//
// Copyright 2018 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.
//

#include <grpc/support/port_platform.h>

#include <stddef.h>
#include <stdint.h>

#include <atomic>
#include <map>
#include <memory>
#include <string>
#include <utility>
#include <vector>

#include "absl/base/thread_annotations.h"
#include "absl/status/status.h"
#include "absl/status/statusor.h"
#include "absl/strings/str_cat.h"
#include "absl/strings/string_view.h"
#include "absl/types/optional.h"
#include "absl/types/variant.h"

#include <grpc/event_engine/event_engine.h>
#include <grpc/impl/connectivity_state.h>
#include <grpc/support/log.h>

#include "src/core/ext/filters/client_channel/lb_policy/backend_metric_data.h"
#include "src/core/ext/filters/client_channel/lb_policy/child_policy_handler.h"
#include "src/core/ext/filters/client_channel/lb_policy/xds/xds_attributes.h"
#include "src/core/ext/filters/client_channel/lb_policy/xds/xds_channel_args.h"
#include "src/core/ext/xds/xds_bootstrap.h"
#include "src/core/ext/xds/xds_bootstrap_grpc.h"
#include "src/core/ext/xds/xds_client.h"
#include "src/core/ext/xds/xds_client_grpc.h"
#include "src/core/ext/xds/xds_client_stats.h"
#include "src/core/ext/xds/xds_endpoint.h"
#include "src/core/lib/channel/channel_args.h"
#include "src/core/lib/config/core_configuration.h"
#include "src/core/lib/debug/trace.h"
#include "src/core/lib/gprpp/debug_location.h"
#include "src/core/lib/gprpp/orphanable.h"
#include "src/core/lib/gprpp/ref_counted.h"
#include "src/core/lib/gprpp/ref_counted_ptr.h"
#include "src/core/lib/gprpp/sync.h"
#include "src/core/lib/gprpp/validation_errors.h"
#include "src/core/lib/iomgr/pollset_set.h"
#include "src/core/lib/json/json.h"
#include "src/core/lib/json/json_args.h"
#include "src/core/lib/json/json_object_loader.h"
#include "src/core/lib/load_balancing/lb_policy.h"
#include "src/core/lib/load_balancing/lb_policy_factory.h"
#include "src/core/lib/load_balancing/lb_policy_registry.h"
#include "src/core/lib/load_balancing/subchannel_interface.h"
#include "src/core/lib/resolver/server_address.h"
#include "src/core/lib/transport/connectivity_state.h"

namespace grpc_core {

TraceFlag grpc_xds_cluster_impl_lb_trace(false, "xds_cluster_impl_lb");

namespace {

//
// global circuit breaker atomic map
//

class CircuitBreakerCallCounterMap {
 public:
  using Key =
      std::pair<std::string /*cluster*/, std::string /*eds_service_name*/>;

  class CallCounter : public RefCounted<CallCounter> {
   public:
    explicit CallCounter(Key key) : key_(std::move(key)) {}
    ~CallCounter() override;

    uint32_t Load() {
      return concurrent_requests_.load(std::memory_order_seq_cst);
    }
    uint32_t Increment() { return concurrent_requests_.fetch_add(1); }
    void Decrement() { concurrent_requests_.fetch_sub(1); }

   private:
    Key key_;
    std::atomic<uint32_t> concurrent_requests_{0};
  };

  RefCountedPtr<CallCounter> GetOrCreate(const std::string& cluster,
                                         const std::string& eds_service_name);

 private:
  Mutex mu_;
  std::map<Key, CallCounter*> map_ ABSL_GUARDED_BY(mu_);
};

CircuitBreakerCallCounterMap* const g_call_counter_map =
    new CircuitBreakerCallCounterMap;

RefCountedPtr<CircuitBreakerCallCounterMap::CallCounter>
CircuitBreakerCallCounterMap::GetOrCreate(const std::string& cluster,
                                          const std::string& eds_service_name) {
  Key key(cluster, eds_service_name);
  RefCountedPtr<CallCounter> result;
  MutexLock lock(&mu_);
  auto it = map_.find(key);
  if (it == map_.end()) {
    it = map_.insert({key, nullptr}).first;
  } else {
    result = it->second->RefIfNonZero();
  }
  if (result == nullptr) {
    result = MakeRefCounted<CallCounter>(std::move(key));
    it->second = result.get();
  }
  return result;
}

CircuitBreakerCallCounterMap::CallCounter::~CallCounter() {
  MutexLock lock(&g_call_counter_map->mu_);
  auto it = g_call_counter_map->map_.find(key_);
  if (it != g_call_counter_map->map_.end() && it->second == this) {
    g_call_counter_map->map_.erase(it);
  }
}

//
// LB policy
//

constexpr absl::string_view kXdsClusterImpl = "xds_cluster_impl_experimental";

// Config for xDS Cluster Impl LB policy.
class XdsClusterImplLbConfig : public LoadBalancingPolicy::Config {
 public:
  XdsClusterImplLbConfig() = default;

  XdsClusterImplLbConfig(const XdsClusterImplLbConfig&) = delete;
  XdsClusterImplLbConfig& operator=(const XdsClusterImplLbConfig&) = delete;

  XdsClusterImplLbConfig(XdsClusterImplLbConfig&& other) = delete;
  XdsClusterImplLbConfig& operator=(XdsClusterImplLbConfig&& other) = delete;

  absl::string_view name() const override { return kXdsClusterImpl; }

  RefCountedPtr<LoadBalancingPolicy::Config> child_policy() const {
    return child_policy_;
  }
  const std::string& cluster_name() const { return cluster_name_; }
  const std::string& eds_service_name() const { return eds_service_name_; }
  const absl::optional<GrpcXdsBootstrap::GrpcXdsServer>&
  lrs_load_reporting_server() const {
    return lrs_load_reporting_server_;
  };
  uint32_t max_concurrent_requests() const { return max_concurrent_requests_; }
  RefCountedPtr<XdsEndpointResource::DropConfig> drop_config() const {
    return drop_config_;
  }

  static const JsonLoaderInterface* JsonLoader(const JsonArgs&);
  void JsonPostLoad(const Json& json, const JsonArgs& args,
                    ValidationErrors* errors);

 private:
  RefCountedPtr<LoadBalancingPolicy::Config> child_policy_;
  std::string cluster_name_;
  std::string eds_service_name_;
  absl::optional<GrpcXdsBootstrap::GrpcXdsServer> lrs_load_reporting_server_;
  uint32_t max_concurrent_requests_;
  RefCountedPtr<XdsEndpointResource::DropConfig> drop_config_;
};

// xDS Cluster Impl LB policy.
class XdsClusterImplLb : public LoadBalancingPolicy {
 public:
  XdsClusterImplLb(RefCountedPtr<XdsClient> xds_client, Args args);

  absl::string_view name() const override { return kXdsClusterImpl; }

  absl::Status UpdateLocked(UpdateArgs args) override;
  void ExitIdleLocked() override;
  void ResetBackoffLocked() override;

 private:
  class StatsSubchannelWrapper : public DelegatingSubchannel {
   public:
    StatsSubchannelWrapper(
        RefCountedPtr<SubchannelInterface> wrapped_subchannel,
        RefCountedPtr<XdsClusterLocalityStats> locality_stats)
        : DelegatingSubchannel(std::move(wrapped_subchannel)),
          locality_stats_(std::move(locality_stats)) {}

    XdsClusterLocalityStats* locality_stats() const {
      return locality_stats_.get();
    }

   private:
    RefCountedPtr<XdsClusterLocalityStats> locality_stats_;
  };

  // A picker that wraps the picker from the child to perform drops.
  class Picker : public SubchannelPicker {
   public:
    Picker(XdsClusterImplLb* xds_cluster_impl_lb,
           RefCountedPtr<SubchannelPicker> picker);

    PickResult Pick(PickArgs args) override;

   private:
    class SubchannelCallTracker;

    RefCountedPtr<CircuitBreakerCallCounterMap::CallCounter> call_counter_;
    uint32_t max_concurrent_requests_;
    RefCountedPtr<XdsEndpointResource::DropConfig> drop_config_;
    RefCountedPtr<XdsClusterDropStats> drop_stats_;
    RefCountedPtr<SubchannelPicker> picker_;
  };

  class Helper : public ChannelControlHelper {
   public:
    explicit Helper(RefCountedPtr<XdsClusterImplLb> xds_cluster_impl_policy)
        : xds_cluster_impl_policy_(std::move(xds_cluster_impl_policy)) {}

    ~Helper() override {
      xds_cluster_impl_policy_.reset(DEBUG_LOCATION, "Helper");
    }

    RefCountedPtr<SubchannelInterface> CreateSubchannel(
        ServerAddress address, const ChannelArgs& args) override;
    void UpdateState(grpc_connectivity_state state, const absl::Status& status,
                     RefCountedPtr<SubchannelPicker> picker) override;
    void RequestReresolution() override;
    absl::string_view GetAuthority() override;
    grpc_event_engine::experimental::EventEngine* GetEventEngine() override;
    void AddTraceEvent(TraceSeverity severity,
                       absl::string_view message) override;

   private:
    RefCountedPtr<XdsClusterImplLb> xds_cluster_impl_policy_;
  };

  ~XdsClusterImplLb() override;

  void ShutdownLocked() override;

  OrphanablePtr<LoadBalancingPolicy> CreateChildPolicyLocked(
      const ChannelArgs& args);
  absl::Status UpdateChildPolicyLocked(
      absl::StatusOr<ServerAddressList> addresses, std::string resolution_note,
      const ChannelArgs& args);

  void MaybeUpdatePickerLocked();

  // Current config from the resolver.
  RefCountedPtr<XdsClusterImplLbConfig> config_;

  // Current concurrent number of requests.
  RefCountedPtr<CircuitBreakerCallCounterMap::CallCounter> call_counter_;

  // Internal state.
  bool shutting_down_ = false;

  // The xds client.
  RefCountedPtr<XdsClient> xds_client_;

  // The stats for client-side load reporting.
  RefCountedPtr<XdsClusterDropStats> drop_stats_;

  OrphanablePtr<LoadBalancingPolicy> child_policy_;

  // Latest state and picker reported by the child policy.
  grpc_connectivity_state state_ = GRPC_CHANNEL_IDLE;
  absl::Status status_;
  RefCountedPtr<SubchannelPicker> picker_;
};

//
// XdsClusterImplLb::Picker::SubchannelCallTracker
//

class XdsClusterImplLb::Picker::SubchannelCallTracker
    : public LoadBalancingPolicy::SubchannelCallTrackerInterface {
 public:
  SubchannelCallTracker(
      std::unique_ptr<LoadBalancingPolicy::SubchannelCallTrackerInterface>
          original_subchannel_call_tracker,
      RefCountedPtr<XdsClusterLocalityStats> locality_stats,
      RefCountedPtr<CircuitBreakerCallCounterMap::CallCounter> call_counter)
      : original_subchannel_call_tracker_(
            std::move(original_subchannel_call_tracker)),
        locality_stats_(std::move(locality_stats)),
        call_counter_(std::move(call_counter)) {}

  ~SubchannelCallTracker() override {
    locality_stats_.reset(DEBUG_LOCATION, "SubchannelCallTracker");
    call_counter_.reset(DEBUG_LOCATION, "SubchannelCallTracker");
    GPR_DEBUG_ASSERT(!started_);
  }

  void Start() override {
    // Increment number of calls in flight.
    call_counter_->Increment();
    // Record a call started.
    if (locality_stats_ != nullptr) {
      locality_stats_->AddCallStarted();
    }
    // Delegate if needed.
    if (original_subchannel_call_tracker_ != nullptr) {
      original_subchannel_call_tracker_->Start();
    }
#ifndef NDEBUG
    started_ = true;
#endif
  }

  void Finish(FinishArgs args) override {
    // Delegate if needed.
    if (original_subchannel_call_tracker_ != nullptr) {
      original_subchannel_call_tracker_->Finish(args);
    }
    // Record call completion for load reporting.
    if (locality_stats_ != nullptr) {
      auto* backend_metric_data =
          args.backend_metric_accessor->GetBackendMetricData();
      const std::map<absl::string_view, double>* named_metrics = nullptr;
      if (backend_metric_data != nullptr) {
        named_metrics = &backend_metric_data->named_metrics;
      }
      locality_stats_->AddCallFinished(named_metrics, !args.status.ok());
    }
    // Decrement number of calls in flight.
    call_counter_->Decrement();
#ifndef NDEBUG
    started_ = false;
#endif
  }

 private:
  std::unique_ptr<LoadBalancingPolicy::SubchannelCallTrackerInterface>
      original_subchannel_call_tracker_;
  RefCountedPtr<XdsClusterLocalityStats> locality_stats_;
  RefCountedPtr<CircuitBreakerCallCounterMap::CallCounter> call_counter_;
#ifndef NDEBUG
  bool started_ = false;
#endif
};

//
// XdsClusterImplLb::Picker
//

XdsClusterImplLb::Picker::Picker(XdsClusterImplLb* xds_cluster_impl_lb,
                                 RefCountedPtr<SubchannelPicker> picker)
    : call_counter_(xds_cluster_impl_lb->call_counter_),
      max_concurrent_requests_(
          xds_cluster_impl_lb->config_->max_concurrent_requests()),
      drop_config_(xds_cluster_impl_lb->config_->drop_config()),
      drop_stats_(xds_cluster_impl_lb->drop_stats_),
      picker_(std::move(picker)) {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO, "[xds_cluster_impl_lb %p] constructed new picker %p",
            xds_cluster_impl_lb, this);
  }
}

LoadBalancingPolicy::PickResult XdsClusterImplLb::Picker::Pick(
    LoadBalancingPolicy::PickArgs args) {
  // Handle EDS drops.
  const std::string* drop_category;
  if (drop_config_->ShouldDrop(&drop_category)) {
    if (drop_stats_ != nullptr) drop_stats_->AddCallDropped(*drop_category);
    return PickResult::Drop(absl::UnavailableError(
        absl::StrCat("EDS-configured drop: ", *drop_category)));
  }
  // Check if we exceeded the max concurrent requests circuit breaking limit.
  // Note: We check the value here, but we don't actually increment the
  // counter for the current request until the channel calls the subchannel
  // call tracker's Start() method.  This means that we may wind up
  // allowing more concurrent requests than the configured limit.
  if (call_counter_->Load() >= max_concurrent_requests_) {
    if (drop_stats_ != nullptr) drop_stats_->AddUncategorizedDrops();
    return PickResult::Drop(absl::UnavailableError("circuit breaker drop"));
  }
  // If we're not dropping the call, we should always have a child picker.
  if (picker_ == nullptr) {  // Should never happen.
    return PickResult::Fail(absl::InternalError(
        "xds_cluster_impl picker not given any child picker"));
  }
  // Not dropping, so delegate to child picker.
  PickResult result = picker_->Pick(args);
  auto* complete_pick = absl::get_if<PickResult::Complete>(&result.result);
  if (complete_pick != nullptr) {
    RefCountedPtr<XdsClusterLocalityStats> locality_stats;
    if (drop_stats_ != nullptr) {  // If load reporting is enabled.
      auto* subchannel_wrapper =
          static_cast<StatsSubchannelWrapper*>(complete_pick->subchannel.get());
      // Handle load reporting.
      locality_stats = subchannel_wrapper->locality_stats()->Ref(
          DEBUG_LOCATION, "SubchannelCallTracker");
      // Unwrap subchannel to pass back up the stack.
      complete_pick->subchannel = subchannel_wrapper->wrapped_subchannel();
    }
    // Inject subchannel call tracker to record call completion.
    complete_pick->subchannel_call_tracker =
        std::make_unique<SubchannelCallTracker>(
            std::move(complete_pick->subchannel_call_tracker),
            std::move(locality_stats),
            call_counter_->Ref(DEBUG_LOCATION, "SubchannelCallTracker"));
  } else {
    // TODO(roth): We should ideally also record call failures here in the case
    // where a pick fails.  This is challenging, because we don't know which
    // picks are for wait_for_ready RPCs or how many times we'll return a
    // failure for the same wait_for_ready RPC.
  }
  return result;
}

//
// XdsClusterImplLb
//

XdsClusterImplLb::XdsClusterImplLb(RefCountedPtr<XdsClient> xds_client,
                                   Args args)
    : LoadBalancingPolicy(std::move(args)), xds_client_(std::move(xds_client)) {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO, "[xds_cluster_impl_lb %p] created -- using xds client %p",
            this, xds_client_.get());
  }
}

XdsClusterImplLb::~XdsClusterImplLb() {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO,
            "[xds_cluster_impl_lb %p] destroying xds_cluster_impl LB policy",
            this);
  }
}

void XdsClusterImplLb::ShutdownLocked() {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO, "[xds_cluster_impl_lb %p] shutting down", this);
  }
  shutting_down_ = true;
  // Remove the child policy's interested_parties pollset_set from the
  // xDS policy.
  if (child_policy_ != nullptr) {
    grpc_pollset_set_del_pollset_set(child_policy_->interested_parties(),
                                     interested_parties());
    child_policy_.reset();
  }
  // Drop our ref to the child's picker, in case it's holding a ref to
  // the child.
  picker_.reset();
  drop_stats_.reset();
  xds_client_.reset(DEBUG_LOCATION, "XdsClusterImpl");
}

void XdsClusterImplLb::ExitIdleLocked() {
  if (child_policy_ != nullptr) child_policy_->ExitIdleLocked();
}

void XdsClusterImplLb::ResetBackoffLocked() {
  // The XdsClient will have its backoff reset by the xds resolver, so we
  // don't need to do it here.
  if (child_policy_ != nullptr) child_policy_->ResetBackoffLocked();
}

absl::Status XdsClusterImplLb::UpdateLocked(UpdateArgs args) {
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO, "[xds_cluster_impl_lb %p] Received update", this);
  }
  // Update config.
  const bool is_initial_update = config_ == nullptr;
  auto old_config = std::move(config_);
  config_ = std::move(args.config);
  // On initial update, create drop stats.
  if (is_initial_update) {
    if (config_->lrs_load_reporting_server().has_value()) {
      drop_stats_ = xds_client_->AddClusterDropStats(
          config_->lrs_load_reporting_server().value(), config_->cluster_name(),
          config_->eds_service_name());
      if (drop_stats_ == nullptr) {
        gpr_log(GPR_ERROR,
                "[xds_cluster_impl_lb %p] Failed to get cluster drop stats for "
                "LRS server %s, cluster %s, EDS service name %s, load "
                "reporting for drops will not be done.",
                this,
                config_->lrs_load_reporting_server()->server_uri().c_str(),
                config_->cluster_name().c_str(),
                config_->eds_service_name().c_str());
      }
    }
    call_counter_ = g_call_counter_map->GetOrCreate(
        config_->cluster_name(), config_->eds_service_name());
  } else {
    // Cluster name, EDS service name, and LRS server name should never
    // change, because the xds_cluster_resolver policy above us should be
    // swapped out if that happens.
    GPR_ASSERT(config_->cluster_name() == old_config->cluster_name());
    GPR_ASSERT(config_->eds_service_name() == old_config->eds_service_name());
    GPR_ASSERT(config_->lrs_load_reporting_server() ==
               old_config->lrs_load_reporting_server());
  }
  // Update picker if max_concurrent_requests has changed.
  if (is_initial_update || config_->max_concurrent_requests() !=
                               old_config->max_concurrent_requests()) {
    MaybeUpdatePickerLocked();
  }
  // Update child policy.
  return UpdateChildPolicyLocked(std::move(args.addresses),
                                 std::move(args.resolution_note), args.args);
}

void XdsClusterImplLb::MaybeUpdatePickerLocked() {
  // If we're dropping all calls, report READY, regardless of what (or
  // whether) the child has reported.
  if (config_->drop_config() != nullptr && config_->drop_config()->drop_all()) {
    auto drop_picker = MakeRefCounted<Picker>(this, picker_);
    if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
      gpr_log(GPR_INFO,
              "[xds_cluster_impl_lb %p] updating connectivity (drop all): "
              "state=READY picker=%p",
              this, drop_picker.get());
    }
    channel_control_helper()->UpdateState(GRPC_CHANNEL_READY, absl::Status(),
                                          std::move(drop_picker));
    return;
  }
  // Otherwise, update only if we have a child picker.
  if (picker_ != nullptr) {
    auto drop_picker = MakeRefCounted<Picker>(this, picker_);
    if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
      gpr_log(GPR_INFO,
              "[xds_cluster_impl_lb %p] updating connectivity: state=%s "
              "status=(%s) picker=%p",
              this, ConnectivityStateName(state_), status_.ToString().c_str(),
              drop_picker.get());
    }
    channel_control_helper()->UpdateState(state_, status_,
                                          std::move(drop_picker));
  }
}

OrphanablePtr<LoadBalancingPolicy> XdsClusterImplLb::CreateChildPolicyLocked(
    const ChannelArgs& args) {
  LoadBalancingPolicy::Args lb_policy_args;
  lb_policy_args.work_serializer = work_serializer();
  lb_policy_args.args = args;
  lb_policy_args.channel_control_helper =
      std::make_unique<Helper>(Ref(DEBUG_LOCATION, "Helper"));
  OrphanablePtr<LoadBalancingPolicy> lb_policy =
      MakeOrphanable<ChildPolicyHandler>(std::move(lb_policy_args),
                                         &grpc_xds_cluster_impl_lb_trace);
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO,
            "[xds_cluster_impl_lb %p] Created new child policy handler %p",
            this, lb_policy.get());
  }
  // Add our interested_parties pollset_set to that of the newly created
  // child policy. This will make the child policy progress upon activity on
  // this policy, which in turn is tied to the application's call.
  grpc_pollset_set_add_pollset_set(lb_policy->interested_parties(),
                                   interested_parties());
  return lb_policy;
}

absl::Status XdsClusterImplLb::UpdateChildPolicyLocked(
    absl::StatusOr<ServerAddressList> addresses, std::string resolution_note,
    const ChannelArgs& args) {
  // Create policy if needed.
  if (child_policy_ == nullptr) {
    child_policy_ = CreateChildPolicyLocked(args);
  }
  // Construct update args.
  UpdateArgs update_args;
  update_args.addresses = std::move(addresses);
  update_args.resolution_note = std::move(resolution_note);
  update_args.config = config_->child_policy();
  update_args.args =
      args.Set(GRPC_ARG_XDS_CLUSTER_NAME, config_->cluster_name());
  // Update the policy.
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO,
            "[xds_cluster_impl_lb %p] Updating child policy handler %p", this,
            child_policy_.get());
  }
  return child_policy_->UpdateLocked(std::move(update_args));
}

//
// XdsClusterImplLb::Helper
//

RefCountedPtr<SubchannelInterface> XdsClusterImplLb::Helper::CreateSubchannel(
    ServerAddress address, const ChannelArgs& args) {
  if (xds_cluster_impl_policy_->shutting_down_) return nullptr;
  // If load reporting is enabled, wrap the subchannel such that it
  // includes the locality stats object, which will be used by the Picker.
  if (xds_cluster_impl_policy_->config_->lrs_load_reporting_server()
          .has_value()) {
    RefCountedPtr<XdsLocalityName> locality_name;
    auto* attribute = address.GetAttribute(kXdsLocalityNameAttributeKey);
    if (attribute != nullptr) {
      const auto* locality_attr =
          static_cast<const XdsLocalityAttribute*>(attribute);
      locality_name = locality_attr->locality_name();
    }
    RefCountedPtr<XdsClusterLocalityStats> locality_stats =
        xds_cluster_impl_policy_->xds_client_->AddClusterLocalityStats(
            xds_cluster_impl_policy_->config_->lrs_load_reporting_server()
                .value(),
            xds_cluster_impl_policy_->config_->cluster_name(),
            xds_cluster_impl_policy_->config_->eds_service_name(),
            std::move(locality_name));
    if (locality_stats != nullptr) {
      return MakeRefCounted<StatsSubchannelWrapper>(
          xds_cluster_impl_policy_->channel_control_helper()->CreateSubchannel(
              std::move(address), args),
          std::move(locality_stats));
    }
    gpr_log(GPR_ERROR,
            "[xds_cluster_impl_lb %p] Failed to get locality stats object for "
            "LRS server %s, cluster %s, EDS service name %s; load reports will "
            "not be generated (not wrapping subchannel)",
            this,
            xds_cluster_impl_policy_->config_->lrs_load_reporting_server()
                ->server_uri()
                .c_str(),
            xds_cluster_impl_policy_->config_->cluster_name().c_str(),
            xds_cluster_impl_policy_->config_->eds_service_name().c_str());
  }
  // Load reporting not enabled, so don't wrap the subchannel.
  return xds_cluster_impl_policy_->channel_control_helper()->CreateSubchannel(
      std::move(address), args);
}

void XdsClusterImplLb::Helper::UpdateState(
    grpc_connectivity_state state, const absl::Status& status,
    RefCountedPtr<SubchannelPicker> picker) {
  if (xds_cluster_impl_policy_->shutting_down_) return;
  if (GRPC_TRACE_FLAG_ENABLED(grpc_xds_cluster_impl_lb_trace)) {
    gpr_log(GPR_INFO,
            "[xds_cluster_impl_lb %p] child connectivity state update: "
            "state=%s (%s) "
            "picker=%p",
            xds_cluster_impl_policy_.get(), ConnectivityStateName(state),
            status.ToString().c_str(), picker.get());
  }
  // Save the state and picker.
  xds_cluster_impl_policy_->state_ = state;
  xds_cluster_impl_policy_->status_ = status;
  xds_cluster_impl_policy_->picker_ = std::move(picker);
  // Wrap the picker and return it to the channel.
  xds_cluster_impl_policy_->MaybeUpdatePickerLocked();
}

void XdsClusterImplLb::Helper::RequestReresolution() {
  if (xds_cluster_impl_policy_->shutting_down_) return;
  xds_cluster_impl_policy_->channel_control_helper()->RequestReresolution();
}

absl::string_view XdsClusterImplLb::Helper::GetAuthority() {
  return xds_cluster_impl_policy_->channel_control_helper()->GetAuthority();
}

grpc_event_engine::experimental::EventEngine*
XdsClusterImplLb::Helper::GetEventEngine() {
  return xds_cluster_impl_policy_->channel_control_helper()->GetEventEngine();
}

void XdsClusterImplLb::Helper::AddTraceEvent(TraceSeverity severity,
                                             absl::string_view message) {
  if (xds_cluster_impl_policy_->shutting_down_) return;
  xds_cluster_impl_policy_->channel_control_helper()->AddTraceEvent(severity,
                                                                    message);
}

//
// factory
//

struct DropCategory {
  std::string category;
  uint32_t requests_per_million;

  static const JsonLoaderInterface* JsonLoader(const JsonArgs&) {
    static const auto* loader =
        JsonObjectLoader<DropCategory>()
            .Field("category", &DropCategory::category)
            .Field("requests_per_million", &DropCategory::requests_per_million)
            .Finish();
    return loader;
  }
};

const JsonLoaderInterface* XdsClusterImplLbConfig::JsonLoader(const JsonArgs&) {
  static const auto* loader =
      JsonObjectLoader<XdsClusterImplLbConfig>()
          // Note: Some fields require custom processing, so they are
          // handled in JsonPostLoad() instead.
          .Field("clusterName", &XdsClusterImplLbConfig::cluster_name_)
          .OptionalField("edsServiceName",
                         &XdsClusterImplLbConfig::eds_service_name_)
          .OptionalField("lrsLoadReportingServer",
                         &XdsClusterImplLbConfig::lrs_load_reporting_server_)
          .OptionalField("maxConcurrentRequests",
                         &XdsClusterImplLbConfig::max_concurrent_requests_)
          .Finish();
  return loader;
}

void XdsClusterImplLbConfig::JsonPostLoad(const Json& json,
                                          const JsonArgs& args,
                                          ValidationErrors* errors) {
  // Parse "childPolicy" field.
  {
    ValidationErrors::ScopedField field(errors, ".childPolicy");
    auto it = json.object().find("childPolicy");
    if (it == json.object().end()) {
      errors->AddError("field not present");
    } else {
      auto lb_config = CoreConfiguration::Get()
                           .lb_policy_registry()
                           .ParseLoadBalancingConfig(it->second);
      if (!lb_config.ok()) {
        errors->AddError(lb_config.status().message());
      } else {
        child_policy_ = std::move(*lb_config);
      }
    }
  }
  // Parse "dropCategories" field.
  {
    auto value = LoadJsonObjectField<std::vector<DropCategory>>(
        json.object(), args, "dropCategories", errors);
    if (value.has_value()) {
      drop_config_ = MakeRefCounted<XdsEndpointResource::DropConfig>();
      for (size_t i = 0; i < value->size(); ++i) {
        DropCategory& drop_category = (*value)[i];
        drop_config_->AddCategory(std::move(drop_category.category),
                                  drop_category.requests_per_million);
      }
    }
  }
}

class XdsClusterImplLbFactory : public LoadBalancingPolicyFactory {
 public:
  OrphanablePtr<LoadBalancingPolicy> CreateLoadBalancingPolicy(
      LoadBalancingPolicy::Args args) const override {
    auto xds_client = args.args.GetObjectRef<GrpcXdsClient>(DEBUG_LOCATION,
                                                            "XdsClusterImplLb");
    if (xds_client == nullptr) {
      gpr_log(GPR_ERROR,
              "XdsClient not present in channel args -- cannot instantiate "
              "xds_cluster_impl LB policy");
      return nullptr;
    }
    return MakeOrphanable<XdsClusterImplLb>(std::move(xds_client),
                                            std::move(args));
  }

  absl::string_view name() const override { return kXdsClusterImpl; }

  absl::StatusOr<RefCountedPtr<LoadBalancingPolicy::Config>>
  ParseLoadBalancingConfig(const Json& json) const override {
    if (json.type() == Json::Type::kNull) {
      // This policy was configured in the deprecated loadBalancingPolicy
      // field or in the client API.
      return absl::InvalidArgumentError(
          "field:loadBalancingPolicy error:xds_cluster_impl policy requires "
          "configuration. Please use loadBalancingConfig field of service "
          "config instead.");
    }
    return LoadRefCountedFromJson<XdsClusterImplLbConfig>(
        json, JsonArgs(),
        "errors validating xds_cluster_impl LB policy config");
  }
};

}  // namespace

void RegisterXdsClusterImplLbPolicy(CoreConfiguration::Builder* builder) {
  builder->lb_policy_registry()->RegisterLoadBalancingPolicyFactory(
      std::make_unique<XdsClusterImplLbFactory>());
}

}  // namespace grpc_core