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GH-54937: [C++] Fix DenseUnionArray::Make for sliced type_ids and value_offsets arrays - #54940
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…nd value_offsets arrays DenseUnionArray::Make used type_ids.offset() as the offset of the union array for both the type ids and the value offsets buffers, and never checked the length of value_offsets. With a value_offsets array sliced differently from type_ids the union read value offsets from the wrong positions (which ValidateFull cannot detect), and a value_offsets array shorter than type_ids was accepted. Reject value_offsets arrays shorter than type_ids, slice both buffers to their logical windows (zero-copy) when the inputs are sliced, and build the union with offset 0. Unsliced inputs keep using the original buffers.
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Rationale for this change
DenseUnionArray::Makeusedtype_ids.offset()as the union offset for both the type ids and the value offsets buffers, and never checked the length ofvalue_offsets. With avalue_offsetsarray that is sliced differently fromtype_idsthe union reads value offsets from the wrong positions (andValidateFullcannot tell), and a too-shortvalue_offsetsarray is accepted. See GH-54937; same root cause as GH-54907 (#54938).What changes are included in this PR?
DenseUnionArray::MakereturnsInvalidwhenvalue_offsetsis shorter thantype_ids. A longervalue_offsetsarray is still tolerated (only the firstlen(type_ids)offsets are used), since existing callers rely on that.type_idsorvalue_offsetsis sliced, its buffer is sliced (zero-copy) to the logical window and the union is built with offset 0. Unsliced inputs keep using the original buffers.TestUnionArrayFactories.TestMakeDenseSliced.Are these changes tested?
Yes. The new test fails on main (sliced
value_offsetsyields[10, 20]instead of[20, 30], and the shortvalue_offsetsarray is accepted) and passes with the fix.arrow-array-test,arrow-ipc-read-write-test,arrow-scalar-testandarrow-compute-scalar-type-testpass locally (Debug, ASan/UBSan).Are there any user-facing changes?
DenseUnionArray::Makenow returnsInvalidwhenvalue_offsetsis shorter thantype_ids, and the resulting array has offset 0 instead oftype_ids.offset().AI usage disclosure: Claude Code assisted with investigating the root cause, preparing the implementation, and developing the regression test. I reviewed the changes and am responsible for their correctness.