src/core/ext/filters/client_channel/resolving_lb_policy.cc - platform/external/grpc-grpc - Git at Google

 /*
  *
  * Copyright 2015 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 "src/core/ext/filters/client_channel/resolving_lb_policy.h"

 #include <inttypes.h>
 #include <limits.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>

 #include <grpc/support/alloc.h>
 #include <grpc/support/log.h>
 #include <grpc/support/string_util.h>
 #include <grpc/support/sync.h>

 #include "src/core/ext/filters/client_channel/backup_poller.h"
 #include "src/core/ext/filters/client_channel/http_connect_handshaker.h"
 #include "src/core/ext/filters/client_channel/lb_policy_registry.h"
 #include "src/core/ext/filters/client_channel/proxy_mapper_registry.h"
 #include "src/core/ext/filters/client_channel/resolver_registry.h"
 #include "src/core/ext/filters/client_channel/retry_throttle.h"
 #include "src/core/ext/filters/client_channel/server_address.h"
 #include "src/core/ext/filters/client_channel/service_config.h"
 #include "src/core/ext/filters/client_channel/subchannel.h"
 #include "src/core/ext/filters/deadline/deadline_filter.h"
 #include "src/core/lib/backoff/backoff.h"
 #include "src/core/lib/channel/channel_args.h"
 #include "src/core/lib/channel/connected_channel.h"
 #include "src/core/lib/channel/status_util.h"
 #include "src/core/lib/gpr/string.h"
 #include "src/core/lib/gprpp/inlined_vector.h"
 #include "src/core/lib/gprpp/manual_constructor.h"
 #include "src/core/lib/gprpp/sync.h"
 #include "src/core/lib/iomgr/combiner.h"
 #include "src/core/lib/iomgr/iomgr.h"
 #include "src/core/lib/iomgr/polling_entity.h"
 #include "src/core/lib/profiling/timers.h"
 #include "src/core/lib/slice/slice_internal.h"
 #include "src/core/lib/slice/slice_string_helpers.h"
 #include "src/core/lib/surface/channel.h"
 #include "src/core/lib/transport/connectivity_state.h"
 #include "src/core/lib/transport/error_utils.h"
 #include "src/core/lib/transport/metadata.h"
 #include "src/core/lib/transport/metadata_batch.h"
 #include "src/core/lib/transport/static_metadata.h"
 #include "src/core/lib/transport/status_metadata.h"

 namespace grpc_core {

 //
 // ResolvingLoadBalancingPolicy::ResolverResultHandler
 //

 class ResolvingLoadBalancingPolicy::ResolverResultHandler
     : public Resolver::ResultHandler {
  public:
   explicit ResolverResultHandler(
       RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
       : parent_(std::move(parent)) {}

   ~ResolverResultHandler() {
     if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
       gpr_log(GPR_INFO, "resolving_lb=%p: resolver shutdown complete",
               parent_.get());
     }
   }

   void ReturnResult(Resolver::Result result) override {
     parent_->OnResolverResultChangedLocked(std::move(result));
   }

   void ReturnError(grpc_error* error) override {
     parent_->OnResolverError(error);
   }

  private:
   RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
 };

 //
 // ResolvingLoadBalancingPolicy::ResolvingControlHelper
 //

 class ResolvingLoadBalancingPolicy::ResolvingControlHelper
     : public LoadBalancingPolicy::ChannelControlHelper {
  public:
   explicit ResolvingControlHelper(
       RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
       : parent_(std::move(parent)) {}

   RefCountedPtr<SubchannelInterface> CreateSubchannel(
       const grpc_channel_args& args) override {
     if (parent_->resolver_ == nullptr) return nullptr;  // Shutting down.
     if (!CalledByCurrentChild() && !CalledByPendingChild()) return nullptr;
     return parent_->channel_control_helper()->CreateSubchannel(args);
   }

   void UpdateState(grpc_connectivity_state state,
                    UniquePtr<SubchannelPicker> picker) override {
     if (parent_->resolver_ == nullptr) return;  // Shutting down.
     // If this request is from the pending child policy, ignore it until
     // it reports READY, at which point we swap it into place.
     if (CalledByPendingChild()) {
       if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
         gpr_log(GPR_INFO,
                 "resolving_lb=%p helper=%p: pending child policy %p reports "
                 "state=%s",
                 parent_.get(), this, child_,
                 grpc_connectivity_state_name(state));
       }
       if (state != GRPC_CHANNEL_READY) return;
       grpc_pollset_set_del_pollset_set(
           parent_->lb_policy_->interested_parties(),
           parent_->interested_parties());
       parent_->lb_policy_ = std::move(parent_->pending_lb_policy_);
     } else if (!CalledByCurrentChild()) {
       // This request is from an outdated child, so ignore it.
       return;
     }
     parent_->channel_control_helper()->UpdateState(state, std::move(picker));
   }

   void RequestReresolution() override {
     // If there is a pending child policy, ignore re-resolution requests
     // from the current child policy (or any outdated child).
     if (parent_->pending_lb_policy_ != nullptr && !CalledByPendingChild()) {
       return;
     }
     if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
       gpr_log(GPR_INFO, "resolving_lb=%p: started name re-resolving",
               parent_.get());
     }
     if (parent_->resolver_ != nullptr) {
       parent_->resolver_->RequestReresolutionLocked();
     }
   }

   void AddTraceEvent(TraceSeverity severity, StringView message) override {}

   void set_child(LoadBalancingPolicy* child) { child_ = child; }

  private:
   bool CalledByPendingChild() const {
     GPR_ASSERT(child_ != nullptr);
     return child_ == parent_->pending_lb_policy_.get();
   }

   bool CalledByCurrentChild() const {
     GPR_ASSERT(child_ != nullptr);
     return child_ == parent_->lb_policy_.get();
   };

   RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
   LoadBalancingPolicy* child_ = nullptr;
 };

 //
 // ResolvingLoadBalancingPolicy
 //

 ResolvingLoadBalancingPolicy::ResolvingLoadBalancingPolicy(
     Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
     ProcessResolverResultCallback process_resolver_result,
     void* process_resolver_result_user_data)
     : LoadBalancingPolicy(std::move(args)),
       tracer_(tracer),
       target_uri_(std::move(target_uri)),
       process_resolver_result_(process_resolver_result),
       process_resolver_result_user_data_(process_resolver_result_user_data) {
   GPR_ASSERT(process_resolver_result != nullptr);
   resolver_ = ResolverRegistry::CreateResolver(
       target_uri_.get(), args.args, interested_parties(), combiner(),
       UniquePtr<Resolver::ResultHandler>(New<ResolverResultHandler>(Ref())));
   // Since the validity of args has been checked when create the channel,
   // CreateResolver() must return a non-null result.
   GPR_ASSERT(resolver_ != nullptr);
   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
     gpr_log(GPR_INFO, "resolving_lb=%p: starting name resolution", this);
   }
   channel_control_helper()->UpdateState(
       GRPC_CHANNEL_CONNECTING,
       UniquePtr<SubchannelPicker>(New<QueuePicker>(Ref())));
   resolver_->StartLocked();
 }

 ResolvingLoadBalancingPolicy::~ResolvingLoadBalancingPolicy() {
   GPR_ASSERT(resolver_ == nullptr);
   GPR_ASSERT(lb_policy_ == nullptr);
 }

 void ResolvingLoadBalancingPolicy::ShutdownLocked() {
   if (resolver_ != nullptr) {
     resolver_.reset();
     if (lb_policy_ != nullptr) {
       if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
         gpr_log(GPR_INFO, "resolving_lb=%p: shutting down lb_policy=%p", this,
                 lb_policy_.get());
       }
       grpc_pollset_set_del_pollset_set(lb_policy_->interested_parties(),
                                        interested_parties());
       lb_policy_.reset();
     }
     if (pending_lb_policy_ != nullptr) {
       if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
         gpr_log(GPR_INFO, "resolving_lb=%p: shutting down pending lb_policy=%p",
                 this, pending_lb_policy_.get());
       }
       grpc_pollset_set_del_pollset_set(pending_lb_policy_->interested_parties(),
                                        interested_parties());
       pending_lb_policy_.reset();
     }
   }
 }

 void ResolvingLoadBalancingPolicy::ExitIdleLocked() {
   if (lb_policy_ != nullptr) {
     lb_policy_->ExitIdleLocked();
     if (pending_lb_policy_ != nullptr) pending_lb_policy_->ExitIdleLocked();
   }
 }

 void ResolvingLoadBalancingPolicy::ResetBackoffLocked() {
   if (resolver_ != nullptr) {
     resolver_->ResetBackoffLocked();
     resolver_->RequestReresolutionLocked();
   }
   if (lb_policy_ != nullptr) lb_policy_->ResetBackoffLocked();
   if (pending_lb_policy_ != nullptr) pending_lb_policy_->ResetBackoffLocked();
 }

 void ResolvingLoadBalancingPolicy::OnResolverError(grpc_error* error) {
   if (resolver_ == nullptr) {
     GRPC_ERROR_UNREF(error);
     return;
   }
   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
     gpr_log(GPR_INFO, "resolving_lb=%p: resolver transient failure: %s", this,
             grpc_error_string(error));
   }
   // If we already have an LB policy from a previous resolution
   // result, then we continue to let it set the connectivity state.
   // Otherwise, we go into TRANSIENT_FAILURE.
   if (lb_policy_ == nullptr) {
     grpc_error* state_error = GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
         "Resolver transient failure", &error, 1);
     channel_control_helper()->UpdateState(
         GRPC_CHANNEL_TRANSIENT_FAILURE,
         UniquePtr<SubchannelPicker>(New<TransientFailurePicker>(state_error)));
   }
   GRPC_ERROR_UNREF(error);
 }

 void ResolvingLoadBalancingPolicy::CreateOrUpdateLbPolicyLocked(
     const char* lb_policy_name,
     RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config,
     Resolver::Result result, TraceStringVector* trace_strings) {
   // If the child policy name changes, we need to create a new child
   // policy.  When this happens, we leave child_policy_ as-is and store
   // the new child policy in pending_child_policy_.  Once the new child
   // policy transitions into state READY, we swap it into child_policy_,
   // replacing the original child policy.  So pending_child_policy_ is
   // non-null only between when we apply an update that changes the child
   // policy name and when the new child reports state READY.
   //
   // Updates can arrive at any point during this transition.  We always
   // apply updates relative to the most recently created child policy,
   // even if the most recent one is still in pending_child_policy_.  This
   // is true both when applying the updates to an existing child policy
   // and when determining whether we need to create a new policy.
   //
   // As a result of this, there are several cases to consider here:
   //
   // 1. We have no existing child policy (i.e., we have started up but
   //    have not yet received a serverlist from the balancer or gone
   //    into fallback mode; in this case, both child_policy_ and
   //    pending_child_policy_ are null).  In this case, we create a
   //    new child policy and store it in child_policy_.
   //
   // 2. We have an existing child policy and have no pending child policy
   //    from a previous update (i.e., either there has not been a
   //    previous update that changed the policy name, or we have already
   //    finished swapping in the new policy; in this case, child_policy_
   //    is non-null but pending_child_policy_ is null).  In this case:
   //    a. If child_policy_->name() equals child_policy_name, then we
   //       update the existing child policy.
   //    b. If child_policy_->name() does not equal child_policy_name,
   //       we create a new policy.  The policy will be stored in
   //       pending_child_policy_ and will later be swapped into
   //       child_policy_ by the helper when the new child transitions
   //       into state READY.
   //
   // 3. We have an existing child policy and have a pending child policy
   //    from a previous update (i.e., a previous update set
   //    pending_child_policy_ as per case 2b above and that policy has
   //    not yet transitioned into state READY and been swapped into
   //    child_policy_; in this case, both child_policy_ and
   //    pending_child_policy_ are non-null).  In this case:
   //    a. If pending_child_policy_->name() equals child_policy_name,
   //       then we update the existing pending child policy.
   //    b. If pending_child_policy->name() does not equal
   //       child_policy_name, then we create a new policy.  The new
   //       policy is stored in pending_child_policy_ (replacing the one
   //       that was there before, which will be immediately shut down)
   //       and will later be swapped into child_policy_ by the helper
   //       when the new child transitions into state READY.
   const bool create_policy =
       // case 1
       lb_policy_ == nullptr ||
       // case 2b
       (pending_lb_policy_ == nullptr &&
        strcmp(lb_policy_->name(), lb_policy_name) != 0) ||
       // case 3b
       (pending_lb_policy_ != nullptr &&
        strcmp(pending_lb_policy_->name(), lb_policy_name) != 0);
   LoadBalancingPolicy* policy_to_update = nullptr;
   if (create_policy) {
     // Cases 1, 2b, and 3b: create a new child policy.
     // If lb_policy_ is null, we set it (case 1), else we set
     // pending_lb_policy_ (cases 2b and 3b).
     if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
       gpr_log(GPR_INFO, "resolving_lb=%p: Creating new %schild policy %s", this,
               lb_policy_ == nullptr ? "" : "pending ", lb_policy_name);
     }
     auto& lb_policy = lb_policy_ == nullptr ? lb_policy_ : pending_lb_policy_;
     lb_policy =
         CreateLbPolicyLocked(lb_policy_name, *result.args, trace_strings);
     policy_to_update = lb_policy.get();
   } else {
     // Cases 2a and 3a: update an existing policy.
     // If we have a pending child policy, send the update to the pending
     // policy (case 3a), else send it to the current policy (case 2a).
     policy_to_update = pending_lb_policy_ != nullptr ? pending_lb_policy_.get()
                                                      : lb_policy_.get();
   }
   GPR_ASSERT(policy_to_update != nullptr);
   // Update the policy.
   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
     gpr_log(GPR_INFO, "resolving_lb=%p: Updating %schild policy %p", this,
             policy_to_update == pending_lb_policy_.get() ? "pending " : "",
             policy_to_update);
   }
   UpdateArgs update_args;
   update_args.addresses = std::move(result.addresses);
   update_args.config = std::move(lb_policy_config);
   // TODO(roth): Once channel args is converted to C++, use std::move() here.
   update_args.args = result.args;
   result.args = nullptr;
   policy_to_update->UpdateLocked(std::move(update_args));
 }

 // Creates a new LB policy.
 // Updates trace_strings to indicate what was done.
 OrphanablePtr<LoadBalancingPolicy>
 ResolvingLoadBalancingPolicy::CreateLbPolicyLocked(
     const char* lb_policy_name, const grpc_channel_args& args,
     TraceStringVector* trace_strings) {
   ResolvingControlHelper* helper = New<ResolvingControlHelper>(Ref());
   LoadBalancingPolicy::Args lb_policy_args;
   lb_policy_args.combiner = combiner();
   lb_policy_args.channel_control_helper =
       UniquePtr<ChannelControlHelper>(helper);
   lb_policy_args.args = &args;
   OrphanablePtr<LoadBalancingPolicy> lb_policy =
       LoadBalancingPolicyRegistry::CreateLoadBalancingPolicy(
           lb_policy_name, std::move(lb_policy_args));
   if (GPR_UNLIKELY(lb_policy == nullptr)) {
     gpr_log(GPR_ERROR, "could not create LB policy \"%s\"", lb_policy_name);
     char* str;
     gpr_asprintf(&str, "Could not create LB policy \"%s\"", lb_policy_name);
     trace_strings->push_back(str);
     return nullptr;
   }
   helper->set_child(lb_policy.get());
   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
     gpr_log(GPR_INFO, "resolving_lb=%p: created new LB policy \"%s\" (%p)",
             this, lb_policy_name, lb_policy.get());
   }
   char* str;
   gpr_asprintf(&str, "Created new LB policy \"%s\"", lb_policy_name);
   trace_strings->push_back(str);
   grpc_pollset_set_add_pollset_set(lb_policy->interested_parties(),
                                    interested_parties());
   return lb_policy;
 }

 void ResolvingLoadBalancingPolicy::MaybeAddTraceMessagesForAddressChangesLocked(
     bool resolution_contains_addresses, TraceStringVector* trace_strings) {
   if (!resolution_contains_addresses &&
       previous_resolution_contained_addresses_) {
     trace_strings->push_back(gpr_strdup("Address list became empty"));
   } else if (resolution_contains_addresses &&
              !previous_resolution_contained_addresses_) {
     trace_strings->push_back(gpr_strdup("Address list became non-empty"));
   }
   previous_resolution_contained_addresses_ = resolution_contains_addresses;
 }

 void ResolvingLoadBalancingPolicy::ConcatenateAndAddChannelTraceLocked(
     TraceStringVector* trace_strings) const {
   if (!trace_strings->empty()) {
     gpr_strvec v;
     gpr_strvec_init(&v);
     gpr_strvec_add(&v, gpr_strdup("Resolution event: "));
     bool is_first = 1;
     for (size_t i = 0; i < trace_strings->size(); ++i) {
       if (!is_first) gpr_strvec_add(&v, gpr_strdup(", "));
       is_first = false;
       gpr_strvec_add(&v, (*trace_strings)[i]);
     }
     size_t len = 0;
     UniquePtr<char> message(gpr_strvec_flatten(&v, &len));
     channel_control_helper()->AddTraceEvent(ChannelControlHelper::TRACE_INFO,
                                             StringView(message.get()));
     gpr_strvec_destroy(&v);
   }
 }

 void ResolvingLoadBalancingPolicy::OnResolverResultChangedLocked(
     Resolver::Result result) {
   // Handle race conditions.
   if (resolver_ == nullptr) return;
   if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
     gpr_log(GPR_INFO, "resolving_lb=%p: got resolver result", this);
   }
   // We only want to trace the address resolution in the follow cases:
   // (a) Address resolution resulted in service config change.
   // (b) Address resolution that causes number of backends to go from
   //     zero to non-zero.
   // (c) Address resolution that causes number of backends to go from
   //     non-zero to zero.
   // (d) Address resolution that causes a new LB policy to be created.
   //
   // We track a list of strings to eventually be concatenated and traced.
   TraceStringVector trace_strings;
   const bool resolution_contains_addresses = result.addresses.size() > 0;
   // Process the resolver result.
   const char* lb_policy_name = nullptr;
   RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config;
   bool service_config_changed = false;
   char* service_config_error_string = nullptr;
   if (process_resolver_result_ != nullptr) {
     grpc_error* service_config_error = GRPC_ERROR_NONE;
     service_config_changed = process_resolver_result_(
         process_resolver_result_user_data_, result, &lb_policy_name,
         &lb_policy_config, &service_config_error);
     if (service_config_error != GRPC_ERROR_NONE) {
       service_config_error_string =
           gpr_strdup(grpc_error_string(service_config_error));
       if (lb_policy_name == nullptr) {
         // Use an empty lb_policy_name as an indicator that we received an
         // invalid service config and we don't have a fallback service config.
         OnResolverError(service_config_error);
       } else {
         GRPC_ERROR_UNREF(service_config_error);
       }
     }
   } else {
     lb_policy_name = child_policy_name_.get();
     lb_policy_config = child_lb_config_;
   }
   if (lb_policy_name != nullptr) {
     // Create or update LB policy, as needed.
     CreateOrUpdateLbPolicyLocked(lb_policy_name, lb_policy_config,
                                  std::move(result), &trace_strings);
   }
   // Add channel trace event.
   if (service_config_changed) {
     // TODO(ncteisen): might be worth somehow including a snippet of the
     // config in the trace, at the risk of bloating the trace logs.
     trace_strings.push_back(gpr_strdup("Service config changed"));
   }
   if (service_config_error_string != nullptr) {
     trace_strings.push_back(service_config_error_string);
     service_config_error_string = nullptr;
   }
   MaybeAddTraceMessagesForAddressChangesLocked(resolution_contains_addresses,
                                                &trace_strings);
   ConcatenateAndAddChannelTraceLocked(&trace_strings);
 }

 }  // namespace grpc_core
	/*
	*
	* Copyright 2015 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 "src/core/ext/filters/client_channel/resolving_lb_policy.h"

	#include <inttypes.h>
	#include <limits.h>
	#include <stdbool.h>
	#include <stdio.h>
	#include <string.h>

	#include <grpc/support/alloc.h>
	#include <grpc/support/log.h>
	#include <grpc/support/string_util.h>
	#include <grpc/support/sync.h>

	#include "src/core/ext/filters/client_channel/backup_poller.h"
	#include "src/core/ext/filters/client_channel/http_connect_handshaker.h"
	#include "src/core/ext/filters/client_channel/lb_policy_registry.h"
	#include "src/core/ext/filters/client_channel/proxy_mapper_registry.h"
	#include "src/core/ext/filters/client_channel/resolver_registry.h"
	#include "src/core/ext/filters/client_channel/retry_throttle.h"
	#include "src/core/ext/filters/client_channel/server_address.h"
	#include "src/core/ext/filters/client_channel/service_config.h"
	#include "src/core/ext/filters/client_channel/subchannel.h"
	#include "src/core/ext/filters/deadline/deadline_filter.h"
	#include "src/core/lib/backoff/backoff.h"
	#include "src/core/lib/channel/channel_args.h"
	#include "src/core/lib/channel/connected_channel.h"
	#include "src/core/lib/channel/status_util.h"
	#include "src/core/lib/gpr/string.h"
	#include "src/core/lib/gprpp/inlined_vector.h"
	#include "src/core/lib/gprpp/manual_constructor.h"
	#include "src/core/lib/gprpp/sync.h"
	#include "src/core/lib/iomgr/combiner.h"
	#include "src/core/lib/iomgr/iomgr.h"
	#include "src/core/lib/iomgr/polling_entity.h"
	#include "src/core/lib/profiling/timers.h"
	#include "src/core/lib/slice/slice_internal.h"
	#include "src/core/lib/slice/slice_string_helpers.h"
	#include "src/core/lib/surface/channel.h"
	#include "src/core/lib/transport/connectivity_state.h"
	#include "src/core/lib/transport/error_utils.h"
	#include "src/core/lib/transport/metadata.h"
	#include "src/core/lib/transport/metadata_batch.h"
	#include "src/core/lib/transport/static_metadata.h"
	#include "src/core/lib/transport/status_metadata.h"

	namespace grpc_core {

	//
	// ResolvingLoadBalancingPolicy::ResolverResultHandler
	//

	class ResolvingLoadBalancingPolicy::ResolverResultHandler
	: public Resolver::ResultHandler {
	public:
	explicit ResolverResultHandler(
	RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
	: parent_(std::move(parent)) {}

	~ResolverResultHandler() {
	if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
	gpr_log(GPR_INFO, "resolving_lb=%p: resolver shutdown complete",
	parent_.get());
	}
	}

	void ReturnResult(Resolver::Result result) override {
	parent_->OnResolverResultChangedLocked(std::move(result));
	}

	void ReturnError(grpc_error* error) override {
	parent_->OnResolverError(error);
	}

	private:
	RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
	};

	//
	// ResolvingLoadBalancingPolicy::ResolvingControlHelper
	//

	class ResolvingLoadBalancingPolicy::ResolvingControlHelper
	: public LoadBalancingPolicy::ChannelControlHelper {
	public:
	explicit ResolvingControlHelper(
	RefCountedPtr<ResolvingLoadBalancingPolicy> parent)
	: parent_(std::move(parent)) {}

	RefCountedPtr<SubchannelInterface> CreateSubchannel(
	const grpc_channel_args& args) override {
	if (parent_->resolver_ == nullptr) return nullptr; // Shutting down.
	if (!CalledByCurrentChild() && !CalledByPendingChild()) return nullptr;
	return parent_->channel_control_helper()->CreateSubchannel(args);
	}

	void UpdateState(grpc_connectivity_state state,
	UniquePtr<SubchannelPicker> picker) override {
	if (parent_->resolver_ == nullptr) return; // Shutting down.
	// If this request is from the pending child policy, ignore it until
	// it reports READY, at which point we swap it into place.
	if (CalledByPendingChild()) {
	if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
	gpr_log(GPR_INFO,
	"resolving_lb=%p helper=%p: pending child policy %p reports "
	"state=%s",
	parent_.get(), this, child_,
	grpc_connectivity_state_name(state));
	}
	if (state != GRPC_CHANNEL_READY) return;
	grpc_pollset_set_del_pollset_set(
	parent_->lb_policy_->interested_parties(),
	parent_->interested_parties());
	parent_->lb_policy_ = std::move(parent_->pending_lb_policy_);
	} else if (!CalledByCurrentChild()) {
	// This request is from an outdated child, so ignore it.
	return;
	}
	parent_->channel_control_helper()->UpdateState(state, std::move(picker));
	}

	void RequestReresolution() override {
	// If there is a pending child policy, ignore re-resolution requests
	// from the current child policy (or any outdated child).
	if (parent_->pending_lb_policy_ != nullptr && !CalledByPendingChild()) {
	return;
	}
	if (GRPC_TRACE_FLAG_ENABLED(*(parent_->tracer_))) {
	gpr_log(GPR_INFO, "resolving_lb=%p: started name re-resolving",
	parent_.get());
	}
	if (parent_->resolver_ != nullptr) {
	parent_->resolver_->RequestReresolutionLocked();
	}
	}

	void AddTraceEvent(TraceSeverity severity, StringView message) override {}

	void set_child(LoadBalancingPolicy* child) { child_ = child; }

	private:
	bool CalledByPendingChild() const {
	GPR_ASSERT(child_ != nullptr);
	return child_ == parent_->pending_lb_policy_.get();
	}

	bool CalledByCurrentChild() const {
	GPR_ASSERT(child_ != nullptr);
	return child_ == parent_->lb_policy_.get();
	};

	RefCountedPtr<ResolvingLoadBalancingPolicy> parent_;
	LoadBalancingPolicy* child_ = nullptr;
	};

	//
	// ResolvingLoadBalancingPolicy
	//

	ResolvingLoadBalancingPolicy::ResolvingLoadBalancingPolicy(
	Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
	ProcessResolverResultCallback process_resolver_result,
	void* process_resolver_result_user_data)
	: LoadBalancingPolicy(std::move(args)),
	tracer_(tracer),
	target_uri_(std::move(target_uri)),
	process_resolver_result_(process_resolver_result),
	process_resolver_result_user_data_(process_resolver_result_user_data) {
	GPR_ASSERT(process_resolver_result != nullptr);
	resolver_ = ResolverRegistry::CreateResolver(
	target_uri_.get(), args.args, interested_parties(), combiner(),
	UniquePtr<Resolver::ResultHandler>(New<ResolverResultHandler>(Ref())));
	// Since the validity of args has been checked when create the channel,
	// CreateResolver() must return a non-null result.
	GPR_ASSERT(resolver_ != nullptr);
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: starting name resolution", this);
	}
	channel_control_helper()->UpdateState(
	GRPC_CHANNEL_CONNECTING,
	UniquePtr<SubchannelPicker>(New<QueuePicker>(Ref())));
	resolver_->StartLocked();
	}

	ResolvingLoadBalancingPolicy::~ResolvingLoadBalancingPolicy() {
	GPR_ASSERT(resolver_ == nullptr);
	GPR_ASSERT(lb_policy_ == nullptr);
	}

	void ResolvingLoadBalancingPolicy::ShutdownLocked() {
	if (resolver_ != nullptr) {
	resolver_.reset();
	if (lb_policy_ != nullptr) {
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: shutting down lb_policy=%p", this,
	lb_policy_.get());
	}
	grpc_pollset_set_del_pollset_set(lb_policy_->interested_parties(),
	interested_parties());
	lb_policy_.reset();
	}
	if (pending_lb_policy_ != nullptr) {
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: shutting down pending lb_policy=%p",
	this, pending_lb_policy_.get());
	}
	grpc_pollset_set_del_pollset_set(pending_lb_policy_->interested_parties(),
	interested_parties());
	pending_lb_policy_.reset();
	}
	}
	}

	void ResolvingLoadBalancingPolicy::ExitIdleLocked() {
	if (lb_policy_ != nullptr) {
	lb_policy_->ExitIdleLocked();
	if (pending_lb_policy_ != nullptr) pending_lb_policy_->ExitIdleLocked();
	}
	}

	void ResolvingLoadBalancingPolicy::ResetBackoffLocked() {
	if (resolver_ != nullptr) {
	resolver_->ResetBackoffLocked();
	resolver_->RequestReresolutionLocked();
	}
	if (lb_policy_ != nullptr) lb_policy_->ResetBackoffLocked();
	if (pending_lb_policy_ != nullptr) pending_lb_policy_->ResetBackoffLocked();
	}

	void ResolvingLoadBalancingPolicy::OnResolverError(grpc_error* error) {
	if (resolver_ == nullptr) {
	GRPC_ERROR_UNREF(error);
	return;
	}
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: resolver transient failure: %s", this,
	grpc_error_string(error));
	}
	// If we already have an LB policy from a previous resolution
	// result, then we continue to let it set the connectivity state.
	// Otherwise, we go into TRANSIENT_FAILURE.
	if (lb_policy_ == nullptr) {
	grpc_error* state_error = GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
	"Resolver transient failure", &error, 1);
	channel_control_helper()->UpdateState(
	GRPC_CHANNEL_TRANSIENT_FAILURE,
	UniquePtr<SubchannelPicker>(New<TransientFailurePicker>(state_error)));
	}
	GRPC_ERROR_UNREF(error);
	}

	void ResolvingLoadBalancingPolicy::CreateOrUpdateLbPolicyLocked(
	const char* lb_policy_name,
	RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config,
	Resolver::Result result, TraceStringVector* trace_strings) {
	// If the child policy name changes, we need to create a new child
	// policy. When this happens, we leave child_policy_ as-is and store
	// the new child policy in pending_child_policy_. Once the new child
	// policy transitions into state READY, we swap it into child_policy_,
	// replacing the original child policy. So pending_child_policy_ is
	// non-null only between when we apply an update that changes the child
	// policy name and when the new child reports state READY.
	//
	// Updates can arrive at any point during this transition. We always
	// apply updates relative to the most recently created child policy,
	// even if the most recent one is still in pending_child_policy_. This
	// is true both when applying the updates to an existing child policy
	// and when determining whether we need to create a new policy.
	//
	// As a result of this, there are several cases to consider here:
	//
	// 1. We have no existing child policy (i.e., we have started up but
	// have not yet received a serverlist from the balancer or gone
	// into fallback mode; in this case, both child_policy_ and
	// pending_child_policy_ are null). In this case, we create a
	// new child policy and store it in child_policy_.
	//
	// 2. We have an existing child policy and have no pending child policy
	// from a previous update (i.e., either there has not been a
	// previous update that changed the policy name, or we have already
	// finished swapping in the new policy; in this case, child_policy_
	// is non-null but pending_child_policy_ is null). In this case:
	// a. If child_policy_->name() equals child_policy_name, then we
	// update the existing child policy.
	// b. If child_policy_->name() does not equal child_policy_name,
	// we create a new policy. The policy will be stored in
	// pending_child_policy_ and will later be swapped into
	// child_policy_ by the helper when the new child transitions
	// into state READY.
	//
	// 3. We have an existing child policy and have a pending child policy
	// from a previous update (i.e., a previous update set
	// pending_child_policy_ as per case 2b above and that policy has
	// not yet transitioned into state READY and been swapped into
	// child_policy_; in this case, both child_policy_ and
	// pending_child_policy_ are non-null). In this case:
	// a. If pending_child_policy_->name() equals child_policy_name,
	// then we update the existing pending child policy.
	// b. If pending_child_policy->name() does not equal
	// child_policy_name, then we create a new policy. The new
	// policy is stored in pending_child_policy_ (replacing the one
	// that was there before, which will be immediately shut down)
	// and will later be swapped into child_policy_ by the helper
	// when the new child transitions into state READY.
	const bool create_policy =
	// case 1
	lb_policy_ == nullptr \|\|
	// case 2b
	(pending_lb_policy_ == nullptr &&
	strcmp(lb_policy_->name(), lb_policy_name) != 0) \|\|
	// case 3b
	(pending_lb_policy_ != nullptr &&
	strcmp(pending_lb_policy_->name(), lb_policy_name) != 0);
	LoadBalancingPolicy* policy_to_update = nullptr;
	if (create_policy) {
	// Cases 1, 2b, and 3b: create a new child policy.
	// If lb_policy_ is null, we set it (case 1), else we set
	// pending_lb_policy_ (cases 2b and 3b).
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: Creating new %schild policy %s", this,
	lb_policy_ == nullptr ? "" : "pending ", lb_policy_name);
	}
	auto& lb_policy = lb_policy_ == nullptr ? lb_policy_ : pending_lb_policy_;
	lb_policy =
	CreateLbPolicyLocked(lb_policy_name, *result.args, trace_strings);
	policy_to_update = lb_policy.get();
	} else {
	// Cases 2a and 3a: update an existing policy.
	// If we have a pending child policy, send the update to the pending
	// policy (case 3a), else send it to the current policy (case 2a).
	policy_to_update = pending_lb_policy_ != nullptr ? pending_lb_policy_.get()
	: lb_policy_.get();
	}
	GPR_ASSERT(policy_to_update != nullptr);
	// Update the policy.
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: Updating %schild policy %p", this,
	policy_to_update == pending_lb_policy_.get() ? "pending " : "",
	policy_to_update);
	}
	UpdateArgs update_args;
	update_args.addresses = std::move(result.addresses);
	update_args.config = std::move(lb_policy_config);
	// TODO(roth): Once channel args is converted to C++, use std::move() here.
	update_args.args = result.args;
	result.args = nullptr;
	policy_to_update->UpdateLocked(std::move(update_args));
	}

	// Creates a new LB policy.
	// Updates trace_strings to indicate what was done.
	OrphanablePtr<LoadBalancingPolicy>
	ResolvingLoadBalancingPolicy::CreateLbPolicyLocked(
	const char* lb_policy_name, const grpc_channel_args& args,
	TraceStringVector* trace_strings) {
	ResolvingControlHelper* helper = New<ResolvingControlHelper>(Ref());
	LoadBalancingPolicy::Args lb_policy_args;
	lb_policy_args.combiner = combiner();
	lb_policy_args.channel_control_helper =
	UniquePtr<ChannelControlHelper>(helper);
	lb_policy_args.args = &args;
	OrphanablePtr<LoadBalancingPolicy> lb_policy =
	LoadBalancingPolicyRegistry::CreateLoadBalancingPolicy(
	lb_policy_name, std::move(lb_policy_args));
	if (GPR_UNLIKELY(lb_policy == nullptr)) {
	gpr_log(GPR_ERROR, "could not create LB policy \"%s\"", lb_policy_name);
	char* str;
	gpr_asprintf(&str, "Could not create LB policy \"%s\"", lb_policy_name);
	trace_strings->push_back(str);
	return nullptr;
	}
	helper->set_child(lb_policy.get());
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: created new LB policy \"%s\" (%p)",
	this, lb_policy_name, lb_policy.get());
	}
	char* str;
	gpr_asprintf(&str, "Created new LB policy \"%s\"", lb_policy_name);
	trace_strings->push_back(str);
	grpc_pollset_set_add_pollset_set(lb_policy->interested_parties(),
	interested_parties());
	return lb_policy;
	}

	void ResolvingLoadBalancingPolicy::MaybeAddTraceMessagesForAddressChangesLocked(
	bool resolution_contains_addresses, TraceStringVector* trace_strings) {
	if (!resolution_contains_addresses &&
	previous_resolution_contained_addresses_) {
	trace_strings->push_back(gpr_strdup("Address list became empty"));
	} else if (resolution_contains_addresses &&
	!previous_resolution_contained_addresses_) {
	trace_strings->push_back(gpr_strdup("Address list became non-empty"));
	}
	previous_resolution_contained_addresses_ = resolution_contains_addresses;
	}

	void ResolvingLoadBalancingPolicy::ConcatenateAndAddChannelTraceLocked(
	TraceStringVector* trace_strings) const {
	if (!trace_strings->empty()) {
	gpr_strvec v;
	gpr_strvec_init(&v);
	gpr_strvec_add(&v, gpr_strdup("Resolution event: "));
	bool is_first = 1;
	for (size_t i = 0; i < trace_strings->size(); ++i) {
	if (!is_first) gpr_strvec_add(&v, gpr_strdup(", "));
	is_first = false;
	gpr_strvec_add(&v, (*trace_strings)[i]);
	}
	size_t len = 0;
	UniquePtr<char> message(gpr_strvec_flatten(&v, &len));
	channel_control_helper()->AddTraceEvent(ChannelControlHelper::TRACE_INFO,
	StringView(message.get()));
	gpr_strvec_destroy(&v);
	}
	}

	void ResolvingLoadBalancingPolicy::OnResolverResultChangedLocked(
	Resolver::Result result) {
	// Handle race conditions.
	if (resolver_ == nullptr) return;
	if (GRPC_TRACE_FLAG_ENABLED(*tracer_)) {
	gpr_log(GPR_INFO, "resolving_lb=%p: got resolver result", this);
	}
	// We only want to trace the address resolution in the follow cases:
	// (a) Address resolution resulted in service config change.
	// (b) Address resolution that causes number of backends to go from
	// zero to non-zero.
	// (c) Address resolution that causes number of backends to go from
	// non-zero to zero.
	// (d) Address resolution that causes a new LB policy to be created.
	//
	// We track a list of strings to eventually be concatenated and traced.
	TraceStringVector trace_strings;
	const bool resolution_contains_addresses = result.addresses.size() > 0;
	// Process the resolver result.
	const char* lb_policy_name = nullptr;
	RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config;
	bool service_config_changed = false;
	char* service_config_error_string = nullptr;
	if (process_resolver_result_ != nullptr) {
	grpc_error* service_config_error = GRPC_ERROR_NONE;
	service_config_changed = process_resolver_result_(
	process_resolver_result_user_data_, result, &lb_policy_name,
	&lb_policy_config, &service_config_error);
	if (service_config_error != GRPC_ERROR_NONE) {
	service_config_error_string =
	gpr_strdup(grpc_error_string(service_config_error));
	if (lb_policy_name == nullptr) {
	// Use an empty lb_policy_name as an indicator that we received an
	// invalid service config and we don't have a fallback service config.
	OnResolverError(service_config_error);
	} else {
	GRPC_ERROR_UNREF(service_config_error);
	}
	}
	} else {
	lb_policy_name = child_policy_name_.get();
	lb_policy_config = child_lb_config_;
	}
	if (lb_policy_name != nullptr) {
	// Create or update LB policy, as needed.
	CreateOrUpdateLbPolicyLocked(lb_policy_name, lb_policy_config,
	std::move(result), &trace_strings);
	}
	// Add channel trace event.
	if (service_config_changed) {
	// TODO(ncteisen): might be worth somehow including a snippet of the
	// config in the trace, at the risk of bloating the trace logs.
	trace_strings.push_back(gpr_strdup("Service config changed"));
	}
	if (service_config_error_string != nullptr) {
	trace_strings.push_back(service_config_error_string);
	service_config_error_string = nullptr;
	}
	MaybeAddTraceMessagesForAddressChangesLocked(resolution_contains_addresses,
	&trace_strings);
	ConcatenateAndAddChannelTraceLocked(&trace_strings);
	}

	} // namespace grpc_core