BUG: Make RangeIndex._data read-only to prevent external mutation

[Benlite777]

What does this fix

When a Series is created from a RangeIndex, the underlying cached _data array can be modified externally, silently corrupting the index. This happens because _data is decorated with @cache_readonly but the returned numpy array is writeable.

import pandas as pd

idx = pd.RangeIndex(3)
ser = pd.Series(idx)
ser.iloc[0] = 99
# idx is now corrupted: RangeIndex(start=0, stop=3, step=1) but _data shows [99, 1, 2]

Fix

Mark the _data array as read-only (data.flags.writeable = False) after construction, consistent with the same pattern used in:

• DatetimeIndex (pandas/core/indexes/datetimelike.py:1041)
• MultiIndex (pandas/core/indexes/multi.py:5133)
• Index.values (pandas/core/indexes/base.py:5256)

Confirmed by @jbrockmendel that read-only is the desired approach.

Test

Added test_rangeindex_data_is_immutable which verifies:

1. Direct modification of _data raises ValueError
2. Creating a Series from a RangeIndex and modifying it does not corrupt the original index

Retargeted from #67060 per @rhshadrach's comment.

Fixes #67060

[rhshadrach]

Apologies @Benlite777 - I didn't triage the original issue deep enough. #49663 already raises on main due to being read-only and should have been closed. Your issue is different. For yours, it's due to RangeIndex._simple_new setting _references=None.

Instead, I believe we need to create a new BlockValueRefs a RangeIndex (lazily, for perf) - you can do this by changing _references to being a @cache_readonly property on RangeIndex, something like:

@cache_readonly
def _references(self) -> BlockValuesRefs:  # type: ignore[override]
    result = BlockValuesRefs()
    result.add_index_reference(self)
    return result

cc @mroeschke for feedback on this proposal.

[rhshadrach]

Will also need to rework the whatsnew slightly too.

[rhshadrach]

With my comment above, this test can be removed and I think you can just add RangeIndex to the parametrization of test_series_from_index

[Benlite777]

@rhshadrach Thanks for the review! I've reworked the fix based on your feedback:

Changes:

• Reverted the _data writeable flag change (not needed, Index.values already handles that)
• Fixed RangeIndex._simple_new to initialize _references with BlockValuesRefs() and register the index reference via add_index_reference, matching the pattern in Index._simple_new
• Updated the test to verify that modifying a Series created from a RangeIndex does not corrupt the original index (CoW triggers a copy)
• Reworked the whatsnew entry accordingly

[Benlite777]

@rhshadrach Thanks for the detailed feedback! I've reworked the fix:

Changes:

• Made _references a @cache_readonly property on RangeIndex (lazy initialization, as you suggested) instead of setting it in _simple_new
• Removed _references = None from _simple_new — the property handles it now
• Removed the standalone test_rangeindex_data_is_immutable test and added RangeIndex to the parametrization of test_series_from_index in test_constructors.py
• Import for RangeIndex added to the test file

Waiting on @mroeschke's feedback on the approach as you mentioned.