/*
 *
 * 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.
 *
 */

#ifndef GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_RESOLVING_LB_POLICY_H
#define GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_RESOLVING_LB_POLICY_H

#include <grpc/support/port_platform.h>

#include "src/core/ext/filters/client_channel/lb_policy.h"
#include "src/core/ext/filters/client_channel/lb_policy_factory.h"
#include "src/core/ext/filters/client_channel/resolver.h"
#include "src/core/lib/channel/channel_args.h"
#include "src/core/lib/channel/channel_stack.h"
#include "src/core/lib/debug/trace.h"
#include "src/core/lib/gprpp/inlined_vector.h"
#include "src/core/lib/gprpp/orphanable.h"
#include "src/core/lib/iomgr/call_combiner.h"
#include "src/core/lib/iomgr/closure.h"
#include "src/core/lib/iomgr/polling_entity.h"
#include "src/core/lib/iomgr/pollset_set.h"
#include "src/core/lib/transport/connectivity_state.h"
#include "src/core/lib/transport/metadata_batch.h"

namespace grpc_core {

// An LB policy that wraps a resolver and a child LB policy to make use
// of the addresses returned by the resolver.
//
// When used in the client_channel code, the resolver will attempt to
// fetch the service config, and the child LB policy name and config
// will be determined based on the service config.
//
// When used in an LB policy implementation that needs to do another
// round of resolution before creating a child policy, the resolver does
// not fetch the service config, and the caller must pre-determine the
// child LB policy and config to use.
class ResolvingLoadBalancingPolicy : public LoadBalancingPolicy {
 public:
  // If error is set when this returns, then construction failed, and
  // the caller may not use the new object.
  ResolvingLoadBalancingPolicy(
      Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
      UniquePtr<char> child_policy_name,
      RefCountedPtr<LoadBalancingPolicy::Config> child_lb_config,
      grpc_error** error);

  // Private ctor, to be used by client_channel only!
  //
  // Synchronous callback that takes the resolver result and sets
  // lb_policy_name and lb_policy_config to point to the right data.
  // Returns true if the service config has changed since the last result.
  // If the returned service_config_error is not none and lb_policy_name is
  // empty, it means that we don't have a valid service config to use, and we
  // should set the channel to be in TRANSIENT_FAILURE.
  typedef bool (*ProcessResolverResultCallback)(
      void* user_data, const Resolver::Result& result,
      const char** lb_policy_name,
      RefCountedPtr<LoadBalancingPolicy::Config>* lb_policy_config,
      grpc_error** service_config_error);
  // If error is set when this returns, then construction failed, and
  // the caller may not use the new object.
  ResolvingLoadBalancingPolicy(
      Args args, TraceFlag* tracer, UniquePtr<char> target_uri,
      ProcessResolverResultCallback process_resolver_result,
      void* process_resolver_result_user_data, grpc_error** error);

  virtual const char* name() const override { return "resolving_lb"; }

  // No-op -- should never get updates from the channel.
  // TODO(roth): Need to support updating child LB policy's config for xds
  // use case.
  void UpdateLocked(UpdateArgs args) override {}

  void ExitIdleLocked() override;

  void ResetBackoffLocked() override;

 private:
  using TraceStringVector = InlinedVector<char*, 3>;

  class ResolverResultHandler;
  class ResolvingControlHelper;

  ~ResolvingLoadBalancingPolicy();

  grpc_error* Init(const grpc_channel_args& args);
  void ShutdownLocked() override;

  void StartResolvingLocked();
  void OnResolverError(grpc_error* error);
  void CreateOrUpdateLbPolicyLocked(
      const char* lb_policy_name,
      RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config,
      Resolver::Result result, TraceStringVector* trace_strings);
  OrphanablePtr<LoadBalancingPolicy> CreateLbPolicyLocked(
      const char* lb_policy_name, const grpc_channel_args& args,
      TraceStringVector* trace_strings);
  void MaybeAddTraceMessagesForAddressChangesLocked(
      bool resolution_contains_addresses, TraceStringVector* trace_strings);
  void ConcatenateAndAddChannelTraceLocked(
      TraceStringVector* trace_strings) const;
  void OnResolverResultChangedLocked(Resolver::Result result);

  // Passed in from caller at construction time.
  TraceFlag* tracer_;
  UniquePtr<char> target_uri_;
  ProcessResolverResultCallback process_resolver_result_ = nullptr;
  void* process_resolver_result_user_data_ = nullptr;
  UniquePtr<char> child_policy_name_;
  RefCountedPtr<LoadBalancingPolicy::Config> child_lb_config_;

  // Resolver and associated state.
  OrphanablePtr<Resolver> resolver_;
  bool started_resolving_ = false;
  bool previous_resolution_contained_addresses_ = false;

  // Child LB policy.
  OrphanablePtr<LoadBalancingPolicy> lb_policy_;
  OrphanablePtr<LoadBalancingPolicy> pending_lb_policy_;
};

}  // namespace grpc_core

#endif /* GRPC_CORE_EXT_FILTERS_CLIENT_CHANNEL_RESOLVING_LB_POLICY_H */