//
// 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.
//

#ifndef GRPC_SRC_CORE_CLIENT_CHANNEL_SUBCHANNEL_H
#define GRPC_SRC_CORE_CLIENT_CHANNEL_SUBCHANNEL_H

#include <grpc/support/port_platform.h>

#include <stddef.h>

#include <functional>
#include <map>
#include <memory>

#include "absl/base/thread_annotations.h"
#include "absl/status/status.h"

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

#include "src/core/client_channel/client_channel_channelz.h"
#include "src/core/client_channel/connector.h"
#include "src/core/client_channel/subchannel_pool_interface.h"
#include "src/core/lib/backoff/backoff.h"
#include "src/core/lib/channel/channel_args.h"
#include "src/core/lib/channel/channel_fwd.h"
#include "src/core/lib/channel/context.h"
#include "src/core/lib/gpr/time_precise.h"
#include "src/core/lib/gprpp/debug_location.h"
#include "src/core/lib/gprpp/dual_ref_counted.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/time.h"
#include "src/core/lib/gprpp/unique_type_name.h"
#include "src/core/lib/gprpp/work_serializer.h"
#include "src/core/lib/iomgr/call_combiner.h"
#include "src/core/lib/iomgr/closure.h"
#include "src/core/lib/iomgr/error.h"
#include "src/core/lib/iomgr/iomgr_fwd.h"
#include "src/core/lib/iomgr/polling_entity.h"
#include "src/core/lib/iomgr/resolved_address.h"
#include "src/core/lib/promise/arena_promise.h"
#include "src/core/lib/resource_quota/arena.h"
#include "src/core/lib/slice/slice.h"
#include "src/core/lib/transport/connectivity_state.h"
#include "src/core/lib/transport/metadata_batch.h"
#include "src/core/lib/transport/transport.h"

namespace grpc_core {

class SubchannelCall;

class ConnectedSubchannel final : public RefCounted<ConnectedSubchannel> {
 public:
  ConnectedSubchannel(
      grpc_channel_stack* channel_stack, const ChannelArgs& args,
      RefCountedPtr<channelz::SubchannelNode> channelz_subchannel);
  ~ConnectedSubchannel() override;

  void StartWatch(grpc_pollset_set* interested_parties,
                  OrphanablePtr<ConnectivityStateWatcherInterface> watcher);

  void Ping(grpc_closure* on_initiate, grpc_closure* on_ack);

  grpc_channel_stack* channel_stack() const { return channel_stack_; }
  const ChannelArgs& args() const { return args_; }
  channelz::SubchannelNode* channelz_subchannel() const {
    return channelz_subchannel_.get();
  }

  size_t GetInitialCallSizeEstimate() const;

  ArenaPromise<ServerMetadataHandle> MakeCallPromise(CallArgs call_args);

 private:
  grpc_channel_stack* channel_stack_;
  ChannelArgs args_;
  // ref counted pointer to the channelz node in this connected subchannel's
  // owning subchannel.
  RefCountedPtr<channelz::SubchannelNode> channelz_subchannel_;
};

// Implements the interface of RefCounted<>.
class SubchannelCall final {
 public:
  struct Args {
    RefCountedPtr<ConnectedSubchannel> connected_subchannel;
    grpc_polling_entity* pollent;
    Slice path;
    gpr_cycle_counter start_time;
    Timestamp deadline;
    Arena* arena;
    grpc_call_context_element* context;
    CallCombiner* call_combiner;
  };
  static RefCountedPtr<SubchannelCall> Create(Args args,
                                              grpc_error_handle* error);

  // Continues processing a transport stream op batch.
  void StartTransportStreamOpBatch(grpc_transport_stream_op_batch* batch);

  // Returns the call stack of the subchannel call.
  grpc_call_stack* GetCallStack();

  // Sets the 'then_schedule_closure' argument for call stack destruction.
  // Must be called once per call.
  void SetAfterCallStackDestroy(grpc_closure* closure);

  // Interface of RefCounted<>.
  GRPC_MUST_USE_RESULT RefCountedPtr<SubchannelCall> Ref();
  GRPC_MUST_USE_RESULT RefCountedPtr<SubchannelCall> Ref(
      const DebugLocation& location, const char* reason);
  // When refcount drops to 0, destroys itself and the associated call stack,
  // but does NOT free the memory because it's in the call arena.
  void Unref();
  void Unref(const DebugLocation& location, const char* reason);

 private:
  // Allow RefCountedPtr<> to access IncrementRefCount().
  template <typename T>
  friend class RefCountedPtr;

  SubchannelCall(Args args, grpc_error_handle* error);

  // If channelz is enabled, intercepts recv_trailing so that we may check the
  // status and associate it to a subchannel.
  void MaybeInterceptRecvTrailingMetadata(
      grpc_transport_stream_op_batch* batch);

  static void RecvTrailingMetadataReady(void* arg, grpc_error_handle error);

  // Interface of RefCounted<>.
  void IncrementRefCount();
  void IncrementRefCount(const DebugLocation& location, const char* reason);

  static void Destroy(void* arg, grpc_error_handle error);

  RefCountedPtr<ConnectedSubchannel> connected_subchannel_;
  grpc_closure* after_call_stack_destroy_ = nullptr;
  // State needed to support channelz interception of recv trailing metadata.
  grpc_closure recv_trailing_metadata_ready_;
  grpc_closure* original_recv_trailing_metadata_ = nullptr;
  grpc_metadata_batch* recv_trailing_metadata_ = nullptr;
  Timestamp deadline_;
};

// A subchannel that knows how to connect to exactly one target address. It
// provides a target for load balancing.
//
// Note that this is the "real" subchannel implementation, whose API is
// different from the SubchannelInterface that is exposed to LB policy
// implementations.  The client channel provides an adaptor class
// (SubchannelWrapper) that "converts" between the two.
class Subchannel final : public DualRefCounted<Subchannel> {
 public:
  // TODO(roth): Once we remove pollset_set, consider whether this can
  // just use the normal AsyncConnectivityStateWatcherInterface API.
  class ConnectivityStateWatcherInterface
      : public RefCounted<ConnectivityStateWatcherInterface> {
   public:
    // Invoked whenever the subchannel's connectivity state changes.
    // There will be only one invocation of this method on a given watcher
    // instance at any given time.
    // A ref to the watcher is passed in here so that the implementation
    // can unref it in the appropriate synchronization context (e.g.,
    // inside a WorkSerializer).
    // TODO(roth): Figure out a cleaner way to guarantee that the ref is
    // released in the right context.
    virtual void OnConnectivityStateChange(
        RefCountedPtr<ConnectivityStateWatcherInterface> self,
        grpc_connectivity_state state, const absl::Status& status) = 0;

    virtual grpc_pollset_set* interested_parties() = 0;
  };

  // A base class for producers of subchannel-specific data.
  // Implementations will typically add their own methods as needed.
  class DataProducerInterface : public DualRefCounted<DataProducerInterface> {
   public:
    // A unique identifier for this implementation.
    // Only one producer may be registered under a given type name on a
    // given subchannel at any given time.
    // Note that we use the pointer address instead of the string
    // contents for uniqueness; all instances for a given implementation
    // are expected to return the same string *instance*, not just the
    // same string contents.
    virtual UniqueTypeName type() const = 0;
  };

  // Creates a subchannel.
  static RefCountedPtr<Subchannel> Create(
      OrphanablePtr<SubchannelConnector> connector,
      const grpc_resolved_address& address, const ChannelArgs& args);

  // The ctor and dtor are not intended to use directly.
  Subchannel(SubchannelKey key, OrphanablePtr<SubchannelConnector> connector,
             const ChannelArgs& args);
  ~Subchannel() override;

  // Throttles keepalive time to \a new_keepalive_time iff \a new_keepalive_time
  // is larger than the subchannel's current keepalive time. The updated value
  // will have an affect when the subchannel creates a new ConnectedSubchannel.
  void ThrottleKeepaliveTime(int new_keepalive_time) ABSL_LOCKS_EXCLUDED(mu_);

  grpc_pollset_set* pollset_set() const { return pollset_set_; }

  channelz::SubchannelNode* channelz_node();

  const grpc_resolved_address& address() const { return key_.address(); }

  // Starts watching the subchannel's connectivity state.
  // The first callback to the watcher will be delivered ~immediately.
  // Subsequent callbacks will be delivered as the subchannel's state
  // changes.
  // The watcher will be destroyed either when the subchannel is
  // destroyed or when CancelConnectivityStateWatch() is called.
  void WatchConnectivityState(
      RefCountedPtr<ConnectivityStateWatcherInterface> watcher)
      ABSL_LOCKS_EXCLUDED(mu_);

  // Cancels a connectivity state watch.
  // If the watcher has already been destroyed, this is a no-op.
  void CancelConnectivityStateWatch(ConnectivityStateWatcherInterface* watcher)
      ABSL_LOCKS_EXCLUDED(mu_);

  RefCountedPtr<ConnectedSubchannel> connected_subchannel()
      ABSL_LOCKS_EXCLUDED(mu_) {
    MutexLock lock(&mu_);
    return connected_subchannel_;
  }

  // Attempt to connect to the backend.  Has no effect if already connected.
  void RequestConnection() ABSL_LOCKS_EXCLUDED(mu_);

  // Resets the connection backoff of the subchannel.
  void ResetBackoff() ABSL_LOCKS_EXCLUDED(mu_);

  // Access to data producer map.
  // We do not hold refs to the data producer; the implementation is
  // expected to register itself upon construction and remove itself
  // upon destruction.
  //
  // Looks up the current data producer for type and invokes get_or_add()
  // with a pointer to that producer in the map.  The get_or_add() function
  // can modify the pointed-to value to update the map.  This provides a
  // way to either re-use an existing producer or register a new one in
  // a non-racy way.
  void GetOrAddDataProducer(
      UniqueTypeName type,
      std::function<void(DataProducerInterface**)> get_or_add)
      ABSL_LOCKS_EXCLUDED(mu_);
  // Removes the data producer from the map, if the current producer for
  // this type is the specified producer.
  void RemoveDataProducer(DataProducerInterface* data_producer)
      ABSL_LOCKS_EXCLUDED(mu_);

  std::shared_ptr<grpc_event_engine::experimental::EventEngine> event_engine() {
    return event_engine_;
  }

 private:
  // Tears down any existing connection, and arranges for destruction
  void Orphaned() override ABSL_LOCKS_EXCLUDED(mu_);

  // A linked list of ConnectivityStateWatcherInterfaces that are monitoring
  // the subchannel's state.
  class ConnectivityStateWatcherList final {
   public:
    explicit ConnectivityStateWatcherList(Subchannel* subchannel)
        : subchannel_(subchannel) {}

    ~ConnectivityStateWatcherList() { Clear(); }

    void AddWatcherLocked(
        RefCountedPtr<ConnectivityStateWatcherInterface> watcher);
    void RemoveWatcherLocked(ConnectivityStateWatcherInterface* watcher);

    // Notifies all watchers in the list about a change to state.
    void NotifyLocked(grpc_connectivity_state state,
                      const absl::Status& status);

    void Clear() { watchers_.clear(); }

    bool empty() const { return watchers_.empty(); }

   private:
    Subchannel* subchannel_;
    // TODO(roth): Once we can use C++-14 heterogeneous lookups, this can
    // be a set instead of a map.
    std::map<ConnectivityStateWatcherInterface*,
             RefCountedPtr<ConnectivityStateWatcherInterface>>
        watchers_;
  };

  class ConnectedSubchannelStateWatcher;

  // Sets the subchannel's connectivity state to \a state.
  void SetConnectivityStateLocked(grpc_connectivity_state state,
                                  const absl::Status& status)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);

  // Methods for connection.
  void OnRetryTimer() ABSL_LOCKS_EXCLUDED(mu_);
  void OnRetryTimerLocked() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);
  void StartConnectingLocked() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);
  static void OnConnectingFinished(void* arg, grpc_error_handle error)
      ABSL_LOCKS_EXCLUDED(mu_);
  void OnConnectingFinishedLocked(grpc_error_handle error)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);
  bool PublishTransportLocked() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_);

  // The subchannel pool this subchannel is in.
  RefCountedPtr<SubchannelPoolInterface> subchannel_pool_;
  // Subchannel key that identifies this subchannel in the subchannel pool.
  const SubchannelKey key_;
  // Actual address to connect to.  May be different than the address in
  // key_ if overridden by proxy mapper.
  grpc_resolved_address address_for_connect_;
  // Channel args.
  ChannelArgs args_;
  // pollset_set tracking who's interested in a connection being setup.
  grpc_pollset_set* pollset_set_;
  // Channelz tracking.
  RefCountedPtr<channelz::SubchannelNode> channelz_node_;
  // Minimum connection timeout.
  Duration min_connect_timeout_;

  // Connection state.
  OrphanablePtr<SubchannelConnector> connector_;
  SubchannelConnector::Result connecting_result_;
  grpc_closure on_connecting_finished_;

  // Protects the other members.
  Mutex mu_;

  bool shutdown_ ABSL_GUARDED_BY(mu_) = false;

  // Connectivity state tracking.
  // Note that the connectivity state implies the state of the
  // Subchannel object:
  // - IDLE: no retry timer pending, can start a connection attempt at any time
  // - CONNECTING: connection attempt in progress
  // - READY: connection attempt succeeded, connected_subchannel_ created
  // - TRANSIENT_FAILURE: connection attempt failed, retry timer pending
  grpc_connectivity_state state_ ABSL_GUARDED_BY(mu_) = GRPC_CHANNEL_IDLE;
  absl::Status status_ ABSL_GUARDED_BY(mu_);
  // The list of connectivity state watchers.
  ConnectivityStateWatcherList watcher_list_ ABSL_GUARDED_BY(mu_);
  // Used for sending connectivity state notifications.
  WorkSerializer work_serializer_;

  // Active connection, or null.
  RefCountedPtr<ConnectedSubchannel> connected_subchannel_ ABSL_GUARDED_BY(mu_);

  // Backoff state.
  BackOff backoff_ ABSL_GUARDED_BY(mu_);
  Timestamp next_attempt_time_ ABSL_GUARDED_BY(mu_);
  grpc_event_engine::experimental::EventEngine::TaskHandle retry_timer_handle_
      ABSL_GUARDED_BY(mu_);

  // Keepalive time period (-1 for unset)
  int keepalive_time_ ABSL_GUARDED_BY(mu_) = -1;

  // Data producer map.
  std::map<UniqueTypeName, DataProducerInterface*> data_producer_map_
      ABSL_GUARDED_BY(mu_);
  std::shared_ptr<grpc_event_engine::experimental::EventEngine> event_engine_;
};

}  // namespace grpc_core

#endif  // GRPC_SRC_CORE_CLIENT_CHANNEL_SUBCHANNEL_H