`IterableWrapper` fails on an SQLite cursor

🐛 Describe the bug

I am wrapping an sqlite3 cursor inside an IterableWrapper for further processing.

import sqlite3

from torchdata.nodes import IterableWrapper, Loader, ParallelMapper

db = sqlite3.connect(':memory:')
db.execute('create table data (value int)')
db.executemany('insert into data values (?)', [(i,) for i in range(100)])

node = IterableWrapper(db.execute('select value from data'))
node = ParallelMapper(node, map_fn=lambda x: x, num_workers=3, method='thread')
loader = Loader(node)
result = list(loader)
print(result)

This raises

sqlite3.ProgrammingError: SQLite objects created in a thread can only be used in that same thread.

I note that

• if I use a Mapper, it works
• surprisingly if I use a Mapper followed by a ParallelMapper, I get the same error -- not sure why ParallelMapper has any relationship with the original sqlite cursor at this point.

For some context, I am trying to reproduce a tensorflow data pipeline based on tf.data.experimental.SqlDataset.

Versions

Collecting environment information...
PyTorch version: 2.6.0+cpu
Is debug build: False
CUDA used to build PyTorch: None
ROCM used to build PyTorch: N/A

OS: Ubuntu 22.04.5 LTS (x86_64)
GCC version: (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Clang version: Could not collect
CMake version: Could not collect
Libc version: glibc-2.35

Python version: 3.10.16 | packaged by conda-forge | (main, Dec 5 2024, 14:16:10) [GCC 13.3.0] (64-bit runtime)
Python platform: Linux-6.8.0-52-generic-x86_64-with-glibc2.35
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
Address sizes: 39 bits physical, 48 bits virtual
Byte Order: Little Endian
CPU(s): 12
On-line CPU(s) list: 0-11
Vendor ID: GenuineIntel
Model name: 12th Gen Intel(R) Core(TM) i7-1250U
CPU family: 6
Model: 154
Thread(s) per core: 2
Core(s) per socket: 10
Socket(s): 1
Stepping: 4
CPU max MHz: 4700,0000
CPU min MHz: 400,0000
BogoMIPS: 3763.20
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf tsc_known_freq pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdseed adx smap clflushopt clwb intel_pt sha_ni xsaveopt xsavec xgetbv1 xsaves split_lock_detect user_shstk avx_vnni dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp hwp_pkg_req hfi vnmi umip pku ospke waitpkg gfni vaes vpclmulqdq rdpid movdiri movdir64b fsrm md_clear serialize arch_lbr ibt flush_l1d arch_capabilities
Virtualization: VT-x
L1d cache: 352 KiB (10 instances)
L1i cache: 576 KiB (10 instances)
L2 cache: 6,5 MiB (4 instances)
L3 cache: 12 MiB (1 instance)
NUMA node(s): 1
NUMA node0 CPU(s): 0-11
Vulnerability Gather data sampling: Not affected
Vulnerability Itlb multihit: Not affected
Vulnerability L1tf: Not affected
Vulnerability Mds: Not affected
Vulnerability Meltdown: Not affected
Vulnerability Mmio stale data: Not affected
Vulnerability Reg file data sampling: Mitigation; Clear Register File
Vulnerability Retbleed: Not affected
Vulnerability Spec rstack overflow: Not affected
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; Enhanced / Automatic IBRS; IBPB conditional; RSB filling; PBRSB-eIBRS SW sequence; BHI BHI_DIS_S
Vulnerability Srbds: Not affected
Vulnerability Tsx async abort: Not affected

Versions of relevant libraries:
[pip3] flake8==3.9.2
[pip3] mypy-boto3-s3==1.34.162
[pip3] mypy-extensions==1.0.0
[pip3] numpy==1.26.4
[pip3] torch==2.6.0+cpu
[pip3] torchdata==0.11.0
[conda] numpy 1.26.4 pypi_0 pypi
[conda] torch 2.6.0+cpu pypi_0 pypi
[conda] torchdata 0.11.0 pypi_0 pypi

Start from the IterableWrapper and ParallelMapper entry points and reproduce the provided SQLite cursor example with the stated torchdata version. Compare the Mapper-only and ParallelMapper paths, then verify that cursor-backed iteration no longer raises the reported thread error while processing the rows.