src/core/client_channel/client_channel_filter.h

//
// 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_CLIENT_CHANNEL_FILTER_H
#define GRPC_SRC_CORE_CLIENT_CHANNEL_CLIENT_CHANNEL_FILTER_H

#include <grpc/support/port_platform.h>

#include <stddef.h>

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

#include "absl/base/thread_annotations.h"
#include "absl/container/flat_hash_set.h"
#include "absl/functional/any_invocable.h"
#include "absl/status/status.h"
#include "absl/strings/string_view.h"
#include "absl/types/optional.h"

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

#include "src/core/client_channel/client_channel_factory.h"
#include "src/core/client_channel/config_selector.h"
#include "src/core/client_channel/dynamic_filters.h"
#include "src/core/client_channel/subchannel.h"
#include "src/core/client_channel/subchannel_pool_interface.h"
#include "src/core/lib/channel/call_tracer.h"
#include "src/core/lib/channel/channel_args.h"
#include "src/core/lib/channel/channel_fwd.h"
#include "src/core/lib/channel/channel_stack.h"
#include "src/core/lib/channel/channelz.h"
#include "src/core/lib/channel/context.h"
#include "src/core/lib/gpr/time_precise.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/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/promise/activity.h"
#include "src/core/lib/promise/arena_promise.h"
#include "src/core/lib/resource_quota/arena.h"
#include "src/core/service_config/service_config.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"
#include "src/core/load_balancing/backend_metric_data.h"
#include "src/core/load_balancing/lb_policy.h"
#include "src/core/resolver/resolver.h"

//
// Client channel filter
//

// A client channel is a channel that begins disconnected, and can connect
// to some endpoint on demand. If that endpoint disconnects, it will be
// connected to again later.
//
// Calls on a disconnected client channel are queued until a connection is
// established.

// Channel arg key for server URI string.
#define GRPC_ARG_SERVER_URI "grpc.server_uri"

// Channel arg containing a pointer to the ClientChannelFilter object.
#define GRPC_ARG_CLIENT_CHANNEL "grpc.internal.client_channel_filter"

// Max number of batches that can be pending on a call at any given
// time.  This includes one batch for each of the following ops:
//   recv_initial_metadata
//   send_initial_metadata
//   recv_message
//   send_message
//   recv_trailing_metadata
//   send_trailing_metadata
#define MAX_PENDING_BATCHES 6

namespace grpc_core {

class ClientChannelFilter final {
 public:
  static const grpc_channel_filter kFilterVtableWithPromises;
  static const grpc_channel_filter kFilterVtableWithoutPromises;

  class LoadBalancedCall;
  class FilterBasedLoadBalancedCall;
  class PromiseBasedLoadBalancedCall;

  // Flag that this object gets stored in channel args as a raw pointer.
  struct RawPointerChannelArgTag {};
  static absl::string_view ChannelArgName() { return GRPC_ARG_CLIENT_CHANNEL; }

  static ArenaPromise<ServerMetadataHandle> MakeCallPromise(
      grpc_channel_element* elem, CallArgs call_args,
      NextPromiseFactory next_promise_factory);

  grpc_connectivity_state CheckConnectivityState(bool try_to_connect);

  // Starts a one-time connectivity state watch.  When the channel's state
  // becomes different from *state, sets *state to the new state and
  // schedules on_complete.  The watcher_timer_init callback is invoked as
  // soon as the watch is actually started (i.e., after hopping into the
  // client channel combiner).  I/O will be serviced via pollent.
  //
  // This is intended to be used when starting a watch from outside of C-core
  // via grpc_channel_watch_connectivity_state().  It should not be used
  // by other callers.
  void AddExternalConnectivityWatcher(grpc_polling_entity pollent,
                                      grpc_connectivity_state* state,
                                      grpc_closure* on_complete,
                                      grpc_closure* watcher_timer_init) {
    new ExternalConnectivityWatcher(this, pollent, state, on_complete,
                                    watcher_timer_init);
  }

  // Cancels a pending external watcher previously added by
  // AddExternalConnectivityWatcher().
  void CancelExternalConnectivityWatcher(grpc_closure* on_complete) {
    ExternalConnectivityWatcher::RemoveWatcherFromExternalWatchersMap(
        this, on_complete, /*cancel=*/true);
  }

  // Starts and stops a connectivity watch.  The watcher will be initially
  // notified as soon as the state changes from initial_state and then on
  // every subsequent state change until either the watch is stopped or
  // it is notified that the state has changed to SHUTDOWN.
  //
  // This is intended to be used when starting watches from code inside of
  // C-core (e.g., for a nested control plane channel for things like xds).
  void AddConnectivityWatcher(
      grpc_connectivity_state initial_state,
      OrphanablePtr<AsyncConnectivityStateWatcherInterface> watcher);
  void RemoveConnectivityWatcher(
      AsyncConnectivityStateWatcherInterface* watcher);

  OrphanablePtr<FilterBasedLoadBalancedCall> CreateLoadBalancedCall(
      const grpc_call_element_args& args, grpc_polling_entity* pollent,
      grpc_closure* on_call_destruction_complete,
      absl::AnyInvocable<void()> on_commit, bool is_transparent_retry);

  ArenaPromise<ServerMetadataHandle> CreateLoadBalancedCallPromise(
      CallArgs call_args, absl::AnyInvocable<void()> on_commit,
      bool is_transparent_retry);

  // Exposed for testing only.
  static ChannelArgs MakeSubchannelArgs(
      const ChannelArgs& channel_args, const ChannelArgs& address_args,
      const RefCountedPtr<SubchannelPoolInterface>& subchannel_pool,
      const std::string& channel_default_authority);

 private:
  class CallData;
  class FilterBasedCallData;
  class PromiseBasedCallData;
  class ResolverResultHandler;
  class SubchannelWrapper;
  class ClientChannelControlHelper;
  class ConnectivityWatcherAdder;
  class ConnectivityWatcherRemover;

  // Represents a pending connectivity callback from an external caller
  // via grpc_client_channel_watch_connectivity_state().
  class ExternalConnectivityWatcher final
      : public ConnectivityStateWatcherInterface {
   public:
    ExternalConnectivityWatcher(ClientChannelFilter* chand,
                                grpc_polling_entity pollent,
                                grpc_connectivity_state* state,
                                grpc_closure* on_complete,
                                grpc_closure* watcher_timer_init);

    ~ExternalConnectivityWatcher() override;

    // Removes the watcher from the external_watchers_ map.
    static void RemoveWatcherFromExternalWatchersMap(ClientChannelFilter* chand,
                                                     grpc_closure* on_complete,
                                                     bool cancel);

    void Notify(grpc_connectivity_state state,
                const absl::Status& /* status */) override;

    void Cancel();

   private:
    // Adds the watcher to state_tracker_. Consumes the ref that is passed to it
    // from Start().
    void AddWatcherLocked()
        ABSL_EXCLUSIVE_LOCKS_REQUIRED(*chand_->work_serializer_);
    void RemoveWatcherLocked()
        ABSL_EXCLUSIVE_LOCKS_REQUIRED(*chand_->work_serializer_);

    ClientChannelFilter* chand_;
    grpc_polling_entity pollent_;
    grpc_connectivity_state initial_state_;
    grpc_connectivity_state* state_;
    grpc_closure* on_complete_;
    grpc_closure* watcher_timer_init_;
    std::atomic<bool> done_{false};
  };

  ClientChannelFilter(grpc_channel_element_args* args,
                      grpc_error_handle* error);
  ~ClientChannelFilter();

  // Filter vtable functions.
  static grpc_error_handle Init(grpc_channel_element* elem,
                                grpc_channel_element_args* args);
  static void Destroy(grpc_channel_element* elem);
  static void StartTransportOp(grpc_channel_element* elem,
                               grpc_transport_op* op);
  static void GetChannelInfo(grpc_channel_element* elem,
                             const grpc_channel_info* info);

  // Note: All methods with "Locked" suffix must be invoked from within
  // work_serializer_.

  void ReprocessQueuedResolverCalls()
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&resolution_mu_);

  void OnResolverResultChangedLocked(Resolver::Result result)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);
  void OnResolverErrorLocked(absl::Status status)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  absl::Status CreateOrUpdateLbPolicyLocked(
      RefCountedPtr<LoadBalancingPolicy::Config> lb_policy_config,
      const absl::optional<std::string>& health_check_service_name,
      Resolver::Result result) ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);
  OrphanablePtr<LoadBalancingPolicy> CreateLbPolicyLocked(
      const ChannelArgs& args) ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  void UpdateStateLocked(grpc_connectivity_state state,
                         const absl::Status& status, const char* reason)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  void UpdateStateAndPickerLocked(
      grpc_connectivity_state state, const absl::Status& status,
      const char* reason,
      RefCountedPtr<LoadBalancingPolicy::SubchannelPicker> picker)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  void UpdateServiceConfigInControlPlaneLocked(
      RefCountedPtr<ServiceConfig> service_config,
      RefCountedPtr<ConfigSelector> config_selector, std::string lb_policy_name)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  void UpdateServiceConfigInDataPlaneLocked()
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  void CreateResolverLocked() ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);
  void DestroyResolverAndLbPolicyLocked()
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  grpc_error_handle DoPingLocked(grpc_transport_op* op)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  void StartTransportOpLocked(grpc_transport_op* op)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  void TryToConnectLocked() ABSL_EXCLUSIVE_LOCKS_REQUIRED(*work_serializer_);

  //
  // Fields set at construction and never modified.
  //
  ChannelArgs channel_args_;
  const bool deadline_checking_enabled_;
  grpc_channel_stack* owning_stack_;
  ClientChannelFactory* client_channel_factory_;
  RefCountedPtr<ServiceConfig> default_service_config_;
  std::string target_uri_;
  std::string uri_to_resolve_;
  std::string default_authority_;
  channelz::ChannelNode* channelz_node_;
  grpc_pollset_set* interested_parties_;
  const size_t service_config_parser_index_;

  //
  // Fields related to name resolution.  Guarded by resolution_mu_.
  //
  mutable Mutex resolution_mu_;
  // List of calls queued waiting for resolver result.
  absl::flat_hash_set<CallData*> resolver_queued_calls_
      ABSL_GUARDED_BY(resolution_mu_);
  // Data from service config.
  absl::Status resolver_transient_failure_error_
      ABSL_GUARDED_BY(resolution_mu_);
  bool received_service_config_data_ ABSL_GUARDED_BY(resolution_mu_) = false;
  RefCountedPtr<ServiceConfig> service_config_ ABSL_GUARDED_BY(resolution_mu_);
  RefCountedPtr<ConfigSelector> config_selector_
      ABSL_GUARDED_BY(resolution_mu_);
  RefCountedPtr<DynamicFilters> dynamic_filters_
      ABSL_GUARDED_BY(resolution_mu_);

  //
  // Fields related to LB picks.  Guarded by lb_mu_.
  //
  mutable Mutex lb_mu_;
  RefCountedPtr<LoadBalancingPolicy::SubchannelPicker> picker_
      ABSL_GUARDED_BY(lb_mu_);
  absl::flat_hash_set<RefCountedPtr<LoadBalancedCall>,
                      RefCountedPtrHash<LoadBalancedCall>,
                      RefCountedPtrEq<LoadBalancedCall>>
      lb_queued_calls_ ABSL_GUARDED_BY(lb_mu_);

  //
  // Fields used in the control plane.  Guarded by work_serializer.
  //
  std::shared_ptr<WorkSerializer> work_serializer_;
  ConnectivityStateTracker state_tracker_ ABSL_GUARDED_BY(*work_serializer_);
  OrphanablePtr<Resolver> resolver_ ABSL_GUARDED_BY(*work_serializer_);
  bool previous_resolution_contained_addresses_
      ABSL_GUARDED_BY(*work_serializer_) = false;
  RefCountedPtr<ServiceConfig> saved_service_config_
      ABSL_GUARDED_BY(*work_serializer_);
  RefCountedPtr<ConfigSelector> saved_config_selector_
      ABSL_GUARDED_BY(*work_serializer_);
  OrphanablePtr<LoadBalancingPolicy> lb_policy_
      ABSL_GUARDED_BY(*work_serializer_);
  RefCountedPtr<SubchannelPoolInterface> subchannel_pool_
      ABSL_GUARDED_BY(*work_serializer_);
  // The number of SubchannelWrapper instances referencing a given Subchannel.
  std::map<Subchannel*, int> subchannel_refcount_map_
      ABSL_GUARDED_BY(*work_serializer_);
  // The set of SubchannelWrappers that currently exist.
  // No need to hold a ref, since the map is updated in the control-plane
  // work_serializer when the SubchannelWrappers are created and destroyed.
  absl::flat_hash_set<SubchannelWrapper*> subchannel_wrappers_
      ABSL_GUARDED_BY(*work_serializer_);
  int keepalive_time_ ABSL_GUARDED_BY(*work_serializer_) = -1;
  grpc_error_handle disconnect_error_ ABSL_GUARDED_BY(*work_serializer_);

  //
  // Fields guarded by a mutex, since they need to be accessed
  // synchronously via get_channel_info().
  //
  Mutex info_mu_;
  std::string info_lb_policy_name_ ABSL_GUARDED_BY(info_mu_);
  std::string info_service_config_json_ ABSL_GUARDED_BY(info_mu_);

  //
  // Fields guarded by a mutex, since they need to be accessed
  // synchronously via grpc_channel_num_external_connectivity_watchers().
  //
  mutable Mutex external_watchers_mu_;
  std::map<grpc_closure*, RefCountedPtr<ExternalConnectivityWatcher>>
      external_watchers_ ABSL_GUARDED_BY(external_watchers_mu_);
};

//
// ClientChannelFilter::LoadBalancedCall
//

// TODO(roth): As part of simplifying cancellation in the filter stack,
// this should no longer need to be ref-counted.
class ClientChannelFilter::LoadBalancedCall
    : public InternallyRefCounted<LoadBalancedCall, UnrefCallDtor> {
 public:
  LoadBalancedCall(ClientChannelFilter* chand,
                   grpc_call_context_element* call_context,
                   absl::AnyInvocable<void()> on_commit,
                   bool is_transparent_retry);
  ~LoadBalancedCall() override;

  void Orphan() override { Unref(); }

  // Called by channel when removing a call from the list of queued calls.
  void RemoveCallFromLbQueuedCallsLocked()
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannelFilter::lb_mu_);

  // Called by the channel for each queued call when a new picker
  // becomes available.
  virtual void RetryPickLocked()
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannelFilter::lb_mu_) = 0;

 protected:
  ClientChannelFilter* chand() const { return chand_; }
  ClientCallTracer::CallAttemptTracer* call_attempt_tracer() const {
    return static_cast<ClientCallTracer::CallAttemptTracer*>(
        call_context_[GRPC_CONTEXT_CALL_TRACER].value);
  }
  ConnectedSubchannel* connected_subchannel() const {
    return connected_subchannel_.get();
  }
  LoadBalancingPolicy::SubchannelCallTrackerInterface*
  lb_subchannel_call_tracker() const {
    return lb_subchannel_call_tracker_.get();
  }

  void Commit() {
    auto on_commit = std::move(on_commit_);
    on_commit();
  }

  // Attempts an LB pick.  The following outcomes are possible:
  // - No pick result is available yet.  The call will be queued and
  //   nullopt will be returned.  The channel will later call
  //   RetryPickLocked() when a new picker is available and the pick
  //   should be retried.
  // - The pick failed.  If the call is not wait_for_ready, a non-OK
  //   status will be returned.  (If the call *is* wait_for_ready,
  //   it will be queued instead.)
  // - The pick completed successfully.  A connected subchannel is
  //   stored and an OK status will be returned.
  absl::optional<absl::Status> PickSubchannel(bool was_queued);

  void RecordCallCompletion(absl::Status status,
                            grpc_metadata_batch* recv_trailing_metadata,
                            grpc_transport_stream_stats* transport_stream_stats,
                            absl::string_view peer_address);

  void RecordLatency();

  grpc_call_context_element* call_context() const { return call_context_; }

 private:
  class LbCallState;
  class Metadata;
  class BackendMetricAccessor;

  virtual Arena* arena() const = 0;
  virtual grpc_polling_entity* pollent() = 0;
  virtual grpc_metadata_batch* send_initial_metadata() const = 0;

  // Helper function for performing an LB pick with a specified picker.
  // Returns true if the pick is complete.
  bool PickSubchannelImpl(LoadBalancingPolicy::SubchannelPicker* picker,
                          grpc_error_handle* error);
  // Adds the call to the channel's list of queued picks if not already present.
  void AddCallToLbQueuedCallsLocked()
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannelFilter::lb_mu_);

  // Called when adding the call to the LB queue.
  virtual void OnAddToQueueLocked()
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannelFilter::lb_mu_) = 0;

  ClientChannelFilter* chand_;

  absl::AnyInvocable<void()> on_commit_;

  gpr_cycle_counter lb_call_start_time_ = gpr_get_cycle_counter();

  RefCountedPtr<ConnectedSubchannel> connected_subchannel_;
  const BackendMetricData* backend_metric_data_ = nullptr;
  std::unique_ptr<LoadBalancingPolicy::SubchannelCallTrackerInterface>
      lb_subchannel_call_tracker_;
  grpc_call_context_element* const call_context_;
};

class ClientChannelFilter::FilterBasedLoadBalancedCall final
    : public ClientChannelFilter::LoadBalancedCall {
 public:
  // If on_call_destruction_complete is non-null, then it will be
  // invoked once the LoadBalancedCall is completely destroyed.
  // If it is null, then the caller is responsible for checking whether
  // the LB call has a subchannel call and ensuring that the
  // on_call_destruction_complete closure passed down from the surface
  // is not invoked until after the subchannel call stack is destroyed.
  FilterBasedLoadBalancedCall(ClientChannelFilter* chand,
                              const grpc_call_element_args& args,
                              grpc_polling_entity* pollent,
                              grpc_closure* on_call_destruction_complete,
                              absl::AnyInvocable<void()> on_commit,
                              bool is_transparent_retry);
  ~FilterBasedLoadBalancedCall() override;

  void Orphan() override;

  void StartTransportStreamOpBatch(grpc_transport_stream_op_batch* batch);

  RefCountedPtr<SubchannelCall> subchannel_call() const {
    return subchannel_call_;
  }

 private:
  class LbQueuedCallCanceller;

  // Work-around for Windows compilers that don't allow nested classes
  // to access protected members of the enclosing class's parent class.
  using LoadBalancedCall::chand;
  using LoadBalancedCall::Commit;

  Arena* arena() const override { return arena_; }
  grpc_polling_entity* pollent() override { return pollent_; }
  grpc_metadata_batch* send_initial_metadata() const override {
    return pending_batches_[0]
        ->payload->send_initial_metadata.send_initial_metadata;
  }

  // Returns the index into pending_batches_ to be used for batch.
  static size_t GetBatchIndex(grpc_transport_stream_op_batch* batch);
  void PendingBatchesAdd(grpc_transport_stream_op_batch* batch);
  static void FailPendingBatchInCallCombiner(void* arg,
                                             grpc_error_handle error);
  // A predicate type and some useful implementations for PendingBatchesFail().
  typedef bool (*YieldCallCombinerPredicate)(
      const CallCombinerClosureList& closures);
  static bool YieldCallCombiner(const CallCombinerClosureList& /*closures*/) {
    return true;
  }
  static bool NoYieldCallCombiner(const CallCombinerClosureList& /*closures*/) {
    return false;
  }
  static bool YieldCallCombinerIfPendingBatchesFound(
      const CallCombinerClosureList& closures) {
    return closures.size() > 0;
  }
  // Fails all pending batches.
  // If yield_call_combiner_predicate returns true, assumes responsibility for
  // yielding the call combiner.
  void PendingBatchesFail(
      grpc_error_handle error,
      YieldCallCombinerPredicate yield_call_combiner_predicate);
  static void ResumePendingBatchInCallCombiner(void* arg,
                                               grpc_error_handle ignored);
  // Resumes all pending batches on subchannel_call_.
  void PendingBatchesResume();

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

  // Called to perform a pick, both when the call is initially started
  // and when it is queued and the channel gets a new picker.
  void TryPick(bool was_queued);

  void OnAddToQueueLocked() override
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannelFilter::lb_mu_);

  void RetryPickLocked() override
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannelFilter::lb_mu_);

  void CreateSubchannelCall();

  // TODO(roth): Instead of duplicating these fields in every filter
  // that uses any one of them, we should store them in the call
  // context.  This will save per-call memory overhead.
  Timestamp deadline_;
  Arena* arena_;
  grpc_call_stack* owning_call_;
  CallCombiner* call_combiner_;
  grpc_polling_entity* pollent_;
  grpc_closure* on_call_destruction_complete_;
  absl::optional<Slice> peer_string_;

  // Set when we get a cancel_stream op.
  grpc_error_handle cancel_error_;

  // Set when we fail inside the LB call.
  grpc_error_handle failure_error_;

  LbQueuedCallCanceller* lb_call_canceller_
      ABSL_GUARDED_BY(&ClientChannelFilter::lb_mu_) = nullptr;

  RefCountedPtr<SubchannelCall> subchannel_call_;

  // For intercepting recv_initial_metadata_ready.
  grpc_metadata_batch* recv_initial_metadata_ = nullptr;
  grpc_closure recv_initial_metadata_ready_;
  grpc_closure* original_recv_initial_metadata_ready_ = nullptr;

  // For intercepting recv_trailing_metadata_ready.
  grpc_metadata_batch* recv_trailing_metadata_ = nullptr;
  grpc_transport_stream_stats* transport_stream_stats_ = nullptr;
  grpc_closure recv_trailing_metadata_ready_;
  grpc_closure* original_recv_trailing_metadata_ready_ = nullptr;

  // Batches are added to this list when received from above.
  // They are removed when we are done handling the batch (i.e., when
  // either we have invoked all of the batch's callbacks or we have
  // passed the batch down to the subchannel call and are not
  // intercepting any of its callbacks).
  grpc_transport_stream_op_batch* pending_batches_[MAX_PENDING_BATCHES] = {};
};

class ClientChannelFilter::PromiseBasedLoadBalancedCall final
    : public ClientChannelFilter::LoadBalancedCall {
 public:
  PromiseBasedLoadBalancedCall(ClientChannelFilter* chand,
                               absl::AnyInvocable<void()> on_commit,
                               bool is_transparent_retry);

  ArenaPromise<ServerMetadataHandle> MakeCallPromise(
      CallArgs call_args, OrphanablePtr<PromiseBasedLoadBalancedCall> lb_call);

 private:
  Arena* arena() const override;
  grpc_polling_entity* pollent() override { return &pollent_; }
  grpc_metadata_batch* send_initial_metadata() const override;

  void RetryPickLocked() override;

  void OnAddToQueueLocked() override
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(&ClientChannelFilter::lb_mu_);

  grpc_polling_entity pollent_;
  ClientMetadataHandle client_initial_metadata_;
  Waker waker_;
  bool was_queued_ = false;
  Slice peer_string_;
};

}  // namespace grpc_core

#endif  // GRPC_SRC_CORE_CLIENT_CHANNEL_CLIENT_CHANNEL_FILTER_H