/* Copyright 2019 The TensorFlow Authors. All Rights Reserved.

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 TENSORFLOW_CORE_DISTRIBUTED_RUNTIME_EAGER_REMOTE_COPY_NODE_H_
#define TENSORFLOW_CORE_DISTRIBUTED_RUNTIME_EAGER_REMOTE_COPY_NODE_H_

#include "tensorflow/core/common_runtime/device.h"
#include "tensorflow/core/common_runtime/eager/eager_executor.h"
#include "tensorflow/core/common_runtime/eager/eager_operation.h"
#include "tensorflow/core/common_runtime/eager/tensor_handle.h"
#include "tensorflow/core/framework/cancellation.h"
#include "tensorflow/core/framework/tensor.h"
#include "tensorflow/core/lib/core/status.h"

namespace tensorflow {
namespace eager {

// This node supports copying a tensor in the following way:
// - Remote -> Local:
//   We don't block on the remote _Send op and start executing the local
//   _Recv immediately after issuing the remote _Send. The local _Recv
//   kernel (or rather the special _Recv handling in KernelAndDeviceOp::Run)
//   blocks until the tensor is received. If the remote _Send (or some op
//   before it) fails, the local callback we give to EnqueueAsync will run
//   and call CancellationManager.StartCancel(). The blocked local _Recv will
//   get this notification and return with a cancelled error.
//
// - Local -> Remote:
//   The local _Send op is synchronous and non-blocking, thus it should complete
//   quickly. We issue remote _Recv RPC only after local _Send completes
//   successfully. At this point, the tensor to be sent is in the local
//   Rendezvous, hence, remote _Recv op will not deadlock waiting for the tensor
//   to appear.
//
// - Remote -> Remote:
//   We could issue both remote ops asynchronously, but if remote _Send (or some
//   op before it) fails, we don't have a good way of cancelling the remote
//   _Recv. The remote _Recv will deadlock in this case. The current approach
//   to deal with this issue is to wait for remote _Send to complete before
//   issuing remote _Recv RPC. Another option is to close the whole streaming
//   RPC that contains the deadlocked remote _Recv. This would not unblock the
//   deadlocked RPC on the remote machine without some extra code. Luckily, the
//   remote -> remote case seems to be fairly rare at this point. So, the
//   current partially synchronous approach seems fine.
//
// To copy a tensor within a host, please use copy_to_device_node instead.
class RemoteCopyNode : public EagerNode {
 public:
  RemoteCopyNode(EagerContext* ctx, EagerExecutor* executor, TensorHandle* src,
                 TensorHandle* dst, Device* recv_device, uint64 recv_op_id);

  ~RemoteCopyNode() override {}

  Status Run() override;

  void Abort(Status status) override;

 private:
  // Runs the _Send operation locally or remotely.
  // An error return value indicates that _Send did not run successfully.
  // An OK return value does NOT necessarily indicate that _Send has completed
  // successfully. It might still fail after this method returns.
  // StartSend() makes sure that captured_state_->send_status_ is set to the
  // final _Send status after captured_state->send_done_.WaitForNotification()
  // returns.
  Status StartSend();

  // Synchronously runs local send `op` and returns its status.
  Status RunLocalSend(EagerOperation* op);

  // Runs the _Recv operation locally or remotely.
  // An error return value indicates that _Recv did not run successfully. It
  // does not indicate that _Send op has completed since StartRecv could have
  // encountered an error before waiting for _Send's completion.
  // An OK return value does NOT necessarily indicate that _Recv has completed
  // successfully (it does now, but won't when streaming RPCs are turned on).
  // StartRecv() makes sure that dst_ tensor handle is handled correctly
  // (potentially after this methods returns); a tensor is set in the local
  // case, a remote shape is set in the remote case, the dst_ handle is
  // poisoned in either case if there is an error.
  Status StartRecv();

  // Synchronously runs local receive `op` and returns its status.
  // Does not wait for the send to complete before running receive.
  Status RunLocalRecv(EagerOperation* op, std::vector<Tensor>* outputs);

  // Waits for send to complete, then issues remote receive `op` and
  // returns its status.
  Status RunRemoteRecv(EagerOperation* op);

  // State that is captured by Send and/or Recv callbacks (depending on which
  // one(s) is remote) and outlives this node in the case of remote->remote
  // copy.
  class CapturedSharedState {
   public:
    explicit CapturedSharedState(TensorHandle* d) : dst_(d) { dst_->Ref(); }
    ~CapturedSharedState() { dst_->Unref(); }

    void SetSendStatus(Status status) {
      send_status_.Update(status);
      send_done_.Notify();
    }

    Status GetSendStatus() {
      send_done_.WaitForNotification();
      return send_status_;
    }

    TensorHandle* dst() { return dst_; }
    CancellationManager* recv_cancellation() { return &recv_cancellation_; }

   private:
    TensorHandle* const dst_;
    CancellationManager recv_cancellation_;
    // send_status_ is safe to read only after send_done_.WaitForNotification()
    // has returned.
    Status send_status_;
    Notification send_done_;
  };

  TensorHandle* const src_;
  EagerContext* const ctx_;
  EagerExecutor* const executor_;
  Device* const send_device_;
  Device* const recv_device_;
  const string wire_id_;
  const uint64 recv_op_id_;

  std::shared_ptr<CapturedSharedState> captured_state_;
};

}  // namespace eager
}  // namespace tensorflow

#endif  // TENSORFLOW_CORE_DISTRIBUTED_RUNTIME_EAGER_REMOTE_COPY_NODE_H_
