feat: initial tracing support (#1623)

1 files changed, 1647 insertions, 0 deletions

diff --git a/vendor/google.golang.org/grpc/clientconn.go b/vendor/google.golang.org/grpc/clientconn.go
new file mode 100644
index 000000000..d607d4e9e
--- /dev/null
+++ b/vendor/google.golang.org/grpc/clientconn.go
@@ -0,0 +1,1647 @@
+/*
+ *
+ * Copyright 2014 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package grpc
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"math"
+	"net/url"
+	"reflect"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/balancer/base"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/connectivity"
+	"google.golang.org/grpc/credentials"
+	"google.golang.org/grpc/internal/backoff"
+	"google.golang.org/grpc/internal/channelz"
+	"google.golang.org/grpc/internal/grpcsync"
+	iresolver "google.golang.org/grpc/internal/resolver"
+	"google.golang.org/grpc/internal/transport"
+	"google.golang.org/grpc/keepalive"
+	"google.golang.org/grpc/resolver"
+	"google.golang.org/grpc/serviceconfig"
+	"google.golang.org/grpc/status"
+
+	_ "google.golang.org/grpc/balancer/roundrobin"           // To register roundrobin.
+	_ "google.golang.org/grpc/internal/resolver/dns"         // To register dns resolver.
+	_ "google.golang.org/grpc/internal/resolver/passthrough" // To register passthrough resolver.
+	_ "google.golang.org/grpc/internal/resolver/unix"        // To register unix resolver.
+)
+
+const (
+	// minimum time to give a connection to complete
+	minConnectTimeout = 20 * time.Second
+	// must match grpclbName in grpclb/grpclb.go
+	grpclbName = "grpclb"
+)
+
+var (
+	// ErrClientConnClosing indicates that the operation is illegal because
+	// the ClientConn is closing.
+	//
+	// Deprecated: this error should not be relied upon by users; use the status
+	// code of Canceled instead.
+	ErrClientConnClosing = status.Error(codes.Canceled, "grpc: the client connection is closing")
+	// errConnDrain indicates that the connection starts to be drained and does not accept any new RPCs.
+	errConnDrain = errors.New("grpc: the connection is drained")
+	// errConnClosing indicates that the connection is closing.
+	errConnClosing = errors.New("grpc: the connection is closing")
+	// invalidDefaultServiceConfigErrPrefix is used to prefix the json parsing error for the default
+	// service config.
+	invalidDefaultServiceConfigErrPrefix = "grpc: the provided default service config is invalid"
+)
+
+// The following errors are returned from Dial and DialContext
+var (
+	// errNoTransportSecurity indicates that there is no transport security
+	// being set for ClientConn. Users should either set one or explicitly
+	// call WithInsecure DialOption to disable security.
+	errNoTransportSecurity = errors.New("grpc: no transport security set (use grpc.WithTransportCredentials(insecure.NewCredentials()) explicitly or set credentials)")
+	// errTransportCredsAndBundle indicates that creds bundle is used together
+	// with other individual Transport Credentials.
+	errTransportCredsAndBundle = errors.New("grpc: credentials.Bundle may not be used with individual TransportCredentials")
+	// errNoTransportCredsInBundle indicated that the configured creds bundle
+	// returned a transport credentials which was nil.
+	errNoTransportCredsInBundle = errors.New("grpc: credentials.Bundle must return non-nil transport credentials")
+	// errTransportCredentialsMissing indicates that users want to transmit
+	// security information (e.g., OAuth2 token) which requires secure
+	// connection on an insecure connection.
+	errTransportCredentialsMissing = errors.New("grpc: the credentials require transport level security (use grpc.WithTransportCredentials() to set)")
+)
+
+const (
+	defaultClientMaxReceiveMessageSize = 1024 * 1024 * 4
+	defaultClientMaxSendMessageSize    = math.MaxInt32
+	// http2IOBufSize specifies the buffer size for sending frames.
+	defaultWriteBufSize = 32 * 1024
+	defaultReadBufSize  = 32 * 1024
+)
+
+// Dial creates a client connection to the given target.
+func Dial(target string, opts ...DialOption) (*ClientConn, error) {
+	return DialContext(context.Background(), target, opts...)
+}
+
+type defaultConfigSelector struct {
+	sc *ServiceConfig
+}
+
+func (dcs *defaultConfigSelector) SelectConfig(rpcInfo iresolver.RPCInfo) (*iresolver.RPCConfig, error) {
+	return &iresolver.RPCConfig{
+		Context:      rpcInfo.Context,
+		MethodConfig: getMethodConfig(dcs.sc, rpcInfo.Method),
+	}, nil
+}
+
+// DialContext creates a client connection to the given target. By default, it's
+// a non-blocking dial (the function won't wait for connections to be
+// established, and connecting happens in the background). To make it a blocking
+// dial, use WithBlock() dial option.
+//
+// In the non-blocking case, the ctx does not act against the connection. It
+// only controls the setup steps.
+//
+// In the blocking case, ctx can be used to cancel or expire the pending
+// connection. Once this function returns, the cancellation and expiration of
+// ctx will be noop. Users should call ClientConn.Close to terminate all the
+// pending operations after this function returns.
+//
+// The target name syntax is defined in
+// https://github.com/grpc/grpc/blob/master/doc/naming.md.
+// e.g. to use dns resolver, a "dns:///" prefix should be applied to the target.
+func DialContext(ctx context.Context, target string, opts ...DialOption) (conn *ClientConn, err error) {
+	cc := &ClientConn{
+		target:            target,
+		csMgr:             &connectivityStateManager{},
+		conns:             make(map[*addrConn]struct{}),
+		dopts:             defaultDialOptions(),
+		blockingpicker:    newPickerWrapper(),
+		czData:            new(channelzData),
+		firstResolveEvent: grpcsync.NewEvent(),
+	}
+	cc.retryThrottler.Store((*retryThrottler)(nil))
+	cc.safeConfigSelector.UpdateConfigSelector(&defaultConfigSelector{nil})
+	cc.ctx, cc.cancel = context.WithCancel(context.Background())
+
+	for _, opt := range extraDialOptions {
+		opt.apply(&cc.dopts)
+	}
+
+	for _, opt := range opts {
+		opt.apply(&cc.dopts)
+	}
+
+	chainUnaryClientInterceptors(cc)
+	chainStreamClientInterceptors(cc)
+
+	defer func() {
+		if err != nil {
+			cc.Close()
+		}
+	}()
+
+	pid := cc.dopts.channelzParentID
+	cc.channelzID = channelz.RegisterChannel(&channelzChannel{cc}, pid, target)
+	ted := &channelz.TraceEventDesc{
+		Desc:     "Channel created",
+		Severity: channelz.CtInfo,
+	}
+	if cc.dopts.channelzParentID != nil {
+		ted.Parent = &channelz.TraceEventDesc{
+			Desc:     fmt.Sprintf("Nested Channel(id:%d) created", cc.channelzID.Int()),
+			Severity: channelz.CtInfo,
+		}
+	}
+	channelz.AddTraceEvent(logger, cc.channelzID, 1, ted)
+	cc.csMgr.channelzID = cc.channelzID
+
+	if cc.dopts.copts.TransportCredentials == nil && cc.dopts.copts.CredsBundle == nil {
+		return nil, errNoTransportSecurity
+	}
+	if cc.dopts.copts.TransportCredentials != nil && cc.dopts.copts.CredsBundle != nil {
+		return nil, errTransportCredsAndBundle
+	}
+	if cc.dopts.copts.CredsBundle != nil && cc.dopts.copts.CredsBundle.TransportCredentials() == nil {
+		return nil, errNoTransportCredsInBundle
+	}
+	transportCreds := cc.dopts.copts.TransportCredentials
+	if transportCreds == nil {
+		transportCreds = cc.dopts.copts.CredsBundle.TransportCredentials()
+	}
+	if transportCreds.Info().SecurityProtocol == "insecure" {
+		for _, cd := range cc.dopts.copts.PerRPCCredentials {
+			if cd.RequireTransportSecurity() {
+				return nil, errTransportCredentialsMissing
+			}
+		}
+	}
+
+	if cc.dopts.defaultServiceConfigRawJSON != nil {
+		scpr := parseServiceConfig(*cc.dopts.defaultServiceConfigRawJSON)
+		if scpr.Err != nil {
+			return nil, fmt.Errorf("%s: %v", invalidDefaultServiceConfigErrPrefix, scpr.Err)
+		}
+		cc.dopts.defaultServiceConfig, _ = scpr.Config.(*ServiceConfig)
+	}
+	cc.mkp = cc.dopts.copts.KeepaliveParams
+
+	if cc.dopts.copts.UserAgent != "" {
+		cc.dopts.copts.UserAgent += " " + grpcUA
+	} else {
+		cc.dopts.copts.UserAgent = grpcUA
+	}
+
+	if cc.dopts.timeout > 0 {
+		var cancel context.CancelFunc
+		ctx, cancel = context.WithTimeout(ctx, cc.dopts.timeout)
+		defer cancel()
+	}
+	defer func() {
+		select {
+		case <-ctx.Done():
+			switch {
+			case ctx.Err() == err:
+				conn = nil
+			case err == nil || !cc.dopts.returnLastError:
+				conn, err = nil, ctx.Err()
+			default:
+				conn, err = nil, fmt.Errorf("%v: %v", ctx.Err(), err)
+			}
+		default:
+		}
+	}()
+
+	scSet := false
+	if cc.dopts.scChan != nil {
+		// Try to get an initial service config.
+		select {
+		case sc, ok := <-cc.dopts.scChan:
+			if ok {
+				cc.sc = &sc
+				cc.safeConfigSelector.UpdateConfigSelector(&defaultConfigSelector{&sc})
+				scSet = true
+			}
+		default:
+		}
+	}
+	if cc.dopts.bs == nil {
+		cc.dopts.bs = backoff.DefaultExponential
+	}
+
+	// Determine the resolver to use.
+	resolverBuilder, err := cc.parseTargetAndFindResolver()
+	if err != nil {
+		return nil, err
+	}
+	cc.authority, err = determineAuthority(cc.parsedTarget.Endpoint(), cc.target, cc.dopts)
+	if err != nil {
+		return nil, err
+	}
+	channelz.Infof(logger, cc.channelzID, "Channel authority set to %q", cc.authority)
+
+	if cc.dopts.scChan != nil && !scSet {
+		// Blocking wait for the initial service config.
+		select {
+		case sc, ok := <-cc.dopts.scChan:
+			if ok {
+				cc.sc = &sc
+				cc.safeConfigSelector.UpdateConfigSelector(&defaultConfigSelector{&sc})
+			}
+		case <-ctx.Done():
+			return nil, ctx.Err()
+		}
+	}
+	if cc.dopts.scChan != nil {
+		go cc.scWatcher()
+	}
+
+	var credsClone credentials.TransportCredentials
+	if creds := cc.dopts.copts.TransportCredentials; creds != nil {
+		credsClone = creds.Clone()
+	}
+	cc.balancerWrapper = newCCBalancerWrapper(cc, balancer.BuildOptions{
+		DialCreds:        credsClone,
+		CredsBundle:      cc.dopts.copts.CredsBundle,
+		Dialer:           cc.dopts.copts.Dialer,
+		Authority:        cc.authority,
+		CustomUserAgent:  cc.dopts.copts.UserAgent,
+		ChannelzParentID: cc.channelzID,
+		Target:           cc.parsedTarget,
+	})
+
+	// Build the resolver.
+	rWrapper, err := newCCResolverWrapper(cc, resolverBuilder)
+	if err != nil {
+		return nil, fmt.Errorf("failed to build resolver: %v", err)
+	}
+	cc.mu.Lock()
+	cc.resolverWrapper = rWrapper
+	cc.mu.Unlock()
+
+	// A blocking dial blocks until the clientConn is ready.
+	if cc.dopts.block {
+		for {
+			cc.Connect()
+			s := cc.GetState()
+			if s == connectivity.Ready {
+				break
+			} else if cc.dopts.copts.FailOnNonTempDialError && s == connectivity.TransientFailure {
+				if err = cc.connectionError(); err != nil {
+					terr, ok := err.(interface {
+						Temporary() bool
+					})
+					if ok && !terr.Temporary() {
+						return nil, err
+					}
+				}
+			}
+			if !cc.WaitForStateChange(ctx, s) {
+				// ctx got timeout or canceled.
+				if err = cc.connectionError(); err != nil && cc.dopts.returnLastError {
+					return nil, err
+				}
+				return nil, ctx.Err()
+			}
+		}
+	}
+
+	return cc, nil
+}
+
+// chainUnaryClientInterceptors chains all unary client interceptors into one.
+func chainUnaryClientInterceptors(cc *ClientConn) {
+	interceptors := cc.dopts.chainUnaryInts
+	// Prepend dopts.unaryInt to the chaining interceptors if it exists, since unaryInt will
+	// be executed before any other chained interceptors.
+	if cc.dopts.unaryInt != nil {
+		interceptors = append([]UnaryClientInterceptor{cc.dopts.unaryInt}, interceptors...)
+	}
+	var chainedInt UnaryClientInterceptor
+	if len(interceptors) == 0 {
+		chainedInt = nil
+	} else if len(interceptors) == 1 {
+		chainedInt = interceptors[0]
+	} else {
+		chainedInt = func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker, opts ...CallOption) error {
+			return interceptors[0](ctx, method, req, reply, cc, getChainUnaryInvoker(interceptors, 0, invoker), opts...)
+		}
+	}
+	cc.dopts.unaryInt = chainedInt
+}
+
+// getChainUnaryInvoker recursively generate the chained unary invoker.
+func getChainUnaryInvoker(interceptors []UnaryClientInterceptor, curr int, finalInvoker UnaryInvoker) UnaryInvoker {
+	if curr == len(interceptors)-1 {
+		return finalInvoker
+	}
+	return func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, opts ...CallOption) error {
+		return interceptors[curr+1](ctx, method, req, reply, cc, getChainUnaryInvoker(interceptors, curr+1, finalInvoker), opts...)
+	}
+}
+
+// chainStreamClientInterceptors chains all stream client interceptors into one.
+func chainStreamClientInterceptors(cc *ClientConn) {
+	interceptors := cc.dopts.chainStreamInts
+	// Prepend dopts.streamInt to the chaining interceptors if it exists, since streamInt will
+	// be executed before any other chained interceptors.
+	if cc.dopts.streamInt != nil {
+		interceptors = append([]StreamClientInterceptor{cc.dopts.streamInt}, interceptors...)
+	}
+	var chainedInt StreamClientInterceptor
+	if len(interceptors) == 0 {
+		chainedInt = nil
+	} else if len(interceptors) == 1 {
+		chainedInt = interceptors[0]
+	} else {
+		chainedInt = func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, streamer Streamer, opts ...CallOption) (ClientStream, error) {
+			return interceptors[0](ctx, desc, cc, method, getChainStreamer(interceptors, 0, streamer), opts...)
+		}
+	}
+	cc.dopts.streamInt = chainedInt
+}
+
+// getChainStreamer recursively generate the chained client stream constructor.
+func getChainStreamer(interceptors []StreamClientInterceptor, curr int, finalStreamer Streamer) Streamer {
+	if curr == len(interceptors)-1 {
+		return finalStreamer
+	}
+	return func(ctx context.Context, desc *StreamDesc, cc *ClientConn, method string, opts ...CallOption) (ClientStream, error) {
+		return interceptors[curr+1](ctx, desc, cc, method, getChainStreamer(interceptors, curr+1, finalStreamer), opts...)
+	}
+}
+
+// connectivityStateManager keeps the connectivity.State of ClientConn.
+// This struct will eventually be exported so the balancers can access it.
+type connectivityStateManager struct {
+	mu         sync.Mutex
+	state      connectivity.State
+	notifyChan chan struct{}
+	channelzID *channelz.Identifier
+}
+
+// updateState updates the connectivity.State of ClientConn.
+// If there's a change it notifies goroutines waiting on state change to
+// happen.
+func (csm *connectivityStateManager) updateState(state connectivity.State) {
+	csm.mu.Lock()
+	defer csm.mu.Unlock()
+	if csm.state == connectivity.Shutdown {
+		return
+	}
+	if csm.state == state {
+		return
+	}
+	csm.state = state
+	channelz.Infof(logger, csm.channelzID, "Channel Connectivity change to %v", state)
+	if csm.notifyChan != nil {
+		// There are other goroutines waiting on this channel.
+		close(csm.notifyChan)
+		csm.notifyChan = nil
+	}
+}
+
+func (csm *connectivityStateManager) getState() connectivity.State {
+	csm.mu.Lock()
+	defer csm.mu.Unlock()
+	return csm.state
+}
+
+func (csm *connectivityStateManager) getNotifyChan() <-chan struct{} {
+	csm.mu.Lock()
+	defer csm.mu.Unlock()
+	if csm.notifyChan == nil {
+		csm.notifyChan = make(chan struct{})
+	}
+	return csm.notifyChan
+}
+
+// ClientConnInterface defines the functions clients need to perform unary and
+// streaming RPCs.  It is implemented by *ClientConn, and is only intended to
+// be referenced by generated code.
+type ClientConnInterface interface {
+	// Invoke performs a unary RPC and returns after the response is received
+	// into reply.
+	Invoke(ctx context.Context, method string, args interface{}, reply interface{}, opts ...CallOption) error
+	// NewStream begins a streaming RPC.
+	NewStream(ctx context.Context, desc *StreamDesc, method string, opts ...CallOption) (ClientStream, error)
+}
+
+// Assert *ClientConn implements ClientConnInterface.
+var _ ClientConnInterface = (*ClientConn)(nil)
+
+// ClientConn represents a virtual connection to a conceptual endpoint, to
+// perform RPCs.
+//
+// A ClientConn is free to have zero or more actual connections to the endpoint
+// based on configuration, load, etc. It is also free to determine which actual
+// endpoints to use and may change it every RPC, permitting client-side load
+// balancing.
+//
+// A ClientConn encapsulates a range of functionality including name
+// resolution, TCP connection establishment (with retries and backoff) and TLS
+// handshakes. It also handles errors on established connections by
+// re-resolving the name and reconnecting.
+type ClientConn struct {
+	ctx    context.Context    // Initialized using the background context at dial time.
+	cancel context.CancelFunc // Cancelled on close.
+
+	// The following are initialized at dial time, and are read-only after that.
+	target          string               // User's dial target.
+	parsedTarget    resolver.Target      // See parseTargetAndFindResolver().
+	authority       string               // See determineAuthority().
+	dopts           dialOptions          // Default and user specified dial options.
+	channelzID      *channelz.Identifier // Channelz identifier for the channel.
+	balancerWrapper *ccBalancerWrapper   // Uses gracefulswitch.balancer underneath.
+
+	// The following provide their own synchronization, and therefore don't
+	// require cc.mu to be held to access them.
+	csMgr              *connectivityStateManager
+	blockingpicker     *pickerWrapper
+	safeConfigSelector iresolver.SafeConfigSelector
+	czData             *channelzData
+	retryThrottler     atomic.Value // Updated from service config.
+
+	// firstResolveEvent is used to track whether the name resolver sent us at
+	// least one update. RPCs block on this event.
+	firstResolveEvent *grpcsync.Event
+
+	// mu protects the following fields.
+	// TODO: split mu so the same mutex isn't used for everything.
+	mu              sync.RWMutex
+	resolverWrapper *ccResolverWrapper         // Initialized in Dial; cleared in Close.
+	sc              *ServiceConfig             // Latest service config received from the resolver.
+	conns           map[*addrConn]struct{}     // Set to nil on close.
+	mkp             keepalive.ClientParameters // May be updated upon receipt of a GoAway.
+
+	lceMu               sync.Mutex // protects lastConnectionError
+	lastConnectionError error
+}
+
+// WaitForStateChange waits until the connectivity.State of ClientConn changes from sourceState or
+// ctx expires. A true value is returned in former case and false in latter.
+//
+// # Experimental
+//
+// Notice: This API is EXPERIMENTAL and may be changed or removed in a
+// later release.
+func (cc *ClientConn) WaitForStateChange(ctx context.Context, sourceState connectivity.State) bool {
+	ch := cc.csMgr.getNotifyChan()
+	if cc.csMgr.getState() != sourceState {
+		return true
+	}
+	select {
+	case <-ctx.Done():
+		return false
+	case <-ch:
+		return true
+	}
+}
+
+// GetState returns the connectivity.State of ClientConn.
+//
+// # Experimental
+//
+// Notice: This API is EXPERIMENTAL and may be changed or removed in a later
+// release.
+func (cc *ClientConn) GetState() connectivity.State {
+	return cc.csMgr.getState()
+}
+
+// Connect causes all subchannels in the ClientConn to attempt to connect if
+// the channel is idle.  Does not wait for the connection attempts to begin
+// before returning.
+//
+// # Experimental
+//
+// Notice: This API is EXPERIMENTAL and may be changed or removed in a later
+// release.
+func (cc *ClientConn) Connect() {
+	cc.balancerWrapper.exitIdle()
+}
+
+func (cc *ClientConn) scWatcher() {
+	for {
+		select {
+		case sc, ok := <-cc.dopts.scChan:
+			if !ok {
+				return
+			}
+			cc.mu.Lock()
+			// TODO: load balance policy runtime change is ignored.
+			// We may revisit this decision in the future.
+			cc.sc = &sc
+			cc.safeConfigSelector.UpdateConfigSelector(&defaultConfigSelector{&sc})
+			cc.mu.Unlock()
+		case <-cc.ctx.Done():
+			return
+		}
+	}
+}
+
+// waitForResolvedAddrs blocks until the resolver has provided addresses or the
+// context expires.  Returns nil unless the context expires first; otherwise
+// returns a status error based on the context.
+func (cc *ClientConn) waitForResolvedAddrs(ctx context.Context) error {
+	// This is on the RPC path, so we use a fast path to avoid the
+	// more-expensive "select" below after the resolver has returned once.
+	if cc.firstResolveEvent.HasFired() {
+		return nil
+	}
+	select {
+	case <-cc.firstResolveEvent.Done():
+		return nil
+	case <-ctx.Done():
+		return status.FromContextError(ctx.Err()).Err()
+	case <-cc.ctx.Done():
+		return ErrClientConnClosing
+	}
+}
+
+var emptyServiceConfig *ServiceConfig
+
+func init() {
+	cfg := parseServiceConfig("{}")
+	if cfg.Err != nil {
+		panic(fmt.Sprintf("impossible error parsing empty service config: %v", cfg.Err))
+	}
+	emptyServiceConfig = cfg.Config.(*ServiceConfig)
+}
+
+func (cc *ClientConn) maybeApplyDefaultServiceConfig(addrs []resolver.Address) {
+	if cc.sc != nil {
+		cc.applyServiceConfigAndBalancer(cc.sc, nil, addrs)
+		return
+	}
+	if cc.dopts.defaultServiceConfig != nil {
+		cc.applyServiceConfigAndBalancer(cc.dopts.defaultServiceConfig, &defaultConfigSelector{cc.dopts.defaultServiceConfig}, addrs)
+	} else {
+		cc.applyServiceConfigAndBalancer(emptyServiceConfig, &defaultConfigSelector{emptyServiceConfig}, addrs)
+	}
+}
+
+func (cc *ClientConn) updateResolverState(s resolver.State, err error) error {
+	defer cc.firstResolveEvent.Fire()
+	cc.mu.Lock()
+	// Check if the ClientConn is already closed. Some fields (e.g.
+	// balancerWrapper) are set to nil when closing the ClientConn, and could
+	// cause nil pointer panic if we don't have this check.
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return nil
+	}
+
+	if err != nil {
+		// May need to apply the initial service config in case the resolver
+		// doesn't support service configs, or doesn't provide a service config
+		// with the new addresses.
+		cc.maybeApplyDefaultServiceConfig(nil)
+
+		cc.balancerWrapper.resolverError(err)
+
+		// No addresses are valid with err set; return early.
+		cc.mu.Unlock()
+		return balancer.ErrBadResolverState
+	}
+
+	var ret error
+	if cc.dopts.disableServiceConfig {
+		channelz.Infof(logger, cc.channelzID, "ignoring service config from resolver (%v) and applying the default because service config is disabled", s.ServiceConfig)
+		cc.maybeApplyDefaultServiceConfig(s.Addresses)
+	} else if s.ServiceConfig == nil {
+		cc.maybeApplyDefaultServiceConfig(s.Addresses)
+		// TODO: do we need to apply a failing LB policy if there is no
+		// default, per the error handling design?
+	} else {
+		if sc, ok := s.ServiceConfig.Config.(*ServiceConfig); s.ServiceConfig.Err == nil && ok {
+			configSelector := iresolver.GetConfigSelector(s)
+			if configSelector != nil {
+				if len(s.ServiceConfig.Config.(*ServiceConfig).Methods) != 0 {
+					channelz.Infof(logger, cc.channelzID, "method configs in service config will be ignored due to presence of config selector")
+				}
+			} else {
+				configSelector = &defaultConfigSelector{sc}
+			}
+			cc.applyServiceConfigAndBalancer(sc, configSelector, s.Addresses)
+		} else {
+			ret = balancer.ErrBadResolverState
+			if cc.sc == nil {
+				// Apply the failing LB only if we haven't received valid service config
+				// from the name resolver in the past.
+				cc.applyFailingLB(s.ServiceConfig)
+				cc.mu.Unlock()
+				return ret
+			}
+		}
+	}
+
+	var balCfg serviceconfig.LoadBalancingConfig
+	if cc.sc != nil && cc.sc.lbConfig != nil {
+		balCfg = cc.sc.lbConfig.cfg
+	}
+	bw := cc.balancerWrapper
+	cc.mu.Unlock()
+
+	uccsErr := bw.updateClientConnState(&balancer.ClientConnState{ResolverState: s, BalancerConfig: balCfg})
+	if ret == nil {
+		ret = uccsErr // prefer ErrBadResolver state since any other error is
+		// currently meaningless to the caller.
+	}
+	return ret
+}
+
+// applyFailingLB is akin to configuring an LB policy on the channel which
+// always fails RPCs. Here, an actual LB policy is not configured, but an always
+// erroring picker is configured, which returns errors with information about
+// what was invalid in the received service config. A config selector with no
+// service config is configured, and the connectivity state of the channel is
+// set to TransientFailure.
+//
+// Caller must hold cc.mu.
+func (cc *ClientConn) applyFailingLB(sc *serviceconfig.ParseResult) {
+	var err error
+	if sc.Err != nil {
+		err = status.Errorf(codes.Unavailable, "error parsing service config: %v", sc.Err)
+	} else {
+		err = status.Errorf(codes.Unavailable, "illegal service config type: %T", sc.Config)
+	}
+	cc.safeConfigSelector.UpdateConfigSelector(&defaultConfigSelector{nil})
+	cc.blockingpicker.updatePicker(base.NewErrPicker(err))
+	cc.csMgr.updateState(connectivity.TransientFailure)
+}
+
+func (cc *ClientConn) handleSubConnStateChange(sc balancer.SubConn, s connectivity.State, err error) {
+	cc.balancerWrapper.updateSubConnState(sc, s, err)
+}
+
+// newAddrConn creates an addrConn for addrs and adds it to cc.conns.
+//
+// Caller needs to make sure len(addrs) > 0.
+func (cc *ClientConn) newAddrConn(addrs []resolver.Address, opts balancer.NewSubConnOptions) (*addrConn, error) {
+	ac := &addrConn{
+		state:        connectivity.Idle,
+		cc:           cc,
+		addrs:        addrs,
+		scopts:       opts,
+		dopts:        cc.dopts,
+		czData:       new(channelzData),
+		resetBackoff: make(chan struct{}),
+	}
+	ac.ctx, ac.cancel = context.WithCancel(cc.ctx)
+	// Track ac in cc. This needs to be done before any getTransport(...) is called.
+	cc.mu.Lock()
+	defer cc.mu.Unlock()
+	if cc.conns == nil {
+		return nil, ErrClientConnClosing
+	}
+
+	var err error
+	ac.channelzID, err = channelz.RegisterSubChannel(ac, cc.channelzID, "")
+	if err != nil {
+		return nil, err
+	}
+	channelz.AddTraceEvent(logger, ac.channelzID, 0, &channelz.TraceEventDesc{
+		Desc:     "Subchannel created",
+		Severity: channelz.CtInfo,
+		Parent: &channelz.TraceEventDesc{
+			Desc:     fmt.Sprintf("Subchannel(id:%d) created", ac.channelzID.Int()),
+			Severity: channelz.CtInfo,
+		},
+	})
+
+	cc.conns[ac] = struct{}{}
+	return ac, nil
+}
+
+// removeAddrConn removes the addrConn in the subConn from clientConn.
+// It also tears down the ac with the given error.
+func (cc *ClientConn) removeAddrConn(ac *addrConn, err error) {
+	cc.mu.Lock()
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return
+	}
+	delete(cc.conns, ac)
+	cc.mu.Unlock()
+	ac.tearDown(err)
+}
+
+func (cc *ClientConn) channelzMetric() *channelz.ChannelInternalMetric {
+	return &channelz.ChannelInternalMetric{
+		State:                    cc.GetState(),
+		Target:                   cc.target,
+		CallsStarted:             atomic.LoadInt64(&cc.czData.callsStarted),
+		CallsSucceeded:           atomic.LoadInt64(&cc.czData.callsSucceeded),
+		CallsFailed:              atomic.LoadInt64(&cc.czData.callsFailed),
+		LastCallStartedTimestamp: time.Unix(0, atomic.LoadInt64(&cc.czData.lastCallStartedTime)),
+	}
+}
+
+// Target returns the target string of the ClientConn.
+//
+// # Experimental
+//
+// Notice: This API is EXPERIMENTAL and may be changed or removed in a
+// later release.
+func (cc *ClientConn) Target() string {
+	return cc.target
+}
+
+func (cc *ClientConn) incrCallsStarted() {
+	atomic.AddInt64(&cc.czData.callsStarted, 1)
+	atomic.StoreInt64(&cc.czData.lastCallStartedTime, time.Now().UnixNano())
+}
+
+func (cc *ClientConn) incrCallsSucceeded() {
+	atomic.AddInt64(&cc.czData.callsSucceeded, 1)
+}
+
+func (cc *ClientConn) incrCallsFailed() {
+	atomic.AddInt64(&cc.czData.callsFailed, 1)
+}
+
+// connect starts creating a transport.
+// It does nothing if the ac is not IDLE.
+// TODO(bar) Move this to the addrConn section.
+func (ac *addrConn) connect() error {
+	ac.mu.Lock()
+	if ac.state == connectivity.Shutdown {
+		if logger.V(2) {
+			logger.Infof("connect called on shutdown addrConn; ignoring.")
+		}
+		ac.mu.Unlock()
+		return errConnClosing
+	}
+	if ac.state != connectivity.Idle {
+		if logger.V(2) {
+			logger.Infof("connect called on addrConn in non-idle state (%v); ignoring.", ac.state)
+		}
+		ac.mu.Unlock()
+		return nil
+	}
+	// Update connectivity state within the lock to prevent subsequent or
+	// concurrent calls from resetting the transport more than once.
+	ac.updateConnectivityState(connectivity.Connecting, nil)
+	ac.mu.Unlock()
+
+	ac.resetTransport()
+	return nil
+}
+
+func equalAddresses(a, b []resolver.Address) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i, v := range a {
+		if !v.Equal(b[i]) {
+			return false
+		}
+	}
+	return true
+}
+
+// tryUpdateAddrs tries to update ac.addrs with the new addresses list.
+//
+// If ac is TransientFailure, it updates ac.addrs and returns true. The updated
+// addresses will be picked up by retry in the next iteration after backoff.
+//
+// If ac is Shutdown or Idle, it updates ac.addrs and returns true.
+//
+// If the addresses is the same as the old list, it does nothing and returns
+// true.
+//
+// If ac is Connecting, it returns false. The caller should tear down the ac and
+// create a new one. Note that the backoff will be reset when this happens.
+//
+// If ac is Ready, it checks whether current connected address of ac is in the
+// new addrs list.
+//   - If true, it updates ac.addrs and returns true. The ac will keep using
+//     the existing connection.
+//   - If false, it does nothing and returns false.
+func (ac *addrConn) tryUpdateAddrs(addrs []resolver.Address) bool {
+	ac.mu.Lock()
+	defer ac.mu.Unlock()
+	channelz.Infof(logger, ac.channelzID, "addrConn: tryUpdateAddrs curAddr: %v, addrs: %v", ac.curAddr, addrs)
+	if ac.state == connectivity.Shutdown ||
+		ac.state == connectivity.TransientFailure ||
+		ac.state == connectivity.Idle {
+		ac.addrs = addrs
+		return true
+	}
+
+	if equalAddresses(ac.addrs, addrs) {
+		return true
+	}
+
+	if ac.state == connectivity.Connecting {
+		return false
+	}
+
+	// ac.state is Ready, try to find the connected address.
+	var curAddrFound bool
+	for _, a := range addrs {
+		a.ServerName = ac.cc.getServerName(a)
+		if reflect.DeepEqual(ac.curAddr, a) {
+			curAddrFound = true
+			break
+		}
+	}
+	channelz.Infof(logger, ac.channelzID, "addrConn: tryUpdateAddrs curAddrFound: %v", curAddrFound)
+	if curAddrFound {
+		ac.addrs = addrs
+	}
+
+	return curAddrFound
+}
+
+// getServerName determines the serverName to be used in the connection
+// handshake. The default value for the serverName is the authority on the
+// ClientConn, which either comes from the user's dial target or through an
+// authority override specified using the WithAuthority dial option. Name
+// resolvers can specify a per-address override for the serverName through the
+// resolver.Address.ServerName field which is used only if the WithAuthority
+// dial option was not used. The rationale is that per-address authority
+// overrides specified by the name resolver can represent a security risk, while
+// an override specified by the user is more dependable since they probably know
+// what they are doing.
+func (cc *ClientConn) getServerName(addr resolver.Address) string {
+	if cc.dopts.authority != "" {
+		return cc.dopts.authority
+	}
+	if addr.ServerName != "" {
+		return addr.ServerName
+	}
+	return cc.authority
+}
+
+func getMethodConfig(sc *ServiceConfig, method string) MethodConfig {
+	if sc == nil {
+		return MethodConfig{}
+	}
+	if m, ok := sc.Methods[method]; ok {
+		return m
+	}
+	i := strings.LastIndex(method, "/")
+	if m, ok := sc.Methods[method[:i+1]]; ok {
+		return m
+	}
+	return sc.Methods[""]
+}
+
+// GetMethodConfig gets the method config of the input method.
+// If there's an exact match for input method (i.e. /service/method), we return
+// the corresponding MethodConfig.
+// If there isn't an exact match for the input method, we look for the service's default
+// config under the service (i.e /service/) and then for the default for all services (empty string).
+//
+// If there is a default MethodConfig for the service, we return it.
+// Otherwise, we return an empty MethodConfig.
+func (cc *ClientConn) GetMethodConfig(method string) MethodConfig {
+	// TODO: Avoid the locking here.
+	cc.mu.RLock()
+	defer cc.mu.RUnlock()
+	return getMethodConfig(cc.sc, method)
+}
+
+func (cc *ClientConn) healthCheckConfig() *healthCheckConfig {
+	cc.mu.RLock()
+	defer cc.mu.RUnlock()
+	if cc.sc == nil {
+		return nil
+	}
+	return cc.sc.healthCheckConfig
+}
+
+func (cc *ClientConn) getTransport(ctx context.Context, failfast bool, method string) (transport.ClientTransport, balancer.PickResult, error) {
+	return cc.blockingpicker.pick(ctx, failfast, balancer.PickInfo{
+		Ctx:            ctx,
+		FullMethodName: method,
+	})
+}
+
+func (cc *ClientConn) applyServiceConfigAndBalancer(sc *ServiceConfig, configSelector iresolver.ConfigSelector, addrs []resolver.Address) {
+	if sc == nil {
+		// should never reach here.
+		return
+	}
+	cc.sc = sc
+	if configSelector != nil {
+		cc.safeConfigSelector.UpdateConfigSelector(configSelector)
+	}
+
+	if cc.sc.retryThrottling != nil {
+		newThrottler := &retryThrottler{
+			tokens: cc.sc.retryThrottling.MaxTokens,
+			max:    cc.sc.retryThrottling.MaxTokens,
+			thresh: cc.sc.retryThrottling.MaxTokens / 2,
+			ratio:  cc.sc.retryThrottling.TokenRatio,
+		}
+		cc.retryThrottler.Store(newThrottler)
+	} else {
+		cc.retryThrottler.Store((*retryThrottler)(nil))
+	}
+
+	var newBalancerName string
+	if cc.sc != nil && cc.sc.lbConfig != nil {
+		newBalancerName = cc.sc.lbConfig.name
+	} else {
+		var isGRPCLB bool
+		for _, a := range addrs {
+			if a.Type == resolver.GRPCLB {
+				isGRPCLB = true
+				break
+			}
+		}
+		if isGRPCLB {
+			newBalancerName = grpclbName
+		} else if cc.sc != nil && cc.sc.LB != nil {
+			newBalancerName = *cc.sc.LB
+		} else {
+			newBalancerName = PickFirstBalancerName
+		}
+	}
+	cc.balancerWrapper.switchTo(newBalancerName)
+}
+
+func (cc *ClientConn) resolveNow(o resolver.ResolveNowOptions) {
+	cc.mu.RLock()
+	r := cc.resolverWrapper
+	cc.mu.RUnlock()
+	if r == nil {
+		return
+	}
+	go r.resolveNow(o)
+}
+
+// ResetConnectBackoff wakes up all subchannels in transient failure and causes
+// them to attempt another connection immediately.  It also resets the backoff
+// times used for subsequent attempts regardless of the current state.
+//
+// In general, this function should not be used.  Typical service or network
+// outages result in a reasonable client reconnection strategy by default.
+// However, if a previously unavailable network becomes available, this may be
+// used to trigger an immediate reconnect.
+//
+// # Experimental
+//
+// Notice: This API is EXPERIMENTAL and may be changed or removed in a
+// later release.
+func (cc *ClientConn) ResetConnectBackoff() {
+	cc.mu.Lock()
+	conns := cc.conns
+	cc.mu.Unlock()
+	for ac := range conns {
+		ac.resetConnectBackoff()
+	}
+}
+
+// Close tears down the ClientConn and all underlying connections.
+func (cc *ClientConn) Close() error {
+	defer cc.cancel()
+
+	cc.mu.Lock()
+	if cc.conns == nil {
+		cc.mu.Unlock()
+		return ErrClientConnClosing
+	}
+	conns := cc.conns
+	cc.conns = nil
+	cc.csMgr.updateState(connectivity.Shutdown)
+
+	rWrapper := cc.resolverWrapper
+	cc.resolverWrapper = nil
+	bWrapper := cc.balancerWrapper
+	cc.mu.Unlock()
+
+	// The order of closing matters here since the balancer wrapper assumes the
+	// picker is closed before it is closed.
+	cc.blockingpicker.close()
+	if bWrapper != nil {
+		bWrapper.close()
+	}
+	if rWrapper != nil {
+		rWrapper.close()
+	}
+
+	for ac := range conns {
+		ac.tearDown(ErrClientConnClosing)
+	}
+	ted := &channelz.TraceEventDesc{
+		Desc:     "Channel deleted",
+		Severity: channelz.CtInfo,
+	}
+	if cc.dopts.channelzParentID != nil {
+		ted.Parent = &channelz.TraceEventDesc{
+			Desc:     fmt.Sprintf("Nested channel(id:%d) deleted", cc.channelzID.Int()),
+			Severity: channelz.CtInfo,
+		}
+	}
+	channelz.AddTraceEvent(logger, cc.channelzID, 0, ted)
+	// TraceEvent needs to be called before RemoveEntry, as TraceEvent may add
+	// trace reference to the entity being deleted, and thus prevent it from being
+	// deleted right away.
+	channelz.RemoveEntry(cc.channelzID)
+
+	return nil
+}
+
+// addrConn is a network connection to a given address.
+type addrConn struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	cc     *ClientConn
+	dopts  dialOptions
+	acbw   balancer.SubConn
+	scopts balancer.NewSubConnOptions
+
+	// transport is set when there's a viable transport (note: ac state may not be READY as LB channel
+	// health checking may require server to report healthy to set ac to READY), and is reset
+	// to nil when the current transport should no longer be used to create a stream (e.g. after GoAway
+	// is received, transport is closed, ac has been torn down).
+	transport transport.ClientTransport // The current transport.
+
+	mu      sync.Mutex
+	curAddr resolver.Address   // The current address.
+	addrs   []resolver.Address // All addresses that the resolver resolved to.
+
+	// Use updateConnectivityState for updating addrConn's connectivity state.
+	state connectivity.State
+
+	backoffIdx   int // Needs to be stateful for resetConnectBackoff.
+	resetBackoff chan struct{}
+
+	channelzID *channelz.Identifier
+	czData     *channelzData
+}
+
+// Note: this requires a lock on ac.mu.
+func (ac *addrConn) updateConnectivityState(s connectivity.State, lastErr error) {
+	if ac.state == s {
+		return
+	}
+	ac.state = s
+	channelz.Infof(logger, ac.channelzID, "Subchannel Connectivity change to %v", s)
+	ac.cc.handleSubConnStateChange(ac.acbw, s, lastErr)
+}
+
+// adjustParams updates parameters used to create transports upon
+// receiving a GoAway.
+func (ac *addrConn) adjustParams(r transport.GoAwayReason) {
+	switch r {
+	case transport.GoAwayTooManyPings:
+		v := 2 * ac.dopts.copts.KeepaliveParams.Time
+		ac.cc.mu.Lock()
+		if v > ac.cc.mkp.Time {
+			ac.cc.mkp.Time = v
+		}
+		ac.cc.mu.Unlock()
+	}
+}
+
+func (ac *addrConn) resetTransport() {
+	ac.mu.Lock()
+	if ac.state == connectivity.Shutdown {
+		ac.mu.Unlock()
+		return
+	}
+
+	addrs := ac.addrs
+	backoffFor := ac.dopts.bs.Backoff(ac.backoffIdx)
+	// This will be the duration that dial gets to finish.
+	dialDuration := minConnectTimeout
+	if ac.dopts.minConnectTimeout != nil {
+		dialDuration = ac.dopts.minConnectTimeout()
+	}
+
+	if dialDuration < backoffFor {
+		// Give dial more time as we keep failing to connect.
+		dialDuration = backoffFor
+	}
+	// We can potentially spend all the time trying the first address, and
+	// if the server accepts the connection and then hangs, the following
+	// addresses will never be tried.
+	//
+	// The spec doesn't mention what should be done for multiple addresses.
+	// https://github.com/grpc/grpc/blob/master/doc/connection-backoff.md#proposed-backoff-algorithm
+	connectDeadline := time.Now().Add(dialDuration)
+
+	ac.updateConnectivityState(connectivity.Connecting, nil)
+	ac.mu.Unlock()
+
+	if err := ac.tryAllAddrs(addrs, connectDeadline); err != nil {
+		ac.cc.resolveNow(resolver.ResolveNowOptions{})
+		// After exhausting all addresses, the addrConn enters
+		// TRANSIENT_FAILURE.
+		ac.mu.Lock()
+		if ac.state == connectivity.Shutdown {
+			ac.mu.Unlock()
+			return
+		}
+		ac.updateConnectivityState(connectivity.TransientFailure, err)
+
+		// Backoff.
+		b := ac.resetBackoff
+		ac.mu.Unlock()
+
+		timer := time.NewTimer(backoffFor)
+		select {
+		case <-timer.C:
+			ac.mu.Lock()
+			ac.backoffIdx++
+			ac.mu.Unlock()
+		case <-b:
+			timer.Stop()
+		case <-ac.ctx.Done():
+			timer.Stop()
+			return
+		}
+
+		ac.mu.Lock()
+		if ac.state != connectivity.Shutdown {
+			ac.updateConnectivityState(connectivity.Idle, err)
+		}
+		ac.mu.Unlock()
+		return
+	}
+	// Success; reset backoff.
+	ac.mu.Lock()
+	ac.backoffIdx = 0
+	ac.mu.Unlock()
+}
+
+// tryAllAddrs tries to creates a connection to the addresses, and stop when at
+// the first successful one. It returns an error if no address was successfully
+// connected, or updates ac appropriately with the new transport.
+func (ac *addrConn) tryAllAddrs(addrs []resolver.Address, connectDeadline time.Time) error {
+	var firstConnErr error
+	for _, addr := range addrs {
+		ac.mu.Lock()
+		if ac.state == connectivity.Shutdown {
+			ac.mu.Unlock()
+			return errConnClosing
+		}
+
+		ac.cc.mu.RLock()
+		ac.dopts.copts.KeepaliveParams = ac.cc.mkp
+		ac.cc.mu.RUnlock()
+
+		copts := ac.dopts.copts
+		if ac.scopts.CredsBundle != nil {
+			copts.CredsBundle = ac.scopts.CredsBundle
+		}
+		ac.mu.Unlock()
+
+		channelz.Infof(logger, ac.channelzID, "Subchannel picks a new address %q to connect", addr.Addr)
+
+		err := ac.createTransport(addr, copts, connectDeadline)
+		if err == nil {
+			return nil
+		}
+		if firstConnErr == nil {
+			firstConnErr = err
+		}
+		ac.cc.updateConnectionError(err)
+	}
+
+	// Couldn't connect to any address.
+	return firstConnErr
+}
+
+// createTransport creates a connection to addr. It returns an error if the
+// address was not successfully connected, or updates ac appropriately with the
+// new transport.
+func (ac *addrConn) createTransport(addr resolver.Address, copts transport.ConnectOptions, connectDeadline time.Time) error {
+	addr.ServerName = ac.cc.getServerName(addr)
+	hctx, hcancel := context.WithCancel(ac.ctx)
+
+	onClose := func(r transport.GoAwayReason) {
+		ac.mu.Lock()
+		defer ac.mu.Unlock()
+		// adjust params based on GoAwayReason
+		ac.adjustParams(r)
+		if ac.state == connectivity.Shutdown {
+			// Already shut down.  tearDown() already cleared the transport and
+			// canceled hctx via ac.ctx, and we expected this connection to be
+			// closed, so do nothing here.
+			return
+		}
+		hcancel()
+		if ac.transport == nil {
+			// We're still connecting to this address, which could error.  Do
+			// not update the connectivity state or resolve; these will happen
+			// at the end of the tryAllAddrs connection loop in the event of an
+			// error.
+			return
+		}
+		ac.transport = nil
+		// Refresh the name resolver on any connection loss.
+		ac.cc.resolveNow(resolver.ResolveNowOptions{})
+		// Always go idle and wait for the LB policy to initiate a new
+		// connection attempt.
+		ac.updateConnectivityState(connectivity.Idle, nil)
+	}
+
+	connectCtx, cancel := context.WithDeadline(ac.ctx, connectDeadline)
+	defer cancel()
+	copts.ChannelzParentID = ac.channelzID
+
+	newTr, err := transport.NewClientTransport(connectCtx, ac.cc.ctx, addr, copts, onClose)
+	if err != nil {
+		if logger.V(2) {
+			logger.Infof("Creating new client transport to %q: %v", addr, err)
+		}
+		// newTr is either nil, or closed.
+		hcancel()
+		channelz.Warningf(logger, ac.channelzID, "grpc: addrConn.createTransport failed to connect to %s. Err: %v", addr, err)
+		return err
+	}
+
+	ac.mu.Lock()
+	defer ac.mu.Unlock()
+	if ac.state == connectivity.Shutdown {
+		// This can happen if the subConn was removed while in `Connecting`
+		// state. tearDown() would have set the state to `Shutdown`, but
+		// would not have closed the transport since ac.transport would not
+		// have been set at that point.
+		//
+		// We run this in a goroutine because newTr.Close() calls onClose()
+		// inline, which requires locking ac.mu.
+		//
+		// The error we pass to Close() is immaterial since there are no open
+		// streams at this point, so no trailers with error details will be sent
+		// out. We just need to pass a non-nil error.
+		go newTr.Close(transport.ErrConnClosing)
+		return nil
+	}
+	if hctx.Err() != nil {
+		// onClose was already called for this connection, but the connection
+		// was successfully established first.  Consider it a success and set
+		// the new state to Idle.
+		ac.updateConnectivityState(connectivity.Idle, nil)
+		return nil
+	}
+	ac.curAddr = addr
+	ac.transport = newTr
+	ac.startHealthCheck(hctx) // Will set state to READY if appropriate.
+	return nil
+}
+
+// startHealthCheck starts the health checking stream (RPC) to watch the health
+// stats of this connection if health checking is requested and configured.
+//
+// LB channel health checking is enabled when all requirements below are met:
+// 1. it is not disabled by the user with the WithDisableHealthCheck DialOption
+// 2. internal.HealthCheckFunc is set by importing the grpc/health package
+// 3. a service config with non-empty healthCheckConfig field is provided
+// 4. the load balancer requests it
+//
+// It sets addrConn to READY if the health checking stream is not started.
+//
+// Caller must hold ac.mu.
+func (ac *addrConn) startHealthCheck(ctx context.Context) {
+	var healthcheckManagingState bool
+	defer func() {
+		if !healthcheckManagingState {
+			ac.updateConnectivityState(connectivity.Ready, nil)
+		}
+	}()
+
+	if ac.cc.dopts.disableHealthCheck {
+		return
+	}
+	healthCheckConfig := ac.cc.healthCheckConfig()
+	if healthCheckConfig == nil {
+		return
+	}
+	if !ac.scopts.HealthCheckEnabled {
+		return
+	}
+	healthCheckFunc := ac.cc.dopts.healthCheckFunc
+	if healthCheckFunc == nil {
+		// The health package is not imported to set health check function.
+		//
+		// TODO: add a link to the health check doc in the error message.
+		channelz.Error(logger, ac.channelzID, "Health check is requested but health check function is not set.")
+		return
+	}
+
+	healthcheckManagingState = true
+
+	// Set up the health check helper functions.
+	currentTr := ac.transport
+	newStream := func(method string) (interface{}, error) {
+		ac.mu.Lock()
+		if ac.transport != currentTr {
+			ac.mu.Unlock()
+			return nil, status.Error(codes.Canceled, "the provided transport is no longer valid to use")
+		}
+		ac.mu.Unlock()
+		return newNonRetryClientStream(ctx, &StreamDesc{ServerStreams: true}, method, currentTr, ac)
+	}
+	setConnectivityState := func(s connectivity.State, lastErr error) {
+		ac.mu.Lock()
+		defer ac.mu.Unlock()
+		if ac.transport != currentTr {
+			return
+		}
+		ac.updateConnectivityState(s, lastErr)
+	}
+	// Start the health checking stream.
+	go func() {
+		err := ac.cc.dopts.healthCheckFunc(ctx, newStream, setConnectivityState, healthCheckConfig.ServiceName)
+		if err != nil {
+			if status.Code(err) == codes.Unimplemented {
+				channelz.Error(logger, ac.channelzID, "Subchannel health check is unimplemented at server side, thus health check is disabled")
+			} else {
+				channelz.Errorf(logger, ac.channelzID, "Health checking failed: %v", err)
+			}
+		}
+	}()
+}
+
+func (ac *addrConn) resetConnectBackoff() {
+	ac.mu.Lock()
+	close(ac.resetBackoff)
+	ac.backoffIdx = 0
+	ac.resetBackoff = make(chan struct{})
+	ac.mu.Unlock()
+}
+
+// getReadyTransport returns the transport if ac's state is READY or nil if not.
+func (ac *addrConn) getReadyTransport() transport.ClientTransport {
+	ac.mu.Lock()
+	defer ac.mu.Unlock()
+	if ac.state == connectivity.Ready {
+		return ac.transport
+	}
+	return nil
+}
+
+// tearDown starts to tear down the addrConn.
+//
+// Note that tearDown doesn't remove ac from ac.cc.conns, so the addrConn struct
+// will leak. In most cases, call cc.removeAddrConn() instead.
+func (ac *addrConn) tearDown(err error) {
+	ac.mu.Lock()
+	if ac.state == connectivity.Shutdown {
+		ac.mu.Unlock()
+		return
+	}
+	curTr := ac.transport
+	ac.transport = nil
+	// We have to set the state to Shutdown before anything else to prevent races
+	// between setting the state and logic that waits on context cancellation / etc.
+	ac.updateConnectivityState(connectivity.Shutdown, nil)
+	ac.cancel()
+	ac.curAddr = resolver.Address{}
+	if err == errConnDrain && curTr != nil {
+		// GracefulClose(...) may be executed multiple times when
+		// i) receiving multiple GoAway frames from the server; or
+		// ii) there are concurrent name resolver/Balancer triggered
+		// address removal and GoAway.
+		// We have to unlock and re-lock here because GracefulClose => Close => onClose, which requires locking ac.mu.
+		ac.mu.Unlock()
+		curTr.GracefulClose()
+		ac.mu.Lock()
+	}
+	channelz.AddTraceEvent(logger, ac.channelzID, 0, &channelz.TraceEventDesc{
+		Desc:     "Subchannel deleted",
+		Severity: channelz.CtInfo,
+		Parent: &channelz.TraceEventDesc{
+			Desc:     fmt.Sprintf("Subchannel(id:%d) deleted", ac.channelzID.Int()),
+			Severity: channelz.CtInfo,
+		},
+	})
+	// TraceEvent needs to be called before RemoveEntry, as TraceEvent may add
+	// trace reference to the entity being deleted, and thus prevent it from
+	// being deleted right away.
+	channelz.RemoveEntry(ac.channelzID)
+	ac.mu.Unlock()
+}
+
+func (ac *addrConn) getState() connectivity.State {
+	ac.mu.Lock()
+	defer ac.mu.Unlock()
+	return ac.state
+}
+
+func (ac *addrConn) ChannelzMetric() *channelz.ChannelInternalMetric {
+	ac.mu.Lock()
+	addr := ac.curAddr.Addr
+	ac.mu.Unlock()
+	return &channelz.ChannelInternalMetric{
+		State:                    ac.getState(),
+		Target:                   addr,
+		CallsStarted:             atomic.LoadInt64(&ac.czData.callsStarted),
+		CallsSucceeded:           atomic.LoadInt64(&ac.czData.callsSucceeded),
+		CallsFailed:              atomic.LoadInt64(&ac.czData.callsFailed),
+		LastCallStartedTimestamp: time.Unix(0, atomic.LoadInt64(&ac.czData.lastCallStartedTime)),
+	}
+}
+
+func (ac *addrConn) incrCallsStarted() {
+	atomic.AddInt64(&ac.czData.callsStarted, 1)
+	atomic.StoreInt64(&ac.czData.lastCallStartedTime, time.Now().UnixNano())
+}
+
+func (ac *addrConn) incrCallsSucceeded() {
+	atomic.AddInt64(&ac.czData.callsSucceeded, 1)
+}
+
+func (ac *addrConn) incrCallsFailed() {
+	atomic.AddInt64(&ac.czData.callsFailed, 1)
+}
+
+type retryThrottler struct {
+	max    float64
+	thresh float64
+	ratio  float64
+
+	mu     sync.Mutex
+	tokens float64 // TODO(dfawley): replace with atomic and remove lock.
+}
+
+// throttle subtracts a retry token from the pool and returns whether a retry
+// should be throttled (disallowed) based upon the retry throttling policy in
+// the service config.
+func (rt *retryThrottler) throttle() bool {
+	if rt == nil {
+		return false
+	}
+	rt.mu.Lock()
+	defer rt.mu.Unlock()
+	rt.tokens--
+	if rt.tokens < 0 {
+		rt.tokens = 0
+	}
+	return rt.tokens <= rt.thresh
+}
+
+func (rt *retryThrottler) successfulRPC() {
+	if rt == nil {
+		return
+	}
+	rt.mu.Lock()
+	defer rt.mu.Unlock()
+	rt.tokens += rt.ratio
+	if rt.tokens > rt.max {
+		rt.tokens = rt.max
+	}
+}
+
+type channelzChannel struct {
+	cc *ClientConn
+}
+
+func (c *channelzChannel) ChannelzMetric() *channelz.ChannelInternalMetric {
+	return c.cc.channelzMetric()
+}
+
+// ErrClientConnTimeout indicates that the ClientConn cannot establish the
+// underlying connections within the specified timeout.
+//
+// Deprecated: This error is never returned by grpc and should not be
+// referenced by users.
+var ErrClientConnTimeout = errors.New("grpc: timed out when dialing")
+
+func (cc *ClientConn) getResolver(scheme string) resolver.Builder {
+	for _, rb := range cc.dopts.resolvers {
+		if scheme == rb.Scheme() {
+			return rb
+		}
+	}
+	return resolver.Get(scheme)
+}
+
+func (cc *ClientConn) updateConnectionError(err error) {
+	cc.lceMu.Lock()
+	cc.lastConnectionError = err
+	cc.lceMu.Unlock()
+}
+
+func (cc *ClientConn) connectionError() error {
+	cc.lceMu.Lock()
+	defer cc.lceMu.Unlock()
+	return cc.lastConnectionError
+}
+
+func (cc *ClientConn) parseTargetAndFindResolver() (resolver.Builder, error) {
+	channelz.Infof(logger, cc.channelzID, "original dial target is: %q", cc.target)
+
+	var rb resolver.Builder
+	parsedTarget, err := parseTarget(cc.target)
+	if err != nil {
+		channelz.Infof(logger, cc.channelzID, "dial target %q parse failed: %v", cc.target, err)
+	} else {
+		channelz.Infof(logger, cc.channelzID, "parsed dial target is: %+v", parsedTarget)
+		rb = cc.getResolver(parsedTarget.URL.Scheme)
+		if rb != nil {
+			cc.parsedTarget = parsedTarget
+			return rb, nil
+		}
+	}
+
+	// We are here because the user's dial target did not contain a scheme or
+	// specified an unregistered scheme. We should fallback to the default
+	// scheme, except when a custom dialer is specified in which case, we should
+	// always use passthrough scheme.
+	defScheme := resolver.GetDefaultScheme()
+	channelz.Infof(logger, cc.channelzID, "fallback to scheme %q", defScheme)
+	canonicalTarget := defScheme + ":///" + cc.target
+
+	parsedTarget, err = parseTarget(canonicalTarget)
+	if err != nil {
+		channelz.Infof(logger, cc.channelzID, "dial target %q parse failed: %v", canonicalTarget, err)
+		return nil, err
+	}
+	channelz.Infof(logger, cc.channelzID, "parsed dial target is: %+v", parsedTarget)
+	rb = cc.getResolver(parsedTarget.URL.Scheme)
+	if rb == nil {
+		return nil, fmt.Errorf("could not get resolver for default scheme: %q", parsedTarget.URL.Scheme)
+	}
+	cc.parsedTarget = parsedTarget
+	return rb, nil
+}
+
+// parseTarget uses RFC 3986 semantics to parse the given target into a
+// resolver.Target struct containing scheme, authority and url. Query
+// params are stripped from the endpoint.
+func parseTarget(target string) (resolver.Target, error) {
+	u, err := url.Parse(target)
+	if err != nil {
+		return resolver.Target{}, err
+	}
+
+	return resolver.Target{
+		Scheme:    u.Scheme,
+		Authority: u.Host,
+		URL:       *u,
+	}, nil
+}
+
+// Determine channel authority. The order of precedence is as follows:
+// - user specified authority override using `WithAuthority` dial option
+// - creds' notion of server name for the authentication handshake
+// - endpoint from dial target of the form "scheme://[authority]/endpoint"
+func determineAuthority(endpoint, target string, dopts dialOptions) (string, error) {
+	// Historically, we had two options for users to specify the serverName or
+	// authority for a channel. One was through the transport credentials
+	// (either in its constructor, or through the OverrideServerName() method).
+	// The other option (for cases where WithInsecure() dial option was used)
+	// was to use the WithAuthority() dial option.
+	//
+	// A few things have changed since:
+	// - `insecure` package with an implementation of the `TransportCredentials`
+	//   interface for the insecure case
+	// - WithAuthority() dial option support for secure credentials
+	authorityFromCreds := ""
+	if creds := dopts.copts.TransportCredentials; creds != nil && creds.Info().ServerName != "" {
+		authorityFromCreds = creds.Info().ServerName
+	}
+	authorityFromDialOption := dopts.authority
+	if (authorityFromCreds != "" && authorityFromDialOption != "") && authorityFromCreds != authorityFromDialOption {
+		return "", fmt.Errorf("ClientConn's authority from transport creds %q and dial option %q don't match", authorityFromCreds, authorityFromDialOption)
+	}
+
+	switch {
+	case authorityFromDialOption != "":
+		return authorityFromDialOption, nil
+	case authorityFromCreds != "":
+		return authorityFromCreds, nil
+	case strings.HasPrefix(target, "unix:") || strings.HasPrefix(target, "unix-abstract:"):
+		// TODO: remove when the unix resolver implements optional interface to
+		// return channel authority.
+		return "localhost", nil
+	case strings.HasPrefix(endpoint, ":"):
+		return "localhost" + endpoint, nil
+	default:
+		// TODO: Define an optional interface on the resolver builder to return
+		// the channel authority given the user's dial target. For resolvers
+		// which don't implement this interface, we will use the endpoint from
+		// "scheme://authority/endpoint" as the default authority.
+		return endpoint, nil
+	}
+}