Fix out-of-bound access in cudf::detail::label_segments

[ttnghia]

In cudf::detail::label_segments, when the input lists column has empty/nulls lists at the end of the column, its offsets column will contain out-of-bound indices. This leads to invalid memory access bug. Such bug is elusive and doesn't show up consistently. Test failures reported in NVIDIA/spark-rapids#6249 are due to this.

The existing unit tests already cover such corner case. Unfortunately, the bug didn't show up until being tested on some systems. Even that, it was very difficult to reproduce it.

Closes #11495.

[ttnghia]

Added DO_NOT_MERGE label, waiting for Spark team to verify.

[upsj]

LGTM!

[bdice]

Looks good. Thanks for the fix.

[bdice]

Can we add a test that catches this?

[ttnghia]

Can we add a test that catches this?

No way to catch this consistently. I had to run tests in Spark with large random input columns to get the test inconsistently failed.

compute-sanitizer doesn't catch anything unless the test failed.

[ttnghia]

In addition, the current list distinct tests already covered this case but they never failed.

[ttnghia]

I guess maybe rmm typically allocates more than enough memory for the column thus reading just off-by-one out-of-bound still be a valid read? Only in some special situations when such off-by-one out-of-bound indices are actually out of valid memory bound then the bug shows up?

[abellina]

I ran this locally with a test that was reproducing and with the patch it doesn't anymore. LGTM

[jbrennan333]

+1 good catch!

[ttnghia]

Rerun tests.

[codecov]

Codecov Report

Merging #11497 (dcff820) into branch-22.08 (622e0f4) will not change coverage.
The diff coverage is n/a.

❗ Current head dcff820 differs from pull request most recent head 0489a3f. Consider uploading reports for the commit 0489a3f to get more accurate results

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  Files               144      144           
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  Hits              19765    19765           
  Misses             3091     3091           

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[mythrocks]

👍 Yikes. This is a good corner case to look out for.