[C++] When Validating, ensure DenseUnionArray offsets are increasing

[asfimport]

#8652 (comment)

The format documentation states that "The respective offsets for each child value array must be in order / increasing."
https://arrow.apache.org/docs/format/Columnar.html#dense-union

However this is not currently checked in Array::ValidateFull

Reporter: Ben Kietzman / @bkietz
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] Simplify IPC writer for dense unions (is related to)

PRs and other links:

• GitHub Pull Request #9794

_{Note: This issue was originally created as ARROW-10580. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
The problem is that we currently have tests that deliberately use non-increasing offsets. @wesm should we fix them?

[asfimport]

Antoine Pitrou / @pitrou:
Something else: if we mandate strictly increasing values, we can't collapse contiguous null or empty slots together.

[asfimport]

Wes McKinney / @wesm:
We should discuss on the mailing list. In principle I don't have an issue with non-monotonic offsets, but this change is fully backward compatible and should be forward compatible in principle but it's a gray area so we should discuss and see if we want to make a change to the language in the columnar specification

[asfimport]

Antoine Pitrou / @pitrou:
Which change? The change to forbid non-increasing offsets?

[asfimport]

Wes McKinney / @wesm:
Possibly allowing non-increasing offsets in the columnar spec

[asfimport]

Antoine Pitrou / @pitrou:
Issue resolved by pull request 9794
#9794