Crash when resetting datapipe with bufffer of filehandles

🐛 Describe the bug

When the datapipe iterator is reset, the multiprocessing reading service tries to pickle the datapipe (why?). In case the data pipe contains a buffer with file handles this fails. A MWE

# curl -O http://storage.googleapis.com/nvdata-openimages/openimages-train-000000.tar

import torchdata.datapipes as dp
from torchdata.dataloader2 import DataLoader2, MultiProcessingReadingService


# strip filehandles
def decode(data):
    return {key: key for key, _value in data.items()}


webds_dp = (
    dp.iter.FileLister(".", masks="openimages*.tar")
    .open_files("rb")
    .load_from_tar()
    .webdataset()
    .shuffle(buffer_size=50)
    .header(5)  # to trigger crash faster
    .map(decode)
)

dl = DataLoader2(
    webds_dp,
    reading_service=MultiProcessingReadingService(4),
)

for _ in dl:
    pass

print("first pass completed")

# reset dataloader
for _ in dl:
    pass

I can avoid the crash decoding the data before feeding them into the buffer, but would like to delay the expensive decode till after the buffer to have the datapipe load faster after initialization.

Traceback

first pass completed
Process ForkProcess-3:
Process ForkProcess-1:
Process ForkProcess-2:
Process ForkProcess-4:
Traceback (most recent call last):
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/eventloop.py", line 133, in DataPipeToQueuesLoop
    for _ in loop:
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/iter.py", line 153, in DataPipeBehindQueues
    source_datapipe = request.reset_fn(source_datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/utils/worker.py", line 164, in process_reset_fn
    graph = traverse_dps(datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 98, in traverse_dps
    return _traverse_helper(datapipe, only_datapipe=True, cache=cache)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 140, in _traverse_helper
    items = _list_connected_datapipes(datapipe, only_datapipe, cache.copy())
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 67, in _list_connected_datapipes
    p.dump(scan_obj)
TypeError: cannot pickle 'ExFileObject' object
Traceback (most recent call last):
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
Traceback (most recent call last):
Traceback (most recent call last):
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/eventloop.py", line 133, in DataPipeToQueuesLoop
    for _ in loop:
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/iter.py", line 153, in DataPipeBehindQueues
    source_datapipe = request.reset_fn(source_datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/utils/worker.py", line 164, in process_reset_fn
    graph = traverse_dps(datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 98, in traverse_dps
    return _traverse_helper(datapipe, only_datapipe=True, cache=cache)
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 314, in _bootstrap
    self.run()
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/eventloop.py", line 133, in DataPipeToQueuesLoop
    for _ in loop:
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/eventloop.py", line 133, in DataPipeToQueuesLoop
    for _ in loop:
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/iter.py", line 153, in DataPipeBehindQueues
    source_datapipe = request.reset_fn(source_datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 140, in _traverse_helper
    items = _list_connected_datapipes(datapipe, only_datapipe, cache.copy())
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/communication/iter.py", line 153, in DataPipeBehindQueues
    source_datapipe = request.reset_fn(source_datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/utils/worker.py", line 164, in process_reset_fn
    graph = traverse_dps(datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 67, in _list_connected_datapipes
    p.dump(scan_obj)
  File "/opt/conda/lib/python3.10/site-packages/torchdata/dataloader2/utils/worker.py", line 164, in process_reset_fn
    graph = traverse_dps(datapipe)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 98, in traverse_dps
    return _traverse_helper(datapipe, only_datapipe=True, cache=cache)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 98, in traverse_dps
    return _traverse_helper(datapipe, only_datapipe=True, cache=cache)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
TypeError: cannot pickle 'ExFileObject' object
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 145, in _traverse_helper
    d[dp_id][1].update(_traverse_helper(item, only_datapipe, cache.copy()))
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 140, in _traverse_helper
    items = _list_connected_datapipes(datapipe, only_datapipe, cache.copy())
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 140, in _traverse_helper
    items = _list_connected_datapipes(datapipe, only_datapipe, cache.copy())
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 67, in _list_connected_datapipes
    p.dump(scan_obj)
  File "/opt/conda/lib/python3.10/site-packages/torch/utils/data/graph.py", line 67, in _list_connected_datapipes
    p.dump(scan_obj)
TypeError: cannot pickle 'ExFileObject' object
TypeError: cannot pickle 'ExFileObject' object

Versions

Test run in pytorch-nightly docker image. (Also fails on pytorch 2.0 with torchdata 0.6.)

PyTorch version: 2.1.0.dev20230514
Is debug build: False
CUDA used to build PyTorch: 11.7
ROCM used to build PyTorch: N/A

OS: Ubuntu 20.04.6 LTS (x86_64)
GCC version: Could not collect
Clang version: Could not collect
CMake version: version 3.22.1
Libc version: glibc-2.31

Python version: 3.10.11 (main, Apr 20 2023, 19:02:41) [GCC 11.2.0] (64-bit runtime)
Python platform: Linux-5.19.0-1024-aws-x86_64-with-glibc2.31
Is CUDA available: False
CUDA runtime version: No CUDA
CUDA_MODULE_LOADING set to: N/A
GPU models and configuration: No CUDA
Nvidia driver version: No CUDA
cuDNN version: No CUDA
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

CPU:
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
Address sizes: 48 bits physical, 48 bits virtual
CPU(s): 64
On-line CPU(s) list: 0-63
Thread(s) per core: 2
Core(s) per socket: 32
Socket(s): 1
NUMA node(s): 1
Vendor ID: AuthenticAMD
CPU family: 23
Model: 49
Model name: AMD EPYC 7R32
Stepping: 0
CPU MHz: 2799.972
BogoMIPS: 5599.94
Hypervisor vendor: KVM
Virtualization type: full
L1d cache: 1 MiB
L1i cache: 1 MiB
L2 cache: 16 MiB
L3 cache: 128 MiB
NUMA node0 CPU(s): 0-63
Vulnerability Itlb multihit: Not affected
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Mmio stale data: Not affected
Vulnerability Retbleed: Mitigation; untrained return thunk; SMT enabled with STIBP protection
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2: Mitigation; Retpolines, IBPB conditional, STIBP always-on, RSB filling, PBRSB-eIBRS Not affected
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Not affected
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf tsc_known_freq pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx f16c rdrand hypervisor lahf_lm cmp_legacy cr8_legacy abm sse4a misalignsse 3dnowprefetch topoext ssbd ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 clzero xsaveerptr rdpru wbnoinvd arat npt nrip_save rdpid

Versions of relevant libraries:
[pip3] numpy==1.24.3
[pip3] torch==2.1.0.dev20230514
[pip3] torchaudio==2.1.0.dev20230514
[pip3] torchdata==0.7.0.dev20230514
[pip3] torchelastic==0.2.2
[pip3] torchtext==0.16.0.dev20230514
[pip3] torchvision==0.16.0.dev20230514
[pip3] triton==2.1.0
[conda] blas 1.0 mkl
[conda] mkl 2023.1.0 h6d00ec8_46342
[conda] mkl-service 2.4.0 py310h5eee18b_1
[conda] mkl_fft 1.3.6 py310h1128e8f_1
[conda] mkl_random 1.2.2 py310h1128e8f_1
[conda] numpy 1.24.3 py310h5f9d8c6_1
[conda] numpy-base 1.24.3 py310hb5e798b_1
[conda] pytorch 2.1.0.dev20230514 py3.10_cuda11.7_cudnn8.5.0_0 pytorch-nightly
[conda] pytorch-cuda 11.7 h778d358_5 pytorch-nightly
[conda] pytorch-mutex 1.0 cuda pytorch-nightly
[conda] torchaudio 2.1.0.dev20230514 py310_cu117 pytorch-nightly
[conda] torchdata 0.7.0.dev20230514 py310 pytorch-nightly
[conda] torchelastic 0.2.2 pypi_0 pypi
[conda] torchtext 0.16.0.dev20230514 py310 pytorch-nightly
[conda] torchtriton 2.1.0+7d1a95b046 py310 pytorch-nightly
[conda] torchvision 0.16.0.dev20230514 py310_cu117 pytorch-nightly

Start with dataloader2/utils/worker.py at process_reset_fn and torch/utils/data/graph.py at traverse_dps and _list_connected_datapipes, then reproduce the provided MWE with MultiProcessingReadingService. Done means resetting the DataLoader2 after the first pass completes without the ExFileObject pickle error.