnGotGoAway indicates that
			// the request should be retried on a new connection.
			cs.abortStreamLocked(errClientConnGotGoAway)
		}
	}
	return nil
}

func (rl *clientConnReadLoop) processSettings(f *SettingsFrame) error {
	cc := rl.cc
	// Locking both mu and wmu here allows frame encoding to read settings with only wmu held.
	// Acquiring wmu when f.IsAck() is unnecessary, but convenient and mostly harmless.
	cc.wmu.Lock()
	defer cc.wmu.Unlock()

	if err := rl.processSettingsNoWrite(f); err != nil {
		return err
	}
	if !f.IsAck() {
		cc.fr.WriteSettingsAck()
		cc.bw.Flush()
	}
	return nil
}

func (rl *clientConnReadLoop) processSettingsNoWrite(f *SettingsFrame) error {
	cc := rl.cc
	defer cc.maybeCallStateHook()
	cc.mu.Lock()
	defer cc.mu.Unlock()

	if f.IsAck() {
		if cc.wantSettingsAck {
			cc.wantSettingsAck = false
			return nil
		}
		return ConnectionError(ErrCodeProtocol)
	}

	var seenMaxConcurrentStreams bool
	err := f.ForeachSetting(func(s Setting) error {
		if err := s.Valid(); err != nil {
			return err
		}
		switch s.ID {
		case SettingMaxFrameSize:
			cc.maxFrameSize = s.Val
		case SettingMaxConcurrentStreams:
			cc.maxConcurrentStreams = s.Val
			seenMaxConcurrentStreams = true
		case SettingMaxHeaderListSize:
			cc.peerMaxHeaderListSize = uint64(s.Val)
		case SettingInitialWindowSize:
			if !cc.flow.changeInitialWindowSize(int64(s.Val)) {
				return ConnectionError(ErrCodeFlowControl)
			}
			cc.cond.Broadcast()
		case SettingHeaderTableSize:
			cc.henc.SetMaxDynamicTableSize(s.Val)
			cc.peerMaxHeaderTableSize = s.Val
		case SettingEnableConnectProtocol:
			// If the peer wants to send us SETTINGS_ENABLE_CONNECT_PROTOCOL,
			// we require that it do so in the first SETTINGS frame.
			//
			// When we attempt to use extended CONNECT, we wait for the first
			// SETTINGS frame to see if the server supports it. If we let the
			// server enable the feature with a later SETTINGS frame, then
			// users will see inconsistent results depending on whether we've
			// seen that frame or not.
			if !cc.seenSettings {
				cc.extendedConnectAllowed = s.Val == 1
			}
		default:
			cc.vlogf("Unhandled Setting: %v", s)
		}
		return nil
	})
	if err != nil {
		return err
	}

	if !cc.seenSettings {
		if !seenMaxConcurrentStreams {
			// This was the servers initial SETTINGS frame and it
			// didn't contain a MAX_CONCURRENT_STREAMS field so
			// increase the number of concurrent streams this
			// connection can establish to our default.
			cc.maxConcurrentStreams = defaultMaxConcurrentStreams
		}
		close(cc.seenSettingsChan)
		cc.seenSettings = true
	}

	return nil
}

func (rl *clientConnReadLoop) processWindowUpdate(f *WindowUpdateFrame) error {
	cc := rl.cc
	cc.mu.Lock()
	defer cc.mu.Unlock()
	if f.StreamID == 0 {
		if !cc.flow.add(int32(f.Increment)) {
			return ConnectionError(ErrCodeFlowControl)
		}
	} else {
		cs := rl.streamByIDLocked(f.StreamID, notHeaderOrDataFrame)
		if cs == nil {
			return nil
		}
		if !cs.flow.add(int32(f.Increment)) {
			if cs.flow.conn.flowErr {
				// This is a lazily-detected connection-level flow control error.
				return ConnectionError(ErrCodeFlowControl)
			}
			// For stream, the sender sends RST_STREAM with
			// an error code of FLOW_CONTROL_ERROR.
			rl.endStreamErrorLocked(cs, StreamError{
				StreamID: f.StreamID,
				Code:     ErrCodeFlowControl,
			})
			return nil
		}
	}
	cc.cond.Broadcast()
	return nil
}

func (rl *clientConnReadLoop) processResetStream(f *RSTStreamFrame) error {
	cs := rl.streamByID(f.StreamID, notHeaderOrDataFrame)
	if cs == nil {
		// TODO: return error if server tries to RST_STREAM an idle stream
		return nil
	}
	serr := streamError(cs.ID, f.ErrCode)
	serr.Cause = errFromPeer
	if f.ErrCode == ErrCodeProtocol {
		rl.cc.SetDoNotReuse()
	}
	if fn := cs.cc.fr.countError; fn != nil {
		fn("recv_rststream_" + f.ErrCode.stringToken())
	}
	cs.abortStream(serr)

	cs.bufPipe.CloseWithError(serr)
	return nil
}

// Ping sends a PING frame to the server and waits for the ack.
func (cc *ClientConn) Ping(ctx context.Context) error {
	c := make(chan struct{})
	// Generate a random payload
	var p [8]byte
	for {
		if _, err := rand.Read(p[:]); err != nil {
			return err
		}
		cc.mu.Lock()
		// check for dup before insert
		if _, found := cc.pings[p]; !found {
			cc.pings[p] = c
			cc.mu.Unlock()
			break
		}
		cc.mu.Unlock()
	}
	var pingError error
	errc := make(chan struct{})
	go func() {
		cc.wmu.Lock()
		defer cc.wmu.Unlock()
		if pingError = cc.fr.WritePing(false, p); pingError != nil {
			close(errc)
			return
		}
		if pingError = cc.bw.Flush(); pingError != nil {
			close(errc)
			return
		}
	}()
	select {
	case <-c:
		return nil
	case <-errc:
		return pingError
	case <-ctx.Done():
		return ctx.Err()
	case <-cc.readerDone:
		// connection closed
		return cc.readerErr
	}
}

func (rl *clientConnReadLoop) processPing(f *PingFrame) error {
	if f.IsAck() {
		cc := rl.cc
		defer cc.maybeCallStateHook()
		cc.mu.Lock()
		defer cc.mu.Unlock()
		// If ack, notify listener if any
		if c, ok := cc.pings[f.Data]; ok {
			close(c)
			delete(cc.pings, f.Data)
		}
		if cc.pendingResets > 0 {
			// See clientStream.cleanupWriteRequest.
			cc.pendingResets = 0
			cc.rstStreamPingsBlocked = true
			cc.cond.Broadcast()
		}
		return nil
	}
	cc := rl.cc
	cc.wmu.Lock()
	defer cc.wmu.Unlock()
	if err := cc.fr.WritePing(true, f.Data); err != nil {
		return err
	}
	return cc.bw.Flush()
}

func (rl *clientConnReadLoop) processPushPromise(f *PushPromiseFrame) error {
	// We told the peer we don't want them.
	// Spec says:
	// "PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH
	// setting of the peer endpoint is set to 0. An endpoint that
	// has set this setting and has received acknowledgement MUST
	// treat the receipt of a PUSH_PROMISE frame as a connection
	// error (Section 5.4.1) of type PROTOCOL_ERROR."
	return ConnectionError(ErrCodeProtocol)
}

// writeStreamReset sends a RST_STREAM frame.
// When ping is true, it also sends a PING frame with a random payload.
func (cc *ClientConn) writeStreamReset(streamID uint32, code ErrCode, ping bool, err error) {
	// TODO: map err to more interesting error codes, once the
	// HTTP community comes up with some. But currently for
	// RST_STREAM there's no equivalent to GOAWAY frame's debug
	// data, and the error codes are all pretty vague ("cancel").
	cc.wmu.Lock()
	cc.fr.WriteRSTStream(streamID, code)
	if ping {
		var payload [8]byte
		rand.Read(payload[:])
		cc.fr.WritePing(false, payload)
	}
	cc.bw.Flush()
	cc.wmu.Unlock()
}

var (
	errResponseHeaderListSize = errors.New("http2: response header list larger than advertised limit")
	errRequestHeaderListSize  = httpcommon.ErrRequestHeaderListSize
)

func (cc *ClientConn) logf(format string, args ...any) {
	cc.t.logf(format, args...)
}

func (cc *ClientConn) vlogf(format string, args ...any) {
	cc.t.vlogf(format, args...)
}

func (t *Transport) vlogf(format string, args ...any) {
	if VerboseLogs {
		t.logf(format, args...)
	}
}

func (t *Transport) logf(format string, args ...any) {
	log.Printf(format, args...)
}

type missingBody struct{}

func (missingBody) Close() error             { return nil }
func (missingBody) Read([]byte) (int, error) { return 0, io.ErrUnexpectedEOF }

type erringRoundTripper struct{ err error }

func (rt erringRoundTripper) RoundTripErr() error                               { return rt.err }
func (rt erringRoundTripper) RoundTrip(*ClientRequest) (*ClientResponse, error) { return nil, rt.err }

var errConcurrentReadOnResBody = errors.New("http2: concurrent read on response body")

// gzipReader wraps a response body so it can lazily
// get gzip.Reader from the pool on the first call to Read.
// After Close is called it puts gzip.Reader to the pool immediately
// if there is no Read in progress or later when Read completes.
type gzipReader struct {
	_    incomparable
	body io.ReadCloser // underlying Response.Body
	mu   sync.Mutex    // guards zr and zerr
	zr   *gzip.Reader  // stores gzip reader from the pool between reads
	zerr error         // sticky gzip reader init error or sentinel value to detect concurrent read and read after close
}

type eofReader struct{}

func (eofReader) Read([]byte) (int, error) { return 0, io.EOF }
func (eofReader) ReadByte() (byte, error)  { return 0, io.EOF }

var gzipPool = sync.Pool{New: func() any { return new(gzip.Reader) }}

// gzipPoolGet gets a gzip.Reader from the pool and resets it to read from r.
func gzipPoolGet(r io.Reader) (*gzip.Reader, error) {
	zr := gzipPool.Get().(*gzip.Reader)
	if err := zr.Reset(r); err != nil {
		gzipPoolPut(zr)
		return nil, err
	}
	return zr, nil
}

// gzipPoolPut puts a gzip.Reader back into the pool.
func gzipPoolPut(zr *gzip.Reader) {
	// Reset will allocate bufio.Reader if we pass it anything
	// other than a flate.Reader, so ensure that it's getting one.
	var r flate.Reader = eofReader{}
	zr.Reset(r)
	gzipPool.Put(zr)
}

// acquire returns a gzip.Reader for reading response body.
// The reader must be released after use.
func (gz *gzipReader) acquire() (*gzip.Reader, error) {
	gz.mu.Lock()
	defer gz.mu.Unlock()
	if gz.zerr != nil {
		return nil, gz.zerr
	}
	if gz.zr == nil {
		// gzipPoolGet might block indefinitely since it reads the gzip header.
		// Therefore, drop mu temporarily when using gzipPoolGet.
		// We set zerr to errConcurrentReadOnResBody to prevent concurrent read
		// even when mu is temporarily dropped.
		gz.zerr = errConcurrentReadOnResBody
		gz.mu.Unlock()
		zr, err := gzipPoolGet(gz.body)
		gz.mu.Lock()
		// Guard against Close being called while gzipPoolGet is running.
		if gz.zerr != errConcurrentReadOnResBody {
			if zr != nil {
				gzipPoolPut(zr)
			}
			return nil, gz.zerr
		}
		gz.zr, gz.zerr = zr, err
		if gz.zerr != nil {
			return nil, gz.zerr
		}
	}
	ret := gz.zr
	gz.zr, gz.zerr = nil, errConcurrentReadOnResBody
	return ret, nil
}

// release returns the gzip.Reader to the pool if Close was called during Read.
func (gz *gzipReader) release(zr *gzip.Reader) {
	gz.mu.Lock()
	defer gz.mu.Unlock()
	if gz.zerr == errConcurrentReadOnResBody {
		gz.zr, gz.zerr = zr, nil
	} else { // fs.ErrClosed
		gzipPoolPut(zr)
	}
}

// close returns the gzip.Reader to the pool immediately or
// signals release to do so after Read completes.
func (gz *gzipReader) close() {
	gz.mu.Lock()
	defer gz.mu.Unlock()
	if gz.zerr == nil && gz.zr != nil {
		gzipPoolPut(gz.zr)
		gz.zr = nil
	}
	gz.zerr = fs.ErrClosed
}

func (gz *gzipReader) Read(p []byte) (n int, err error) {
	zr, err := gz.acquire()
	if err != nil {
		return 0, err
	}
	defer gz.release(zr)

	return zr.Read(p)
}

func (gz *gzipReader) Close() error {
	gz.close()

	return gz.body.Close()
}

// isConnectionCloseRequest reports whether req should use its own
// connection for a single request and then close the connection.
func isConnectionCloseRequest(req *ClientRequest) bool {
	return req.Close || httpguts.HeaderValuesContainsToken(req.Header["Connection"], "close")
}

// netHTTPClientConn wraps ClientConn and implements the interface net/http expects from
// the RoundTripper returned by NewClientConn.
type NetHTTPClientConn struct {
	cc *ClientConn
}

func (cc NetHTTPClientConn) RoundTrip(req *ClientRequest) (*ClientResponse, error) {
	return cc.cc.RoundTrip(req)
}

func (cc NetHTTPClientConn) Close() error {
	return cc.cc.Close()
}

func (cc NetHTTPClientConn) Err() error {
	cc.cc.mu.Lock()
	defer cc.cc.mu.Unlock()
	if cc.cc.closed {
		return errors.New("connection closed")
	}
	return nil
}

func (cc NetHTTPClientConn) Reserve() error {
	defer cc.cc.maybeCallStateHook()
	cc.cc.mu.Lock()
	defer cc.cc.mu.Unlock()
	if !cc.cc.canReserveLocked() {
		return errors.New("connection is unavailable")
	}
	cc.cc.streamsReserved++
	return nil
}

func (cc NetHTTPClientConn) Release() {
	defer cc.cc.maybeCallStateHook()
	cc.cc.mu.Lock()
	defer cc.cc.mu.Unlock()
	// We don't complain if streamsReserved is 0.
	//
	// This is consistent with RoundTrip: both Release and RoundTrip will
	// consume a reservation iff one exists.
	if cc.cc.streamsReserved > 0 {
		cc.cc.streamsReserved--
	}
}

func (cc NetHTTPClientConn) Available() int {
	cc.cc.mu.Lock()
	defer cc.cc.mu.Unlock()
	return cc.cc.availableLocked()
}

func (cc NetHTTPClientConn) InFlight() int {
	cc.cc.mu.Lock()
	defer cc.cc.mu.Unlock()
	return cc.cc.currentRequestCountLocked()
}

func (cc NetHTTPClientConn) Ping(ctx context.Context) error {
	return cc.cc.Ping(ctx)
}

func (cc *ClientConn) maybeCallStateHook() {
	if cc.internalStateHook != nil {
		cc.internalStateHook()
	}
}

func (t *Transport) idleConnTimeout() time.Duration {
	if t.t1 != nil {
		return t.t1.IdleConnTimeout()
	}

	return 0
}

func traceGetConn(req *ClientRequest, hostPort string) {
	trace := httptrace.ContextClientTrace(req.Context)
	if trace == nil || trace.GetConn == nil {
		return
	}
	trace.GetConn(hostPort)
}

func traceGotConn(req *ClientRequest, cc *ClientConn, reused bool) {
	trace := httptrace.ContextClientTrace(req.Context)
	if trace == nil || trace.GotConn == nil {
		return
	}
	ci := httptrace.GotConnInfo{Conn: cc.tconn}
	ci.Reused = reused
	cc.mu.Lock()
	ci.WasIdle = len(cc.streams) == 0 && reused
	if ci.WasIdle && !cc.lastActive.IsZero() {
		ci.IdleTime = time.Since(cc.lastActive)
	}
	cc.mu.Unlock()

	trace.GotConn(ci)
}

func traceWroteHeaders(trace *httptrace.ClientTrace) {
	if trace != nil && trace.WroteHeaders != nil {
		trace.WroteHeaders()
	}
}

func traceGot100Continue(trace *httptrace.ClientTrace) {
	if trace != nil && trace.Got100Continue != nil {
		trace.Got100Continue()
	}
}

func traceWait100Continue(trace *httptrace.ClientTrace) {
	if trace != nil && trace.Wait100Continue != nil {
		trace.Wait100Continue()
	}
}

func traceWroteRequest(trace *httptrace.ClientTrace, err error) {
	if trace != nil && trace.WroteRequest != nil {
		trace.WroteRequest(httptrace.WroteRequestInfo{Err: err})
	}
}

func traceFirstResponseByte(trace *httptrace.ClientTrace) {
	if trace != nil && trace.GotFirstResponseByte != nil {
		trace.GotFirstResponseByte()
	}
}

func traceGot1xxResponseFunc(trace *httptrace.ClientTrace) func(int, textproto.MIMEHeader) error {
	if trace != nil {
		return trace.Got1xxResponse
	}
	return nil
}

// dialTLSWithContext uses tls.Dialer, added in Go 1.15, to open a TLS
// connection.
func (t *Transport) dialTLSWithContext(ctx context.Context, network, addr string, cfg *tls.Config) (*tls.Conn, error) {
	dialer := &tls.Dialer{
		Config: cfg,
	}
	cn, err := dialer.DialContext(ctx, network, addr)
	if err != nil {
		return nil, err
	}
	tlsCn := cn.(*tls.Conn) // DialContext comment promises this will always succeed
	return tlsCn, nil
}

The pages are generated with Golds v0.8.4. (GOOS=linux GOARCH=amd64)