/* Copyright 2017 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.
==============================================================================*/
#include "tensorflow/core/kernels/data/cache_dataset_ops.h"

#include <atomic>
#include <cstddef>
#include <cstdint>
#include <memory>
#include <string>
#include <utility>
#include <vector>

#include "absl/status/status.h"
#include "absl/strings/str_format.h"
#include "tensorflow/core/data/global_shuffle_utils.h"
#include "tensorflow/core/data/name_utils.h"
#include "tensorflow/core/data/serialization_utils.h"
#include "tensorflow/core/framework/dataset.h"
#include "tensorflow/core/framework/dataset_options.pb.h"
#include "tensorflow/core/framework/partial_tensor_shape.h"
#include "tensorflow/core/framework/resource_mgr.h"
#include "tensorflow/core/framework/tensor.h"
#include "tensorflow/core/kernels/data/cache_ops.h"
#include "tensorflow/core/kernels/data/iterator_ops.h"
#include "tensorflow/core/lib/core/errors.h"
#include "tensorflow/core/lib/strings/stringprintf.h"
#include "tensorflow/core/platform/env.h"
#include "tensorflow/core/platform/errors.h"
#include "tensorflow/core/platform/refcount.h"
#include "tensorflow/core/util/tensor_bundle/naming.h"
#include "tensorflow/core/util/tensor_bundle/tensor_bundle.h"

namespace tensorflow {
namespace data {

// See documentation in ../../ops/dataset_ops.cc for a high-level description of
// the following op.

/* static */ constexpr const char* const CacheDatasetOp::kDatasetType;
/* static */ constexpr const char* const CacheDatasetOp::kInputDataset;
/* static */ constexpr const char* const CacheDatasetOp::kFileName;
/* static */ constexpr const char* const CacheDatasetOp::kOutputTypes;
/* static */ constexpr const char* const CacheDatasetOp::kOutputShapes;

namespace {

constexpr char kKeyStrFormat[] = "%%%zuzu_%%%zuzu";
constexpr char kPaddingSizeStrFormat[] = "%zu";
constexpr char kFileDatasetPrefix[] = "File";
constexpr char kMode[] = "Mode";
constexpr char kLockFileSuffix[] = ".lockfile";
constexpr char kIterationCompleted[] = "iteration_completed";
constexpr char kCurIndex[] = "cur_index";
constexpr char kShardId[] = "shard_id";
constexpr char kCreatedAt[] = "Created at";
constexpr char kMemoryDatasetPrefix[] = "Memory";
constexpr char kMemoryCache[] = "MemoryCache";
constexpr char kCacheCompleted[] = "cache_completed";
constexpr char kIndex[] = "index";
constexpr char kImpl[] = "Impl";
constexpr char kCacheDataset[] = "CacheDataset";
constexpr char kIncompleteCacheErrorMessage[] =
    "The calling iterator did not fully read the dataset being cached. In "
    "order to avoid unexpected truncation of the dataset, the partially cached "
    "contents of the dataset  will be discarded. This can happen if you have "
    "an input pipeline similar to `dataset.cache().take(k).repeat()`. You "
    "should use `dataset.take(k).cache().repeat()` instead.";
}  // namespace

class DatasetRandomAccessCache {
 public:
  explicit DatasetRandomAccessCache(const DatasetBase* dataset)
      : input_(dataset) {}

  // Extends the temporary cache up to a given index and then updates
  // out_tensors with the element at that index.
  absl::Status Get(OpKernelContext* ctx, int64_t index,
                   std::vector<Tensor>* out_tensors) {
    if (!iter_resource_) {
      TF_ASSIGN_OR_RETURN(iter_resource_,
                          GetIteratorResourceFromDataset(ctx, input_));
      TF_RETURN_IF_ERROR(iter_resource_->SetIteratorFromDataset(ctx, input_));
    }
    if (index >= cache_.size()) {
      TF_RETURN_IF_ERROR(ExtendTempCacheToIndex(index, ctx));
    }
    *out_tensors = cache_.at(index);
    return absl::OkStatus();
  }

  // Returns the data which has been cached up to this point.
  std::vector<std::vector<Tensor>> GetCacheData() { return cache_; }

 private:
  absl::Status ExtendTempCacheToIndex(int64_t index, OpKernelContext* ctx) {
    bool end_of_sequence;
    while (cache_.size() <= index) {
      std::vector<Tensor> out_tensors;
      TF_RETURN_IF_ERROR(
          iter_resource_->GetNext(ctx, &out_tensors, &end_of_sequence));
      if (end_of_sequence) {
        return tensorflow::errors::OutOfRange("Index out of range [0, ",
                                              cache_.size(), "):", index);
      }
      cache_.push_back(out_tensors);
    }
    return absl::OkStatus();
  }

  absl::StatusOr<core::RefCountPtr<IteratorResource>>
  GetIteratorResourceFromDataset(OpKernelContext* ctx,
                                 const DatasetBase* dataset) {
    FunctionLibraryRuntime* flr;
    std::unique_ptr<DeviceMgr> device_mgr(nullptr);
    std::unique_ptr<FunctionLibraryDefinition> flib_def(nullptr);
    std::unique_ptr<ProcessFunctionLibraryRuntime> plfr(nullptr);
    TF_RETURN_IF_ERROR(
        ctx->function_library()->Clone(&flib_def, &plfr, &flr, true));

    core::RefCountPtr<IteratorResource> iter_resource(new IteratorResource(
        ctx->env(), dataset->output_dtypes(), dataset->output_shapes(),
        std::move(device_mgr), std::move(flib_def), std::move(plfr), flr));
    return iter_resource;
  }

  const DatasetBase* input_;  // Not owned.
  core::RefCountPtr<IteratorResource> iter_resource_;
  std::vector<std::vector<Tensor>> cache_;
};

// Caches dataset elements when global shuffling is enabled.
class IteratorRandomAccessCache {
 public:
  explicit IteratorRandomAccessCache(const DatasetBase* input)
      : input_(input) {}

  absl::Status Get(AnyContext ctx, size_t element_position,
                   std::vector<Tensor>* out_tensors) {
    if (element_position < cache_.size() && !cache_[element_position].empty()) {
      *out_tensors = cache_[element_position];
      return absl::OkStatus();
    }

    TF_RETURN_IF_ERROR(input_->Get(ctx, element_position, out_tensors));
    if (element_position >= cache_.size()) {
      cache_.resize(element_position + 1);
    }
    cache_[element_position] = *out_tensors;
    return absl::OkStatus();
  }

 private:
  const DatasetBase* input_ = nullptr;
  std::vector<std::vector<Tensor>> cache_;
};

class CacheDatasetOp::FileDatasetBase : public DatasetBase {
 public:
  FileDatasetBase(OpKernelContext* ctx, const DatasetBase* input,
                  std::string filename, Env* env)
      : DatasetBase(DatasetContext(ctx)),
        input_(input),
        filename_(std::move(filename)),
        env_(env),
        num_tensors_(input->output_dtypes().size()),
        tensor_index_padding_size_(StringPaddingSize(num_tensors_)),
        item_index_padding_size_(StringPaddingSize(kMaxItems)) {
    input_->Ref();
    DCHECK_EQ(item_index_padding_size_, 7);
  }

  ~FileDatasetBase() override { input_->Unref(); }

  std::unique_ptr<IteratorBase> MakeIteratorInternal(
      const std::string& prefix) const override {
    name_utils::IteratorPrefixParams params;
    params.dataset_prefix = kFileDatasetPrefix;
    return std::make_unique<FileIterator>(FileIterator::Params{
        this, name_utils::IteratorPrefix(kDatasetType, prefix, params)});
  }

  const DataTypeVector& output_dtypes() const override {
    return input_->output_dtypes();
  }

  const std::vector<PartialTensorShape>& output_shapes() const override {
    return input_->output_shapes();
  }

  std::string DebugString() const override {
    name_utils::DatasetDebugStringParams params;
    params.dataset_prefix = kFileDatasetPrefix;
    return name_utils::DatasetDebugString(kDatasetType, params);
  }

  int64_t CardinalityInternal(CardinalityOptions options) const override {
    return input_->Cardinality(options);
  }

  absl::Status InputDatasets(
      std::vector<const DatasetBase*>* inputs) const override {
    inputs->push_back(input_);
    return absl::OkStatus();
  }

  absl::Status CheckExternalState() const override {
    return input_->CheckExternalState();
  }

 protected:
  const DatasetBase* const input_;
  const tstring filename_;

 private:
  static size_t StringPaddingSize(size_t num_tensors) {
    return absl::StrFormat(kPaddingSizeStrFormat, num_tensors - 1).size();
  }

  std::string FormatName(size_t item_index, size_t tensor_index) const {
    return absl::StrFormat("%*zu_%*zu", item_index_padding_size_, item_index,
                           tensor_index_padding_size_, tensor_index);
  }

  class FileIterator : public DatasetIterator<FileDatasetBase> {
   public:
    explicit FileIterator(const Params& params)
        : DatasetIterator<FileDatasetBase>(params) {
      if (params.dataset->env_
              ->FileExists(MetaFilename(params.dataset->filename_))
              .ok()) {
        mode_ = Mode::read;
      } else {
        mode_ = Mode::write;
      }
    }

    absl::Status Initialize(IteratorContext* ctx) override {
      mutex_lock l(mu_);
      return InitializeIterator(ctx);
    }

    absl::Status GetNextInternal(IteratorContext* ctx,
                                 std::vector<Tensor>* out_tensors,
                                 bool* end_of_sequence) override {
      mutex_lock l(mu_);
      return iterator_->GetNext(ctx, out_tensors, end_of_sequence);
    }

   protected:
    std::shared_ptr<model::Node> CreateNode(
        IteratorContext* ctx, model::Node::Args args) const override {
      return model::MakeKnownRatioNode(std::move(args),
                                       /*ratio=*/1);
    }

    absl::Status SaveInternal(SerializationContext* ctx,
                              IteratorStateWriter* writer) override {
      mutex_lock l(mu_);
      TF_RETURN_IF_ERROR(writer->WriteScalar(prefix(), kMode, mode_));
      return SaveInput(ctx, writer, iterator_);
    }
    absl::Status RestoreInternal(IteratorContext* ctx,
                                 IteratorStateReader* reader) override {
      mutex_lock l(mu_);
      {
        int64_t temp;
        TF_RETURN_IF_ERROR(reader->ReadScalar(prefix(), kMode, &temp));
        mode_ = static_cast<Mode>(temp);
      }
      if (mode_ == Mode::write &&
          dataset()
              ->env_->FileExists(MetaFilename(dataset()->filename_))
              .ok()) {
        // This could happen if the cache was completely written after the
        // checkpoint was saved.
        LOG(WARNING)
            << "It looks like the cache was already completely written("
            << MetaFilename(dataset()->filename_)
            << ") after the last checkpoint was saved. Attempting to read "
            << "the cache instead of continuing to write. If this is a "
            << "mistake, please remove the above file and try running again.";
        mode_ = Mode::read;
      }
      TF_RETURN_IF_ERROR(InitializeIterator(ctx));
      return RestoreInput(ctx, reader, iterator_);
    }

   private:
    // FileWriterIterator passes through and caches items from the input
    // FileDatasetBase.
    //
    // This iterator is used when the cache directory is not found on disk. It
    // creates the cache directory, and passes on the underlying iterator's
    // elements.
    //
    // Caching is performed by writing the input tensors to disk using the
    // `BundleWriter`. Note that the cache gets fully flushed to disk only
    // after the input iterator has been fully exhausted. If the program
    // exits, before completion of an epoch, the cached state would be lost.
    // To ensure that the partial cache persists across sessions, one should
    // checkpoint the input pipeline. On each call to `SaveInternal` the
    // partial cache gets flushed to disk in files with prefix
    // <filename>_<shard_id> where shard_id is unique for each checkpoint.
    // When all elements have been produced, these shards get coalesced.
    class FileWriterIterator : public DatasetIterator<FileDatasetBase> {
     public:
      explicit FileWriterIterator(const Params& params)
          : DatasetIterator<FileDatasetBase>(params),
            cur_index_(0),
            shard_id_(0),
            filename_(absl::StrCat(params.dataset->filename_, "_", shard_id_)),
            lockfile_(absl::StrCat(filename_, kLockFileSuffix)),
            lockfile_created_(false),
            iteration_completed_(false) {}

      ~FileWriterIterator() override {
        if (!dataset()->env_->FileExists(MetaFilename(filename_)).ok()) {
          LOG(WARNING) << kIncompleteCacheErrorMessage;
          std::vector<std::string> cache_files;
          absl::Status s = dataset()->env_->GetMatchingPaths(
              absl::StrCat(filename_, "*"), &cache_files);
          if (!s.ok()) {
            LOG(WARNING) << "Failed to get matching files on " << filename_
                         << "* : " << s.ToString();
          }
          for (const std::string& path : cache_files) {
            s = dataset()->env_->DeleteFile(path);
            if (!s.ok()) {
              LOG(WARNING) << "Failed to delete " << path << " : "
                           << s.ToString();
            }
          }
        }
      }

      absl::Status Initialize(IteratorContext* ctx) override {
        return dataset()->input_->MakeIterator(ctx, this, prefix(),
                                               &input_impl_);
      }

      absl::Status GetNextInternal(IteratorContext* ctx,
                                   std::vector<Tensor>* out_tensors,
                                   bool* end_of_sequence) override {
        mutex_lock l(mu_);
        *end_of_sequence = false;
        TF_RETURN_IF_ERROR(EnsureLockFileExists(end_of_sequence));
        if (*end_of_sequence) {
          return absl::OkStatus();
        }
        TF_RETURN_IF_ERROR(writer_->status());
        if (cur_index_ >= kMaxItems) {
          // As a courtesy, close the [truncated] cache file.
          absl::Status s = Finish();
          if (!s.ok()) {
            LOG(ERROR) << s;
          }
          return errors::InvalidArgument(
              "Upstream iterator is producing more than ", kMaxItems,
              " items, which is more than the cache limit.");
        }

        TF_RETURN_IF_ERROR(
            input_impl_->GetNext(ctx, out_tensors, end_of_sequence));
        if (*end_of_sequence && out_tensors->empty()) {
          TF_RETURN_IF_ERROR(Finish());
          cur_index_++;
          return absl::OkStatus();
        }
        if (out_tensors->size() != dataset()->num_tensors_) {
          return errors::Internal(
              "Upstream iterator returned invalid number of tensors. "
              "Expected ",
              dataset()->num_tensors_, " got: ", out_tensors->size());
        }
        size_t tensor_index = 0;
        for (const Tensor& t : *out_tensors) {
          DCHECK_LT(tensor_index, dataset()->num_tensors_);
          std::string key = dataset()->FormatName(cur_index_, tensor_index++);
          TF_RETURN_IF_ERROR(writer_->Add(key, t));
        }
        if (*end_of_sequence) {
          TF_RETURN_IF_ERROR(Finish());
        }
        cur_index_++;
        return absl::OkStatus();
      }

     protected:
      std::shared_ptr<model::Node> CreateNode(
          IteratorContext* ctx, model::Node::Args args) const override {
        return model::MakeKnownRatioNode(std::move(args),
                                         /*ratio=*/1);
      }

      absl::Status SaveInternal(SerializationContext* ctx,
                                IteratorStateWriter* writer) override {
        mutex_lock l(mu_);
        TF_RETURN_IF_ERROR(
            writer->WriteScalar(prefix(), kCurIndex, cur_index_));

        if (iteration_completed_) {
          TF_RETURN_IF_ERROR(
              writer->WriteScalar(prefix(), kIterationCompleted, ""));
          return absl::OkStatus();
        }

        // lockfile is created on the first call to GetNextInternal. The
        // absence of a lockfile means that GetNextInternal was not called
        // and hence nothing was written to cache. So we don't need to worry
        // about flushing the current shard. This ensures that we never write
        // empty shards.
        if (lockfile_created_) {
          // Flush the current bundle.
          TF_RETURN_IF_ERROR(writer_->Finish());

          // Note: We do not delete the lockfile here. We keep lockfiles of
          // all shards around until the entire cache has been written to
          // prevent concurrent iterators from corrupting any of the shards.

          // Start caching to a new shard.
          shard_id_++;
          filename_ = absl::StrCat(dataset()->filename_, "_", shard_id_);
          lockfile_ = absl::StrCat(filename_, kLockFileSuffix);
          lockfile_created_ = false;
        }
        TF_RETURN_IF_ERROR(SaveInput(ctx, writer, input_impl_));
        TF_RETURN_IF_ERROR(writer->WriteScalar(prefix(), kShardId, shard_id_));
        return absl::OkStatus();
      }

      absl::Status RestoreInternal(IteratorContext* ctx,
                                   IteratorStateReader* reader) override {
        mutex_lock l(mu_);
        int64_t temp;
        // TODO(b/78048575): Update this when saving size_t tensors directly
        // is supported.
        {
          TF_RETURN_IF_ERROR(reader->ReadScalar(prefix(), kCurIndex, &temp));
          cur_index_ = static_cast<size_t>(temp);
          if (cur_index_ != temp) {
            return errors::Internal("Invalid value for cur_index ", temp);
          }
        }

        if (reader->Contains(prefix(), kIterationCompleted)) {
          iteration_completed_ = true;
          return absl::OkStatus();
        }

        TF_RETURN_IF_ERROR(RestoreInput(ctx, reader, input_impl_));

        // TODO(b/78048575): Update this when saving size_t tensors directly
        // is supported.
        {
          TF_RETURN_IF_ERROR(reader->ReadScalar(prefix(), kShardId, &temp));
          shard_id_ = static_cast<size_t>(temp);
          if (shard_id_ != temp) {
            return errors::Internal("Invalid value for shard_id ", temp);
          }
        }
        filename_ = absl::StrCat(dataset()->filename_, "_", shard_id_);
        lockfile_ = absl::StrCat(filename_, kLockFileSuffix);
        writer_ = std::make_unique<BundleWriter>(dataset()->env_, filename_);
        return absl::OkStatus();
      }

     private:
      absl::Status EnsureLockFileExists(bool* end_of_sequence)
          TF_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
        if (iteration_completed_) {
          *end_of_sequence = true;
          return absl::OkStatus();
        }
        if (lockfile_created_) {
          return absl::OkStatus();
        }

        // Perform rudimentary locking to help catch concurrent writes to the
        // same cache files.

        // 1. Check that a checkpoint for the shard has not already been
        // written.
        if (dataset()->env_->FileExists(MetaFilename(filename_)).ok()) {
          return errors::AlreadyExists("Existing cache files found: \n",
                                       MetaFilename(filename_), "\n",
                                       DataFilename(filename_, 0, 1), "\n",
                                       "To continue delete the above files.");
        }

        // 2. Check that there isn't a concurrent iterator that is writing
        // to cache.
        if (dataset()->env_->FileExists(lockfile_).ok()) {
          // Attempt to read the contents of the lockfile.
          char contents_scratch[151] = {0};  // Initialize all to 0.
          absl::string_view contents;
          std::unique_ptr<RandomAccessFile> file;
          if (dataset()->env_->NewRandomAccessFile(lockfile_, &file).ok()) {
            file->Read(0, contents, absl::MakeSpan(contents_scratch, 150))
                .IgnoreError();
          }
          return errors::AlreadyExists(
              "There appears to be a concurrent caching iterator running - "
              "cache lockfile already exists ('",
              lockfile_,
              "'). If you are sure no other running TF computations are "
              "using this cache prefix, delete the lockfile and "
              "re-initialize the iterator. Lockfile contents: ",
              contents);
        }
        // Create the file, and write some basic contents.
        std::unique_ptr<WritableFile> lockfile;
        TF_RETURN_IF_ERROR(
            dataset()->env_->NewWritableFile(lockfile_, &lockfile));
        TF_RETURN_IF_ERROR(lockfile->Append(
            absl::StrCat(kCreatedAt, ": ", EnvTime::NowSeconds())));

        // At this point we know that
        // 1. There is no conflicting checkpoint with prefix `filename_`.
        // 2. There is no concurrent session that is trying to write a ckpt
        //    to filename.
        // So it is safe to create a BundleWriter here. Note that it is
        // unsafe to initialize the BundleWriter anywhere the above
        // conditions are not met since BundleWriter's constructor creates
        // new temp files which can delete the temp files created by a
        // BundleWriter in another Session.
        writer_ = std::make_unique<BundleWriter>(dataset()->env_, filename_);
        lockfile_created_ = true;
        return absl::OkStatus();
      }

      absl::Status Finish() TF_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
        iteration_completed_ = true;
        // Flush the current bundle.
        TF_RETURN_IF_ERROR(writer_->Finish());
        // Merge all the bundles.
        // Currently there are `shard_id_ + 1` bundles, one for each
        // checkpoint. Each bundle has prefix <filename>_<id> where `id` is an
        // integer starting at 0 and incremented by 1 for each new checkpoint.
        // We merge all these bundles into a bundle with prefix <filename> so
        // that the next call to `MakeIterator` can build a
        // `FileReaderIterator`.
        {
          std::vector<tstring> prefixes;
          prefixes.reserve(shard_id_ + 1);
          for (size_t i = 0; i <= shard_id_; ++i) {
            prefixes.emplace_back(absl::StrCat(dataset()->filename_, "_", i));
          }
          TF_RETURN_IF_ERROR(
              MergeBundles(dataset()->env_, prefixes, dataset()->filename_));
        }
        // Delete all lockfiles.
        for (size_t i = 0; i <= shard_id_; ++i) {
          TF_RETURN_IF_ERROR(dataset()->env_->DeleteFile(
              absl::StrCat(dataset()->filename_, "_", i, kLockFileSuffix)));
        }
        return absl::OkStatus();
      }

      mutex mu_;
      size_t cur_index_ TF_GUARDED_BY(mu_);
      // Index of the current shard. This gets incremented whenever a new
      // cache shard is saved.
      size_t shard_id_ TF_GUARDED_BY(mu_);
      std::unique_ptr<IteratorBase> input_impl_ TF_GUARDED_BY(mu_);
      // The current prefix for the cache file. This is equal to
      // `StrCat(dataset()->filename_, "_", shard_id_)`.
      std::string filename_;
      std::unique_ptr<BundleWriter> writer_ TF_GUARDED_BY(mu_);
      std::string lockfile_ TF_GUARDED_BY(mu_);
      bool lockfile_created_ TF_GUARDED_BY(mu_);
      bool iteration_completed_ TF_GUARDED_BY(mu_);
    };  // FileWriterIterator

    class FileReaderIterator : public DatasetIterator<FileDatasetBase> {
     public:
      explicit FileReaderIterator(const Params& params)
          : DatasetIterator<FileDatasetBase>(params),
            cur_index_(0),
            reader_(dataset()->env_, dataset()->filename_),
            iterator_restored_(false) {}

      absl::Status GetNextInternal(IteratorContext* ctx,
                                   std::vector<Tensor>* out_tensors,
                                   bool* end_of_sequence) override {
        mutex_lock l(mu_);
        *end_of_sequence = false;
        TF_RETURN_IF_ERROR(reader_.status());
        if (!reader_.Valid()) {
          *end_of_sequence = true;
          return absl::OkStatus();
        }
        out_tensors->clear();
        out_tensors->resize(dataset()->num_tensors_);

        for (size_t i = 0; i < dataset()->num_tensors_; ++i) {
          // When the iterator is restored from the checkpoint, `reader_` is
          // already pointing at `key` so we do not need to skip the header
          // entry.
          if (!iterator_restored_) {
            reader_.Next();  // The first entry in the table is a header.
          } else {
            iterator_restored_ = false;
          }
          if (!reader_.Valid()) {
            out_tensors->clear();
            *end_of_sequence = true;
            return absl::OkStatus();
          }
          absl::string_view key = reader_.key();
          DCHECK_EQ(key, dataset()->FormatName(cur_index_, i));
          TF_RETURN_IF_ERROR(reader_.ReadCurrent(&(*out_tensors)[i]));
          TF_RETURN_IF_ERROR(reader_.status());
        }
        cur_index_++;
        return absl::OkStatus();
      }

     protected:
      std::shared_ptr<model::Node> CreateNode(
          IteratorContext* ctx, model::Node::Args args) const override {
        return model::MakeKnownRatioNode(std::move(args),
                                         /*ratio=*/1);
      }

      absl::Status SaveInternal(SerializationContext* ctx,
                                IteratorStateWriter* writer) override {
        mutex_lock l(mu_);
        TF_RETURN_IF_ERROR(
            writer->WriteScalar(prefix(), kCurIndex, cur_index_));
        return absl::OkStatus();
      }

      absl::Status RestoreInternal(
          IteratorContext* ctx,
          IteratorStateReader* iterator_state_reader) override {
        mutex_lock l(mu_);
        {
          // TODO(b/78048575): Update this when saving size_t tensors directly
          // is supported.
          int64_t temp;
          TF_RETURN_IF_ERROR(
              iterator_state_reader->ReadScalar(prefix(), kCurIndex, &temp));
          cur_index_ = static_cast<size_t>(temp);
          if (cur_index_ != temp) {
            return errors::Internal("Invalid value for cur_index ", temp);
          }
        }
        if (!reader_.Valid()) {
          return errors::Internal("Error initializing BundleReader.");
        }
        reader_.Seek(dataset()->FormatName(cur_index_, 0));
        iterator_restored_ = true;
        return absl::OkStatus();
      }

     private:
      mutex mu_;
      size_t cur_index_ TF_GUARDED_BY(mu_);
      BundleReader reader_ TF_GUARDED_BY(mu_);
      bool iterator_restored_ TF_GUARDED_BY(mu_);
    };  // FileReaderIterator

    absl::Status InitializeIterator(IteratorContext* ctx)
        TF_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
      // We intentionally use the same prefix for both `FileReaderIterator` and
      // `FileWriterIterator`. Since at any time there will be at most one of
      // them alive, there should be no conflicts. This allows both iterators to
      // use a common key for `cur_index`. We leverage this in the corner case
      // when this iterator is restored from an old checkpoint in `write` mode
      // and the cache has been completely flushed to disk since then. In that
      // case we simply build a `FileReaderIterator` and seek to the
      // `cur_index`.
      switch (mode_) {
        case Mode::read:
          iterator_ =
              std::make_unique<FileReaderIterator>(FileReaderIterator::Params{
                  dataset(), absl::StrCat(prefix(), kImpl)});
          break;
        case Mode::write:
          iterator_ =
              std::make_unique<FileWriterIterator>(FileWriterIterator::Params{
                  dataset(), absl::StrCat(prefix(), kImpl)});
      }
      TF_RETURN_IF_ERROR(iterator_->InitializeBase(ctx, this));
      return iterator_->Initialize(ctx);
    }

    mutex mu_;
    enum Mode { read, write };
    Mode mode_ TF_GUARDED_BY(mu_);
    std::unique_ptr<IteratorBase> iterator_ TF_GUARDED_BY(mu_);
  };  // FileIterator

  Env* const env_;
  const size_t num_tensors_;
  const size_t tensor_index_padding_size_;
  static constexpr size_t kMaxItems = 10000000;  // 10 million
  const size_t item_index_padding_size_;
};  // FileDatasetBase

class CacheDatasetOp::FileDataset : public CacheDatasetOp::FileDatasetBase {
 public:
  using FileDatasetBase::FileDatasetBase;

 protected:
  absl::Status AsGraphDefInternal(SerializationContext* ctx,
                                  DatasetGraphDefBuilder* b,
                                  Node** output) const override {
    Node* input_graph = nullptr;
    TF_RETURN_IF_ERROR(b->AddInputDataset(ctx, input_, &input_graph));
    Node* filename = nullptr;
    TF_RETURN_IF_ERROR(b->AddScalar(filename_, &filename));
    TF_RETURN_IF_ERROR(b->AddDataset(this, {input_graph, filename}, output));
    return absl::OkStatus();
  }
};

class CacheDatasetOp::FileDatasetV2 : public CacheDatasetOp::FileDatasetBase {
 public:
  explicit FileDatasetV2(OpKernelContext* ctx, const DatasetBase* input,
                         std::string filename, Env* env,
                         const Tensor& resource_handle)
      : FileDatasetBase(ctx, input, filename, env),
        resource_handle_(resource_handle) {}

 protected:
  absl::Status AsGraphDefInternal(SerializationContext* ctx,
                                  DatasetGraphDefBuilder* b,
                                  Node** output) const override {
    Node* input_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddInputDataset(ctx, input_, &input_node));
    Node* filename_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddScalar(filename_, &filename_node));
    Node* resource_handle_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddTensor(resource_handle_, &resource_handle_node));
    TF_RETURN_IF_ERROR(b->AddDataset(
        this, {input_node, filename_node, resource_handle_node}, output));
    return absl::OkStatus();
  }

 private:
  const Tensor resource_handle_;
};

class CacheDatasetOp::MemoryDatasetBase : public DatasetBase {
 public:
  explicit MemoryDatasetBase(OpKernelContext* ctx, const DatasetBase* input,
                             std::shared_ptr<MemoryCache> cache)
      : DatasetBase(DatasetContext(ctx)),
        input_(input),
        cache_(std::move(cache)) {
    input_->Ref();
    random_indexing_compatible_ = input_->RandomIndexingCompatible();
  }

  ~MemoryDatasetBase() override { input_->Unref(); }

  std::unique_ptr<IteratorBase> MakeIteratorInternal(
      const std::string& prefix) const override {
    name_utils::IteratorPrefixParams params;
    params.dataset_prefix = kMemoryDatasetPrefix;
    return std::make_unique<MemoryIterator>(
        MemoryIterator::Params{
            this, name_utils::IteratorPrefix(kDatasetType, prefix, params)},
        cache_.get());
  }

  const DataTypeVector& output_dtypes() const override {
    return input_->output_dtypes();
  }

  const std::vector<PartialTensorShape>& output_shapes() const override {
    return input_->output_shapes();
  }

  std::string DebugString() const override {
    name_utils::DatasetDebugStringParams params;
    params.dataset_prefix = kMemoryDatasetPrefix;
    return name_utils::DatasetDebugString(kDatasetType, params);
  }

  int64_t CardinalityInternal(CardinalityOptions options) const override {
    return input_->Cardinality(options);
  };

  absl::Status Get(OpKernelContext* ctx, int64_t index,
                   std::vector<Tensor>* out_tensors) const override {
    mutex_lock l(mu_);

    CardinalityOptions options;
    options.set_compute_level(CardinalityOptions::CARDINALITY_COMPUTE_LOW);
    int64_t cardinality = Cardinality(options);

    if (cardinality != kUnknownCardinality &&
        cardinality != kInfiniteCardinality && index >= cardinality) {
      return errors::OutOfRange("Index out of range [0, ", cardinality,
                                "):", index);
    }
    if (!dataset_random_access_cache_) {
      dataset_random_access_cache_ =
          std::make_unique<DatasetRandomAccessCache>(input_);
    }
    return dataset_random_access_cache_->Get(ctx, index, out_tensors);
  }

  absl::Status Get(AnyContext ctx, int64_t index,
                   std::vector<Tensor>* out_tensors) const override {
    mutex_lock l(mu_);
    if (!iterator_random_access_cache_) {
      iterator_random_access_cache_ =
          std::make_unique<IteratorRandomAccessCache>(input_);
    }
    return iterator_random_access_cache_->Get(ctx, index, out_tensors);
  }

  absl::Status InputDatasets(
      std::vector<const DatasetBase*>* inputs) const override {
    inputs->push_back(input_);
    return absl::OkStatus();
  }

  absl::Status CheckExternalState() const override {
    return input_->CheckExternalState();
  }

  absl::Status RandomIndexingCompatible() const override {
    return random_indexing_compatible_;
  }

 protected:
  class MemoryIterator : public DatasetIterator<MemoryDatasetBase> {
   public:
    explicit MemoryIterator(const Params& params, MemoryCache* cache)
        : DatasetIterator<MemoryDatasetBase>(params),
          cache_(cache),
          global_shuffle_iterator_(dataset()) {}

    absl::Status Initialize(IteratorContext* ctx) override {
      mutex_lock l(mu_);
      return InitializeIterator(ctx);
    }

    absl::Status GetNextInternal(IteratorContext* ctx,
                                 std::vector<Tensor>* out_tensors,
                                 bool* end_of_sequence) override {
      if (ctx->index_mapper() != nullptr) {
        return global_shuffle_iterator_.GetNext(ctx, out_tensors,
                                                end_of_sequence);
      }
      mutex_lock l(mu_);
      return iterator_->GetNext(ctx, out_tensors, end_of_sequence);
    }

   protected:
    std::shared_ptr<model::Node> CreateNode(
        IteratorContext* ctx, model::Node::Args args) const override {
      return model::MakeKnownRatioNode(std::move(args),
                                       /*ratio=*/1);
    }

    absl::Status SaveInternal(SerializationContext* ctx,
                              IteratorStateWriter* writer) override {
      mutex_lock l(mu_);
      if (cache_->IsCompleted()) {
        TF_RETURN_IF_ERROR(writer->WriteScalar(prefix(), kCacheCompleted, ""));
        TF_RETURN_IF_ERROR(
            WriteElementsToCheckpoint(writer, prefix(), cache_->data()));
      }
      TF_RETURN_IF_ERROR(global_shuffle_iterator_.Save(prefix(), ctx, writer));
      return SaveInput(ctx, writer, iterator_);
    }

    absl::Status RestoreInternal(IteratorContext* ctx,
                                 IteratorStateReader* reader) override {
      if (ctx->restored_element_count().has_value()) {
        return global_shuffle_iterator_.Restore(prefix(), ctx, reader);
      }
      mutex_lock l(mu_);
      iterator_.reset();
      cache_->Reset();
      if (reader->Contains(prefix(), kCacheCompleted)) {
        std::vector<std::vector<Tensor>> temp_cache;
        TF_RETURN_IF_ERROR(
            ReadElementsFromCheckpoint(ctx, reader, prefix(), &temp_cache));
        cache_->Complete(std::move(temp_cache));
      }
      TF_RETURN_IF_ERROR(InitializeIterator(ctx));
      return RestoreInput(ctx, reader, iterator_);
    }

   private:
    class MemoryWriterIterator : public DatasetIterator<MemoryDatasetBase> {
     public:
      explicit MemoryWriterIterator(const Params& params, MemoryCache* cache)
          : DatasetIterator<MemoryDatasetBase>(params), cache_(cache) {}

      ~MemoryWriterIterator() override {
        mutex_lock l(mu_);
        if (!temp_cache_.empty() && !cache_->IsCompleted()) {
          LOG(WARNING) << kIncompleteCacheErrorMessage;
          cache_->Reset();
        }
      }

      absl::Status Initialize(IteratorContext* ctx) override {
        return dataset()->input_->MakeIterator(ctx, this, prefix(),
                                               &input_impl_);
      }

      absl::Status GetNextInternal(IteratorContext* ctx,
                                   std::vector<Tensor>* out_tensors,
                                   bool* end_of_sequence) override {
        mutex_lock l(mu_);
        TF_RETURN_IF_ERROR(
            input_impl_->GetNext(ctx, out_tensors, end_of_sequence));
        if (*end_of_sequence) {
          if (!cache_->IsCompleted()) {
            VLOG(2) << "Finalizing the cache because EOF has been reached.";
            cache_->Complete(std::move(temp_cache_));
          }
          return absl::OkStatus();
        }
        RecordBufferEnqueue(ctx, *out_tensors);
        temp_cache_.emplace_back(*out_tensors);
        if (temp_cache_.size() == dataset()->input_->Cardinality()) {
          VLOG(2) << "Finalizing the cache because its size matches the "
                     "expected input cardinality.";
          cache_->Complete(std::move(temp_cache_));
        }
        return absl::OkStatus();
      }

     protected:
      std::shared_ptr<model::Node> CreateNode(
          IteratorContext* ctx, model::Node::Args args) const override {
        return model::MakeKnownRatioNode(std::move(args),
                                         /*ratio=*/1);
      }

      absl::Status SaveInternal(SerializationContext* ctx,
                                IteratorStateWriter* writer) override {
        mutex_lock l(mu_);
        if (!cache_->IsCompleted()) {
          TF_RETURN_IF_ERROR(
              WriteElementsToCheckpoint(writer, prefix(), temp_cache_));
        }
        return SaveInput(ctx, writer, input_impl_);
      }

      absl::Status RestoreInternal(IteratorContext* ctx,
                                   IteratorStateReader* reader) override {
        mutex_lock l(mu_);
        if (!reader->Contains(prefix(), kCacheCompleted)) {
          TF_RETURN_IF_ERROR(
              ReadElementsFromCheckpoint(ctx, reader, prefix(), &temp_cache_));
        }
        return RestoreInput(ctx, reader, input_impl_);
      }

     private:
      mutex mu_;
      std::unique_ptr<IteratorBase> input_impl_ TF_GUARDED_BY(mu_);
      MemoryCache* const cache_ TF_GUARDED_BY(mu_);  // not owned.
      std::vector<std::vector<Tensor>> temp_cache_ TF_GUARDED_BY(mu_);
    };  // MemoryWriterIterator

    class MemoryReaderIterator : public DatasetIterator<MemoryDatasetBase> {
     public:
      explicit MemoryReaderIterator(const Params& params, MemoryCache* cache)
          : DatasetIterator<MemoryDatasetBase>(params),
            cache_(cache),
            index_(0) {}

      absl::Status Initialize(IteratorContext* ctx) override {
        // The memory allocated for the cache is owned by the parent
        // dataset but performance modeling uses the iterator abstraction and
        // thus we record the memory allocated for the cache here. The caveat
        // is that this is incorrect if there are concurrent instances of this
        // iterator.
        tf_shared_lock l(mu_);
        for (size_t i = 0; i < cache_->size(); ++i) {
          RecordBufferEnqueue(ctx, cache_->at(i));
        }
        return absl::OkStatus();
      }

      absl::Status GetNextInternal(IteratorContext* ctx,
                                   std::vector<Tensor>* out_tensors,
                                   bool* end_of_sequence) override {
        mutex_lock l(mu_);
        if (index_ < cache_->size()) {
          const std::vector<Tensor>& cache_tensors = cache_->at(index_);
          out_tensors->insert(out_tensors->begin(), cache_tensors.begin(),
                              cache_tensors.end());
          index_++;
          *end_of_sequence = false;
          return absl::OkStatus();
        } else {
          *end_of_sequence = true;
          return absl::OkStatus();
        }
      }

     protected:
      std::shared_ptr<model::Node> CreateNode(
          IteratorContext* ctx, model::Node::Args args) const override {
        return model::MakeKnownRatioNode(std::move(args),
                                         /*ratio=*/1);
      }

      absl::Status SaveInternal(SerializationContext* ctx,
                                IteratorStateWriter* writer) override {
        mutex_lock l(mu_);
        TF_RETURN_IF_ERROR(writer->WriteScalar(prefix(), kIndex, index_));
        return absl::OkStatus();
      }

      absl::Status RestoreInternal(IteratorContext* ctx,
                                   IteratorStateReader* reader) override {
        mutex_lock l(mu_);
        {
          // kIndex will not be set if we are restoring from a checkpoint
          // written by a MemoryWriterIterator that has completed its cache.
          int64_t temp = cache_->size();
          if (reader->Contains(prefix(), kIndex)) {
            TF_RETURN_IF_ERROR(reader->ReadScalar(prefix(), kIndex, &temp));
          }
          index_ = static_cast<size_t>(temp);
        }
        return absl::OkStatus();
      }

     private:
      mutex mu_;
      MemoryCache* const cache_ TF_GUARDED_BY(mu_);  // not owned.
      size_t index_ TF_GUARDED_BY(mu_);
    };  // MemoryReaderIterator

    absl::Status InitializeIterator(IteratorContext* ctx)
        TF_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
      if (cache_->IsCompleted()) {
        iterator_ = std::make_unique<MemoryReaderIterator>(
            MemoryReaderIterator::Params{dataset(),
                                         absl::StrCat(prefix(), kImpl)},
            cache_);
      } else {
        iterator_ = std::make_unique<MemoryWriterIterator>(
            MemoryWriterIterator::Params{dataset(),
                                         absl::StrCat(prefix(), kImpl)},
            cache_);
      }
      TF_RETURN_IF_ERROR(iterator_->InitializeBase(ctx, this));
      return iterator_->Initialize(ctx);
    }

    mutex mu_;
    MemoryCache* cache_ TF_GUARDED_BY(mu_);  // not owned.
    std::unique_ptr<IteratorBase> iterator_ TF_GUARDED_BY(mu_);
    GlobalShuffleIterator global_shuffle_iterator_;
  };  // MemoryIterator

  mutable mutex mu_;
  const DatasetBase* const input_;
  const std::shared_ptr<MemoryCache> cache_;
  mutable std::unique_ptr<DatasetRandomAccessCache> dataset_random_access_cache_
      TF_GUARDED_BY(mu_);
  mutable std::unique_ptr<IteratorRandomAccessCache>
      iterator_random_access_cache_;
  absl::Status random_indexing_compatible_ = absl::OkStatus();
};  // MemoryDatasetBase

// This version of memory dataset has an exclusive ownership of the memory cache
// resource. It supports sharing of the cache across different iterations of the
// `repeat` transformation but not across different iterators.
class CacheDatasetOp::MemoryDataset : public CacheDatasetOp::MemoryDatasetBase {
 public:
  MemoryDataset(OpKernelContext* ctx, const DatasetBase* input,
                MemoryCacheManager* manager, ResourceHandle&& resource_handle)
      : MemoryDatasetBase(ctx, input, manager->get()),
        manager_(manager),
        resource_handle_(std::move(resource_handle)),
        resource_mgr_(ctx->resource_manager()) {}

  ~MemoryDataset() override {
    manager_->Unref();
    absl::Status s = resource_mgr_->Delete<MemoryCacheManager>(
        resource_handle_.container(), resource_handle_.name());
    if (!s.ok()) {
      LOG(WARNING) << "Failed to delete cache resource: " << s.ToString();
    }
  }

 protected:
  absl::Status AsGraphDefInternal(SerializationContext* ctx,
                                  DatasetGraphDefBuilder* b,
                                  Node** output) const override {
    Node* input_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddInputDataset(ctx, input_, &input_node));
    Node* filename_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddScalar(tstring(""), &filename_node));
    TF_RETURN_IF_ERROR(
        b->AddDataset(this, {input_node, filename_node}, output));
    return absl::OkStatus();
  }

 private:
  MemoryCacheManager* const manager_;  // Owned.
  const ResourceHandle resource_handle_;
  ResourceMgr* const resource_mgr_;  // Not owned.
};

// This version of memory dataset has a shared ownership of the memory cache
// resource. It supports sharing of the cache across different iterations of
// the `repeat` transformation and also across different iterators.
class CacheDatasetOp::MemoryDatasetV2
    : public CacheDatasetOp::MemoryDatasetBase {
 public:
  MemoryDatasetV2(OpKernelContext* ctx, const DatasetBase* input,
                  MemoryCacheManager* manager, ResourceHandle&& resource_handle,
                  bool owns_resource)
      : MemoryDatasetBase(ctx, input, manager->get()),
        manager_(manager),
        owns_resource_(owns_resource),
        resource_handle_(std::move(resource_handle)),
        resource_mgr_(ctx->resource_manager()) {}

  ~MemoryDatasetV2() override {
    manager_->Unref();
    if (owns_resource_) {
      absl::Status s = resource_mgr_->Delete<MemoryCacheManager>(
          resource_handle_.container(), resource_handle_.name());
      if (!s.ok()) {
        LOG(WARNING) << "Failed to delete cache resource: " << s.ToString();
      }
    }
  }

 protected:
  absl::Status AsGraphDefInternal(SerializationContext* ctx,
                                  DatasetGraphDefBuilder* b,
                                  Node** output) const override {
    Node* input_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddInputDataset(ctx, input_, &input_node));
    Node* filename_node = nullptr;
    TF_RETURN_IF_ERROR(b->AddScalar(tstring(""), &filename_node));
    Node* resource_handle_node = nullptr;
    Tensor handle(DT_RESOURCE, TensorShape({}));
    handle.scalar<ResourceHandle>()() = resource_handle_;
    TF_RETURN_IF_ERROR(b->AddTensor(handle, &resource_handle_node));
    TF_RETURN_IF_ERROR(b->AddDataset(
        this, {input_node, filename_node, resource_handle_node}, output));
    return absl::OkStatus();
  }

 private:
  MemoryCacheManager* const manager_;  // Owned.
  const bool owns_resource_;
  const ResourceHandle resource_handle_;
  ResourceMgr* const resource_mgr_;  // Not owned.
};

CacheDatasetOp::CacheDatasetOp(OpKernelConstruction* ctx)
    : UnaryDatasetOpKernel(ctx),
      op_version_(ctx->def().op() == kCacheDataset ? 1 : 2) {}

void CacheDatasetOp::MakeDataset(OpKernelContext* ctx, DatasetBase* input,
                                 DatasetBase** output) {
  // Parse out the filenames tensor.
  tstring filename;
  OP_REQUIRES_OK(ctx, ParseScalarArgument<tstring>(ctx, kFileName, &filename));
  if (filename.empty()) {
    static std::atomic<int64_t> resource_id_counter(0);
    const std::string& container = ctx->resource_manager()->default_container();
    auto name = strings::StrCat(ctx->op_kernel().name(), "/", kMemoryCache, "_",
                                resource_id_counter.fetch_add(1));
    if (op_version_ == 2) {
      bool owns_resource = false;
      MemoryCacheManager* manager = nullptr;
      auto handle = HandleFromInput(ctx, 2);
      absl::Status s = ctx->resource_manager()->Lookup<MemoryCacheManager>(
          handle.container(), handle.name(), &manager);
      if (absl::IsNotFound(s)) {
        owns_resource = true;
        OP_REQUIRES_OK(
            ctx,
            ctx->resource_manager()->LookupOrCreate<MemoryCacheManager>(
                container, name, &manager, [](MemoryCacheManager** manager) {
                  *manager = new MemoryCacheManager();
                  return absl::OkStatus();
                }));
        handle = MakeResourceHandle<MemoryCacheManager>(ctx, container, name);
      } else {
        OP_REQUIRES_OK(ctx, s);
      }
      // Ownership of manager is transferred onto `MemoryDatasetV2`.
      *output = new MemoryDatasetV2(ctx, input, manager, std::move(handle),
                                    owns_resource);
    } else {
      MemoryCacheManager* manager;
      OP_REQUIRES_OK(
          ctx, ctx->resource_manager()->LookupOrCreate<MemoryCacheManager>(
                   container, name, &manager, [](MemoryCacheManager** manager) {
                     *manager = new MemoryCacheManager();
                     return absl::OkStatus();
                   }));
      auto handle =
          MakeResourceHandle<MemoryCacheManager>(ctx, container, name);
      // Ownership of manager is transferred onto `MemoryDataset`.
      *output = new MemoryDataset(ctx, input, manager, std::move(handle));
    }
  } else {
    if (op_version_ == 2) {
      *output =
          new FileDatasetV2(ctx, input, filename, ctx->env(), ctx->input(2));
    } else {
      *output = new FileDataset(ctx, input, filename, ctx->env());
    }
  }
}

namespace {
REGISTER_KERNEL_BUILDER(Name("CacheDataset").Device(DEVICE_CPU),
                        CacheDatasetOp);
REGISTER_KERNEL_BUILDER(Name("CacheDatasetV2").Device(DEVICE_CPU),
                        CacheDatasetOp);
}  // namespace
}  // namespace data
}  // namespace tensorflow
