stateful_dataloader.py

# Copyright (c) Meta Platforms, Inc. and affiliates.
# All rights reserved.
#
# This source code is licensed under the BSD-style license found in the
# LICENSE file in the root directory of this source tree.

r"""Definition of the StatefulDataLoader and associated iterators.

This file is a stand-in for torch.utils.data.dataloader, and includes a
StatefulDataLoader, which inherits from DataLoader and adds
state_dict/load_state_dict methods, as well as implementations for
single and multi-process iterators which are also stateful.

Where possible, we import the original definitions from torch.utils.data.dataloader,
and use inheritance for base classes only (StatefulDataLoader, _StatefulBaseDataLoaderIter).

For the single and multi-process iterator implementations, we fork the code to avoid a
diamond-shaped multiple-inheritance scheme.
"""

import collections
import functools
import itertools
import logging
import queue
import threading

from typing import Any, Dict, Iterable, List, Optional, TypeVar, Union

import torch
import torch.multiprocessing as multiprocessing
import torch.utils.data._utils.worker
import torch.utils.data.graph_settings

from torch._utils import ExceptionWrapper

from torch.utils.data import (
    _utils,
    DataLoader,
    Dataset,
    IterableDataset,
    IterDataPipe,
    MapDataPipe,
    Sampler,
    SequentialSampler,
)

from torch.utils.data.dataloader import _BaseDataLoaderIter, _InfiniteConstantSampler
from torch.utils.data.datapipes.datapipe import _IterDataPipeSerializationWrapper, _MapDataPipeSerializationWrapper

from .incremental_state import (
    _DATASET_ITER_STATE,
    _DATASET_STATE,
    _FETCHER_ENDED,
    _FETCHER_STATE,
    _IncrementalWorkerState,
    _WORKER_ID,
)
from .sampler import BatchSampler, RandomSampler
from .stateful import Stateful

from .worker import _AckStartup, _worker_loop, try_to_deserialize, try_to_serialize

__all__ = [
    "StatefulDataLoader",
    "get_worker_info",
    "default_collate",
    "default_convert",
]

from torch.utils.data.dataloader import (
    _collate_fn_t,
    _DatasetKind,
    _sharding_worker_init_fn,
    _worker_init_fn_t,
    default_collate,
    default_convert,
    get_worker_info,
)

_T_co = TypeVar("_T_co", covariant=True)

logger = logging.getLogger(__name__)

_INDEX_SAMPLER_STATE = "_index_sampler_state"
_SAMPLER_ITER_STATE = "_sampler_iter_state"
_SAMPLER_ITER_YIELDED = "_sampler_iter_yielded"
_ITERABLEDATASET_LEN_CALLED = "_IterableDataset_len_called"
_SHARED_SEED = "_shared_seed"
_ITERATOR_FINISHED = "_iterator_finished"


class StatefulDataLoader(DataLoader[_T_co]):
    r"""
    This is a drop in replacement for ``torch.utils.data.DataLoader``
    that implements state_dict and load_state_dict methods, enabling mid-epoch
    checkpointing.

    All arguments are identical to ``torch.utils.data.DataLoader``, with
    a new kwarg: ``snapshot_every_n_steps``.

    Args:
        dataset (Dataset): dataset from which to load the data.
        batch_size (int, optional): how many samples per batch to load
            (default: ``1``).
        shuffle (bool, optional): set to ``True`` to have the data reshuffled
            at every epoch (default: ``False``).
        sampler (Sampler or Iterable, optional): defines the strategy to draw
            samples from the dataset. Can be any ``Iterable`` with ``__len__``
            implemented. If specified, :attr:`shuffle` must not be specified.
        batch_sampler (Sampler or Iterable, optional): like :attr:`sampler`, but
            returns a batch of indices at a time. Mutually exclusive with
            :attr:`batch_size`, :attr:`shuffle`, :attr:`sampler`,
            and :attr:`drop_last`.
        num_workers (int, optional): how many subprocesses to use for data
            loading. ``0`` means that the data will be loaded in the main process.
            (default: ``0``)
        collate_fn (Callable, optional): merges a list of samples to form a
            mini-batch of Tensor(s).  Used when using batched loading from a
            map-style dataset.
        pin_memory (bool, optional): If ``True``, the data loader will copy Tensors
            into device/CUDA pinned memory before returning them.  If your data elements
            are a custom type, or your :attr:`collate_fn` returns a batch that is a custom type,
            see the example below.
        drop_last (bool, optional): set to ``True`` to drop the last incomplete batch,
            if the dataset size is not divisible by the batch size. If ``False`` and
            the size of dataset is not divisible by the batch size, then the last batch
            will be smaller. (default: ``False``)
        timeout (numeric, optional): if positive, the timeout value for collecting a batch
            from workers. Should always be non-negative. (default: ``0``)
        worker_init_fn (Callable, optional): If not ``None``, this will be called on each
            worker subprocess with the worker id (an int in ``[0, num_workers - 1]``) as
            input, after seeding and before data loading. (default: ``None``)
        multiprocessing_context (str or multiprocessing.context.BaseContext, optional): If
            ``None``, the default `multiprocessing context`_ of your operating system will
            be used. (default: ``None``)
        generator (torch.Generator, optional): If not ``None``, this RNG will be used
            by RandomSampler to generate random indexes and multiprocessing to generate
            ``base_seed`` for workers. (default: ``None``)
        prefetch_factor (int, optional, keyword-only arg): Number of batches loaded
            in advance by each worker. ``2`` means there will be a total of
            2 * num_workers batches prefetched across all workers. (default value depends
            on the set value for num_workers. If value of num_workers=0 default is ``None``.
            Otherwise, if value of ``num_workers > 0`` default is ``2``).
        persistent_workers (bool, optional): If ``True``, the data loader will not shut down
            the worker processes after a dataset has been consumed once. This allows to
            maintain the workers `Dataset` instances alive. (default: ``False``)
        pin_memory_device (str, optional): the device to :attr:`pin_memory` to if ``pin_memory`` is
            ``True``.
        in_order (bool, optional): If ``False``, the data loader will not enforce that batches
            are returned in a first-in, first-out order. Only applies when ``num_workers > 0``. (default: ``True``)
        snapshot_every_n_steps (int, optional): Defines how often the state is
            transferred from the dataloader workers to the dataloader. By default, it is set to ``1``, i.e., state is transferred every step. If the state is large, this value can be increased (and ideally set to the frequency of training checkpointing) to reduce the overhead of transferring state every step.


    .. warning:: If the ``spawn`` start method is used, :attr:`worker_init_fn`
                 cannot be an unpicklable object, e.g., a lambda function. See
                 `multiprocessing-best-practices <https://pytorch.org/docs/stable/notes/multiprocessing.html#multiprocessing-best-practices>`_ on more details related
                 to multiprocessing in PyTorch.

    .. warning:: ``len(dataloader)`` heuristic is based on the length of the sampler used.
                 When :attr:`dataset` is an :class:`~torch.utils.data.IterableDataset`,
                 it instead returns an estimate based on ``len(dataset) / batch_size``, with proper
                 rounding depending on :attr:`drop_last`, regardless of multi-process loading
                 configurations. This represents the best guess PyTorch can make because PyTorch
                 trusts user :attr:`dataset` code in correctly handling multi-process
                 loading to avoid duplicate data.

                 However, if sharding results in multiple workers having incomplete last batches,
                 this estimate can still be inaccurate, because (1) an otherwise complete batch can
                 be broken into multiple ones and (2) more than one batch worth of samples can be
                 dropped when :attr:`drop_last` is set. Unfortunately, PyTorch can not detect such
                 cases in general.

                 See `Dataset Types <https://pytorch.org/docs/stable/data.html>`_ for more details on these two types of datasets and how
                 :class:`~torch.utils.data.IterableDataset` interacts with
                 `Multi-process data loading <https://pytorch.org/docs/stable/data.html#multi-process-data-loading>`_.

    .. warning:: See `Reproducibility <https://pytorch.org/docs/stable/notes/randomness.html#reproducibility>`_, and `Dataloader-workers-random-seed <https://pytorch.org/docs/stable/notes/faq.html#dataloader-workers-random-seed>`_, and
                 `Data-loading-randomness <https://pytorch.org/docs/stable/data.html#data-loading-randomness>`_ notes for random seed related questions.

    .. warning:: Setting `in_order` to `False` can harm reproducibility and may lead to a skewed data distribution being fed to the trainer in cases with imbalanced data.

    .. warning:: Setting `in_order` to `False` currently has no guarantees for state management.

    .. _multiprocessing context:
        https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods
    """

    _iterator: Optional["_StatefulBaseDataLoaderIter"]

    def __init__(
        self,
        dataset: Dataset[_T_co],
        batch_size: Optional[int] = 1,
        shuffle: Optional[bool] = None,
        sampler: Union[Sampler, Iterable, None] = None,
        batch_sampler: Union[Sampler[List], Iterable[List], None] = None,
        num_workers: int = 0,
        collate_fn: Optional[_collate_fn_t] = None,
        pin_memory: bool = False,
        drop_last: bool = False,
        timeout: float = 0,
        worker_init_fn: Optional[_worker_init_fn_t] = None,
        multiprocessing_context=None,
        generator=None,
        *,
        prefetch_factor: Optional[int] = None,
        persistent_workers: bool = False,
        pin_memory_device: str = "",
        in_order: bool = True,
        snapshot_every_n_steps: Optional[int] = 1,
    ):
        torch._C._log_api_usage_once("python.stateful_data_loader")

        if num_workers < 0:
            raise ValueError(
                "num_workers option should be non-negative; " "use num_workers=0 to disable multiprocessing."
            )

        if timeout < 0:
            raise ValueError("timeout option should be non-negative")

        if num_workers == 0 and prefetch_factor is not None:
            raise ValueError(
                "prefetch_factor option could only be specified in multiprocessing."
                "let num_workers > 0 to enable multiprocessing, otherwise set prefetch_factor to None."
            )
        elif num_workers > 0 and prefetch_factor is None:
            prefetch_factor = 2
        elif prefetch_factor is not None and prefetch_factor < 0:
            raise ValueError("prefetch_factor option should be non-negative")

        if persistent_workers and num_workers == 0:
            raise ValueError("persistent_workers option needs num_workers > 0")

        if num_workers > 0 and not in_order:
            # TODO: remove warning log when state management is supported with in_order=False
            logger.warning(
                "using in_order=False with multiple workers does not give any guarantees for state management "
                "and loading from a checkpoint may not work as expected."
            )

        self.dataset = dataset
        self.num_workers = num_workers
        self.prefetch_factor = prefetch_factor
        self.pin_memory = pin_memory
        self.pin_memory_device = pin_memory_device
        self.timeout = timeout
        self.worker_init_fn = worker_init_fn
        self.multiprocessing_context = multiprocessing_context
        self.in_order = in_order

        # Adds forward compatibilities so classic DataLoader can work with DataPipes:
        #   _DataPipeSerializationWrapper container makes it easier to serialize without redefining pickler
        if isinstance(self.dataset, IterDataPipe):
            self.dataset = _IterDataPipeSerializationWrapper(self.dataset)
        elif isinstance(self.dataset, MapDataPipe):
            self.dataset = _MapDataPipeSerializationWrapper(self.dataset)

        # Arg-check dataset related before checking samplers because we want to
        # tell users that iterable-style datasets are incompatible with custom
        # samplers first, so that they don't learn that this combo doesn't work
        # after spending time fixing the custom sampler errors.
        if isinstance(dataset, IterableDataset):
            self._dataset_kind = _DatasetKind.Iterable
            # NOTE [ Custom Samplers and IterableDataset ]
            #
            # `IterableDataset` does not support custom `batch_sampler` or
            # `sampler` since the key is irrelevant (unless we support
            # generator-style dataset one day...).
            #
            # For `sampler`, we always create a dummy sampler. This is an
            # infinite sampler even when the dataset may have an implemented
            # finite `__len__` because in multi-process data loading, naive
            # settings will return duplicated data (which may be desired), and
            # thus using a sampler with length matching that of dataset will
            # cause data lost (you may have duplicates of the first couple
            # batches, but never see anything afterwards). Therefore,
            # `Iterabledataset` always uses an infinite sampler, an instance of
            # `_InfiniteConstantSampler` defined above.
            #
            # A custom `batch_sampler` essentially only controls the batch size.
            # However, it is unclear how useful it would be since an iterable-style
            # dataset can handle that within itself. Moreover, it is pointless
            # in multi-process data loading as the assignment order of batches
            # to workers is an implementation detail so users can not control
            # how to batchify each worker's iterable. Thus, we disable this
            # option. If this turns out to be useful in future, we can re-enable
            # this, and support custom samplers that specify the assignments to
            # specific workers.
            if isinstance(dataset, IterDataPipe):
                if shuffle is not None:
                    dataset = torch.utils.data.graph_settings.apply_shuffle_settings(dataset, shuffle=shuffle)
            # We cannot check `shuffle is not None` here, since previously `shuffle=False` was the default.
            elif shuffle not in {False, None}:
                raise ValueError(
                    f"DataLoader with IterableDataset: expected unspecified shuffle option, but got shuffle={shuffle}"
                )

            if sampler is not None:
                # See NOTE [ Custom Samplers and IterableDataset ]
                raise ValueError(
                    f"DataLoader with IterableDataset: expected unspecified sampler option, but got sampler={sampler}"
                )
            elif batch_sampler is not None:
                # See NOTE [ Custom Samplers and IterableDataset ]
                raise ValueError(
                    "DataLoader with IterableDataset: expected unspecified "
                    f"batch_sampler option, but got batch_sampler={batch_sampler}"
                )
        else:
            shuffle = bool(shuffle)
            self._dataset_kind = _DatasetKind.Map

        if sampler is not None and shuffle:
            raise ValueError("sampler option is mutually exclusive with " "shuffle")

        if batch_sampler is not None:
            # auto_collation with custom batch_sampler
            if batch_size != 1 or shuffle or sampler is not None or drop_last:
                raise ValueError(
                    "batch_sampler option is mutually exclusive " "with batch_size, shuffle, sampler, and " "drop_last"
                )
            batch_size = None
            drop_last = False
        elif batch_size is None:
            # no auto_collation
            if drop_last:
                raise ValueError(
                    "batch_size=None option disables auto-batching " "and is mutually exclusive with drop_last"
                )

        if sampler is None:  # give default samplers
            if self._dataset_kind == _DatasetKind.Iterable:
                # See NOTE [ Custom Samplers and IterableDataset ]
                sampler = _InfiniteConstantSampler()
            else:  # map-style
                if shuffle:
                    sampler = RandomSampler(dataset, generator=generator)  # type: ignore[arg-type]
                else:
                    sampler = SequentialSampler(dataset)  # type: ignore[arg-type]

        if batch_size is not None and batch_sampler is None:
            # auto_collation without custom batch_sampler
            batch_sampler = BatchSampler(sampler, batch_size, drop_last)

        self.batch_size = batch_size
        self.drop_last = drop_last
        self.sampler = sampler
        self.batch_sampler = batch_sampler
        self.generator = generator

        if collate_fn is None:
            if self._auto_collation:
                collate_fn = _utils.collate.default_collate
            else:
                collate_fn = _utils.collate.default_convert

        self.collate_fn = collate_fn
        self.persistent_workers = persistent_workers

        # set DataLoader's __initialized attribute.
        self._DataLoader__initialized = True
        self._IterableDataset_len_called = None  # See NOTE [ IterableDataset and __len__ ]

        self._iterator = None

        self.check_worker_number_rationality()

        self.snapshot_every_n_steps = snapshot_every_n_steps
        self.next_iter_state: Optional[Dict[str, Any]] = None
        # When a state_dict is requested before __iter__ is called,
        # we create the __iter__ so we can get a copy of the initial state from
        # its workers. In those cases, we can avoid creating a new multiprocessing
        # iterator on the next __iter__ call, and this flag is used for those cases.
        self._initial_iter_for_state_dict = False

        torch.set_vital("Dataloader", "enabled", "True")  # type: ignore[attr-defined]

    def _get_iterator(self) -> "_StatefulBaseDataLoaderIter":
        it: _StatefulBaseDataLoaderIter
        if self.num_workers == 0:
            it = _StatefulSingleProcessDataLoaderIter(self, self.next_iter_state)
        else:
            self.check_worker_number_rationality()
            it = _StatefulMultiProcessingDataLoaderIter(self, self.next_iter_state)
        self.next_iter_state = None
        return it

    def __iter__(self) -> "_BaseDataLoaderIter":
        # When using a single worker the returned iterator should be
        # created everytime to avoid resetting its state
        # However, in the case of a multiple workers iterator
        # the iterator is only created once in the lifetime of the
        # DataLoader object so that workers can be reused
        if self._initial_iter_for_state_dict:
            self._initial_iter_for_state_dict = False
            assert self._iterator is not None
        elif self.persistent_workers and self.num_workers > 0:
            if self._iterator is None:
                self._iterator = self._get_iterator()
            else:
                self._iterator._reset(self)
        else:
            self._iterator = self._get_iterator()
        if self._iterator._finished:
            if self.persistent_workers:
                self._iterator._reset(self)
            else:
                self._iterator = self._get_iterator()

        return self._iterator

    def state_dict(self) -> Dict[str, Any]:
        if self._iterator is None:
            self._iterator = self._get_iterator()
            self._initial_iter_for_state_dict = True
        return self._iterator.state_dict()

    def load_state_dict(self, state_dict: Dict[str, Any]) -> None:
        self._iterator = None
        self._initial_iter_for_state_dict = False
        if state_dict == {}:
            return
        self.next_iter_state = state_dict


class _StatefulBaseDataLoaderIter(_BaseDataLoaderIter):
    def __init__(self, loader: StatefulDataLoader) -> None:
        super().__init__(loader)
        self._sampler_iter_yielded = 0
        self._finished = False

    def _reset(self, loader, first_iter=False):
        super()._reset(loader, first_iter)
        self._sampler_iter_yielded = 0
        self._finished = False

    def _next_index(self):
        idx = super()._next_index()  # may raise StopIteration
        self._sampler_iter_yielded += 1
        return idx

    def state_dict(self):
        pass

    def __next__(self):
        try:
            return super().__next__()
        except StopIteration:
            self._finished = True
            raise


class _StatefulSingleProcessDataLoaderIter(_StatefulBaseDataLoaderIter):
    """We avoid using inheritance here to share code because we quickly run into
    a diamond which becomes difficult to reason about, so instead we fork the
    code from torch.utils.data.dataloader for _SingleProcessDataLoaderIter and
    _MultiProcessDataLoaderIter. This allows us to satisfy the original
    dataloader __iter__'s return type of _BaseDataLoaderIter (since
    _StatefulBaseDataLoader inherits from _BaseDataLoaderIter).
    """

    _NUM_YIELDED = "_num_yielded"

    def __init__(self, loader, next_iter_state=None):
        super().__init__(loader)
        assert self._timeout == 0
        assert self._num_workers == 0

        # Adds forward compatibilities so classic DataLoader can work with DataPipes:
        #   Taking care of distributed sharding
        if isinstance(self._dataset, (IterDataPipe, MapDataPipe)):
            # For BC, use default SHARDING_PRIORITIES
            torch.utils.data.graph_settings.apply_sharding(self._dataset, self._world_size, self._rank)

        if next_iter_state is not None:
            self.load_state_dict(next_iter_state)
        else:
            self._dataset_fetcher = _DatasetKind.create_fetcher(
                self._dataset_kind,
                self._dataset,
                self._auto_collation,
                self._collate_fn,
                self._drop_last,
            )

    def _next_data(self):
        index = self._next_index()  # may raise StopIteration
        data = self._dataset_fetcher.fetch(index)  # may raise StopIteration
        if self._pin_memory:
            data = _utils.pin_memory.pin_memory(data, self._pin_memory_device)
        return data

    def state_dict(self):
        if self._dataset_kind == _DatasetKind.Iterable:
            fetcher_state = {
                _DATASET_ITER_STATE: try_to_serialize(self._dataset_fetcher.dataset_iter),
                _FETCHER_ENDED: self._dataset_fetcher.ended,
            }
            dataset_state = None
            if self._dataset_fetcher.dataset_iter is not self._dataset_fetcher.dataset:
                dataset_state = try_to_serialize(self._dataset_fetcher.dataset)
        else:
            fetcher_state = None
            dataset_state = try_to_serialize(self._dataset_fetcher.dataset)

        state_dict = {
            _INDEX_SAMPLER_STATE: try_to_serialize(self._index_sampler),
            _SAMPLER_ITER_STATE: try_to_serialize(self._sampler_iter),
            _SAMPLER_ITER_YIELDED: self._sampler_iter_yielded,
            self._NUM_YIELDED: self._num_yielded,
            _ITERABLEDATASET_LEN_CALLED: self._IterableDataset_len_called,
            _SHARED_SEED: self._shared_seed,
            _FETCHER_STATE: fetcher_state,
            _DATASET_STATE: dataset_state,
            _ITERATOR_FINISHED: self._finished,
        }
        return state_dict

    def load_state_dict(self, state_dict):
        assert (
            self._NUM_YIELDED in state_dict
        ), f"State doesn't contain key '{self._NUM_YIELDED}' expected for single process dataloader"
        self._sampler_iter_yielded = state_dict[_SAMPLER_ITER_YIELDED]

        # Try to restore from either _index_sampler state_dict or _sampler_iter state_dict
        if isinstance(self._index_sampler, Stateful) or isinstance(self._sampler_iter, Stateful):
            self._index_sampler = try_to_deserialize(self._index_sampler, state_dict[_INDEX_SAMPLER_STATE])
            self._sampler_iter = iter(self._index_sampler)
            if state_dict[_SAMPLER_ITER_STATE] is not None:
                self._sampler_iter = try_to_deserialize(self._sampler_iter, state_dict[_SAMPLER_ITER_STATE])
        else:
            if not isinstance(
                self._index_sampler,
                torch.utils.data.dataloader._InfiniteConstantSampler,
            ):
                # Fallback to fastforward
                self._sampler_iter = itertools.islice(self._index_sampler, self._sampler_iter_yielded, None)
        self._num_yielded = state_dict[self._NUM_YIELDED]
        self._IterableDataset_len_called = state_dict[_ITERABLEDATASET_LEN_CALLED]
        self._shared_seed = state_dict[_SHARED_SEED]

        # Always restore in this order:
        #  1. try to restore dataset state
        #  2. generate dataset iterator
        #  3. try to restore iterator state
        if state_dict[_DATASET_STATE] is not None and isinstance(self._dataset, Stateful):
            self._dataset = try_to_deserialize(self._dataset, state_dict[_DATASET_STATE])
        self._dataset_fetcher = _DatasetKind.create_fetcher(
            self._dataset_kind,
            self._dataset,
            self._auto_collation,
            self._collate_fn,
            self._drop_last,
        )
        if self._dataset_kind == _DatasetKind.Iterable:
            # If either dataset or it's iter is stateful, we don't fast-forward
            if isinstance(self._dataset, Stateful) or isinstance(self._dataset_fetcher.dataset_iter, Stateful):
                if state_dict[_FETCHER_STATE] is not None:
                    if state_dict[_FETCHER_STATE][_DATASET_ITER_STATE] is not None:
                        self._dataset_fetcher.dataset_iter = try_to_deserialize(
                            self._dataset_fetcher.dataset_iter,
                            state_dict[_FETCHER_STATE][_DATASET_ITER_STATE],
                        )
                    self._dataset_fetcher.ended = state_dict[_FETCHER_STATE][_FETCHER_ENDED]
            else:
                # No state, just try to fastforward
                if self._num_yielded > 0:
                    logger.warning(
                        f"Neither dataset nor iter(dataset) defines state_dict/load_state_dict so we are "
                        f"naively fast-forwarding your dataset by {self._num_yielded} steps. For more efficient "
                        f"resumes, please implement `state_dict` and `load_state_dict` in your IterableDataset and/or iterator."
                    )
                    for _ in range(self._num_yielded):
                        next(self)
        self._finished = state_dict[_ITERATOR_FINISHED]


class _StatefulMultiProcessingDataLoaderIter(_StatefulBaseDataLoaderIter):
    r"""Iterates once over the DataLoader's dataset, as specified by the sampler."""

    # NOTE [ Data Loader Multiprocessing Shutdown Logic ]
    #
    # Preliminary:
    #
    # Our data model looks like this (queues are indicated with curly brackets):
    #
    #                main process                              ||
    #                     |                                    ||
    #               {index_queue}                              ||
    #                     |                                    ||
    #              worker processes                            ||     DATA
    #                     |                                    ||
    #            {worker_result_queue}                         ||     FLOW
    #                     |                                    ||
    #      pin_memory_thread of main process                   ||   DIRECTION
    #                     |                                    ||
    #               {data_queue}                               ||
    #                     |                                    ||
    #                data output                               \/
    #
    # P.S. `worker_result_queue` and `pin_memory_thread` part may be omitted if
    #      `pin_memory=False`.
    #
    #
    # Terminating multiprocessing logic requires very careful design. In
    # particular, we need to make sure that
    #
    #   1. The iterator gracefully exits the workers when its last reference is
    #      gone or it is depleted.
    #
    #      In this case, the workers should be gracefully exited because the
    #      main process may still need to continue to run, and we want cleaning
    #      up code in the workers to be executed (e.g., releasing GPU memory).
    #      Naturally, we implement the shutdown logic in `__del__` of
    #      DataLoaderIterator.
    #
    #      We delay the discussion on the logic in this case until later.
    #
    #   2. The iterator exits the workers when the loader process and/or worker
    #      processes exits normally or with error.
    #
    #      We set all workers and `pin_memory_thread` to have `daemon=True`.
    #
    #      You may ask, why can't we make the workers non-daemonic, and
    #      gracefully exit using the same logic as we have in `__del__` when the
    #      iterator gets deleted (see 1 above)?
    #
    #      First of all, `__del__` is **not** guaranteed to be called when
    #      interpreter exits. Even if it is called, by the time it executes,
    #      many Python core library resources may already be freed, and even
    #      simple things like acquiring an internal lock of a queue may hang.
    #      Therefore, in this case, we actually need to prevent `__del__` from
    #      being executed, and rely on the automatic termination of daemonic
    #      children.
    #
    #      Thus, we register an `atexit` hook that sets a global flag
    #      `_utils.python_exit_status`. Since `atexit` hooks are executed in the
    #      reverse order of registration, we are guaranteed that this flag is
    #      set before library resources we use are freed (which, at least in
    #      CPython, is done via an `atexit` handler defined in
    #      `multiprocessing/util.py`
    #      https://github.com/python/cpython/blob/c606624af8d4cb3b4a052fb263bb983b3f87585b/Lib/multiprocessing/util.py#L320-L362
    #      registered when an object requiring this mechanism is first
    #      created, e.g., `mp.Queue`
    #      https://github.com/python/cpython/blob/c606624af8d4cb3b4a052fb263bb983b3f87585b/Lib/multiprocessing/context.py#L100-L103
    #      https://github.com/python/cpython/blob/c606624af8d4cb3b4a052fb263bb983b3f87585b/Lib/multiprocessing/queues.py#L29
    #      )
    #
    #      So in `__del__`, we check if `_utils.python_exit_status` is set or
    #      `None` (freed), and perform no-op if so.
    #
    #      However, simply letting library clean-up codes run can also be bad,
    #      because such codes (i.e., `multiprocessing.util._exit_function()`)
    #      include join putting threads for `mp.Queue`, which can be blocking.
    #      Hence, the main process putting threads are called with
    #      `cancel_join_thread` at creation.  See later section
    #      [ 3b. A process won't hang when putting into a queue; ]
    #      for more details.
    #
    #      Here are two example cases where library clean-up codes can run
    #      before `__del__` is called:
    #
    #        1. If we hold onto a reference to the iterator, it more often
    #           than not tries to do `multiprocessing` library cleaning before
    #           clearing the alive referenced objects (https://github.com/pytorch/pytorch/issues/48666)
    #           and thus prevents our cleaning-up code to run first.
    #
    #        2. A similar issue araises when a `DataLoader` is used in a subprocess.
    #           When a process ends, it shuts the all its daemonic children
    #           down with a SIGTERM (instead of joining them without a timeout).
    #           Simiarly for threads, but by a different mechanism. This fact,
    #           together with a few implementation details of multiprocessing, forces
    #           us to make workers daemonic. All of our problems arise when a
    #           DataLoader is used in a subprocess, and are caused by multiprocessing
    #           code which looks more or less like this:
    #
    #               try:
    #                   your_function_using_a_dataloader()
    #               finally:
    #                   multiprocessing.util._exit_function()
    #
    #           The joining/termination mentioned above happens inside
    #           `_exit_function()`. Now, if `your_function_using_a_dataloader()`
    #           throws, the stack trace stored in the exception will prevent the
    #           frame which uses `DataLoaderIter` to be freed. If the frame has any
    #           reference to the `DataLoaderIter` (e.g., in a method of the iter),
    #           its  `__del__`, which starts the shutdown procedure, will not be
    #           called. That, in turn, means that workers aren't notified. Attempting
    #           to join in `_exit_function` will then result in a hang.
    #
    #           For context, `_exit_function` is also registered as an `atexit` call.
    #           So it is unclear to me (@ssnl) why this is needed in a finally block.
    #           The code dates back to 2008 and there is no comment on the original
    #           PEP 371 or patch https://bugs.python.org/issue3050 (containing both
    #           the finally block and the `atexit` registration) that explains this.
    #
    #
    #      Finally, another choice is to just shutdown workers with logic in 1
    #      above whenever we see an error in `next`. This isn't ideal because
    #        a. It prevents users from using try-catch to resume data loading.
    #        b. It doesn't prevent hanging if users have references to the
    #           iterator.
    #
    #   3. All processes exit if any of them die unexpectedly by fatal signals.
    #
    #      As shown above, the workers are set as daemonic children of the main
    #      process. However, automatic cleaning-up of such child processes only
    #      happens if the parent process exits gracefully (e.g., not via fatal
    #      signals like SIGKILL). So we must ensure that each process will exit
    #      even the process that should send/receive data to/from it were
    #      killed, i.e.,
    #
    #        a. A process won't hang when getting from a queue.
    #
    #           Even with carefully designed data dependencies (i.e., a `put()`
    #           always corresponding to a `get()`), hanging on `get()` can still
    #           happen when data in queue is corrupted (e.g., due to
    #           `cancel_join_thread` or unexpected exit).
    #
    #           For child exit, we set a timeout whenever we try to get data
    #           from `data_queue`, and check the workers' status on each timeout
    #           and error.
    #           See `_DataLoaderiter._get_batch()` and
    #           `_DataLoaderiter._try_get_data()` for details.
    #
    #           Additionally, for child exit on non-Windows platforms, we also
    #           register a SIGCHLD handler (which is supported on Windows) on
    #           the main process, which checks if any of the workers fail in the
    #           (Python) handler. This is more efficient and faster in detecting
    #           worker failures, compared to only using the above mechanism.
    #           See `DataLoader.cpp` and `_utils/signal_handling.py` for details.
    #
    #           For `.get()` calls where the sender(s) is not the workers, we
    #           guard them with timeouts, and check the status of the sender
    #           when timeout happens:
    #             + in the workers, the `_utils.worker.ManagerWatchdog` class
    #               checks the status of the main process.
    #             + if `pin_memory=True`, when getting from `pin_memory_thread`,
    #               check `pin_memory_thread` status periodically until `.get()`
    #               returns or see that `pin_memory_thread` died.
    #
    #        b. A process won't hang when putting into a queue;
    #
    #           We use `mp.Queue` which has a separate background thread to put
    #           objects from an unbounded buffer array. The background thread is
    #           daemonic and usually automatically joined when the process
    #           *exits*.
    #
    #           In case that the receiver has ended abruptly while
    #           reading from the pipe, the join will hang forever.  The usual
    #           solution for this in Python is calling  `q.cancel_join_thread`,
    #           which prevents automatically joining it when finalizing
    #           (exiting).
    #
    #           Nonetheless, `cancel_join_thread` must only be called when the
    #           queue is **not** going to be read from or write into by another
    #           process, because it may hold onto a lock or leave corrupted data
    #           in the queue, leading other readers/writers to hang.
    #
    #           Hence,
    #             + For worker processes, we only do so (for their output
    #               queues, i.e., `worker_result_queue`) before exiting.
    #             + For `pin_memory_thread`, its output queue `data_queue` is a
    #               `queue.Queue` that does blocking `put` if the queue is full.
    #               So there is no above problem, but as a result, in
    #               `_pin_memory_loop`, we do need to  wrap the `put` in a loop
    #               that breaks not only upon success, but also when the main
    #               process stops reading, i.e., is shutting down.
    #             + For loader process, we `cancel_join_thread()` for all
    #               `_index_queues` because the whole purpose of workers and
    #               `pin_memory_thread` is to serve the loader process.  If
    #               loader process is already exiting, we don't really care if
    #               the queues are corrupted.
    #
    #
    # Now let's get back to 1:
    #   how we gracefully exit the workers when the last reference to the
    #   iterator is gone.
    #
    # To achieve this, we implement the following logic along with the design
    # choices mentioned above:
    #
    # `workers_done_event`:
    #   A `multiprocessing.Event` shared among the main process and all worker
    #   processes. This is used to signal the workers that the iterator is
    #   shutting down. After it is set, they will not send processed data to
    #   queues anymore, and only wait for the final `None` before exiting.
    #   `done_event` isn't strictly needed. I.e., we can just check for `None`
    #   from the input queue, but it allows us to skip wasting resources
    #   processing data if we are already shutting down.
    #
    # `pin_memory_thread_done_event`:
    #   A `threading.Event` for a similar purpose to that of
    #   `workers_done_event`, but is for the `pin_memory_thread`. The reason
    #   that separate events are needed is that `pin_memory_thread` reads from
    #   the output queue of the workers. But the workers, upon seeing that
    #   `workers_done_event` is set, only wants to see the final `None`, and is
    #   not required to flush all data in the output queue (e.g., it may call
    #   `cancel_join_thread` on that queue if its `IterableDataset` iterator
    #   happens to exhaust coincidentally, which is out of the control of the
    #   main process). Thus, since we will exit `pin_memory_thread` before the
    #   workers (see below), two separete events are used.
    #
    # NOTE: In short, the protocol is that the main process will set these
    #       `done_event`s and then the corresponding processes/threads a `None`,
    #       and that they may exit at any time after receiving the `None`.
    #
    # NOTE: Using `None` as the final signal is valid, since normal data will
    #       always be a 2-tuple with the 1st element being the index of the data
    #       transferred (different from dataset index/key), and the 2nd being
    #       either the dataset key or the data sample (depending on which part
    #       of the data model the queue is at).
    #
    # [ worker processes ]
    #   While loader process is alive:
    #     Get from `index_queue`.
    #       If get anything else,
    #          Check `workers_done_event`.
    #            If set, continue to next iteration
    #                    i.e., keep getting until see the `None`, then exit.
    #            Otherwise, process data:
    #                If is fetching from an `IterableDataset` and the iterator
    #                    is exhausted, send an `_IterableDatasetStopIteration`
    #                    object to signal iteration end. The main process, upon
    #                    receiving such an object, will send `None` to this
    #                    worker and not use the corresponding `index_queue`
    #                    anymore.
    #       If timed out,
    #          No matter `workers_done_event` is set (still need to see `None`)
    #          or not, must continue to next iteration.
    #   (outside loop)
    #   If `workers_done_event` is set,  (this can be False with `IterableDataset`)
    #     `data_queue.cancel_join_thread()`.  (Everything is ending here:
    #                                          main process won't read from it;
    #                                          other workers will also call
    #                                          `cancel_join_thread`.)
    #
    # [ pin_memory_thread ]
    #   # No need to check main thread. If this thread is alive, the main loader
    #   # thread must be alive, because this thread is set as daemonic.
    #   While `pin_memory_thread_done_event` is not set:
    #     Get from `worker_result_queue`.
    #       If timed out, continue to get in the next iteration.
    #       Otherwise, process data.
    #       While `pin_memory_thread_done_event` is not set:
    #         Put processed data to `data_queue` (a `queue.Queue` with blocking put)
    #         If timed out, continue to put in the next iteration.
    #         Otherwise, break, i.e., continuing to the out loop.
    #
    #   NOTE: we don't check the status of the main thread because
    #           1. if the process is killed by fatal signal, `pin_memory_thread`
    #              ends.
    #           2. in other cases, either the cleaning-up in __del__ or the
    #              automatic exit of daemonic thread will take care of it.
    #              This won't busy-wait either because `.get(timeout)` does not
    #              busy-wait.
    #
    # [ main process ]
    #   In the DataLoader Iter's `__del__`
    #     b. Exit `pin_memory_thread`
    #          i.   Set `pin_memory_thread_done_event`.
    #          ii   Put `None` in `worker_result_queue`.
    #          iii. Join the `pin_memory_thread`.
    #          iv.  `worker_result_queue.cancel_join_thread()`.
    #
    #     c. Exit the workers.
    #          i.   Set `workers_done_event`.
    #          ii.  Put `None` in each worker's `index_queue`.
    #          iii. Join the workers.
    #          iv.  Call `.cancel_join_thread()` on each worker's `index_queue`.
    #
    #        NOTE: (c) is better placed after (b) because it may leave corrupted
    #              data in `worker_result_queue`, which `pin_memory_thread`
    #              reads from, in which case the `pin_memory_thread` can only
    #              happen at timing out, which is slow. Nonetheless, same thing
    #              happens if a worker is killed by signal at unfortunate times,
    #              but in other cases, we are better off having a non-corrupted
    #              `worker_result_queue` for `pin_memory_thread`.
    #
    #   NOTE: If `pin_memory=False`, there is no `pin_memory_thread` and (b)
    #         can be omitted
    #
    # NB: `done_event`s isn't strictly needed. E.g., we can just check for
    #     `None` from `index_queue`, but it allows us to skip wasting resources
    #     processing indices already in `index_queue` if we are already shutting
    #     down.

    _last_yielded_worker_id: int
    _NUM_WORKERS = "_num_workers"
    _SNAPSHOT = "_snapshot"
    _MAIN_SNAPSHOT = "_main_snapshot"
    _WORKER_SNAPSHOTS = "_worker_snapshots"
    _SNAPSHOT_STEP = "_snapshot_step"
    _STEPS_SINCE_SNAPSHOT = "_steps_since_snapshot"
    _LAST_YIELDED_WORKER_ID = "_last_yielded_worker_id"
    _BASE_SEED = "_base_seed"

    def __init__(self, loader, next_iter_state):
        super().__init__(loader)
        self._snapshot_interval = loader.snapshot_every_n_steps
        self._prefetch_factor = loader.prefetch_factor
        self._in_order = loader.in_order

        assert self._num_workers > 0
        assert self._prefetch_factor > 0

        if loader.multiprocessing_context is None:
            multiprocessing_context = multiprocessing
        else:
            multiprocessing_context = loader.multiprocessing_context

        self._worker_init_fn = loader.worker_init_fn

        # Adds forward compatibilities so classic DataLoader can work with DataPipes:
        #   Additional worker init function will take care of sharding in MP and Distributed
        if isinstance(self._dataset, (IterDataPipe, MapDataPipe)):
            self._worker_init_fn = functools.partial(
                _sharding_worker_init_fn,
                self._worker_init_fn,
                self._world_size,
                self._rank,
            )

        # No certainty which module multiprocessing_context is
        self._worker_result_queue = multiprocessing_context.Queue()  # type: ignore[var-annotated]
        self._worker_pids_set = False
        self._shutdown = False
        self._workers_done_event = multiprocessing_context.Event()

        self._index_queues = []
        self._workers = []

        worker_states = {self._worker_key(i): None for i in range(self._num_workers)}
        if next_iter_state is not None:
            assert (
                self._SNAPSHOT in next_iter_state
            ), f"State doesn't contain key '{self._SNAPSHOT}' expected for multiprocess dataloader"
            wstates = next_iter_state[self._SNAPSHOT].get(self._WORKER_SNAPSHOTS, {})
            assert set(map(self._worker_key, range(len(wstates)))) == set(wstates.keys()), (
                len(wstates),
                wstates.keys(),
            )
            for worker_key, sd in wstates.items():
                worker_states[worker_key] = sd
            self._base_seed = next_iter_state[self._SNAPSHOT][self._MAIN_SNAPSHOT].get(self._BASE_SEED, self._base_seed)
            self._shared_seed = next_iter_state[self._SNAPSHOT][self._MAIN_SNAPSHOT].get(
                _SHARED_SEED, self._shared_seed
            )

        for i in range(self._num_workers):
            # No certainty which module multiprocessing_context is
            index_queue = multiprocessing_context.Queue()  # type: ignore[var-annotated]
            # Need to `cancel_join_thread` here!
            # See sections (2) and (3b) above.
            index_queue.cancel_join_thread()

            w = multiprocessing_context.Process(
                target=_worker_loop,
                args=(
                    self._dataset_kind,
                    self._dataset,
                    index_queue,
                    self._worker_result_queue,
                    self._workers_done_event,
                    self._auto_collation,
                    self._collate_fn,
                    self._drop_last,
                    self._base_seed,
                    self._worker_init_fn,
                    i,
                    self._num_workers,
                    self._persistent_workers,
                    self._shared_seed,
                    worker_states[self._worker_key(i)],
                ),
            )
            w.daemon = True
            # NB: Process.start() actually take some time as it needs to
            #     start a process and pass the arguments over via a pipe.
            #     Therefore, we only add a worker to self._workers list after
            #     it started, so that we do not call .join() if program dies
            #     before it starts, and __del__ tries to join but will get:
            #     AssertionError: can only join a started process.
            w.start()
            self._index_queues.append(index_queue)
            self._workers.append(w)

        if self._pin_memory:
            self._pin_memory_thread_done_event = threading.Event()

            # Queue is not type-annotated
            self._data_queue = queue.Queue()  # type: ignore[var-annotated]
            if self._pin_memory_device == "xpu":
                current_device = torch.xpu.current_device()  # type: ignore[attr-defined]
            elif self._pin_memory_device == torch._C._get_privateuse1_backend_name():
                custom_device_mod = getattr(torch, torch._C._get_privateuse1_backend_name())
                current_device = custom_device_mod.current_device()
            else:
                current_device = torch.cuda.current_device()  # choose cuda for default
            pin_memory_thread = threading.Thread(
                target=_utils.pin_memory._pin_memory_loop,
                args=(
                    self._worker_result_queue,
                    self._data_queue,
                    current_device,
                    self._pin_memory_thread_done_event,
                    self._pin_memory_device,
                ),
            )
            pin_memory_thread.daemon = True
            pin_memory_thread.start()
            # Similar to workers (see comment above), we only register
            # pin_memory_thread once it is started.
            self._pin_memory_thread = pin_memory_thread
        else:
            self._data_queue = self._worker_result_queue  # type: ignore[assignment]

        # In some rare cases, persistent workers (daemonic processes)
        # would be terminated before `__del__` of iterator is invoked
        # when main process exits
        # It would cause failure when pin_memory_thread tries to read
        # corrupted data from worker_result_queue
        # atexit is used to shutdown thread and child processes in the
        # right sequence before main process exits
        if self._persistent_workers and self._pin_memory:
            import atexit

            for w in self._workers:
                atexit.register(_StatefulMultiProcessingDataLoaderIter._clean_up_worker, w)

        # .pid can be None only before process is spawned (not the case, so ignore)
        _utils.signal_handling._set_worker_pids(id(self), tuple(w.pid for w in self._workers))  # type: ignore[misc]
        _utils.signal_handling._set_SIGCHLD_handler()
        self._worker_pids_set = True
        self._snapshot, self._main_snapshots = {}, collections.deque()  # type: ignore[var-annotated]
        # NOTE [ Incremental Worker State ]
        # We only send deltas between incremental worker state to the main process. We synchronize
        # the initial states on worker startup, when it sends an _AckStartup signal back with the initial
        # worker states, and if persistent_workers is True, then the worker sends back an initial
        # state after acking the _ResumeIteration signal.
        #
        # We need to send initial worker state back to the main process to handle state_dict() requests
        # before n >= num_workers steps are taken.
        # self._worker_snapshots: Dict[str, _IncrementalWorkerState] = {}
        self._worker_snapshots = {key: _IncrementalWorkerState(state) for key, state in worker_states.items()}
        self._reset(loader, first_iter=True, prime_prefetch=next_iter_state is None)

        # Try to restore main state
        if next_iter_state is not None:
            self._restore_main_state(next_iter_state[self._SNAPSHOT][self._MAIN_SNAPSHOT])
            self._num_yielded = next_iter_state[self._SNAPSHOT][self._SNAPSHOT_STEP]

            self._update_snapshot(
                snapshot_step=next_iter_state[self._SNAPSHOT][self._SNAPSHOT_STEP],
                last_yielded_worker_id=next_iter_state[self._SNAPSHOT][self._LAST_YIELDED_WORKER_ID],
                num_workers=self._num_workers,
                main_snapshot=next_iter_state[self._SNAPSHOT][self._MAIN_SNAPSHOT],
                worker_snapshots=self._worker_snapshots,
            )

            fast_forward = False
            if self._dataset_kind == _DatasetKind.Iterable:
                for state in worker_states.values():
                    if state is None:
                        continue
                    if state[_DATASET_STATE] is None and state[_FETCHER_STATE][_DATASET_ITER_STATE] is None:
                        fast_forward = True
                    break

            if fast_forward:
                # If neither dataset / dataset iter are stateful, we will fast-forward
                for _ in range(self._prefetch_factor * self._num_workers):
                    self._try_put_index()
                if self._num_yielded > 0:
                    logger.warning(
                        f"Neither dataset nor iter(dataset) defines state_dict/load_state_dict so we are "
                        f"naively fast-forwarding your dataset by {self._num_yielded} steps. For more efficient "
                        f"resumes, please implement `state_dict` and `load_state_dict` in your IterableDataset and/or iterator."
                    )
                    for _ in range(self._num_yielded):
                        next(self)
                # Check if last_yielded_worker_id matches
                if self._last_yielded_worker_id != next_iter_state[self._SNAPSHOT][self._LAST_YIELDED_WORKER_ID]:
                    raise ValueError("last_yielded_worker_id does not match, the dataset may have changed")
            else:
                self._last_yielded_worker_id = next_iter_state[self._SNAPSHOT][self._LAST_YIELDED_WORKER_ID]
                for _ in range(self._last_yielded_worker_id + 1):
                    next(self._worker_queue_idx_cycle)
                for _ in range(self._prefetch_factor * self._num_workers):
                    self._try_put_index()

            for _ in range(next_iter_state[self._STEPS_SINCE_SNAPSHOT]):
                next(self)
            self._finished = next_iter_state[_ITERATOR_FINISHED]

    def _reset(self, loader, first_iter=False, prime_prefetch=True):
        super()._reset(loader, first_iter)
        self._send_idx = 0  # idx of the next task to be sent to workers
        self._rcvd_idx = 0  # idx of the next task to be returned in __next__
        # information about data not yet yielded, i.e., tasks w/ indices in range [rcvd_idx, send_idx).
        # map: task idx => - (worker_id,)        if data isn't fetched (outstanding)
        #                  \ (worker_id, data)   if data is already fetched (out-of-order)
        self._task_info = {}
        self._tasks_outstanding = 0  # always equal to count(v for v in task_info.values() if len(v) == 1)
        # A list of booleans representing whether each worker still has work to
        # do, i.e., not having exhausted its iterable dataset object. It always
        # contains all `True`s if not using an iterable-style dataset
        # (i.e., if kind != Iterable).
        # Not that this indicates that a worker still has work to do *for this epoch*.
        # It does not mean that a worker is dead. In case of `_persistent_workers`,
        # the worker will be reset to available in the next epoch.
        self._workers_status = [True for i in range(self._num_workers)]
        # A list of integers representing how many tasks are outstanding for each worker
        # Incremented when a task is dispatched to the worker
        # Decremented when that data has been given to the main thread
        # Each worker should have at most self._prefetch_factor tasks outstanding
        self._workers_num_tasks = [0 for i in range(self._num_workers)]
        # Reset the worker queue cycle so it resumes next epoch at worker 0
        self._worker_queue_idx_cycle = itertools.cycle(range(self._num_workers))
        remaining = self._num_workers
        if first_iter:
            # Request the initial state_dict
            for i in range(self._num_workers):
                self._index_queues[i].put(_AckStartup(i, None))  # type: ignore[arg-type]

            while remaining > 0:
                _, data = self._get_data()
                if not all(self._workers_status):
                    raise ValueError(f"A worker has failed during startup! {self._workers_status}")
                elif isinstance(data, _AckStartup):
                    if isinstance(data.initial_state, ExceptionWrapper):
                        data.initial_state.reraise()

                    if data.is_delta:
                        self._worker_snapshots[self._worker_key(data.worker_id)].apply_delta(data.initial_state)  # type: ignore[arg-type]
                    else:
                        self._worker_snapshots[self._worker_key(data.worker_id)] = _IncrementalWorkerState(
                            data.initial_state  # type: ignore[arg-type]
                        )
                    remaining -= 1
                else:
                    raise ValueError(f"Invalid response from worker after startup: {data}")
        else:
            # We resume the prefetching in case it was enabled
            for idx in range(self._num_workers):
                self._index_queues[idx].put(_utils.worker._ResumeIteration(self._shared_seed))
            resume_iteration_cnt = self._num_workers
            while resume_iteration_cnt > 0:
                return_idx, data = self._get_data()
                if not all(self._workers_status):
                    raise ValueError(f"A worker has failed during Resume! {self._workers_status}")
                if isinstance(return_idx, _utils.worker._ResumeIteration):
                    assert isinstance(data, _AckStartup), (return_idx, data)
                    if isinstance(data.initial_state, ExceptionWrapper):
                        data.initial_state.reraise()
                    assert data.initial_state is not None, data
                    self._worker_snapshots[self._worker_key(data.worker_id)] = _IncrementalWorkerState(
                        data.initial_state  # type: ignore[arg-type]
                    )
                    resume_iteration_cnt -= 1

        # Reset state variables
        self._main_snapshots = collections.deque()
        self._last_yielded_worker_id = self._num_workers - 1
        self._update_snapshot(
            snapshot_step=0,
            last_yielded_worker_id=self._num_workers - 1,
            num_workers=self._num_workers,
            main_snapshot=self._get_main_state(),
            worker_snapshots=self._worker_snapshots,
        )

        if prime_prefetch:
            # prime the prefetch loop
            for _ in range(self._prefetch_factor * self._num_workers):
                self._try_put_index()

    def _update_worker_snapshot(self, worker_key, state_dict):
        if state_dict is None:
            return
        self._worker_snapshots[worker_key].apply_delta(state_dict)

    def state_dict(self):
        if not self._in_order:
            # TODO: remove warning log when state management is supported with in_order=False
            logger.warning(
                "using in_order=False with multiple workers does not give any guarantees for state management "
                "and loading from a checkpoint may not work as expected."
            )
        steps_since_snapshot = self._num_yielded - self._snapshot[self._SNAPSHOT_STEP]
        state_dict = {
            self._SNAPSHOT: self._snapshot,
            self._STEPS_SINCE_SNAPSHOT: steps_since_snapshot,
            _ITERATOR_FINISHED: self._finished,
        }

        return state_dict

    def _try_get_data(self, timeout=_utils.MP_STATUS_CHECK_INTERVAL):
        # Tries to fetch data from `self._data_queue` once for a given timeout.
        # This can also be used as inner loop of fetching without timeout, with
        # the sender status as the loop condition.
        #
        # This raises a `RuntimeError` if any worker died expectedly. This error
        # can come from either the SIGCHLD handler in `_utils/signal_handling.py`
        # (only for non-Windows platforms), or the manual check below on errors
        # and timeouts.
        #
        # Returns a 2-tuple:
        #   (bool: whether successfully get data, any: data if successful else None)
        try:
            data = self._data_queue.get(timeout=timeout)
            return (True, data)
        except Exception as e:
            # At timeout and error, we manually check whether any worker has
            # failed. Note that this is the only mechanism for Windows to detect
            # worker failures.
            failed_workers = []
            for worker_id, w in enumerate(self._workers):
                if self._workers_status[worker_id] and not w.is_alive():
                    failed_workers.append(w)
                    self._mark_worker_as_unavailable(worker_id)
            if len(failed_workers) > 0:
                pids_str = ", ".join(str(w.pid) for w in failed_workers)
                raise RuntimeError(f"DataLoader worker (pid(s) {pids_str}) exited unexpectedly") from e
            if isinstance(e, queue.Empty):
                return (False, None)
            import errno
            import tempfile

            try:
                # Raise an exception if we are this close to the FDs limit.
                # Apparently, trying to open only one file is not a sufficient
                # test.
                # See NOTE [ DataLoader on Linux and open files limit ]
                fds_limit_margin = 10
                fs = [tempfile.NamedTemporaryFile() for i in range(fds_limit_margin)]  # noqa(F841)
            except OSError as e:
                if e.errno == errno.EMFILE:
                    raise RuntimeError(
                        "Too many open files. Communication with the"
                        " workers is no longer possible. Please increase the"
                        " limit using `ulimit -n` in the shell or change the"
                        " sharing strategy by calling"
                        " `torch.multiprocessing.set_sharing_strategy('file_system')`"
                        " at the beginning of your code"
                    ) from None
            raise

    # NOTE [ DataLoader on Linux and open files limit ]
    #
    # On Linux when DataLoader is used with multiprocessing we pass the data between
    # the root process and the workers through SHM files. We remove those files from
    # the filesystem as soon as they are created and keep them alive by
    # passing around their file descriptors through AF_UNIX sockets. (See
    # docs/source/multiprocessing.rst and 'Multiprocessing Technical Notes` in
    # the wiki (https://github.com/pytorch/pytorch/wiki).)
    #
    # This sometimes leads us to exceeding the open files limit. When that happens,
    # and the offending file descriptor is coming over a socket, the `socket` Python
    # package silently strips the file descriptor from the message, setting only the
    # `MSG_CTRUNC` flag (which might be a bit misleading since the manpage says that
    # it _indicates that some control data were discarded due to lack of space in
    # the buffer for ancillary data_). This might reflect the C implementation of
    # AF_UNIX sockets.
    #
    # This behaviour can be reproduced with the script and instructions at the
    # bottom of this note.
    #
    # When that happens, the standard Python `multiprocessing` (and not
    # `torch.multiprocessing`) raises a `RuntimeError: received 0 items of ancdata`
    #
    # Sometimes, instead of the FD being stripped, you may get an `OSError:
    # Too many open files`, both in the script below and in DataLoader. However,
    # this is rare and seems to be nondeterministic.
    #
    #
    #   #!/usr/bin/env python3
    #   import sys
    #   import socket
    #   import os
    #   import array
    #   import shutil
    #   import socket
    #
    #
    #   if len(sys.argv) != 4:
    #       print("Usage: ", sys.argv[0], " tmp_dirname iteration (send|recv)")
    #       sys.exit(1)
    #
    #   if __name__ == '__main__':
    #       dirname = sys.argv[1]
    #       sock_path = dirname + "/sock"
    #       iterations = int(sys.argv[2])
    #       def dummy_path(i):
    #           return dirname + "/" + str(i) + ".dummy"
    #
    #
    #       if sys.argv[3] == 'send':
    #           while not os.path.exists(sock_path):
    #               pass
    #           client = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
    #           client.connect(sock_path)
    #           for i in range(iterations):
    #               fd = os.open(dummy_path(i), os.O_WRONLY | os.O_CREAT)
    #               ancdata = array.array('i', [fd])
    #               msg = bytes([i % 256])
    #               print("Sending fd ", fd, " (iteration #", i, ")")
    #               client.sendmsg([msg], [(socket.SOL_SOCKET, socket.SCM_RIGHTS, ancdata)])
    #
    #
    #       else:
    #           assert sys.argv[3] == 'recv'
    #
    #           if os.path.exists(dirname):
    #               raise Exception("Directory exists")
    #
    #           os.mkdir(dirname)
    #
    #           print("Opening socket...")
    #           server = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM)
    #           server.bind(sock_path)
    #
    #           print("Listening...")
    #           for i in range(iterations):
    #               a = array.array('i')
    #               msg, ancdata, flags, addr = server.recvmsg(1, socket.CMSG_SPACE(a.itemsize))
    #               assert(len(ancdata) == 1)
    #               cmsg_level, cmsg_type, cmsg_data = ancdata[0]
    #               a.frombytes(cmsg_data)
    #               print("Received fd ", a[0], " (iteration #", i, ")")
    #
    #           shutil.rmtree(dirname)
    #
    # Steps to reproduce:
    #
    # 1. Run two shells and set lower file descriptor limit in the receiving one:
    # (shell1) ulimit -n 1020
    # (shell2) ulimit -n 1022
    #
    # 2. Run the script above with the `recv` option in the first shell
    # (shell1) ./test_socket.py sock_tmp 1017 recv
    #
    # 3. Run the script with the `send` option in the second shell:
    # (shell2) ./test_socket.py sock_tmp 1017 send

    def _get_data(self):
        # Fetches data from `self._data_queue`.
        #
        # We check workers' status every `MP_STATUS_CHECK_INTERVAL` seconds,
        # which we achieve by running `self._try_get_data(timeout=MP_STATUS_CHECK_INTERVAL)`
        # in a loop. This is the only mechanism to detect worker failures for
        # Windows. For other platforms, a SIGCHLD handler is also used for
        # worker failure detection.
        #
        # If `pin_memory=True`, we also need check if `pin_memory_thread` had
        # died at timeouts.
        if self._timeout > 0:
            success, data = self._try_get_data(self._timeout)
            if success:
                return data
            else:
                raise RuntimeError(f"DataLoader timed out after {self._timeout} seconds")
        elif self._pin_memory:
            while self._pin_memory_thread.is_alive():
                success, data = self._try_get_data()
                if success:
                    return data
            else:
                # while condition is false, i.e., pin_memory_thread died.
                raise RuntimeError("Pin memory thread exited unexpectedly")
            # In this case, `self._data_queue` is a `queue.Queue`,. But we don't
            # need to call `.task_done()` because we don't use `.join()`.
        else:
            while True:
                success, data = self._try_get_data()
                if success:
                    return data

    def _worker_key(self, worker_id: int) -> str:
        return f"worker_{worker_id}"

    def _next_data(self):
        while True:
            # If the worker responsible for `self._rcvd_idx` has already ended
            # and was unable to fulfill this task (due to exhausting an `IterableDataset`),
            # we try to advance `self._rcvd_idx` to find the next valid index.
            #
            # This part needs to run in the loop because both the `self._get_data()`
            # call and `_IterableDatasetStopIteration` check below can mark
            # extra worker(s) as dead.
            while self._rcvd_idx < self._send_idx:
                info = self._task_info.get(self._rcvd_idx, None)
                if info:
                    worker_id = info[0]
                    if len(info) == 2 or self._workers_status[worker_id]:  # has data or is still active
                        break
                    del self._task_info[self._rcvd_idx]
                self._rcvd_idx += 1
            else:
                # no valid `self._rcvd_idx` is found (i.e., didn't break)
                if not self._persistent_workers:
                    self._shutdown_workers()
                raise StopIteration

            # Now `self._rcvd_idx` is the batch index we want to fetch

            # Check if the next sample has already been generated
            if len(self._task_info[self._rcvd_idx]) == 2:
                data, worker_id, state_dict = self._task_info.pop(self._rcvd_idx)[1]
                if isinstance(data, _utils.worker._IterableDatasetStopIteration):
                    self._update_worker_snapshot(self._worker_key(data.worker_id), state_dict)
                    self._rcvd_idx += 1
                    continue
                else:
                    self._rcvd_idx += 1
                    return self._process_data(data, worker_id, state_dict)

            assert not self._shutdown and self._tasks_outstanding > 0
            idx, (data, worker_id, state_dict) = self._get_data()
            self._tasks_outstanding -= 1
            if self._dataset_kind == _DatasetKind.Iterable:
                # Check for _IterableDatasetStopIteration
                if isinstance(data, _utils.worker._IterableDatasetStopIteration):
                    if self._persistent_workers:
                        self._workers_status[data.worker_id] = False
                    else:
                        self._mark_worker_as_unavailable(data.worker_id)
                    assert state_dict is not None, "StopIteration should always be accompanied by a state_dict"
                    self._try_put_index()
                    # We want to process states until we get to that position
                    # in the worker cycle, therefore if out-of-order we want
                    # to store the StopIteration and process it later

            if idx != self._rcvd_idx:
                # store out-of-order samples
                if not self._in_order:
                    # don't store it for later, process now
                    if isinstance(data, _utils.worker._IterableDatasetStopIteration):
                        self._update_worker_snapshot(self._worker_key(data.worker_id), state_dict)
                        continue
                    del self._task_info[idx]
                    return self._process_data(data, worker_id, state_dict)
                self._task_info[idx] += ((data, worker_id, state_dict),)
            else:
                del self._task_info[idx]
                if isinstance(data, _utils.worker._IterableDatasetStopIteration):
                    self._update_worker_snapshot(self._worker_key(data.worker_id), state_dict)
                    self._rcvd_idx += 1
                    continue
                else:
                    self._rcvd_idx += 1
                    return self._process_data(data, worker_id, state_dict)

    def _get_main_state(self):
        return {
            self._NUM_WORKERS: self._num_workers,
            _SAMPLER_ITER_STATE: try_to_serialize(self._sampler_iter),
            _INDEX_SAMPLER_STATE: try_to_serialize(self._index_sampler),
            _SAMPLER_ITER_YIELDED: self._sampler_iter_yielded,
            _ITERABLEDATASET_LEN_CALLED: self._IterableDataset_len_called,
            _SHARED_SEED: self._shared_seed,
            self._BASE_SEED: self._base_seed,
        }

    def _restore_main_state(self, state_dict):
        assert self._num_workers == state_dict[self._NUM_WORKERS]
        # Try to restore from either _index_sampler state_dict or _sampler_iter state_dict
        self._sampler_iter_yielded = state_dict[_SAMPLER_ITER_YIELDED]
        if isinstance(self._index_sampler, Stateful) or isinstance(self._sampler_iter, Stateful):
            self._index_sampler = try_to_deserialize(self._index_sampler, state_dict[_INDEX_SAMPLER_STATE])
            self._sampler_iter = iter(self._index_sampler)
            if state_dict[_SAMPLER_ITER_STATE] is not None:
                self._sampler_iter = try_to_deserialize(self._sampler_iter, state_dict[_SAMPLER_ITER_STATE])
        else:
            if not isinstance(
                self._index_sampler,
                torch.utils.data.dataloader._InfiniteConstantSampler,
            ):
                # Fallback to fastforward
                self._sampler_iter = itertools.islice(self._index_sampler, self._sampler_iter_yielded, None)
        self._IterableDataset_len_called = state_dict[_ITERABLEDATASET_LEN_CALLED]
        self._shared_seed = state_dict[_SHARED_SEED]
        self._base_seed = state_dict[self._BASE_SEED]

    def _try_put_index(self):
        max_tasks = self._prefetch_factor * self._num_workers
        assert self._tasks_outstanding < max_tasks

        try:
            index = self._next_index()
            snapshot_main = False
            snapshot = False
            if not self._snapshot_interval:
                pass
            elif self._dataset_kind == _DatasetKind.Iterable:
                x = self._num_yielded % self._snapshot_interval
                hi = x + 1 + self._num_workers * self._prefetch_factor
                if hi >= self._snapshot_interval:
                    snapshot_main = True
                if hi + self._num_workers >= self._snapshot_interval:
                    snapshot = True
            else:
                if self._sampler_iter_yielded % self._snapshot_interval == 0:
                    # Snapshot sampler
                    snapshot_main = True
                if (
                    (self._sampler_iter_yielded - 1) % self._snapshot_interval
                ) + self._num_workers >= self._snapshot_interval:
                    snapshot = True
        except StopIteration:
            return
        for _ in range(self._num_workers):  # find the next active worker, if any
            worker_queue_idx = next(self._worker_queue_idx_cycle)
            if self._workers_status[worker_queue_idx]:
                if self._in_order:
                    break
                elif self._workers_num_tasks[worker_queue_idx] < max_tasks // sum(self._workers_status):
                    # when self._in_order is False, distribute work to a worker if it has capacity
                    # _workers_status is updated only in this thread, so the sum is guaranteed > 0
                    break
        else:
            # not found (i.e., didn't break)
            return

        if snapshot_main:
            assert snapshot
            self._main_snapshots.append((self._send_idx, self._get_main_state()))

        self._index_queues[worker_queue_idx].put((self._send_idx, (index, snapshot)))  # type: ignore[possibly-undefined]
        self._task_info[self._send_idx] = (worker_queue_idx,)
        self._workers_num_tasks[worker_queue_idx] += 1
        self._tasks_outstanding += 1
        self._send_idx += 1

    def _process_data(self, data, worker_id, state_dict):
        self._workers_num_tasks[worker_id] -= 1
        self._try_put_index()
        if isinstance(data, ExceptionWrapper):
            data.reraise()
        self._last_yielded_worker_id = worker_id
        # Update latest worker state
        if state_dict is not None:
            self._update_worker_snapshot(self._worker_key(state_dict[_WORKER_ID]), state_dict)
        if self._snapshot_interval and ((self._num_yielded + 1) % self._snapshot_interval == 0):
            self._take_snapshot()
        return data

    def _take_snapshot(self):
        main_snapshot_idx = None
        while len(self._main_snapshots) and (self._main_snapshots[0][0] <= self._rcvd_idx - 1):
            main_snapshot_idx, main_snapshot = self._main_snapshots.popleft()
        if not self._in_order and main_snapshot_idx is None:
            # in_order is False and no main snapshot is available as we're ahead of rcvd_idx
            # we can't take a snapshot with the current implementation
            return
        assert main_snapshot_idx == self._rcvd_idx - 1, (
            main_snapshot_idx,
            self._rcvd_idx - 1,
        )
        self._update_snapshot(
            self._num_yielded + 1,
            self._last_yielded_worker_id,
            self._num_workers,
            main_snapshot,
            self._worker_snapshots,
        )

    def _update_snapshot(
        self,
        snapshot_step: int,
        last_yielded_worker_id: int,
        num_workers: int,
        main_snapshot: Dict[str, Any],
        worker_snapshots: Dict[str, _IncrementalWorkerState],
    ):
        self._snapshot = {
            self._SNAPSHOT_STEP: snapshot_step,
            self._LAST_YIELDED_WORKER_ID: last_yielded_worker_id,
            self._MAIN_SNAPSHOT: main_snapshot,
            self._WORKER_SNAPSHOTS: {key: worker_state.get_state() for key, worker_state in worker_snapshots.items()},
        }

    def _mark_worker_as_unavailable(self, worker_id, shutdown=False):
        # Mark a worker as having finished its work e.g., due to
        # exhausting an `IterableDataset`. This should be used only when this
        # `_MultiProcessingDataLoaderIter` is going to continue running.

        assert self._workers_status[worker_id] or self._persistent_workers or shutdown

        # Signal termination to that specific worker.
        q = self._index_queues[worker_id]
        # Indicate that no more data will be put on this queue by the current
        # process.
        q.put(None)

        # Note that we don't actually join the worker here, nor do we remove the
        # worker's pid from C side struct because (1) joining may be slow, and
        # (2) since we don't join, the worker may still raise error, and we
        # prefer capturing those, rather than ignoring them, even though they
        # are raised after the worker has finished its job.
        # Joinning is deferred to `_shutdown_workers`, which it is called when
        # all workers finish their jobs (e.g., `IterableDataset` replicas) or
        # when this iterator is garbage collected.

        self._workers_status[worker_id] = False

        assert self._workers_done_event.is_set() == shutdown

    def _shutdown_workers(self):
        # Called when shutting down this `_MultiProcessingDataLoaderIter`.
        # See NOTE [ Data Loader Multiprocessing Shutdown Logic ] for details on
        # the logic of this function.
        if _utils is None or _utils.python_exit_status is True or _utils.python_exit_status is None:
            # See (2) of the note. If Python is shutting down, do no-op.
            return
        # Normal exit when last reference is gone / iterator is depleted.
        # See (1) and the second half of the note.
        if not self._shutdown:
            self._shutdown = True
            try:
                # Normal exit when last reference is gone / iterator is depleted.
                # See (1) and the second half of the note.

                # Exit `pin_memory_thread` first because exiting workers may leave
                # corrupted data in `worker_result_queue` which `pin_memory_thread`
                # reads from.
                if hasattr(self, "_pin_memory_thread"):
                    # Use hasattr in case error happens before we set the attribute.
                    self._pin_memory_thread_done_event.set()
                    # Send something to pin_memory_thread in case it is waiting
                    # so that it can wake up and check `pin_memory_thread_done_event`
                    self._worker_result_queue.put((None, None))
                    self._pin_memory_thread.join()
                    self._worker_result_queue.cancel_join_thread()
                    self._worker_result_queue.close()

                # Exit workers now.
                self._workers_done_event.set()
                for worker_id in range(len(self._workers)):
                    # Get number of workers from `len(self._workers)` instead of
                    # `self._num_workers` in case we error before starting all
                    # workers.
                    # If we are using workers_status with persistent_workers
                    # we have to shut it down because the worker is paused
                    if self._persistent_workers or self._workers_status[worker_id]:
                        self._mark_worker_as_unavailable(worker_id, shutdown=True)
                for w in self._workers:
                    # We should be able to join here, but in case anything went
                    # wrong, we set a timeout and if the workers fail to join,
                    # they are killed in the `finally` block.
                    w.join(timeout=_utils.MP_STATUS_CHECK_INTERVAL)
                for q in self._index_queues:
                    q.cancel_join_thread()
                    q.close()
            finally:
                # Even though all this function does is putting into queues that
                # we have called `cancel_join_thread` on, weird things can
                # happen when a worker is killed by a signal, e.g., hanging in
                # `Event.set()`. So we need to guard this with SIGCHLD handler,
                # and remove pids from the C side data structure only at the
                # end.
                #
                # FIXME: Unfortunately, for Windows, we are missing a worker
                #        error detection mechanism here in this function, as it
                #        doesn't provide a SIGCHLD handler.
                if self._worker_pids_set:
                    _utils.signal_handling._remove_worker_pids(id(self))
                    self._worker_pids_set = False
                for w in self._workers:
                    if w.is_alive():
                        # Existing mechanisms try to make the workers exit
                        # peacefully, but in case that we unfortunately reach
                        # here, which we shouldn't, (e.g., pytorch/pytorch#39570),
                        # we kill the worker.
                        w.terminate()

    # staticmethod is used to remove reference to `_MultiProcessingDataLoaderIter`
    @staticmethod
    def _clean_up_worker(w):
        try:
            w.join(timeout=_utils.MP_STATUS_CHECK_INTERVAL)
        finally:
            if w.is_alive():
                w.terminate()

    def __del__(self):
        self._shutdown_workers()