test: cover the narrowing direction of cast_and_stamp_schema - #5285
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andygrove wants to merge 1 commit into
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test: cover the narrowing direction of cast_and_stamp_schema#5285andygrove wants to merge 1 commit into
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The doc on cast_and_stamp_schema claims that narrowing a nullable nested child to non-null is not silently lossy because arrow rejects unmasked nulls under a non-nullable field, but no test pinned that. Add one that stamps a nullable-child array against a non-null-declared schema both with and without a real null, so the flag alone does not decide the outcome.
parthchandra
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Aug 7, 2026
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thanks for adding the test @andygrove
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Which issue does this PR close?
Follow-up to review feedback on #5138: #5138 (comment)
Rationale for this change
The doc comment on
cast_and_stamp_schemaclaims that reconciliation follows the declared schema in both directions, and that narrowing a nullable nested child to non-null is not silently lossy because arrow'sStructArray::try_newrejects unmasked nulls under a non-nullable field. That claim was not covered by a test, so nothing would catch it if the narrowing direction ever started producing an array whose nullability flag misreports its own contents.What changes are included in this PR?
One test in
native/common/src/schema.rs,narrowing_a_child_errors_only_when_it_holds_a_null. It stamps aList(Struct(nullable Boolean))array against a schema declaring the child non-null, twice:c0.element.flagThe two cases together pin that the child's
nullableflag alone does not decide the outcome: the data does.No production code changes.
How are these changes tested?
cargo test -p datafusion-comet-common schema::(10 passed).