const (
    // CredsBundleModeFallback switches GoogleDefaultCreds to fallback mode.
    CredsBundleModeFallback = "fallback"
    // CredsBundleModeBalancer switches GoogleDefaultCreds to grpclb balancer
    // mode.
    CredsBundleModeBalancer = "balancer"
    // CredsBundleModeBackendFromBalancer switches GoogleDefaultCreds to mode
    // that supports backend returned by grpclb balancer.
    CredsBundleModeBackendFromBalancer = "backend-from-balancer"
)

var (
    // WithHealthCheckFunc is set by dialoptions.go
    WithHealthCheckFunc interface{} // func (HealthChecker) DialOption
    // HealthCheckFunc is used to provide client-side LB channel health checking
    HealthCheckFunc HealthChecker
    // BalancerUnregister is exported by package balancer to unregister a balancer.
    BalancerUnregister func(name string)
    // KeepaliveMinPingTime is the minimum ping interval.  This must be 10s by
    // default, but tests may wish to set it lower for convenience.
    KeepaliveMinPingTime = 10 * time.Second
    // NewRequestInfoContext creates a new context based on the argument context attaching
    // the passed in RequestInfo to the new context.
    NewRequestInfoContext interface{} // func(context.Context, credentials.RequestInfo) context.Context
    // NewClientHandshakeInfoContext returns a copy of the input context with
    // the passed in ClientHandshakeInfo struct added to it.
    NewClientHandshakeInfoContext interface{} // func(context.Context, credentials.ClientHandshakeInfo) context.Context
    // ParseServiceConfigForTesting is for creating a fake
    // ClientConn for resolver testing only
    ParseServiceConfigForTesting interface{} // func(string) *serviceconfig.ParseResult
    // EqualServiceConfigForTesting is for testing service config generation and
    // parsing. Both a and b should be returned by ParseServiceConfigForTesting.
    // This function compares the config without rawJSON stripped, in case the
    // there's difference in white space.
    EqualServiceConfigForTesting func(a, b serviceconfig.Config) bool
    // GetCertificateProviderBuilder returns the registered builder for the
    // given name. This is set by package certprovider for use from xDS
    // bootstrap code while parsing certificate provider configs in the
    // bootstrap file.
    GetCertificateProviderBuilder interface{} // func(string) certprovider.Builder
    // GetXDSHandshakeInfoForTesting returns a pointer to the xds.HandshakeInfo
    // stored in the passed in attributes. This is set by
    // credentials/xds/xds.go.
    GetXDSHandshakeInfoForTesting interface{} // func (*attributes.Attributes) *xds.HandshakeInfo
    // GetServerCredentials returns the transport credentials configured on a
    // gRPC server. An xDS-enabled server needs to know what type of credentials
    // is configured on the underlying gRPC server. This is set by server.go.
    GetServerCredentials interface{} // func (*grpc.Server) credentials.TransportCredentials
)

type HealthChecker func(ctx context.Context, newStream func(string) (interface{}, error), setConnectivityState func(connectivity.State, error), serviceName string) error