GH-49470: [C++][Gandiva] Fix crashes in substring_index and truncate with extreme integer values

[dmitry-chirkov-dremio]

Rationale for this change

Two Gandiva functions crash when called with extreme integer parameter values:

1. substring_index(VARCHAR, VARCHAR, INT) crashes with SIGBUS when count is INT_MIN
2. truncate(BIGINT, INT) crashes with SIGSEGV when scale is INT_MAX or INT_MIN

What changes are included in this PR?

substring_index fix (gdv_string_function_stubs.cc):

• Replace abs(cnt) with safe int64_t computation to avoid undefined behavior when cnt == INT_MIN

truncate fix (precompiled/extended_math_ops.cc):

• Return input unchanged for positive scales (no-op for integers)
• Return 0 for scales < -38 to prevent out-of-bounds access in GetScaleMultiplier

Are these changes tested?

Yes. Added coverage for INT_MAX/INT_MIN values in gdv_function_stubs_test.cc and extended_math_ops_test.cc.

Are there any user-facing changes?

No.

This PR contains a "Critical Fix". These changes fix crashes caused by:

• abs(INT_MIN) triggering undefined behavior (integer overflow) in substring_index
• Out-of-bounds array access in GetScaleMultiplier when truncate receives extreme scale values

• GitHub Issue: [C++][Gandiva] Functions substring_index and truncate crash with extreme integer values #49470

[github-actions]

⚠️ GitHub issue #49470 has been automatically assigned in GitHub to PR creator.

[lriggs]

Is it possible to check the function parameter for the bad size and exit early like in the truncation fix? That seems simpler.

[dmitry-chirkov-dremio]

I'd leave as is:

1. The int64_t fix is more robust - it handles ALL negative values correctly, not just INT_MIN
2. It's the same number of lines - early exit adds 5 lines, current fix adds 3 lines
3. The current fix also simplifies the existing code - removes redundant static_cast<int32_t> casts

The early-exit approach only guards against the specific crash values, while the int64_t approach fixes the underlying type-safety issue.