[JonSnow1807]

Rationale for this change

StringDataTransform reuses the input offsets buffer whenever input.offset == 0, but it writes the transformed characters to a new data buffer starting at byte 0 (it allocates offsets[length] - offsets[0] bytes). When the first value offset is not 0 (a valid array, ValidateFull() accepts it) the reused offsets point past the end of the output data and the result is invalid: Offset invariant failure: offset for slot 1 out of bounds: 8 > 3. Reproduced with the issue's snippet for string and large_string.

What changes are included in this PR?

Only reuse the input offsets buffer when the first offset is 0. Otherwise fall into the existing branch that allocates new offsets and rebases them by offsets[0], which is already what sliced inputs do. The common case (first offset 0, unsliced input) is unchanged and still reuses the buffer without copying.

Are these changes tested?

Yes. TestStringKernels.AsciiCaseTransformNonZeroFirstOffset (typed over string and large_string) builds an array with offsets [5, 8, 8, 8, 10], a null, an empty string and a leading unused data prefix, checks it validates and equals ["abC", null, "", "De"], then checks all three kernels on the full array and on slices with a non-zero array offset. Without the fix the test fails with the offset invariant failure above; with it arrow-compute-scalar-type-test passes (350 tests, Debug + ASan/UBSan).

Out of scope: for a length-0 input whose offsets buffer is absent, the reuse branch already copied sizeof(offset_type) bytes from a null pointer before this change; this PR does not change that path.

Are there any user-facing changes?

The three kernels now return valid arrays for such inputs. No API change.

This PR contains a "Critical Fix". Before this change the three kernels returned arrays whose reused offsets pointed past the new data buffer, i.e. invalid data, for any input whose first value offset is not 0.

• GitHub Issue: [C++][Compute] ascii_upper / ascii_lower / ascii_swapcase return an invalid array when the first offset is not 0 #54906

AI usage: the fix and the test were written with an AI coding assistant (Claude Code) under my direction; I reviewed every line and ran the tests locally.

Was AI used for this PR?

In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.

PR code and description written by:

• Human
• AI

Reviewed before submission by:

• Human
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• Not reviewed

Code and tests were drafted with an AI coding assistant and then reviewed, built and tested by me; the description is mine.

[github-actions]

⚠️ GitHub issue #54906 has been automatically assigned in GitHub to PR creator.

[dmitry-chirkov-dremio]

LGTM. Clean fix and for the clear test.

Could you update the PR description? This fixes a bug that produced invalid data, so please add the "Critical Fix" line from the template.

[JonSnow1807]

Done, added the "Critical Fix" line to the description. Thanks for the review.