grpc/include/grpcpp/impl/codegen/client_callback.h

/*
 *
 * 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 GRPCPP_IMPL_CODEGEN_CLIENT_CALLBACK_H
#define GRPCPP_IMPL_CODEGEN_CLIENT_CALLBACK_H

#include <atomic>
#include <functional>

#include <grpcpp/impl/codegen/call.h>
#include <grpcpp/impl/codegen/call_op_set.h>
#include <grpcpp/impl/codegen/callback_common.h>
#include <grpcpp/impl/codegen/channel_interface.h>
#include <grpcpp/impl/codegen/config.h>
#include <grpcpp/impl/codegen/core_codegen_interface.h>
#include <grpcpp/impl/codegen/status.h>

namespace grpc_impl {
class Channel;
class ClientContext;
}  // namespace grpc_impl

namespace grpc {

namespace internal {
class RpcMethod;

/// Perform a callback-based unary call
/// TODO(vjpai): Combine as much as possible with the blocking unary call code
template <class InputMessage, class OutputMessage>
void CallbackUnaryCall(ChannelInterface* channel, const RpcMethod& method,
                       ClientContext* context, const InputMessage* request,
                       OutputMessage* result,
                       std::function<void(Status)> on_completion) {
  CallbackUnaryCallImpl<InputMessage, OutputMessage> x(
      channel, method, context, request, result, on_completion);
}

template <class InputMessage, class OutputMessage>
class CallbackUnaryCallImpl {
 public:
  CallbackUnaryCallImpl(ChannelInterface* channel, const RpcMethod& method,
                        ClientContext* context, const InputMessage* request,
                        OutputMessage* result,
                        std::function<void(Status)> on_completion) {
    CompletionQueue* cq = channel->CallbackCQ();
    GPR_CODEGEN_ASSERT(cq != nullptr);
    Call call(channel->CreateCall(method, context, cq));

    using FullCallOpSet =
        CallOpSet<CallOpSendInitialMetadata, CallOpSendMessage,
                  CallOpRecvInitialMetadata, CallOpRecvMessage<OutputMessage>,
                  CallOpClientSendClose, CallOpClientRecvStatus>;

    auto* ops = new (g_core_codegen_interface->grpc_call_arena_alloc(
        call.call(), sizeof(FullCallOpSet))) FullCallOpSet;

    auto* tag = new (g_core_codegen_interface->grpc_call_arena_alloc(
        call.call(), sizeof(CallbackWithStatusTag)))
        CallbackWithStatusTag(call.call(), on_completion, ops);

    // TODO(vjpai): Unify code with sync API as much as possible
    Status s = ops->SendMessagePtr(request);
    if (!s.ok()) {
      tag->force_run(s);
      return;
    }
    ops->SendInitialMetadata(&context->send_initial_metadata_,
                             context->initial_metadata_flags());
    ops->RecvInitialMetadata(context);
    ops->RecvMessage(result);
    ops->AllowNoMessage();
    ops->ClientSendClose();
    ops->ClientRecvStatus(context, tag->status_ptr());
    ops->set_core_cq_tag(tag);
    call.PerformOps(ops);
  }
};
}  // namespace internal

namespace experimental {

// Forward declarations
template <class Request, class Response>
class ClientBidiReactor;
template <class Response>
class ClientReadReactor;
template <class Request>
class ClientWriteReactor;
class ClientUnaryReactor;

// NOTE: The streaming objects are not actually implemented in the public API.
//       These interfaces are provided for mocking only. Typical applications
//       will interact exclusively with the reactors that they define.
template <class Request, class Response>
class ClientCallbackReaderWriter {
 public:
  virtual ~ClientCallbackReaderWriter() {}
  virtual void StartCall() = 0;
  virtual void Write(const Request* req, WriteOptions options) = 0;
  virtual void WritesDone() = 0;
  virtual void Read(Response* resp) = 0;
  virtual void AddHold(int holds) = 0;
  virtual void RemoveHold() = 0;

 protected:
  void BindReactor(ClientBidiReactor<Request, Response>* reactor) {
    reactor->BindStream(this);
  }
};

template <class Response>
class ClientCallbackReader {
 public:
  virtual ~ClientCallbackReader() {}
  virtual void StartCall() = 0;
  virtual void Read(Response* resp) = 0;
  virtual void AddHold(int holds) = 0;
  virtual void RemoveHold() = 0;

 protected:
  void BindReactor(ClientReadReactor<Response>* reactor) {
    reactor->BindReader(this);
  }
};

template <class Request>
class ClientCallbackWriter {
 public:
  virtual ~ClientCallbackWriter() {}
  virtual void StartCall() = 0;
  void Write(const Request* req) { Write(req, WriteOptions()); }
  virtual void Write(const Request* req, WriteOptions options) = 0;
  void WriteLast(const Request* req, WriteOptions options) {
    Write(req, options.set_last_message());
  }
  virtual void WritesDone() = 0;

  virtual void AddHold(int holds) = 0;
  virtual void RemoveHold() = 0;

 protected:
  void BindReactor(ClientWriteReactor<Request>* reactor) {
    reactor->BindWriter(this);
  }
};

class ClientCallbackUnary {
 public:
  virtual ~ClientCallbackUnary() {}
  virtual void StartCall() = 0;

 protected:
  void BindReactor(ClientUnaryReactor* reactor);
};

// The following classes are the reactor interfaces that are to be implemented
// by the user. They are passed in to the library as an argument to a call on a
// stub (either a codegen-ed call or a generic call). The streaming RPC is
// activated by calling StartCall, possibly after initiating StartRead,
// StartWrite, or AddHold operations on the streaming object. Note that none of
// the classes are pure; all reactions have a default empty reaction so that the
// user class only needs to override those classes that it cares about.
// The reactor must be passed to the stub invocation before any of the below
// operations can be called.

/// \a ClientBidiReactor is the interface for a bidirectional streaming RPC.
template <class Request, class Response>
class ClientBidiReactor {
 public:
  virtual ~ClientBidiReactor() {}

  /// Activate the RPC and initiate any reads or writes that have been Start'ed
  /// before this call. All streaming RPCs issued by the client MUST have
  /// StartCall invoked on them (even if they are canceled) as this call is the
  /// activation of their lifecycle.
  void StartCall() { stream_->StartCall(); }

  /// Initiate a read operation (or post it for later initiation if StartCall
  /// has not yet been invoked).
  ///
  /// \param[out] resp Where to eventually store the read message. Valid when
  ///                  the library calls OnReadDone
  void StartRead(Response* resp) { stream_->Read(resp); }

  /// Initiate a write operation (or post it for later initiation if StartCall
  /// has not yet been invoked).
  ///
  /// \param[in] req The message to be written. The library takes temporary
  ///                ownership until OnWriteDone, at which point the application
  ///                regains ownership of msg.
  void StartWrite(const Request* req) { StartWrite(req, WriteOptions()); }

  /// Initiate/post a write operation with specified options.
  ///
  /// \param[in] req The message to be written. The library takes temporary
  ///                ownership until OnWriteDone, at which point the application
  ///                regains ownership of msg.
  /// \param[in] options The WriteOptions to use for writing this message
  void StartWrite(const Request* req, WriteOptions options) {
    stream_->Write(req, std::move(options));
  }

  /// Initiate/post a write operation with specified options and an indication
  /// that this is the last write (like StartWrite and StartWritesDone, merged).
  /// Note that calling this means that no more calls to StartWrite,
  /// StartWriteLast, or StartWritesDone are allowed.
  ///
  /// \param[in] req The message to be written. The library takes temporary
  ///                ownership until OnWriteDone, at which point the application
  ///                regains ownership of msg.
  /// \param[in] options The WriteOptions to use for writing this message
  void StartWriteLast(const Request* req, WriteOptions options) {
    StartWrite(req, std::move(options.set_last_message()));
  }

  /// Indicate that the RPC will have no more write operations. This can only be
  /// issued once for a given RPC. This is not required or allowed if
  /// StartWriteLast is used since that already has the same implication.
  /// Note that calling this means that no more calls to StartWrite,
  /// StartWriteLast, or StartWritesDone are allowed.
  void StartWritesDone() { stream_->WritesDone(); }

  /// Holds are needed if (and only if) this stream has operations that take
  /// place on it after StartCall but from outside one of the reactions
  /// (OnReadDone, etc). This is _not_ a common use of the streaming API.
  ///
  /// Holds must be added before calling StartCall. If a stream still has a hold
  /// in place, its resources will not be destroyed even if the status has
  /// already come in from the wire and there are currently no active callbacks
  /// outstanding. Similarly, the stream will not call OnDone if there are still
  /// holds on it.
  ///
  /// For example, if a StartRead or StartWrite operation is going to be
  /// initiated from elsewhere in the application, the application should call
  /// AddHold or AddMultipleHolds before StartCall.  If there is going to be,
  /// for example, a read-flow and a write-flow taking place outside the
  /// reactions, then call AddMultipleHolds(2) before StartCall. When the
  /// application knows that it won't issue any more read operations (such as
  /// when a read comes back as not ok), it should issue a RemoveHold(). It
  /// should also call RemoveHold() again after it does StartWriteLast or
  /// StartWritesDone that indicates that there will be no more write ops.
  /// The number of RemoveHold calls must match the total number of AddHold
  /// calls plus the number of holds added by AddMultipleHolds.
  void AddHold() { AddMultipleHolds(1); }
  void AddMultipleHolds(int holds) { stream_->AddHold(holds); }
  void RemoveHold() { stream_->RemoveHold(); }

  /// Notifies the application that all operations associated with this RPC
  /// have completed and provides the RPC status outcome.
  ///
  /// \param[in] s The status outcome of this RPC
  virtual void OnDone(const Status& s) {}

  /// Notifies the application that a read of initial metadata from the
  /// server is done. If the application chooses not to implement this method,
  /// it can assume that the initial metadata has been read before the first
  /// call of OnReadDone or OnDone.
  ///
  /// \param[in] ok Was the initial metadata read successfully? If false, no
  ///               further read-side operation will succeed.
  virtual void OnReadInitialMetadataDone(bool ok) {}

  /// Notifies the application that a StartRead operation completed.
  ///
  /// \param[in] ok Was it successful? If false, no further read-side operation
  ///               will succeed.
  virtual void OnReadDone(bool ok) {}

  /// Notifies the application that a StartWrite operation completed.
  ///
  /// \param[in] ok Was it successful? If false, no further write-side operation
  ///               will succeed.
  virtual void OnWriteDone(bool ok) {}

  /// Notifies the application that a StartWritesDone operation completed. Note
  /// that this is only used on explicit StartWritesDone operations and not for
  /// those that are implicitly invoked as part of a StartWriteLast.
  ///
  /// \param[in] ok Was it successful? If false, the application will later see
  ///               the failure reflected as a bad status in OnDone.
  virtual void OnWritesDoneDone(bool ok) {}

 private:
  friend class ClientCallbackReaderWriter<Request, Response>;
  void BindStream(ClientCallbackReaderWriter<Request, Response>* stream) {
    stream_ = stream;
  }
  ClientCallbackReaderWriter<Request, Response>* stream_;
};

/// \a ClientReadReactor is the interface for a server-streaming RPC.
/// All public methods behave as in ClientBidiReactor.
template <class Response>
class ClientReadReactor {
 public:
  virtual ~ClientReadReactor() {}

  void StartCall() { reader_->StartCall(); }
  void StartRead(Response* resp) { reader_->Read(resp); }

  void AddHold() { AddMultipleHolds(1); }
  void AddMultipleHolds(int holds) { reader_->AddHold(holds); }
  void RemoveHold() { reader_->RemoveHold(); }

  virtual void OnDone(const Status& s) {}
  virtual void OnReadInitialMetadataDone(bool ok) {}
  virtual void OnReadDone(bool ok) {}

 private:
  friend class ClientCallbackReader<Response>;
  void BindReader(ClientCallbackReader<Response>* reader) { reader_ = reader; }
  ClientCallbackReader<Response>* reader_;
};

/// \a ClientWriteReactor is the interface for a client-streaming RPC.
/// All public methods behave as in ClientBidiReactor.
template <class Request>
class ClientWriteReactor {
 public:
  virtual ~ClientWriteReactor() {}

  void StartCall() { writer_->StartCall(); }
  void StartWrite(const Request* req) { StartWrite(req, WriteOptions()); }
  void StartWrite(const Request* req, WriteOptions options) {
    writer_->Write(req, std::move(options));
  }
  void StartWriteLast(const Request* req, WriteOptions options) {
    StartWrite(req, std::move(options.set_last_message()));
  }
  void StartWritesDone() { writer_->WritesDone(); }

  void AddHold() { AddMultipleHolds(1); }
  void AddMultipleHolds(int holds) { writer_->AddHold(holds); }
  void RemoveHold() { writer_->RemoveHold(); }

  virtual void OnDone(const Status& s) {}
  virtual void OnReadInitialMetadataDone(bool ok) {}
  virtual void OnWriteDone(bool ok) {}
  virtual void OnWritesDoneDone(bool ok) {}

 private:
  friend class ClientCallbackWriter<Request>;
  void BindWriter(ClientCallbackWriter<Request>* writer) { writer_ = writer; }
  ClientCallbackWriter<Request>* writer_;
};

/// \a ClientUnaryReactor is a reactor-style interface for a unary RPC.
/// This is _not_ a common way of invoking a unary RPC. In practice, this
/// option should be used only if the unary RPC wants to receive initial
/// metadata without waiting for the response to complete. Most deployments of
/// RPC systems do not use this option, but it is needed for generality.
/// All public methods behave as in ClientBidiReactor.
/// StartCall is included for consistency with the other reactor flavors: even
/// though there are no StartRead or StartWrite operations to queue before the
/// call (that is part of the unary call itself) and there is no reactor object
/// being created as a result of this call, we keep a consistent 2-phase
/// initiation API among all the reactor flavors.
class ClientUnaryReactor {
 public:
  virtual ~ClientUnaryReactor() {}

  void StartCall() { call_->StartCall(); }
  virtual void OnDone(const Status& s) {}
  virtual void OnReadInitialMetadataDone(bool ok) {}

 private:
  friend class ClientCallbackUnary;
  void BindCall(ClientCallbackUnary* call) { call_ = call; }
  ClientCallbackUnary* call_;
};

// Define function out-of-line from class to avoid forward declaration issue
inline void ClientCallbackUnary::BindReactor(ClientUnaryReactor* reactor) {
  reactor->BindCall(this);
}

}  // namespace experimental

namespace internal {

// Forward declare factory classes for friendship
template <class Request, class Response>
class ClientCallbackReaderWriterFactory;
template <class Response>
class ClientCallbackReaderFactory;
template <class Request>
class ClientCallbackWriterFactory;

template <class Request, class Response>
class ClientCallbackReaderWriterImpl
    : public ::grpc::experimental::ClientCallbackReaderWriter<Request,
                                                              Response> {
 public:
  // always allocated against a call arena, no memory free required
  static void operator delete(void* ptr, std::size_t size) {
    assert(size == sizeof(ClientCallbackReaderWriterImpl));
  }

  // This operator should never be called as the memory should be freed as part
  // of the arena destruction. It only exists to provide a matching operator
  // delete to the operator new so that some compilers will not complain (see
  // https://github.com/grpc/grpc/issues/11301) Note at the time of adding this
  // there are no tests catching the compiler warning.
  static void operator delete(void*, void*) { assert(0); }

  void MaybeFinish() {
    if (GPR_UNLIKELY(callbacks_outstanding_.fetch_sub(
                         1, std::memory_order_acq_rel) == 1)) {
      Status s = std::move(finish_status_);
      auto* reactor = reactor_;
      auto* call = call_.call();
      this->~ClientCallbackReaderWriterImpl();
      g_core_codegen_interface->grpc_call_unref(call);
      reactor->OnDone(s);
    }
  }

  void StartCall() override {
    // This call initiates two batches, plus any backlog, each with a callback
    // 1. Send initial metadata (unless corked) + recv initial metadata
    // 2. Any read backlog
    // 3. Any write backlog
    // 4. Recv trailing metadata, on_completion callback
    started_ = true;

    start_tag_.Set(call_.call(),
                   [this](bool ok) {
                     reactor_->OnReadInitialMetadataDone(ok);
                     MaybeFinish();
                   },
                   &start_ops_);
    if (!start_corked_) {
      start_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                     context_->initial_metadata_flags());
    }
    start_ops_.RecvInitialMetadata(context_);
    start_ops_.set_core_cq_tag(&start_tag_);
    call_.PerformOps(&start_ops_);

    // Also set up the read and write tags so that they don't have to be set up
    // each time
    write_tag_.Set(call_.call(),
                   [this](bool ok) {
                     reactor_->OnWriteDone(ok);
                     MaybeFinish();
                   },
                   &write_ops_);
    write_ops_.set_core_cq_tag(&write_tag_);

    read_tag_.Set(call_.call(),
                  [this](bool ok) {
                    reactor_->OnReadDone(ok);
                    MaybeFinish();
                  },
                  &read_ops_);
    read_ops_.set_core_cq_tag(&read_tag_);
    if (read_ops_at_start_) {
      call_.PerformOps(&read_ops_);
    }

    if (write_ops_at_start_) {
      call_.PerformOps(&write_ops_);
    }

    if (writes_done_ops_at_start_) {
      call_.PerformOps(&writes_done_ops_);
    }

    finish_tag_.Set(call_.call(), [this](bool ok) { MaybeFinish(); },
                    &finish_ops_);
    finish_ops_.ClientRecvStatus(context_, &finish_status_);
    finish_ops_.set_core_cq_tag(&finish_tag_);
    call_.PerformOps(&finish_ops_);
  }

  void Read(Response* msg) override {
    read_ops_.RecvMessage(msg);
    callbacks_outstanding_.fetch_add(1, std::memory_order_relaxed);
    if (started_) {
      call_.PerformOps(&read_ops_);
    } else {
      read_ops_at_start_ = true;
    }
  }

  void Write(const Request* msg, WriteOptions options) override {
    if (start_corked_) {
      write_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                     context_->initial_metadata_flags());
      start_corked_ = false;
    }

    if (options.is_last_message()) {
      options.set_buffer_hint();
      write_ops_.ClientSendClose();
    }
    // TODO(vjpai): don't assert
    GPR_CODEGEN_ASSERT(write_ops_.SendMessagePtr(msg, options).ok());
    callbacks_outstanding_.fetch_add(1, std::memory_order_relaxed);
    if (started_) {
      call_.PerformOps(&write_ops_);
    } else {
      write_ops_at_start_ = true;
    }
  }
  void WritesDone() override {
    if (start_corked_) {
      writes_done_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                           context_->initial_metadata_flags());
      start_corked_ = false;
    }
    writes_done_ops_.ClientSendClose();
    writes_done_tag_.Set(call_.call(),
                         [this](bool ok) {
                           reactor_->OnWritesDoneDone(ok);
                           MaybeFinish();
                         },
                         &writes_done_ops_);
    writes_done_ops_.set_core_cq_tag(&writes_done_tag_);
    callbacks_outstanding_.fetch_add(1, std::memory_order_relaxed);
    if (started_) {
      call_.PerformOps(&writes_done_ops_);
    } else {
      writes_done_ops_at_start_ = true;
    }
  }

  void AddHold(int holds) override {
    callbacks_outstanding_.fetch_add(holds, std::memory_order_relaxed);
  }
  void RemoveHold() override { MaybeFinish(); }

 private:
  friend class ClientCallbackReaderWriterFactory<Request, Response>;

  ClientCallbackReaderWriterImpl(
      Call call, ClientContext* context,
      ::grpc::experimental::ClientBidiReactor<Request, Response>* reactor)
      : context_(context),
        call_(call),
        reactor_(reactor),
        start_corked_(context_->initial_metadata_corked_) {
    this->BindReactor(reactor);
  }

  ClientContext* const context_;
  Call call_;
  ::grpc::experimental::ClientBidiReactor<Request, Response>* const reactor_;

  CallOpSet<CallOpSendInitialMetadata, CallOpRecvInitialMetadata> start_ops_;
  CallbackWithSuccessTag start_tag_;
  bool start_corked_;

  CallOpSet<CallOpClientRecvStatus> finish_ops_;
  CallbackWithSuccessTag finish_tag_;
  Status finish_status_;

  CallOpSet<CallOpSendInitialMetadata, CallOpSendMessage, CallOpClientSendClose>
      write_ops_;
  CallbackWithSuccessTag write_tag_;
  bool write_ops_at_start_{false};

  CallOpSet<CallOpSendInitialMetadata, CallOpClientSendClose> writes_done_ops_;
  CallbackWithSuccessTag writes_done_tag_;
  bool writes_done_ops_at_start_{false};

  CallOpSet<CallOpRecvMessage<Response>> read_ops_;
  CallbackWithSuccessTag read_tag_;
  bool read_ops_at_start_{false};

  // Minimum of 2 callbacks to pre-register for start and finish
  std::atomic<intptr_t> callbacks_outstanding_{2};
  bool started_{false};
};

template <class Request, class Response>
class ClientCallbackReaderWriterFactory {
 public:
  static void Create(
      ChannelInterface* channel, const ::grpc::internal::RpcMethod& method,
      ClientContext* context,
      ::grpc::experimental::ClientBidiReactor<Request, Response>* reactor) {
    Call call = channel->CreateCall(method, context, channel->CallbackCQ());

    g_core_codegen_interface->grpc_call_ref(call.call());
    new (g_core_codegen_interface->grpc_call_arena_alloc(
        call.call(), sizeof(ClientCallbackReaderWriterImpl<Request, Response>)))
        ClientCallbackReaderWriterImpl<Request, Response>(call, context,
                                                          reactor);
  }
};

template <class Response>
class ClientCallbackReaderImpl
    : public ::grpc::experimental::ClientCallbackReader<Response> {
 public:
  // always allocated against a call arena, no memory free required
  static void operator delete(void* ptr, std::size_t size) {
    assert(size == sizeof(ClientCallbackReaderImpl));
  }

  // This operator should never be called as the memory should be freed as part
  // of the arena destruction. It only exists to provide a matching operator
  // delete to the operator new so that some compilers will not complain (see
  // https://github.com/grpc/grpc/issues/11301) Note at the time of adding this
  // there are no tests catching the compiler warning.
  static void operator delete(void*, void*) { assert(0); }

  void MaybeFinish() {
    if (GPR_UNLIKELY(callbacks_outstanding_.fetch_sub(
                         1, std::memory_order_acq_rel) == 1)) {
      Status s = std::move(finish_status_);
      auto* reactor = reactor_;
      auto* call = call_.call();
      this->~ClientCallbackReaderImpl();
      g_core_codegen_interface->grpc_call_unref(call);
      reactor->OnDone(s);
    }
  }

  void StartCall() override {
    // This call initiates two batches, plus any backlog, each with a callback
    // 1. Send initial metadata (unless corked) + recv initial metadata
    // 2. Any backlog
    // 3. Recv trailing metadata, on_completion callback
    started_ = true;

    start_tag_.Set(call_.call(),
                   [this](bool ok) {
                     reactor_->OnReadInitialMetadataDone(ok);
                     MaybeFinish();
                   },
                   &start_ops_);
    start_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                   context_->initial_metadata_flags());
    start_ops_.RecvInitialMetadata(context_);
    start_ops_.set_core_cq_tag(&start_tag_);
    call_.PerformOps(&start_ops_);

    // Also set up the read tag so it doesn't have to be set up each time
    read_tag_.Set(call_.call(),
                  [this](bool ok) {
                    reactor_->OnReadDone(ok);
                    MaybeFinish();
                  },
                  &read_ops_);
    read_ops_.set_core_cq_tag(&read_tag_);
    if (read_ops_at_start_) {
      call_.PerformOps(&read_ops_);
    }

    finish_tag_.Set(call_.call(), [this](bool ok) { MaybeFinish(); },
                    &finish_ops_);
    finish_ops_.ClientRecvStatus(context_, &finish_status_);
    finish_ops_.set_core_cq_tag(&finish_tag_);
    call_.PerformOps(&finish_ops_);
  }

  void Read(Response* msg) override {
    read_ops_.RecvMessage(msg);
    callbacks_outstanding_.fetch_add(1, std::memory_order_relaxed);
    if (started_) {
      call_.PerformOps(&read_ops_);
    } else {
      read_ops_at_start_ = true;
    }
  }

  void AddHold(int holds) override {
    callbacks_outstanding_.fetch_add(holds, std::memory_order_relaxed);
  }
  void RemoveHold() override { MaybeFinish(); }

 private:
  friend class ClientCallbackReaderFactory<Response>;

  template <class Request>
  ClientCallbackReaderImpl(
      Call call, ClientContext* context, Request* request,
      ::grpc::experimental::ClientReadReactor<Response>* reactor)
      : context_(context), call_(call), reactor_(reactor) {
    this->BindReactor(reactor);
    // TODO(vjpai): don't assert
    GPR_CODEGEN_ASSERT(start_ops_.SendMessagePtr(request).ok());
    start_ops_.ClientSendClose();
  }

  ClientContext* const context_;
  Call call_;
  ::grpc::experimental::ClientReadReactor<Response>* const reactor_;

  CallOpSet<CallOpSendInitialMetadata, CallOpSendMessage, CallOpClientSendClose,
            CallOpRecvInitialMetadata>
      start_ops_;
  CallbackWithSuccessTag start_tag_;

  CallOpSet<CallOpClientRecvStatus> finish_ops_;
  CallbackWithSuccessTag finish_tag_;
  Status finish_status_;

  CallOpSet<CallOpRecvMessage<Response>> read_ops_;
  CallbackWithSuccessTag read_tag_;
  bool read_ops_at_start_{false};

  // Minimum of 2 callbacks to pre-register for start and finish
  std::atomic<intptr_t> callbacks_outstanding_{2};
  bool started_{false};
};

template <class Response>
class ClientCallbackReaderFactory {
 public:
  template <class Request>
  static void Create(
      ChannelInterface* channel, const ::grpc::internal::RpcMethod& method,
      ClientContext* context, const Request* request,
      ::grpc::experimental::ClientReadReactor<Response>* reactor) {
    Call call = channel->CreateCall(method, context, channel->CallbackCQ());

    g_core_codegen_interface->grpc_call_ref(call.call());
    new (g_core_codegen_interface->grpc_call_arena_alloc(
        call.call(), sizeof(ClientCallbackReaderImpl<Response>)))
        ClientCallbackReaderImpl<Response>(call, context, request, reactor);
  }
};

template <class Request>
class ClientCallbackWriterImpl
    : public ::grpc::experimental::ClientCallbackWriter<Request> {
 public:
  // always allocated against a call arena, no memory free required
  static void operator delete(void* ptr, std::size_t size) {
    assert(size == sizeof(ClientCallbackWriterImpl));
  }

  // This operator should never be called as the memory should be freed as part
  // of the arena destruction. It only exists to provide a matching operator
  // delete to the operator new so that some compilers will not complain (see
  // https://github.com/grpc/grpc/issues/11301) Note at the time of adding this
  // there are no tests catching the compiler warning.
  static void operator delete(void*, void*) { assert(0); }

  void MaybeFinish() {
    if (GPR_UNLIKELY(callbacks_outstanding_.fetch_sub(
                         1, std::memory_order_acq_rel) == 1)) {
      Status s = std::move(finish_status_);
      auto* reactor = reactor_;
      auto* call = call_.call();
      this->~ClientCallbackWriterImpl();
      g_core_codegen_interface->grpc_call_unref(call);
      reactor->OnDone(s);
    }
  }

  void StartCall() override {
    // This call initiates two batches, plus any backlog, each with a callback
    // 1. Send initial metadata (unless corked) + recv initial metadata
    // 2. Any backlog
    // 3. Recv trailing metadata, on_completion callback
    started_ = true;

    start_tag_.Set(call_.call(),
                   [this](bool ok) {
                     reactor_->OnReadInitialMetadataDone(ok);
                     MaybeFinish();
                   },
                   &start_ops_);
    if (!start_corked_) {
      start_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                     context_->initial_metadata_flags());
    }
    start_ops_.RecvInitialMetadata(context_);
    start_ops_.set_core_cq_tag(&start_tag_);
    call_.PerformOps(&start_ops_);

    // Also set up the read and write tags so that they don't have to be set up
    // each time
    write_tag_.Set(call_.call(),
                   [this](bool ok) {
                     reactor_->OnWriteDone(ok);
                     MaybeFinish();
                   },
                   &write_ops_);
    write_ops_.set_core_cq_tag(&write_tag_);

    if (write_ops_at_start_) {
      call_.PerformOps(&write_ops_);
    }

    if (writes_done_ops_at_start_) {
      call_.PerformOps(&writes_done_ops_);
    }

    finish_tag_.Set(call_.call(), [this](bool ok) { MaybeFinish(); },
                    &finish_ops_);
    finish_ops_.ClientRecvStatus(context_, &finish_status_);
    finish_ops_.set_core_cq_tag(&finish_tag_);
    call_.PerformOps(&finish_ops_);
  }

  void Write(const Request* msg, WriteOptions options) override {
    if (start_corked_) {
      write_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                     context_->initial_metadata_flags());
      start_corked_ = false;
    }

    if (options.is_last_message()) {
      options.set_buffer_hint();
      write_ops_.ClientSendClose();
    }
    // TODO(vjpai): don't assert
    GPR_CODEGEN_ASSERT(write_ops_.SendMessagePtr(msg, options).ok());
    callbacks_outstanding_.fetch_add(1, std::memory_order_relaxed);
    if (started_) {
      call_.PerformOps(&write_ops_);
    } else {
      write_ops_at_start_ = true;
    }
  }
  void WritesDone() override {
    if (start_corked_) {
      writes_done_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                           context_->initial_metadata_flags());
      start_corked_ = false;
    }
    writes_done_ops_.ClientSendClose();
    writes_done_tag_.Set(call_.call(),
                         [this](bool ok) {
                           reactor_->OnWritesDoneDone(ok);
                           MaybeFinish();
                         },
                         &writes_done_ops_);
    writes_done_ops_.set_core_cq_tag(&writes_done_tag_);
    callbacks_outstanding_.fetch_add(1, std::memory_order_relaxed);
    if (started_) {
      call_.PerformOps(&writes_done_ops_);
    } else {
      writes_done_ops_at_start_ = true;
    }
  }

  void AddHold(int holds) override {
    callbacks_outstanding_.fetch_add(holds, std::memory_order_relaxed);
  }
  void RemoveHold() override { MaybeFinish(); }

 private:
  friend class ClientCallbackWriterFactory<Request>;

  template <class Response>
  ClientCallbackWriterImpl(
      Call call, ClientContext* context, Response* response,
      ::grpc::experimental::ClientWriteReactor<Request>* reactor)
      : context_(context),
        call_(call),
        reactor_(reactor),
        start_corked_(context_->initial_metadata_corked_) {
    this->BindReactor(reactor);
    finish_ops_.RecvMessage(response);
    finish_ops_.AllowNoMessage();
  }

  ClientContext* const context_;
  Call call_;
  ::grpc::experimental::ClientWriteReactor<Request>* const reactor_;

  CallOpSet<CallOpSendInitialMetadata, CallOpRecvInitialMetadata> start_ops_;
  CallbackWithSuccessTag start_tag_;
  bool start_corked_;

  CallOpSet<CallOpGenericRecvMessage, CallOpClientRecvStatus> finish_ops_;
  CallbackWithSuccessTag finish_tag_;
  Status finish_status_;

  CallOpSet<CallOpSendInitialMetadata, CallOpSendMessage, CallOpClientSendClose>
      write_ops_;
  CallbackWithSuccessTag write_tag_;
  bool write_ops_at_start_{false};

  CallOpSet<CallOpSendInitialMetadata, CallOpClientSendClose> writes_done_ops_;
  CallbackWithSuccessTag writes_done_tag_;
  bool writes_done_ops_at_start_{false};

  // Minimum of 2 callbacks to pre-register for start and finish
  std::atomic<intptr_t> callbacks_outstanding_{2};
  bool started_{false};
};

template <class Request>
class ClientCallbackWriterFactory {
 public:
  template <class Response>
  static void Create(
      ChannelInterface* channel, const ::grpc::internal::RpcMethod& method,
      ClientContext* context, Response* response,
      ::grpc::experimental::ClientWriteReactor<Request>* reactor) {
    Call call = channel->CreateCall(method, context, channel->CallbackCQ());

    g_core_codegen_interface->grpc_call_ref(call.call());
    new (g_core_codegen_interface->grpc_call_arena_alloc(
        call.call(), sizeof(ClientCallbackWriterImpl<Request>)))
        ClientCallbackWriterImpl<Request>(call, context, response, reactor);
  }
};

class ClientCallbackUnaryImpl final
    : public ::grpc::experimental::ClientCallbackUnary {
 public:
  // always allocated against a call arena, no memory free required
  static void operator delete(void* ptr, std::size_t size) {
    assert(size == sizeof(ClientCallbackUnaryImpl));
  }

  // This operator should never be called as the memory should be freed as part
  // of the arena destruction. It only exists to provide a matching operator
  // delete to the operator new so that some compilers will not complain (see
  // https://github.com/grpc/grpc/issues/11301) Note at the time of adding this
  // there are no tests catching the compiler warning.
  static void operator delete(void*, void*) { assert(0); }

  void StartCall() override {
    // This call initiates two batches, each with a callback
    // 1. Send initial metadata + write + writes done + recv initial metadata
    // 2. Read message, recv trailing metadata
    started_ = true;

    start_tag_.Set(call_.call(),
                   [this](bool ok) {
                     reactor_->OnReadInitialMetadataDone(ok);
                     MaybeFinish();
                   },
                   &start_ops_);
    start_ops_.SendInitialMetadata(&context_->send_initial_metadata_,
                                   context_->initial_metadata_flags());
    start_ops_.RecvInitialMetadata(context_);
    start_ops_.set_core_cq_tag(&start_tag_);
    call_.PerformOps(&start_ops_);

    finish_tag_.Set(call_.call(), [this](bool ok) { MaybeFinish(); },
                    &finish_ops_);
    finish_ops_.ClientRecvStatus(context_, &finish_status_);
    finish_ops_.set_core_cq_tag(&finish_tag_);
    call_.PerformOps(&finish_ops_);
  }

  void MaybeFinish() {
    if (GPR_UNLIKELY(callbacks_outstanding_.fetch_sub(
                         1, std::memory_order_acq_rel) == 1)) {
      Status s = std::move(finish_status_);
      auto* reactor = reactor_;
      auto* call = call_.call();
      this->~ClientCallbackUnaryImpl();
      g_core_codegen_interface->grpc_call_unref(call);
      reactor->OnDone(s);
    }
  }

 private:
  friend class ClientCallbackUnaryFactory;

  template <class Request, class Response>
  ClientCallbackUnaryImpl(Call call, ClientContext* context, Request* request,
                          Response* response,
                          ::grpc::experimental::ClientUnaryReactor* reactor)
      : context_(context), call_(call), reactor_(reactor) {
    this->BindReactor(reactor);
    // TODO(vjpai): don't assert
    GPR_CODEGEN_ASSERT(start_ops_.SendMessagePtr(request).ok());
    start_ops_.ClientSendClose();
    finish_ops_.RecvMessage(response);
    finish_ops_.AllowNoMessage();
  }

  ClientContext* const context_;
  Call call_;
  ::grpc::experimental::ClientUnaryReactor* const reactor_;

  CallOpSet<CallOpSendInitialMetadata, CallOpSendMessage, CallOpClientSendClose,
            CallOpRecvInitialMetadata>
      start_ops_;
  CallbackWithSuccessTag start_tag_;

  CallOpSet<CallOpGenericRecvMessage, CallOpClientRecvStatus> finish_ops_;
  CallbackWithSuccessTag finish_tag_;
  Status finish_status_;

  // This call will have 2 callbacks: start and finish
  std::atomic<intptr_t> callbacks_outstanding_{2};
  bool started_{false};
};

class ClientCallbackUnaryFactory {
 public:
  template <class Request, class Response>
  static void Create(ChannelInterface* channel,
                     const ::grpc::internal::RpcMethod& method,
                     ClientContext* context, const Request* request,
                     Response* response,
                     ::grpc::experimental::ClientUnaryReactor* reactor) {
    Call call = channel->CreateCall(method, context, channel->CallbackCQ());

    g_core_codegen_interface->grpc_call_ref(call.call());

    new (g_core_codegen_interface->grpc_call_arena_alloc(
        call.call(), sizeof(ClientCallbackUnaryImpl)))
        ClientCallbackUnaryImpl(call, context, request, response, reactor);
  }
};

}  // namespace internal
}  // namespace grpc

#endif  // GRPCPP_IMPL_CODEGEN_CLIENT_CALLBACK_H