path: root/contrib/libs/grpc/src/cpp/server/server_cc.cc

/*
 * 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 <grpcpp/server.h>

#include <cstdlib>
#include <sstream>
#include <type_traits> 
#include <utility>

#include <grpc/grpc.h>
#include <grpc/impl/codegen/grpc_types.h>
#include <grpc/support/alloc.h>
#include <grpc/support/log.h>
#include <grpcpp/completion_queue.h>
#include <grpcpp/generic/async_generic_service.h>
#include <grpcpp/impl/codegen/async_unary_call.h>
#include <grpcpp/impl/codegen/byte_buffer.h> 
#include <grpcpp/impl/codegen/call.h>
#include <grpcpp/impl/codegen/completion_queue_tag.h>
#include <grpcpp/impl/codegen/method_handler.h> 
#include <grpcpp/impl/codegen/server_interceptor.h>
#include <grpcpp/impl/grpc_library.h>
#include <grpcpp/impl/rpc_service_method.h>
#include <grpcpp/impl/server_initializer.h>
#include <grpcpp/impl/service_type.h>
#include <grpcpp/security/server_credentials.h>
#include <grpcpp/server_context.h>
#include <grpcpp/support/time.h>

#include "src/core/ext/transport/inproc/inproc_transport.h"
#include "src/core/lib/iomgr/exec_ctx.h"
#include "src/core/lib/profiling/timers.h"
#include "src/core/lib/surface/call.h"
#include "src/core/lib/surface/completion_queue.h"
#include "src/core/lib/surface/server.h"
#include "src/cpp/client/create_channel_internal.h"
#include "src/cpp/server/external_connection_acceptor_impl.h" 
#include "src/cpp/server/health/default_health_check_service.h"
#include "src/cpp/thread_manager/thread_manager.h"

#include <util/stream/str.h>

namespace grpc {
namespace {

// The default value for maximum number of threads that can be created in the
// sync server. This value of INT_MAX is chosen to match the default behavior if
// no ResourceQuota is set. To modify the max number of threads in a sync
// server, pass a custom ResourceQuota object  (with the desired number of
// max-threads set) to the server builder.
#define DEFAULT_MAX_SYNC_SERVER_THREADS INT_MAX

class DefaultGlobalCallbacks final : public Server::GlobalCallbacks {
 public:
  ~DefaultGlobalCallbacks() override {}
  void PreSynchronousRequest(ServerContext* /*context*/) override {} 
  void PostSynchronousRequest(ServerContext* /*context*/) override {} 
};

std::shared_ptr<Server::GlobalCallbacks> g_callbacks = nullptr;
gpr_once g_once_init_callbacks = GPR_ONCE_INIT;

void InitGlobalCallbacks() {
  if (!g_callbacks) {
    g_callbacks.reset(new DefaultGlobalCallbacks());
  }
}

class ShutdownTag : public internal::CompletionQueueTag {
 public:
  bool FinalizeResult(void** /*tag*/, bool* /*status*/) { return false; } 
};

class DummyTag : public internal::CompletionQueueTag {
 public:
  bool FinalizeResult(void** /*tag*/, bool* /*status*/) { return true; } 
};

class UnimplementedAsyncRequestContext {
 protected:
  UnimplementedAsyncRequestContext() : generic_stream_(&server_context_) {}

  GenericServerContext server_context_;
  GenericServerAsyncReaderWriter generic_stream_;
};

// TODO(vjpai): Just for this file, use some contents of the experimental
// namespace here to make the code easier to read below. Remove this when
// de-experimentalized fully.
#ifndef GRPC_CALLBACK_API_NONEXPERIMENTAL
using ::grpc::experimental::CallbackGenericService;
using ::grpc::experimental::CallbackServerContext;
using ::grpc::experimental::GenericCallbackServerContext;
#endif

}  // namespace

ServerInterface::BaseAsyncRequest::BaseAsyncRequest( 
    ServerInterface* server, ServerContext* context, 
    internal::ServerAsyncStreamingInterface* stream, CompletionQueue* call_cq, 
    ServerCompletionQueue* notification_cq, void* tag, bool delete_on_finalize) 
    : server_(server), 
      context_(context), 
      stream_(stream), 
      call_cq_(call_cq), 
      notification_cq_(notification_cq), 
      tag_(tag), 
      delete_on_finalize_(delete_on_finalize), 
      call_(nullptr), 
      done_intercepting_(false) { 
  /* Set up interception state partially for the receive ops. call_wrapper_ is 
   * not filled at this point, but it will be filled before the interceptors are 
   * run. */ 
  interceptor_methods_.SetCall(&call_wrapper_); 
  interceptor_methods_.SetReverse(); 
  call_cq_->RegisterAvalanching();  // This op will trigger more ops 
} 
 
ServerInterface::BaseAsyncRequest::~BaseAsyncRequest() { 
  call_cq_->CompleteAvalanching(); 
} 
 
bool ServerInterface::BaseAsyncRequest::FinalizeResult(void** tag, 
                                                       bool* status) { 
  if (done_intercepting_) { 
    *tag = tag_; 
    if (delete_on_finalize_) { 
      delete this; 
    } 
    return true; 
  } 
  context_->set_call(call_); 
  context_->cq_ = call_cq_; 
  if (call_wrapper_.call() == nullptr) { 
    // Fill it since it is empty. 
    call_wrapper_ = internal::Call( 
        call_, server_, call_cq_, server_->max_receive_message_size(), nullptr); 
  } 
 
  // just the pointers inside call are copied here 
  stream_->BindCall(&call_wrapper_); 
 
  if (*status && call_ && call_wrapper_.server_rpc_info()) { 
    done_intercepting_ = true; 
    // Set interception point for RECV INITIAL METADATA 
    interceptor_methods_.AddInterceptionHookPoint( 
        experimental::InterceptionHookPoints::POST_RECV_INITIAL_METADATA); 
    interceptor_methods_.SetRecvInitialMetadata(&context_->client_metadata_); 
    if (interceptor_methods_.RunInterceptors( 
            [this]() { ContinueFinalizeResultAfterInterception(); })) { 
      // There are no interceptors to run. Continue 
    } else { 
      // There were interceptors to be run, so 
      // ContinueFinalizeResultAfterInterception will be run when interceptors 
      // are done. 
      return false; 
    } 
  } 
  if (*status && call_) { 
    context_->BeginCompletionOp(&call_wrapper_, nullptr, nullptr); 
  } 
  *tag = tag_; 
  if (delete_on_finalize_) { 
    delete this; 
  } 
  return true; 
} 
 
void ServerInterface::BaseAsyncRequest:: 
    ContinueFinalizeResultAfterInterception() { 
  context_->BeginCompletionOp(&call_wrapper_, nullptr, nullptr); 
  // Queue a tag which will be returned immediately 
  grpc_core::ExecCtx exec_ctx; 
  grpc_cq_begin_op(notification_cq_->cq(), this); 
  grpc_cq_end_op( 
      notification_cq_->cq(), this, GRPC_ERROR_NONE, 
      [](void* /*arg*/, grpc_cq_completion* completion) { delete completion; }, 
      nullptr, new grpc_cq_completion()); 
} 
 
ServerInterface::RegisteredAsyncRequest::RegisteredAsyncRequest( 
    ServerInterface* server, ServerContext* context, 
    internal::ServerAsyncStreamingInterface* stream, CompletionQueue* call_cq, 
    ServerCompletionQueue* notification_cq, void* tag, const char* name, 
    internal::RpcMethod::RpcType type) 
    : BaseAsyncRequest(server, context, stream, call_cq, notification_cq, tag, 
                       true), 
      name_(name), 
      type_(type) {} 
 
void ServerInterface::RegisteredAsyncRequest::IssueRequest( 
    void* registered_method, grpc_byte_buffer** payload, 
    ServerCompletionQueue* notification_cq) { 
  // The following call_start_batch is internally-generated so no need for an 
  // explanatory log on failure. 
  GPR_ASSERT(grpc_server_request_registered_call( 
                 server_->server(), registered_method, &call_, 
                 &context_->deadline_, context_->client_metadata_.arr(), 
                 payload, call_cq_->cq(), notification_cq->cq(), 
                 this) == GRPC_CALL_OK); 
} 
 
ServerInterface::GenericAsyncRequest::GenericAsyncRequest( 
    ServerInterface* server, GenericServerContext* context, 
    internal::ServerAsyncStreamingInterface* stream, CompletionQueue* call_cq, 
    ServerCompletionQueue* notification_cq, void* tag, bool delete_on_finalize) 
    : BaseAsyncRequest(server, context, stream, call_cq, notification_cq, tag, 
                       delete_on_finalize) { 
  grpc_call_details_init(&call_details_); 
  GPR_ASSERT(notification_cq); 
  GPR_ASSERT(call_cq); 
  // The following call_start_batch is internally-generated so no need for an 
  // explanatory log on failure. 
  GPR_ASSERT(grpc_server_request_call(server->server(), &call_, &call_details_, 
                                      context->client_metadata_.arr(), 
                                      call_cq->cq(), notification_cq->cq(), 
                                      this) == GRPC_CALL_OK); 
} 
 
bool ServerInterface::GenericAsyncRequest::FinalizeResult(void** tag, 
                                                          bool* status) { 
  // If we are done intercepting, there is nothing more for us to do 
  if (done_intercepting_) { 
    return BaseAsyncRequest::FinalizeResult(tag, status); 
  } 
  // TODO(yangg) remove the copy here. 
  if (*status) { 
    static_cast<GenericServerContext*>(context_)->method_ = 
        StringFromCopiedSlice(call_details_.method); 
    static_cast<GenericServerContext*>(context_)->host_ = 
        StringFromCopiedSlice(call_details_.host); 
    context_->deadline_ = call_details_.deadline; 
  } 
  grpc_slice_unref(call_details_.method); 
  grpc_slice_unref(call_details_.host); 
  call_wrapper_ = internal::Call( 
      call_, server_, call_cq_, server_->max_receive_message_size(), 
      context_->set_server_rpc_info( 
          static_cast<GenericServerContext*>(context_)->method_.c_str(), 
          internal::RpcMethod::BIDI_STREAMING, 
          *server_->interceptor_creators())); 
  return BaseAsyncRequest::FinalizeResult(tag, status); 
} 
 
namespace { 
class ShutdownCallback : public grpc_experimental_completion_queue_functor { 
 public: 
  ShutdownCallback() { 
    functor_run = &ShutdownCallback::Run; 
    // Set inlineable to true since this callback is trivial and thus does not 
    // need to be run from the executor (triggering a thread hop). This should 
    // only be used by internal callbacks like this and not by user application 
    // code. 
    inlineable = true; 
  } 
  // TakeCQ takes ownership of the cq into the shutdown callback 
  // so that the shutdown callback will be responsible for destroying it 
  void TakeCQ(CompletionQueue* cq) { cq_ = cq; } 
 
  // The Run function will get invoked by the completion queue library 
  // when the shutdown is actually complete 
  static void Run(grpc_experimental_completion_queue_functor* cb, int) { 
    auto* callback = static_cast<ShutdownCallback*>(cb); 
    delete callback->cq_; 
    delete callback; 
  } 
 
 private: 
  CompletionQueue* cq_ = nullptr; 
}; 
}  // namespace 
 
/// Use private inheritance rather than composition only to establish order
/// of construction, since the public base class should be constructed after the
/// elements belonging to the private base class are constructed. This is not
/// possible using true composition.
class Server::UnimplementedAsyncRequest final
    : private grpc::UnimplementedAsyncRequestContext, 
      public GenericAsyncRequest {
 public:
  UnimplementedAsyncRequest(ServerInterface* server,
                            grpc::ServerCompletionQueue* cq)
      : GenericAsyncRequest(server, &server_context_, &generic_stream_, cq, cq,
                            nullptr, false) {}

  bool FinalizeResult(void** tag, bool* status) override;

  grpc::ServerContext* context() { return &server_context_; } 
  grpc::GenericServerAsyncReaderWriter* stream() { return &generic_stream_; } 
};

/// UnimplementedAsyncResponse should not post user-visible completions to the
/// C++ completion queue, but is generated as a CQ event by the core
class Server::UnimplementedAsyncResponse final
    : public grpc::internal::CallOpSet< 
          grpc::internal::CallOpSendInitialMetadata, 
          grpc::internal::CallOpServerSendStatus> { 
 public:
  UnimplementedAsyncResponse(UnimplementedAsyncRequest* request);
  ~UnimplementedAsyncResponse() { delete request_; }

  bool FinalizeResult(void** tag, bool* status) override {
    if (grpc::internal::CallOpSet< 
            grpc::internal::CallOpSendInitialMetadata, 
            grpc::internal::CallOpServerSendStatus>::FinalizeResult(tag, 
                                                                    status)) { 
      delete this;
    } else {
      // The tag was swallowed due to interception. We will see it again.
    }
    return false;
  }

 private:
  UnimplementedAsyncRequest* const request_;
};

class Server::SyncRequest final : public grpc::internal::CompletionQueueTag { 
 public:
  SyncRequest(grpc::internal::RpcServiceMethod* method, void* method_tag) 
      : method_(method),
        method_tag_(method_tag),
        in_flight_(false),
        has_request_payload_(method->method_type() == 
                                 grpc::internal::RpcMethod::NORMAL_RPC || 
                             method->method_type() == 
                                 grpc::internal::RpcMethod::SERVER_STREAMING), 
        call_details_(nullptr),
        cq_(nullptr) {
    grpc_metadata_array_init(&request_metadata_);
  }

  ~SyncRequest() {
    if (call_details_) {
      delete call_details_;
    }
    grpc_metadata_array_destroy(&request_metadata_);
  }

  void SetupRequest() { cq_ = grpc_completion_queue_create_for_pluck(nullptr); }

  void TeardownRequest() {
    grpc_completion_queue_destroy(cq_);
    cq_ = nullptr;
  }

  void Request(grpc_server* server, grpc_completion_queue* notify_cq) {
    GPR_ASSERT(cq_ && !in_flight_);
    in_flight_ = true;
    if (method_tag_) {
      if (grpc_server_request_registered_call(
              server, method_tag_, &call_, &deadline_, &request_metadata_,
              has_request_payload_ ? &request_payload_ : nullptr, cq_,
              notify_cq, this) != GRPC_CALL_OK) {
        TeardownRequest();
        return;
      }
    } else {
      if (!call_details_) {
        call_details_ = new grpc_call_details;
        grpc_call_details_init(call_details_);
      }
      if (grpc_server_request_call(server, &call_, call_details_,
                                   &request_metadata_, cq_, notify_cq,
                                   this) != GRPC_CALL_OK) {
        TeardownRequest();
        return;
      }
    }
  }

  void PostShutdownCleanup() {
    if (call_) {
      grpc_call_unref(call_);
      call_ = nullptr;
    }
    if (cq_) {
      grpc_completion_queue_destroy(cq_);
      cq_ = nullptr;
    }
  }

  bool FinalizeResult(void** /*tag*/, bool* status) override { 
    if (!*status) {
      grpc_completion_queue_destroy(cq_);
      cq_ = nullptr;
    }
    if (call_details_) {
      deadline_ = call_details_->deadline;
      grpc_call_details_destroy(call_details_);
      grpc_call_details_init(call_details_);
    }
    return true;
  }

  // The CallData class represents a call that is "active" as opposed
  // to just being requested. It wraps and takes ownership of the cq from
  // the call request
  class CallData final {
   public:
    explicit CallData(Server* server, SyncRequest* mrd)
        : cq_(mrd->cq_),
          ctx_(mrd->deadline_, &mrd->request_metadata_),
          has_request_payload_(mrd->has_request_payload_),
          request_payload_(has_request_payload_ ? mrd->request_payload_
                                                : nullptr),
          request_(nullptr),
          method_(mrd->method_),
          call_(
              mrd->call_, server, &cq_, server->max_receive_message_size(),
              ctx_.set_server_rpc_info(method_->name(), method_->method_type(),
                                       server->interceptor_creators_)),
          server_(server),
          global_callbacks_(nullptr),
          resources_(false) {
      ctx_.set_call(mrd->call_);
      ctx_.cq_ = &cq_;
      GPR_ASSERT(mrd->in_flight_);
      mrd->in_flight_ = false;
      mrd->request_metadata_.count = 0;
    }

    ~CallData() {
      if (has_request_payload_ && request_payload_) {
        grpc_byte_buffer_destroy(request_payload_);
      }
    }

    void Run(const std::shared_ptr<GlobalCallbacks>& global_callbacks,
             bool resources) {
      global_callbacks_ = global_callbacks;
      resources_ = resources;

      interceptor_methods_.SetCall(&call_);
      interceptor_methods_.SetReverse();
      // Set interception point for RECV INITIAL METADATA
      interceptor_methods_.AddInterceptionHookPoint(
          grpc::experimental::InterceptionHookPoints:: 
              POST_RECV_INITIAL_METADATA); 
      interceptor_methods_.SetRecvInitialMetadata(&ctx_.client_metadata_);

      if (has_request_payload_) {
        // Set interception point for RECV MESSAGE
        auto* handler = resources_ ? method_->handler()
                                   : server_->resource_exhausted_handler_.get();
        request_ = handler->Deserialize(call_.call(), request_payload_,
                                        &request_status_, nullptr); 

        request_payload_ = nullptr;
        interceptor_methods_.AddInterceptionHookPoint(
            grpc::experimental::InterceptionHookPoints::POST_RECV_MESSAGE); 
        interceptor_methods_.SetRecvMessage(request_, nullptr);
      }

      if (interceptor_methods_.RunInterceptors(
              [this]() { ContinueRunAfterInterception(); })) {
        ContinueRunAfterInterception();
      } else {
        // There were interceptors to be run, so ContinueRunAfterInterception
        // will be run when interceptors are done.
      }
    }

    void ContinueRunAfterInterception() {
      {
        ctx_.BeginCompletionOp(&call_, nullptr, nullptr);
        global_callbacks_->PreSynchronousRequest(&ctx_);
        auto* handler = resources_ ? method_->handler()
                                   : server_->resource_exhausted_handler_.get();
        handler->RunHandler(grpc::internal::MethodHandler::HandlerParameter( 
            &call_, &ctx_, request_, request_status_, nullptr, nullptr)); 
        request_ = nullptr;
        global_callbacks_->PostSynchronousRequest(&ctx_);

        cq_.Shutdown();

        grpc::internal::CompletionQueueTag* op_tag = ctx_.GetCompletionOpTag(); 
        cq_.TryPluck(op_tag, gpr_inf_future(GPR_CLOCK_REALTIME));

        /* Ensure the cq_ is shutdown */
        grpc::DummyTag ignored_tag; 
        GPR_ASSERT(cq_.Pluck(&ignored_tag) == false);
      }
      delete this;
    }

   private:
    grpc::CompletionQueue cq_; 
    grpc::ServerContext ctx_; 
    const bool has_request_payload_;
    grpc_byte_buffer* request_payload_;
    void* request_;
    grpc::Status request_status_; 
    grpc::internal::RpcServiceMethod* const method_; 
    grpc::internal::Call call_; 
    Server* server_;
    std::shared_ptr<GlobalCallbacks> global_callbacks_;
    bool resources_;
    grpc::internal::InterceptorBatchMethodsImpl interceptor_methods_; 
  };

 private:
  grpc::internal::RpcServiceMethod* const method_; 
  void* const method_tag_;
  bool in_flight_;
  const bool has_request_payload_;
  grpc_call* call_;
  grpc_call_details* call_details_;
  gpr_timespec deadline_;
  grpc_metadata_array request_metadata_;
  grpc_byte_buffer* request_payload_;
  grpc_completion_queue* cq_;
};

template <class ServerContextType> 
class Server::CallbackRequest final
    : public grpc::internal::CompletionQueueTag {
 public: 
  static_assert(
      std::is_base_of<grpc::CallbackServerContext, ServerContextType>::value,
      "ServerContextType must be derived from CallbackServerContext");
 
  // For codegen services, the value of method represents the defined
  // characteristics of the method being requested. For generic services, method
  // is nullptr since these services don't have pre-defined methods.
  CallbackRequest(Server* server, grpc::internal::RpcServiceMethod* method,
                  grpc::CompletionQueue* cq,
                  grpc_core::Server::RegisteredCallAllocation* data)
      : server_(server),
        method_(method),
        has_request_payload_(method->method_type() ==
                                 grpc::internal::RpcMethod::NORMAL_RPC ||
                             method->method_type() ==
                                 grpc::internal::RpcMethod::SERVER_STREAMING),
        cq_(cq),
        tag_(this) {
    CommonSetup(server, data);
    data->deadline = &deadline_;
    data->optional_payload = has_request_payload_ ? &request_payload_ : nullptr;
  }

  // For generic services, method is nullptr since these services don't have
  // pre-defined methods.
  CallbackRequest(Server* server, grpc::CompletionQueue* cq,
                  grpc_core::Server::BatchCallAllocation* data)
      : server_(server),
        method_(nullptr),
        has_request_payload_(false),
        call_details_(new grpc_call_details),
        cq_(cq),
        tag_(this) {
    CommonSetup(server, data);
    grpc_call_details_init(call_details_);
    data->details = call_details_;
  }

  ~CallbackRequest() {
    delete call_details_;
    grpc_metadata_array_destroy(&request_metadata_);
    if (has_request_payload_ && request_payload_) {
      grpc_byte_buffer_destroy(request_payload_);
    }
    server_->UnrefWithPossibleNotify();
  }

  // Needs specialization to account for different processing of metadata 
  // in generic API 
  bool FinalizeResult(void** tag, bool* status) override; 

 private:
  // method_name needs to be specialized between named method and generic 
  const char* method_name() const; 
 
  class CallbackCallTag : public grpc_experimental_completion_queue_functor {
   public:
    CallbackCallTag(Server::CallbackRequest<ServerContextType>* req) 
        : req_(req) { 
      functor_run = &CallbackCallTag::StaticRun;
      // Set inlineable to true since this callback is internally-controlled 
      // without taking any locks, and thus does not need to be run from the 
      // executor (which triggers a thread hop). This should only be used by 
      // internal callbacks like this and not by user application code. The work 
      // here is actually non-trivial, but there is no chance of having user 
      // locks conflict with each other so it's ok to run inlined. 
      inlineable = true; 
    }

    // force_run can not be performed on a tag if operations using this tag
    // have been sent to PerformOpsOnCall. It is intended for error conditions
    // that are detected before the operations are internally processed.
    void force_run(bool ok) { Run(ok); }

   private:
    Server::CallbackRequest<ServerContextType>* req_; 
    grpc::internal::Call* call_; 

    static void StaticRun(grpc_experimental_completion_queue_functor* cb,
                          int ok) {
      static_cast<CallbackCallTag*>(cb)->Run(static_cast<bool>(ok));
    }
    void Run(bool ok) {
      void* ignored = req_;
      bool new_ok = ok;
      GPR_ASSERT(!req_->FinalizeResult(&ignored, &new_ok));
      GPR_ASSERT(ignored == req_);

      if (!ok) {
        // The call has been shutdown.
        // Delete its contents to free up the request.
        delete req_;
        return;
      }

      // Bind the call, deadline, and metadata from what we got
      req_->ctx_.set_call(req_->call_);
      req_->ctx_.cq_ = req_->cq_;
      req_->ctx_.BindDeadlineAndMetadata(req_->deadline_,
                                         &req_->request_metadata_);
      req_->request_metadata_.count = 0;

      // Create a C++ Call to control the underlying core call
      call_ = 
          new (grpc_call_arena_alloc(req_->call_, sizeof(grpc::internal::Call))) 
              grpc::internal::Call( 
                  req_->call_, req_->server_, req_->cq_, 
                  req_->server_->max_receive_message_size(), 
                  req_->ctx_.set_server_rpc_info( 
                      req_->method_name(), 
                      (req_->method_ != nullptr) 
                          ? req_->method_->method_type() 
                          : grpc::internal::RpcMethod::BIDI_STREAMING, 
                      req_->server_->interceptor_creators_)); 

      req_->interceptor_methods_.SetCall(call_);
      req_->interceptor_methods_.SetReverse();
      // Set interception point for RECV INITIAL METADATA
      req_->interceptor_methods_.AddInterceptionHookPoint(
          grpc::experimental::InterceptionHookPoints:: 
              POST_RECV_INITIAL_METADATA); 
      req_->interceptor_methods_.SetRecvInitialMetadata(
          &req_->ctx_.client_metadata_);

      if (req_->has_request_payload_) {
        // Set interception point for RECV MESSAGE
        req_->request_ = req_->method_->handler()->Deserialize(
            req_->call_, req_->request_payload_, &req_->request_status_, 
            &req_->handler_data_); 
        req_->request_payload_ = nullptr;
        req_->interceptor_methods_.AddInterceptionHookPoint(
            grpc::experimental::InterceptionHookPoints::POST_RECV_MESSAGE); 
        req_->interceptor_methods_.SetRecvMessage(req_->request_, nullptr);
      }

      if (req_->interceptor_methods_.RunInterceptors(
              [this] { ContinueRunAfterInterception(); })) {
        ContinueRunAfterInterception();
      } else {
        // There were interceptors to be run, so ContinueRunAfterInterception
        // will be run when interceptors are done.
      }
    }
    void ContinueRunAfterInterception() {
      auto* handler = (req_->method_ != nullptr) 
                          ? req_->method_->handler() 
                          : req_->server_->generic_handler_.get(); 
      handler->RunHandler(grpc::internal::MethodHandler::HandlerParameter( 
          call_, &req_->ctx_, req_->request_, req_->request_status_, 
          req_->handler_data_, [this] { delete req_; }));
    }
  };

  template <class CallAllocation>
  void CommonSetup(Server* server, CallAllocation* data) {
    server->Ref();
    grpc_metadata_array_init(&request_metadata_);
    data->tag = &tag_;
    data->call = &call_;
    data->initial_metadata = &request_metadata_;
  }

  Server* const server_;
  grpc::internal::RpcServiceMethod* const method_; 
  const bool has_request_payload_;
  grpc_byte_buffer* request_payload_ = nullptr;
  void* request_ = nullptr;
  void* handler_data_ = nullptr;
  grpc::Status request_status_; 
  grpc_call_details* const call_details_ = nullptr;
  grpc_call* call_;
  gpr_timespec deadline_;
  grpc_metadata_array request_metadata_;
  grpc::CompletionQueue* const cq_;
  CallbackCallTag tag_;
  ServerContextType ctx_; 
  grpc::internal::InterceptorBatchMethodsImpl interceptor_methods_; 
};

template <> 
bool Server::CallbackRequest<grpc::CallbackServerContext>::FinalizeResult(
    void** /*tag*/, bool* /*status*/) {
  return false; 
} 
 
template <> 
bool Server::CallbackRequest<
    grpc::GenericCallbackServerContext>::FinalizeResult(void** /*tag*/,
                                                        bool* status) {
  if (*status) { 
    deadline_ = call_details_->deadline;
    // TODO(yangg) remove the copy here 
    ctx_.method_ = grpc::StringFromCopiedSlice(call_details_->method); 
    ctx_.host_ = grpc::StringFromCopiedSlice(call_details_->host); 
  } 
  grpc_slice_unref(call_details_->method); 
  grpc_slice_unref(call_details_->host); 
  return false; 
} 
 
template <> 
const char* Server::CallbackRequest<grpc::CallbackServerContext>::method_name()
    const {
  return method_->name(); 
} 
 
template <> 
const char* Server::CallbackRequest< 
    grpc::GenericCallbackServerContext>::method_name() const {
  return ctx_.method().c_str(); 
} 
 
// Implementation of ThreadManager. Each instance of SyncRequestThreadManager
// manages a pool of threads that poll for incoming Sync RPCs and call the
// appropriate RPC handlers
class Server::SyncRequestThreadManager : public grpc::ThreadManager { 
 public:
  SyncRequestThreadManager(Server* server, grpc::CompletionQueue* server_cq, 
                           std::shared_ptr<GlobalCallbacks> global_callbacks,
                           grpc_resource_quota* rq, int min_pollers,
                           int max_pollers, int cq_timeout_msec)
      : ThreadManager("SyncServer", rq, min_pollers, max_pollers),
        server_(server),
        server_cq_(server_cq),
        cq_timeout_msec_(cq_timeout_msec),
        global_callbacks_(std::move(global_callbacks)) {}

  WorkStatus PollForWork(void** tag, bool* ok) override {
    *tag = nullptr;
    // TODO(ctiller): workaround for GPR_TIMESPAN based deadlines not working
    // right now
    gpr_timespec deadline =
        gpr_time_add(gpr_now(GPR_CLOCK_MONOTONIC),
                     gpr_time_from_millis(cq_timeout_msec_, GPR_TIMESPAN));

    switch (server_cq_->AsyncNext(tag, ok, deadline)) {
      case grpc::CompletionQueue::TIMEOUT: 
        return TIMEOUT;
      case grpc::CompletionQueue::SHUTDOWN: 
        return SHUTDOWN;
      case grpc::CompletionQueue::GOT_EVENT: 
        return WORK_FOUND;
    }

    GPR_UNREACHABLE_CODE(return TIMEOUT);
  }

  void DoWork(void* tag, bool ok, bool resources) override {
    SyncRequest* sync_req = static_cast<SyncRequest*>(tag);

    if (!sync_req) {
      // No tag. Nothing to work on. This is an unlikley scenario and possibly a
      // bug in RPC Manager implementation.
      gpr_log(GPR_ERROR, "Sync server. DoWork() was called with NULL tag");
      return;
    }

    if (ok) {
      // Calldata takes ownership of the completion queue and interceptors
      // inside sync_req
      auto* cd = new SyncRequest::CallData(server_, sync_req);
      // Prepare for the next request
      if (!IsShutdown()) {
        sync_req->SetupRequest();  // Create new completion queue for sync_req
        sync_req->Request(server_->c_server(), server_cq_->cq());
      }

      GPR_TIMER_SCOPE("cd.Run()", 0);
      cd->Run(global_callbacks_, resources);
    }
    // TODO (sreek) If ok is false here (which it isn't in case of
    // grpc_request_registered_call), we should still re-queue the request
    // object
  }

  void AddSyncMethod(grpc::internal::RpcServiceMethod* method, void* tag) { 
    sync_requests_.emplace_back(new SyncRequest(method, tag));
  }

  void AddUnknownSyncMethod() {
    if (!sync_requests_.empty()) {
      unknown_method_.reset(new grpc::internal::RpcServiceMethod( 
          "unknown", grpc::internal::RpcMethod::BIDI_STREAMING, 
          new grpc::internal::UnknownMethodHandler)); 
      sync_requests_.emplace_back(
          new SyncRequest(unknown_method_.get(), nullptr));
    }
  }

  void Shutdown() override {
    ThreadManager::Shutdown();
    server_cq_->Shutdown();
  }

  void Wait() override {
    ThreadManager::Wait();
    // Drain any pending items from the queue
    void* tag;
    bool ok;
    while (server_cq_->Next(&tag, &ok)) {
      if (ok) {
        // If a request was pulled off the queue, it means that the thread
        // handling the request added it to the completion queue after shutdown
        // was called - because the thread had already started and checked the
        // shutdown flag before shutdown was called. In this case, we simply
        // clean it up here, *after* calling wait on all the worker threads, at
        // which point we are certain no in-flight requests will add more to the
        // queue. This fixes an intermittent memory leak on shutdown.
        SyncRequest* sync_req = static_cast<SyncRequest*>(tag);
        sync_req->PostShutdownCleanup();
      }
    }
  }

  void Start() {
    if (!sync_requests_.empty()) {
      for (const auto& value : sync_requests_) { 
        value->SetupRequest(); 
        value->Request(server_->c_server(), server_cq_->cq()); 
      }

      Initialize();  // ThreadManager's Initialize()
    }
  }

 private:
  Server* server_;
  grpc::CompletionQueue* server_cq_; 
  int cq_timeout_msec_;
  std::vector<std::unique_ptr<SyncRequest>> sync_requests_;
  std::unique_ptr<grpc::internal::RpcServiceMethod> unknown_method_; 
  std::shared_ptr<Server::GlobalCallbacks> global_callbacks_;
};

static grpc::internal::GrpcLibraryInitializer g_gli_initializer; 
Server::Server(
    grpc::ChannelArguments* args,
    std::shared_ptr<std::vector<std::unique_ptr<grpc::ServerCompletionQueue>>> 
        sync_server_cqs,
    int min_pollers, int max_pollers, int sync_cq_timeout_msec,
    std::vector<std::shared_ptr<grpc::internal::ExternalConnectionAcceptorImpl>> 
        acceptors, 
    grpc_resource_quota* server_rq,
    std::vector<
        std::unique_ptr<grpc::experimental::ServerInterceptorFactoryInterface>> 
        interceptor_creators)
    : acceptors_(std::move(acceptors)), 
      interceptor_creators_(std::move(interceptor_creators)), 
      max_receive_message_size_(INT_MIN),
      sync_server_cqs_(std::move(sync_server_cqs)),
      started_(false),
      shutdown_(false),
      shutdown_notified_(false),
      server_(nullptr),
      server_initializer_(new ServerInitializer(this)),
      health_check_service_disabled_(false) {
  g_gli_initializer.summon();
  gpr_once_init(&grpc::g_once_init_callbacks, grpc::InitGlobalCallbacks); 
  global_callbacks_ = grpc::g_callbacks; 
  global_callbacks_->UpdateArguments(args);

  if (sync_server_cqs_ != nullptr) {
    bool default_rq_created = false;
    if (server_rq == nullptr) {
      server_rq = grpc_resource_quota_create("SyncServer-default-rq");
      grpc_resource_quota_set_max_threads(server_rq,
                                          DEFAULT_MAX_SYNC_SERVER_THREADS);
      default_rq_created = true;
    }

    for (const auto& it : *sync_server_cqs_) {
      sync_req_mgrs_.emplace_back(new SyncRequestThreadManager(
          this, it.get(), global_callbacks_, server_rq, min_pollers,
          max_pollers, sync_cq_timeout_msec));
    }

    if (default_rq_created) {
      grpc_resource_quota_unref(server_rq);
    }
  }

  for (auto& acceptor : acceptors_) { 
    acceptor->SetToChannelArgs(args); 
  } 
 
  grpc_channel_args channel_args;
  args->SetChannelArgs(&channel_args);

  for (size_t i = 0; i < channel_args.num_args; i++) {
    if (0 == strcmp(channel_args.args[i].key, 
                    grpc::kHealthCheckServiceInterfaceArg)) { 
      if (channel_args.args[i].value.pointer.p == nullptr) {
        health_check_service_disabled_ = true;
      } else {
        health_check_service_.reset( 
            static_cast<grpc::HealthCheckServiceInterface*>( 
                channel_args.args[i].value.pointer.p)); 
      }
    }
    if (0 ==
        strcmp(channel_args.args[i].key, GRPC_ARG_MAX_RECEIVE_MESSAGE_LENGTH)) {
      max_receive_message_size_ = channel_args.args[i].value.integer;
    }
  }
  server_ = grpc_server_create(&channel_args, nullptr);
}

Server::~Server() {
  {
    grpc::internal::ReleasableMutexLock lock(&mu_); 
    if (started_ && !shutdown_) {
      lock.Unlock(); 
      Shutdown();
    } else if (!started_) {
      // Shutdown the completion queues
      for (const auto& value : sync_req_mgrs_) { 
        value->Shutdown(); 
      }
      if (callback_cq_ != nullptr) { 
        callback_cq_->Shutdown(); 
        callback_cq_ = nullptr; 
      } 
    }
  }
  // Destroy health check service before we destroy the C server so that
  // it does not call grpc_server_request_registered_call() after the C
  // server has been destroyed.
  health_check_service_.reset();
  grpc_server_destroy(server_);
}

void Server::SetGlobalCallbacks(GlobalCallbacks* callbacks) {
  GPR_ASSERT(!grpc::g_callbacks); 
  GPR_ASSERT(callbacks);
  grpc::g_callbacks.reset(callbacks); 
}

grpc_server* Server::c_server() { return server_; }

std::shared_ptr<grpc::Channel> Server::InProcessChannel( 
    const grpc::ChannelArguments& args) { 
  grpc_channel_args channel_args = args.c_channel_args();
  return grpc::CreateChannelInternal( 
      "inproc", grpc_inproc_channel_create(server_, &channel_args, nullptr),
      std::vector<std::unique_ptr< 
          grpc::experimental::ClientInterceptorFactoryInterface>>()); 
}

std::shared_ptr<grpc::Channel> 
Server::experimental_type::InProcessChannelWithInterceptors(
    const grpc::ChannelArguments& args, 
    std::vector<
        std::unique_ptr<grpc::experimental::ClientInterceptorFactoryInterface>> 
        interceptor_creators) {
  grpc_channel_args channel_args = args.c_channel_args();
  return grpc::CreateChannelInternal( 
      "inproc",
      grpc_inproc_channel_create(server_->server_, &channel_args, nullptr),
      std::move(interceptor_creators));
}

static grpc_server_register_method_payload_handling PayloadHandlingForMethod(
    grpc::internal::RpcServiceMethod* method) { 
  switch (method->method_type()) {
    case grpc::internal::RpcMethod::NORMAL_RPC: 
    case grpc::internal::RpcMethod::SERVER_STREAMING: 
      return GRPC_SRM_PAYLOAD_READ_INITIAL_BYTE_BUFFER;
    case grpc::internal::RpcMethod::CLIENT_STREAMING: 
    case grpc::internal::RpcMethod::BIDI_STREAMING: 
      return GRPC_SRM_PAYLOAD_NONE;
  }
  GPR_UNREACHABLE_CODE(return GRPC_SRM_PAYLOAD_NONE;);
}

bool Server::RegisterService(const TString* host, grpc::Service* service) {
  bool has_async_methods = service->has_async_methods();
  if (has_async_methods) {
    GPR_ASSERT(service->server_ == nullptr &&
               "Can only register an asynchronous service against one server.");
    service->server_ = this;
  }

  const char* method_name = nullptr;

  for (const auto& method : service->methods_) { 
    if (method.get() == nullptr) {  // Handled by generic service if any. 
      continue;
    }

    void* method_registration_tag = grpc_server_register_method(
        server_, method->name(), host ? host->c_str() : nullptr,
        PayloadHandlingForMethod(method.get()), 0); 
    if (method_registration_tag == nullptr) {
      gpr_log(GPR_DEBUG, "Attempt to register %s multiple times",
              method->name());
      return false;
    }

    if (method->handler() == nullptr) {  // Async method without handler
      method->set_server_tag(method_registration_tag);
    } else if (method->api_type() ==
               grpc::internal::RpcServiceMethod::ApiType::SYNC) { 
      for (const auto& value : sync_req_mgrs_) { 
        value->AddSyncMethod(method.get(), method_registration_tag); 
      }
    } else {
      has_callback_methods_ = true;
      grpc::internal::RpcServiceMethod* method_value = method.get();
      grpc::CompletionQueue* cq = CallbackCQ();
      server_->core_server->SetRegisteredMethodAllocator(
          cq->cq(), method_registration_tag, [this, cq, method_value] {
            grpc_core::Server::RegisteredCallAllocation result;
            new CallbackRequest<grpc::CallbackServerContext>(this, method_value,
                                                             cq, &result);
            return result;
          });
    }

    method_name = method->name();
  }

  // Parse service name.
  if (method_name != nullptr) {
    std::stringstream ss(method_name);
    std::string service_name;
    if (std::getline(ss, service_name, '/') &&
        std::getline(ss, service_name, '/')) {
      services_.push_back(service_name.c_str());
    }
  }
  return true;
}

void Server::RegisterAsyncGenericService(grpc::AsyncGenericService* service) { 
  GPR_ASSERT(service->server_ == nullptr &&
             "Can only register an async generic service against one server.");
  service->server_ = this;
  has_async_generic_service_ = true; 
}

void Server::RegisterCallbackGenericService( 
    grpc::CallbackGenericService* service) {
  GPR_ASSERT( 
      service->server_ == nullptr && 
      "Can only register a callback generic service against one server."); 
  service->server_ = this; 
  has_callback_generic_service_ = true; 
  generic_handler_.reset(service->Handler()); 
 
  grpc::CompletionQueue* cq = CallbackCQ();
  server_->core_server->SetBatchMethodAllocator(cq->cq(), [this, cq] {
    grpc_core::Server::BatchCallAllocation result;
    new CallbackRequest<grpc::GenericCallbackServerContext>(this, cq, &result);
    return result;
  });
} 
 
int Server::AddListeningPort(const TString& addr,
                             grpc::ServerCredentials* creds) { 
  GPR_ASSERT(!started_);
  int port = creds->AddPortToServer(addr, server_);
  global_callbacks_->AddPort(this, addr, creds, port);
  return port;
}

void Server::Ref() {
  shutdown_refs_outstanding_.fetch_add(1, std::memory_order_relaxed);
}

void Server::UnrefWithPossibleNotify() {
  if (GPR_UNLIKELY(shutdown_refs_outstanding_.fetch_sub(
                       1, std::memory_order_acq_rel) == 1)) {
    // No refs outstanding means that shutdown has been initiated and no more
    // callback requests are outstanding.
    grpc::internal::MutexLock lock(&mu_);
    GPR_ASSERT(shutdown_);
    shutdown_done_ = true;
    shutdown_done_cv_.Signal();
  }
}

void Server::UnrefAndWaitLocked() {
  if (GPR_UNLIKELY(shutdown_refs_outstanding_.fetch_sub(
                       1, std::memory_order_acq_rel) == 1)) {
    shutdown_done_ = true;
    return;  // no need to wait on CV since done condition already set
  }
  shutdown_done_cv_.WaitUntil(&mu_, [this] { return shutdown_done_; });
}

void Server::Start(grpc::ServerCompletionQueue** cqs, size_t num_cqs) { 
  GPR_ASSERT(!started_);
  global_callbacks_->PreServerStart(this);
  started_ = true;

  // Only create default health check service when user did not provide an
  // explicit one.
  grpc::ServerCompletionQueue* health_check_cq = nullptr; 
  grpc::DefaultHealthCheckService::HealthCheckServiceImpl* 
      default_health_check_service_impl = nullptr;
  if (health_check_service_ == nullptr && !health_check_service_disabled_ &&
      grpc::DefaultHealthCheckServiceEnabled()) { 
    auto* default_hc_service = new grpc::DefaultHealthCheckService; 
    health_check_service_.reset(default_hc_service);
    // We create a non-polling CQ to avoid impacting application
    // performance.  This ensures that we don't introduce thread hops
    // for application requests that wind up on this CQ, which is polled
    // in its own thread.
    health_check_cq = new grpc::ServerCompletionQueue( 
        GRPC_CQ_NEXT, GRPC_CQ_NON_POLLING, nullptr); 
    grpc_server_register_completion_queue(server_, health_check_cq->cq(),
                                          nullptr);
    default_health_check_service_impl =
        default_hc_service->GetHealthCheckService(
            std::unique_ptr<grpc::ServerCompletionQueue>(health_check_cq)); 
    RegisterService(nullptr, default_health_check_service_impl);
  }

  for (auto& acceptor : acceptors_) { 
    acceptor->GetCredentials()->AddPortToServer(acceptor->name(), server_); 
  } 
 
  // If this server uses callback methods, then create a callback generic 
  // service to handle any unimplemented methods using the default reactor 
  // creator 
  if (has_callback_methods_ && !has_callback_generic_service_) {
    unimplemented_service_.reset(new grpc::CallbackGenericService);
    RegisterCallbackGenericService(unimplemented_service_.get()); 
  } 
 
#ifndef NDEBUG
  for (size_t i = 0; i < num_cqs; i++) {
    cq_list_.push_back(cqs[i]);
  }
#endif

  grpc_server_start(server_);

  if (!has_async_generic_service_ && !has_callback_generic_service_) { 
    for (const auto& value : sync_req_mgrs_) { 
      value->AddUnknownSyncMethod(); 
    }

    for (size_t i = 0; i < num_cqs; i++) {
      if (cqs[i]->IsFrequentlyPolled()) {
        new UnimplementedAsyncRequest(this, cqs[i]);
      }
    }
    if (health_check_cq != nullptr) {
      new UnimplementedAsyncRequest(this, health_check_cq);
    }
  }

  // If this server has any support for synchronous methods (has any sync
  // server CQs), make sure that we have a ResourceExhausted handler
  // to deal with the case of thread exhaustion
  if (sync_server_cqs_ != nullptr && !sync_server_cqs_->empty()) {
    resource_exhausted_handler_.reset( 
        new grpc::internal::ResourceExhaustedHandler); 
  }

  for (const auto& value : sync_req_mgrs_) { 
    value->Start(); 
  }

  if (default_health_check_service_impl != nullptr) {
    default_health_check_service_impl->StartServingThread();
  }
 
  for (auto& acceptor : acceptors_) { 
    acceptor->Start(); 
  } 
}

void Server::ShutdownInternal(gpr_timespec deadline) {
  grpc::internal::MutexLock lock(&mu_); 
  if (shutdown_) {
    return;
  }

  shutdown_ = true;

  for (auto& acceptor : acceptors_) { 
    acceptor->Shutdown(); 
  } 
 
  /// The completion queue to use for server shutdown completion notification
  grpc::CompletionQueue shutdown_cq; 
  grpc::ShutdownTag shutdown_tag;  // Dummy shutdown tag 
  grpc_server_shutdown_and_notify(server_, shutdown_cq.cq(), &shutdown_tag);

  shutdown_cq.Shutdown();

  void* tag;
  bool ok;
  grpc::CompletionQueue::NextStatus status = 
      shutdown_cq.AsyncNext(&tag, &ok, deadline);

  // If this timed out, it means we are done with the grace period for a clean
  // shutdown. We should force a shutdown now by cancelling all inflight calls
  if (status == grpc::CompletionQueue::NextStatus::TIMEOUT) { 
    grpc_server_cancel_all_calls(server_);
  }
  // Else in case of SHUTDOWN or GOT_EVENT, it means that the server has
  // successfully shutdown

  // Shutdown all ThreadManagers. This will try to gracefully stop all the
  // threads in the ThreadManagers (once they process any inflight requests)
  for (const auto& value : sync_req_mgrs_) { 
    value->Shutdown();  // ThreadManager's Shutdown() 
  }

  // Wait for threads in all ThreadManagers to terminate
  for (const auto& value : sync_req_mgrs_) { 
    value->Wait(); 
  }

  // Drop the shutdown ref and wait for all other refs to drop as well.
  UnrefAndWaitLocked();

  // Shutdown the callback CQ. The CQ is owned by its own shutdown tag, so it 
  // will delete itself at true shutdown. 
  if (callback_cq_ != nullptr) { 
    callback_cq_->Shutdown(); 
    callback_cq_ = nullptr; 
  } 
 
  // Drain the shutdown queue (if the previous call to AsyncNext() timed out
  // and we didn't remove the tag from the queue yet)
  while (shutdown_cq.Next(&tag, &ok)) {
    // Nothing to be done here. Just ignore ok and tag values
  }

  shutdown_notified_ = true;
  shutdown_cv_.Broadcast(); 

#ifndef NDEBUG
  // Unregister this server with the CQs passed into it by the user so that
  // those can be checked for properly-ordered shutdown.
  for (auto* cq : cq_list_) {
    cq->UnregisterServer(this);
  }
  cq_list_.clear();
#endif
}

void Server::Wait() {
  grpc::internal::MutexLock lock(&mu_); 
  while (started_ && !shutdown_notified_) {
    shutdown_cv_.Wait(&mu_); 
  }
}

void Server::PerformOpsOnCall(grpc::internal::CallOpSetInterface* ops, 
                              grpc::internal::Call* call) { 
  ops->FillOps(call);
}

bool Server::UnimplementedAsyncRequest::FinalizeResult(void** tag,
                                                       bool* status) {
  if (GenericAsyncRequest::FinalizeResult(tag, status)) {
    // We either had no interceptors run or we are done intercepting
    if (*status) {
      // Create a new request/response pair using the server and CQ values
      // stored in this object's base class.
      new UnimplementedAsyncRequest(server_, notification_cq_);
      new UnimplementedAsyncResponse(this);
    } else {
      delete this;
    }
  } else {
    // The tag was swallowed due to interception. We will see it again.
  }
  return false;
}

Server::UnimplementedAsyncResponse::UnimplementedAsyncResponse(
    UnimplementedAsyncRequest* request)
    : request_(request) {
  grpc::Status status(grpc::StatusCode::UNIMPLEMENTED, ""); 
  grpc::internal::UnknownMethodHandler::FillOps(request_->context(), this); 
  request_->stream()->call_.PerformOps(this);
}

grpc::ServerInitializer* Server::initializer() { 
  return server_initializer_.get(); 
} 

grpc::CompletionQueue* Server::CallbackCQ() { 
  // TODO(vjpai): Consider using a single global CQ for the default CQ
  // if there is no explicit per-server CQ registered
  grpc::internal::MutexLock l(&mu_); 
  if (callback_cq_ != nullptr) {
    return callback_cq_;
  }
  auto* shutdown_callback = new grpc::ShutdownCallback;
  callback_cq_ = new grpc::CompletionQueue(grpc_completion_queue_attributes{
      GRPC_CQ_CURRENT_VERSION, GRPC_CQ_CALLBACK, GRPC_CQ_DEFAULT_POLLING,
      shutdown_callback});

  // Transfer ownership of the new cq to its own shutdown callback
  shutdown_callback->TakeCQ(callback_cq_);

  return callback_cq_;
} 

}  // namespace grpc