text: //
  // 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 <inttypes.h>
  #include <limits.h>
  #include <string.h>
  
  #include "absl/container/inlined_vector.h"
  #include "absl/strings/str_cat.h"
  
  #include <grpc/grpc.h>
  
  #include "src/core/ext/filters/client_channel/lb_policy.h"
  #include "src/core/ext/filters/client_channel/lb_policy/address_filtering.h"
  #include "src/core/ext/filters/client_channel/lb_policy/child_policy_handler.h"
  #include "src/core/ext/filters/client_channel/lb_policy_factory.h"
  #include "src/core/ext/filters/client_channel/lb_policy_registry.h"
  #include "src/core/lib/channel/channel_args.h"
  #include "src/core/lib/gpr/string.h"
  #include "src/core/lib/gprpp/orphanable.h"
  #include "src/core/lib/gprpp/ref_counted_ptr.h"
  #include "src/core/lib/iomgr/timer.h"
  #include "src/core/lib/iomgr/work_serializer.h"
  #include "src/core/lib/transport/error_utils.h"
  
  namespace grpc_core {
  
  TraceFlag grpc_lb_weighted_target_trace(false, "weighted_target_lb");
  
  namespace {
  
  constexpr char kWeightedTarget[] = "weighted_target_experimental";
  
  // How long we keep a child around for after it has been removed from
  // the config.
  constexpr int kChildRetentionIntervalMs = 15 * 60 * 1000;
  
  // Config for weighted_target LB policy.
  class WeightedTargetLbConfig : public LoadBalancingPolicy::Config {
   public:
    struct ChildConfig {
      uint32_t weight;
      RefCountedPtr<LoadBalancingPolicy::Config> config;
    };
  
    using TargetMap = std::map<std::string, ChildConfig>;
  
    explicit WeightedTargetLbConfig(TargetMap target_map)
        : target_map_(std::move(target_map)) {}
  
    const char* name() const override { return kWeightedTarget; }
  
    const TargetMap& target_map() const { return target_map_; }
  
   private:
    TargetMap target_map_;
  };
  
  // weighted_target LB policy.
  class WeightedTargetLb : public LoadBalancingPolicy {
   public:
    explicit WeightedTargetLb(Args args);
  
    const char* name() const override { return kWeightedTarget; }
  
    void UpdateLocked(UpdateArgs args) override;
    void ResetBackoffLocked() override;
  
   private:
    // A simple wrapper for ref-counting a picker from the child policy.
    class ChildPickerWrapper : public RefCounted<ChildPickerWrapper> {
     public:
      explicit ChildPickerWrapper(std::unique_ptr<SubchannelPicker> picker)
          : picker_(std::move(picker)) {}
      PickResult Pick(PickArgs args) { return picker_->Pick(args); }
  
     private:
      std::unique_ptr<SubchannelPicker> picker_;
    };
  
    // Picks a child using stateless WRR and then delegates to that
    // child's picker.
    class WeightedPicker : public SubchannelPicker {
     public:
      // Maintains a weighted list of pickers from each child that is in
      // ready state. The first element in the pair represents the end of a
      // range proportional to the child's weight. The start of the range
      // is the previous value in the vector and is 0 for the first element.
      using PickerList = absl::InlinedVector<
          std::pair<uint32_t, RefCountedPtr<ChildPickerWrapper>>, 1>;
  
      explicit WeightedPicker(PickerList pickers)
          : pickers_(std::move(pickers)) {}
  
      PickResult Pick(PickArgs args) override;
  
     private:
      PickerList pickers_;
    };
  
    // Each WeightedChild holds a ref to its parent WeightedTargetLb.
    class WeightedChild : public InternallyRefCounted<WeightedChild> {
     public:
      WeightedChild(RefCountedPtr<WeightedTargetLb> weighted_target_policy,
                    const std::string& name);
      ~WeightedChild() override;
  
      void Orphan() override;
  
      void UpdateLocked(const WeightedTargetLbConfig::ChildConfig& config,
                        ServerAddressList addresses,
                        const grpc_channel_args* args);
      void ResetBackoffLocked();
      void DeactivateLocked();
  
      uint32_t weight() const { return weight_; }
      grpc_connectivity_state connectivity_state() const {
        return connectivity_state_;
      }
      RefCountedPtr<ChildPickerWrapper> picker_wrapper() const {
        return picker_wrapper_;
      }
  
     private:
      class Helper : public ChannelControlHelper {
       public:
        explicit Helper(RefCountedPtr<WeightedChild> weighted_child)
            : weighted_child_(std::move(weighted_child)) {}
  
        ~Helper() override { weighted_child_.reset(DEBUG_LOCATION, "Helper"); }
  
        RefCountedPtr<SubchannelInterface> CreateSubchannel(
            ServerAddress address, const grpc_channel_args& args) override;
        void UpdateState(grpc_connectivity_state state,
                         const absl::Status& status,
                         std::unique_ptr<SubchannelPicker> picker) override;
        void RequestReresolution() override;
        absl::string_view GetAuthority() override;
        void AddTraceEvent(TraceSeverity severity,
                           absl::string_view message) override;
  
       private:
        RefCountedPtr<WeightedChild> weighted_child_;
      };
  
      // Methods for dealing with the child policy.
      OrphanablePtr<LoadBalancingPolicy> CreateChildPolicyLocked(
          const grpc_channel_args* args);
  
      void OnConnectivityStateUpdateLocked(
          grpc_connectivity_state state, const absl::Status& status,
          std::unique_ptr<SubchannelPicker> picker);
  
      static void OnDelayedRemovalTimer(void* arg, grpc_error_handle error);
      void OnDelayedRemovalTimerLocked(grpc_error_handle error);
  
      // The owning LB policy.
      RefCountedPtr<WeightedTargetLb> weighted_target_policy_;
  
      const std::string name_;
  
      uint32_t weight_;
  
      OrphanablePtr<LoadBalancingPolicy> child_policy_;
  
      RefCountedPtr<ChildPickerWrapper> picker_wrapper_;
      grpc_connectivity_state connectivity_state_ = GRPC_CHANNEL_CONNECTING;
      bool seen_failure_since_ready_ = false;
  
      // States for delayed removal.
      grpc_timer delayed_removal_timer_;
      grpc_closure on_delayed_removal_timer_;
      bool delayed_removal_timer_callback_pending_ = false;
      bool shutdown_ = false;
    };
  
    ~WeightedTargetLb() override;
  
    void ShutdownLocked() override;
  
    void UpdateStateLocked();
  
    // Current config from the resolver.
    RefCountedPtr<WeightedTargetLbConfig> config_;
  
    // Internal state.
    bool shutting_down_ = false;
  
    // Children.
    std::map<std::string, OrphanablePtr<WeightedChild>> targets_;
  };
  
  //
  // WeightedTargetLb::WeightedPicker
  //
  
  WeightedTargetLb::PickResult WeightedTargetLb::WeightedPicker::Pick(
      PickArgs args) {
    // Generate a random number in [0, total weight).
    const uint32_t key = rand() % pickers_[pickers_.size() - 1].first;
    // Find the index in pickers_ corresponding to key.
    size_t mid = 0;
    size_t start_index = 0;
    size_t end_index = pickers_.size() - 1;
    size_t index = 0;
    while (end_index > start_index) {
      mid = (start_index + end_index) / 2;
      if (pickers_[mid].first > key) {
        end_index = mid;
      } else if (pickers_[mid].first < key) {
        start_index = mid + 1;
      } else {
        index = mid + 1;
        break;
      }
    }
    if (index == 0) index = start_index;
    GPR_ASSERT(pickers_[index].first > key);
    // Delegate to the child picker.
    return pickers_[index].second->Pick(args);
  }
  
  //
  // WeightedTargetLb
  //
  
  WeightedTargetLb::WeightedTargetLb(Args args)
      : LoadBalancingPolicy(std::move(args)) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO, "[weighted_target_lb %p] created", this);
    }
  }
  
  WeightedTargetLb::~WeightedTargetLb() {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] destroying weighted_target LB policy",
              this);
    }
  }
  
  void WeightedTargetLb::ShutdownLocked() {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO, "[weighted_target_lb %p] shutting down", this);
    }
    shutting_down_ = true;
    targets_.clear();
  }
  
  void WeightedTargetLb::ResetBackoffLocked() {
    for (auto& p : targets_) p.second->ResetBackoffLocked();
  }
  
  void WeightedTargetLb::UpdateLocked(UpdateArgs args) {
    if (shutting_down_) return;
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO, "[weighted_target_lb %p] Received update", this);
    }
    // Update config.
    config_ = std::move(args.config);
    // Deactivate the targets not in the new config.
    for (const auto& p : targets_) {
      const std::string& name = p.first;
      WeightedChild* child = p.second.get();
      if (config_->target_map().find(name) == config_->target_map().end()) {
        child->DeactivateLocked();
      }
    }
    // Create any children that don't already exist.
    // Note that we add all children before updating any of them, because
    // an update may trigger a child to immediately update its
    // connectivity state (e.g., reporting TRANSIENT_FAILURE immediately when
    // receiving an empty address list), and we don't want to return an
    // overall state with incomplete data.
    for (const auto& p : config_->target_map()) {
      const std::string& name = p.first;
      auto it = targets_.find(name);
      if (it == targets_.end()) {
        targets_.emplace(name, MakeOrphanable<WeightedChild>(
                                   Ref(DEBUG_LOCATION, "WeightedChild"), name));
      }
    }
    // Update all children.
    HierarchicalAddressMap address_map =
        MakeHierarchicalAddressMap(args.addresses);
    for (const auto& p : config_->target_map()) {
      const std::string& name = p.first;
      const WeightedTargetLbConfig::ChildConfig& config = p.second;
      targets_[name]->UpdateLocked(config, std::move(address_map[name]),
                                   args.args);
    }
    UpdateStateLocked();
  }
  
  void WeightedTargetLb::UpdateStateLocked() {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] scanning children to determine "
              "connectivity state",
              this);
    }
    // Construct a new picker which maintains a map of all child pickers
    // that are ready. Each child is represented by a portion of the range
    // proportional to its weight, such that the total range is the sum of the
    // weights of all children.
    WeightedPicker::PickerList picker_list;
    uint32_t end = 0;
    // Also count the number of children in each state, to determine the
    // overall state.
    size_t num_connecting = 0;
    size_t num_idle = 0;
    size_t num_transient_failures = 0;
    for (const auto& p : targets_) {
      const std::string& child_name = p.first;
      const WeightedChild* child = p.second.get();
      // Skip the targets that are not in the latest update.
      if (config_->target_map().find(child_name) == config_->target_map().end()) {
        continue;
      }
      if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
        gpr_log(GPR_INFO,
                "[weighted_target_lb %p]   child=%s state=%s weight=%d picker=%p",
                this, child_name.c_str(),
                ConnectivityStateName(child->connectivity_state()),
                child->weight(), child->picker_wrapper().get());
      }
      switch (child->connectivity_state()) {
        case GRPC_CHANNEL_READY: {
          end += child->weight();
          picker_list.push_back(std::make_pair(end, child->picker_wrapper()));
          break;
        }
        case GRPC_CHANNEL_CONNECTING: {
          ++num_connecting;
          break;
        }
        case GRPC_CHANNEL_IDLE: {
          ++num_idle;
          break;
        }
        case GRPC_CHANNEL_TRANSIENT_FAILURE: {
          ++num_transient_failures;
          break;
        }
        default:
          GPR_UNREACHABLE_CODE(return );
      }
    }
    // Determine aggregated connectivity state.
    grpc_connectivity_state connectivity_state;
    if (!picker_list.empty()) {
      connectivity_state = GRPC_CHANNEL_READY;
    } else if (num_connecting > 0) {
      connectivity_state = GRPC_CHANNEL_CONNECTING;
    } else if (num_idle > 0) {
      connectivity_state = GRPC_CHANNEL_IDLE;
    } else {
      connectivity_state = GRPC_CHANNEL_TRANSIENT_FAILURE;
    }
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO, "[weighted_target_lb %p] connectivity changed to %s",
              this, ConnectivityStateName(connectivity_state));
    }
    std::unique_ptr<SubchannelPicker> picker;
    absl::Status status;
    switch (connectivity_state) {
      case GRPC_CHANNEL_READY:
        picker = absl::make_unique<WeightedPicker>(std::move(picker_list));
        break;
      case GRPC_CHANNEL_CONNECTING:
      case GRPC_CHANNEL_IDLE:
        picker =
            absl::make_unique<QueuePicker>(Ref(DEBUG_LOCATION, "QueuePicker"));
        break;
      default:
        status = absl::UnavailableError(
            "weighted_target: all children report state TRANSIENT_FAILURE");
        picker = absl::make_unique<TransientFailurePicker>(status);
    }
    channel_control_helper()->UpdateState(connectivity_state, status,
                                          std::move(picker));
  }
  
  //
  // WeightedTargetLb::WeightedChild
  //
  
  WeightedTargetLb::WeightedChild::WeightedChild(
      RefCountedPtr<WeightedTargetLb> weighted_target_policy,
      const std::string& name)
      : weighted_target_policy_(std::move(weighted_target_policy)), name_(name) {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO, "[weighted_target_lb %p] created WeightedChild %p for %s",
              weighted_target_policy_.get(), this, name_.c_str());
    }
    GRPC_CLOSURE_INIT(&on_delayed_removal_timer_, OnDelayedRemovalTimer, this,
                      grpc_schedule_on_exec_ctx);
  }
  
  WeightedTargetLb::WeightedChild::~WeightedChild() {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] WeightedChild %p %s: destroying child",
              weighted_target_policy_.get(), this, name_.c_str());
    }
    weighted_target_policy_.reset(DEBUG_LOCATION, "WeightedChild");
  }
  
  void WeightedTargetLb::WeightedChild::Orphan() {
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] WeightedChild %p %s: shutting down child",
              weighted_target_policy_.get(), this, name_.c_str());
    }
    // Remove the child policy's interested_parties pollset_set from the
    // xDS policy.
    grpc_pollset_set_del_pollset_set(
        child_policy_->interested_parties(),
        weighted_target_policy_->interested_parties());
    child_policy_.reset();
    // Drop our ref to the child's picker, in case it's holding a ref to
    // the child.
    picker_wrapper_.reset();
    if (delayed_removal_timer_callback_pending_) {
      delayed_removal_timer_callback_pending_ = false;
      grpc_timer_cancel(&delayed_removal_timer_);
    }
    shutdown_ = true;
    Unref();
  }
  
  OrphanablePtr<LoadBalancingPolicy>
  WeightedTargetLb::WeightedChild::CreateChildPolicyLocked(
      const grpc_channel_args* args) {
    LoadBalancingPolicy::Args lb_policy_args;
    lb_policy_args.work_serializer = weighted_target_policy_->work_serializer();
    lb_policy_args.args = args;
    lb_policy_args.channel_control_helper =
        absl::make_unique<Helper>(this->Ref(DEBUG_LOCATION, "Helper"));
    OrphanablePtr<LoadBalancingPolicy> lb_policy =
        MakeOrphanable<ChildPolicyHandler>(std::move(lb_policy_args),
                                           &grpc_lb_weighted_target_trace);
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] WeightedChild %p %s: Created new child "
              "policy handler %p",
              weighted_target_policy_.get(), this, name_.c_str(),
              lb_policy.get());
    }
    // Add the xDS's interested_parties pollset_set to that of the newly created
    // child policy. This will make the child policy progress upon activity on
    // xDS LB, which in turn is tied to the application's call.
    grpc_pollset_set_add_pollset_set(
        lb_policy->interested_parties(),
        weighted_target_policy_->interested_parties());
    return lb_policy;
  }
  
  void WeightedTargetLb::WeightedChild::UpdateLocked(
      const WeightedTargetLbConfig::ChildConfig& config,
      ServerAddressList addresses, const grpc_channel_args* args) {
    if (weighted_target_policy_->shutting_down_) return;
    // Update child weight.
    weight_ = config.weight;
    // Reactivate if needed.
    if (delayed_removal_timer_callback_pending_) {
      if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
        gpr_log(GPR_INFO,
                "[weighted_target_lb %p] WeightedChild %p %s: reactivating",
                weighted_target_policy_.get(), this, name_.c_str());
      }
      delayed_removal_timer_callback_pending_ = false;
      grpc_timer_cancel(&delayed_removal_timer_);
    }
    // Create child policy if needed.
    if (child_policy_ == nullptr) {
      child_policy_ = CreateChildPolicyLocked(args);
    }
    // Construct update args.
    UpdateArgs update_args;
    update_args.config = config.config;
    update_args.addresses = std::move(addresses);
    update_args.args = grpc_channel_args_copy(args);
    // Update the policy.
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] WeightedChild %p %s: Updating child "
              "policy handler %p",
              weighted_target_policy_.get(), this, name_.c_str(),
              child_policy_.get());
    }
    child_policy_->UpdateLocked(std::move(update_args));
  }
  
  void WeightedTargetLb::WeightedChild::ResetBackoffLocked() {
    child_policy_->ResetBackoffLocked();
  }
  
  void WeightedTargetLb::WeightedChild::OnConnectivityStateUpdateLocked(
      grpc_connectivity_state state, const absl::Status& status,
      std::unique_ptr<SubchannelPicker> picker) {
    // Cache the picker in the WeightedChild.
    picker_wrapper_ = MakeRefCounted<ChildPickerWrapper>(std::move(picker));
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] WeightedChild %p %s: connectivity "
              "state update: state=%s (%s) picker_wrapper=%p",
              weighted_target_policy_.get(), this, name_.c_str(),
              ConnectivityStateName(state), status.ToString().c_str(),
              picker_wrapper_.get());
    }
    // If the child reports IDLE, immediately tell it to exit idle.
    if (state == GRPC_CHANNEL_IDLE) child_policy_->ExitIdleLocked();
    // Decide what state to report for aggregation purposes.
    // If we haven't seen a failure since the last time we were in state
    // READY, then we report the state change as-is.  However, once we do see
    // a failure, we report TRANSIENT_FAILURE and ignore any subsequent state
    // changes until we go back into state READY.
    if (!seen_failure_since_ready_) {
      if (state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
        seen_failure_since_ready_ = true;
      }
    } else {
      if (state != GRPC_CHANNEL_READY) return;
      seen_failure_since_ready_ = false;
    }
    connectivity_state_ = state;
    // Notify the LB policy.
    weighted_target_policy_->UpdateStateLocked();
  }
  
  void WeightedTargetLb::WeightedChild::DeactivateLocked() {
    // If already deactivated, don't do that again.
    if (weight_ == 0) return;
    if (GRPC_TRACE_FLAG_ENABLED(grpc_lb_weighted_target_trace)) {
      gpr_log(GPR_INFO,
              "[weighted_target_lb %p] WeightedChild %p %s: deactivating",
              weighted_target_policy_.get(), this, name_.c_str());
    }
    // Set the child weight to 0 so that future picker won't contain this child.
    weight_ = 0;
    // Start a timer to delete the child.
    Ref(DEBUG_LOCATION, "WeightedChild+timer").release();
    delayed_removal_timer_callback_pending_ = true;
    grpc_timer_init(&delayed_removal_timer_,
                    ExecCtx::Get()->Now() + kChildRetentionIntervalMs,
                    &on_delayed_removal_timer_);
  }
  
  void WeightedTargetLb::WeightedChild::OnDelayedRemovalTimer(
      void* arg, grpc_error_handle error) {
    WeightedChild* self = static_cast<WeightedChild*>(arg);
    GRPC_ERROR_REF(error);  // ref owned by lambda
    self->weighted_target_policy_->work_serializer()->Run(
        [self, error]() { self->OnDelayedRemovalTimerLocked(error); },
        DEBUG_LOCATION);
  }
  
  void WeightedTargetLb::WeightedChild::OnDelayedRemovalTimerLocked(
      grpc_error_handle error) {
    if (error == GRPC_ERROR_NONE && delayed_removal_timer_callback_pending_ &&
        !shutdown_ && weight_ == 0) {
      delayed_removal_timer_callback_pending_ = false;
      weighted_target_policy_->targets_.erase(name_);
    }
    Unref(DEBUG_LOCATION, "WeightedChild+timer");
    GRPC_ERROR_UNREF(error);
  }
  
  //
  // WeightedTargetLb::WeightedChild::Helper
  //
  
  RefCountedPtr<SubchannelInterface>
  WeightedTargetLb::WeightedChild::Helper::CreateSubchannel(
      ServerAddress address, const grpc_channel_args& args) {
    if (weighted_child_->weighted_target_policy_->shutting_down_) return nullptr;
    return weighted_child_->weighted_target_policy_->channel_control_helper()
        ->CreateSubchannel(std::move(address), args);
  }
  
  void WeightedTargetLb::WeightedChild::Helper::UpdateState(
      grpc_connectivity_state state, const absl::Status& status,
      std::unique_ptr<SubchannelPicker> picker) {
    if (weighted_child_->weighted_target_policy_->shutting_down_) return;
    weighted_child_->OnConnectivityStateUpdateLocked(state, status,
                                                     std::move(picker));
  }
  
  void WeightedTargetLb::WeightedChild::Helper::RequestReresolution() {
    if (weighted_child_->weighted_target_policy_->shutting_down_) return;
    weighted_child_->weighted_target_policy_->channel_control_helper()
        ->RequestReresolution();
  }
  
  absl::string_view WeightedTargetLb::WeightedChild::Helper::GetAuthority() {
    return weighted_child_->weighted_target_policy_->channel_control_helper()
        ->GetAuthority();
  }
  
  void WeightedTargetLb::WeightedChild::Helper::AddTraceEvent(
      TraceSeverity severity, absl::string_view message) {
    if (weighted_child_->weighted_target_policy_->shutting_down_) return;
    weighted_child_->weighted_target_policy_->channel_control_helper()
        ->AddTraceEvent(severity, message);
  }
  
  //
  // factory
  //
  
  class WeightedTargetLbFactory : public LoadBalancingPolicyFactory {
   public:
    OrphanablePtr<LoadBalancingPolicy> CreateLoadBalancingPolicy(
        LoadBalancingPolicy::Args args) const override {
      return MakeOrphanable<WeightedTargetLb>(std::move(args));
    }
  
    const char* name() const override { return kWeightedTarget; }
  
    RefCountedPtr<LoadBalancingPolicy::Config> ParseLoadBalancingConfig(
        const Json& json, grpc_error_handle* error) const override {
      GPR_DEBUG_ASSERT(error != nullptr && *error == GRPC_ERROR_NONE);
      if (json.type() == Json::Type::JSON_NULL) {
        // weighted_target was mentioned as a policy in the deprecated
        // loadBalancingPolicy field or in the client API.
        *error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
            "field:loadBalancingPolicy error:weighted_target policy requires "
            "configuration.  Please use loadBalancingConfig field of service "
            "config instead.");
        return nullptr;
      }
      std::vector<grpc_error_handle> error_list;
      // Weight map.
      WeightedTargetLbConfig::TargetMap target_map;
      auto it = json.object_value().find("targets");
      if (it == json.object_value().end()) {
        error_list.push_back(GRPC_ERROR_CREATE_FROM_STATIC_STRING(
            "field:targets error:required field not present"));
      } else if (it->second.type() != Json::Type::OBJECT) {
        error_list.push_back(GRPC_ERROR_CREATE_FROM_STATIC_STRING(
            "field:targets error:type should be object"));
      } else {
        for (const auto& p : it->second.object_value()) {
          WeightedTargetLbConfig::ChildConfig child_config;
          std::vector<grpc_error_handle> child_errors =
              ParseChildConfig(p.second, &child_config);
          if (!child_errors.empty()) {
            error_list.push_back(GRPC_ERROR_CREATE_FROM_VECTOR_AND_CPP_STRING(
                absl::StrCat("field:targets key:", p.first), &child_errors));
          } else {
            target_map[p.first] = std::move(child_config);
          }
        }
      }
      if (!error_list.empty()) {
        *error = GRPC_ERROR_CREATE_FROM_VECTOR(
            "weighted_target_experimental LB policy config", &error_list);
        return nullptr;
      }
      return MakeRefCounted<WeightedTargetLbConfig>(std::move(target_map));
    }
  
   private:
    static std::vector<grpc_error_handle> ParseChildConfig(
        const Json& json, WeightedTargetLbConfig::ChildConfig* child_config) {
      std::vector<grpc_error_handle> error_list;
      if (json.type() != Json::Type::OBJECT) {
        error_list.push_back(GRPC_ERROR_CREATE_FROM_STATIC_STRING(
            "value should be of type object"));
        return error_list;
      }
      // Weight.
      auto it = json.object_value().find("weight");
      if (it == json.object_value().end()) {
        error_list.push_back(GRPC_ERROR_CREATE_FROM_STATIC_STRING(
            "required field \"weight\" not specified"));
      } else if (it->second.type() != Json::Type::NUMBER) {
        error_list.push_back(GRPC_ERROR_CREATE_FROM_STATIC_STRING(
            "field:weight error:must be of type number"));
      } else {
        int weight = gpr_parse_nonnegative_int(it->second.string_value().c_str());
        if (weight == -1) {
          error_list.push_back(GRPC_ERROR_CREATE_FROM_STATIC_STRING(
              "field:weight error:unparseable value"));
        } else if (weight == 0) {
          error_list.push_back(GRPC_ERROR_CREATE_FROM_STATIC_STRING(
              "field:weight error:value must be greater than zero"));
        } else {
          child_config->weight = weight;
        }
      }
      // Child policy.
      it = json.object_value().find("childPolicy");
      if (it != json.object_value().end()) {
        grpc_error_handle parse_error = GRPC_ERROR_NONE;
        child_config->config =
            LoadBalancingPolicyRegistry::ParseLoadBalancingConfig(it->second,
                                                                  &parse_error);
        if (child_config->config == nullptr) {
          GPR_DEBUG_ASSERT(parse_error != GRPC_ERROR_NONE);
          std::vector<grpc_error_handle> child_errors;
          child_errors.push_back(parse_error);
          error_list.push_back(
              GRPC_ERROR_CREATE_FROM_VECTOR("field:childPolicy", &child_errors));
        }
      }
      return error_list;
    }
  };
  
  }  // namespace
  
  }  // namespace grpc_core
  
  //
  // Plugin registration
  //
  
  void grpc_lb_policy_weighted_target_init() {
    grpc_core::LoadBalancingPolicyRegistry::Builder::
        RegisterLoadBalancingPolicyFactory(
            absl::make_unique<grpc_core::WeightedTargetLbFactory>());
  }
  
  void grpc_lb_policy_weighted_target_shutdown() {}
  