[io] Relatively rare crash of ML Data Loader

[dpiparo]

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Description

The test pyunittests-bindings-pyroot-pythonizations-batchgen sometimes fails for branches <= 6.38 with this error:

2026-03-03T01:17:49.1635690Z ======================================================================
2026-03-03T01:17:49.1636020Z FAIL: test14_big_data (rbatchgenerator_completeness.RBatchGeneratorMultipleFiles.test14_big_data)
2026-03-03T01:17:49.1636480Z ----------------------------------------------------------------------
2026-03-03T01:17:49.1636700Z Traceback (most recent call last):
2026-03-03T01:17:49.1637100Z   File "/Users/sftnight/ROOT-CI/src/bindings/pyroot/pythonizations/test/rbatchgenerator_completeness.py", line 1017, in test14_big_data
2026-03-03T01:17:49.1637530Z     test(batch_size, chunk_size, entries_in_rdf)
2026-03-03T01:17:49.1637930Z   File "/Users/sftnight/ROOT-CI/src/bindings/pyroot/pythonizations/test/rbatchgenerator_completeness.py", line 1010, in test
2026-03-03T01:17:49.1638480Z     self.assertTrue(set(collect_x) == set(i for i in range(num_of_entries)), f"collected length: {len(set(collect_x))}\
2026-03-03T01:17:49.1638960Z AssertionError: False is not true : collected length: 18742                                 generated length 18963

Reproducer

pyunittests-bindings-pyroot-pythonizations-batchgen

ROOT version

<= 6.38

Installation method

all

Operating system

linux, mac

Additional context

No response