[jira] [Created] (FLINK-40616) Make BITMAP and VARIANT conversion handling consistent and remove redundant checks

["Ramin Gharib (Jira)" <ji...@apache.org>]

Ramin Gharib created FLINK-40616:
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             Summary: Make BITMAP and VARIANT conversion handling consistent and remove redundant checks
                 Key: FLINK-40616
                 URL: https://issues.apache.org/jira/browse/FLINK-40616
             Project: Flink
          Issue Type: Improvement
          Components: API / Type Serialization System, Table SQL / Runtime
            Reporter: Ramin Gharib
            Assignee: Ramin Gharib


BITMAP and VARIANT are the two sealed built-in binary types. Each has a single canonical internal representation, RoaringBitmapData and BinaryVariant. Their handling across the type system had drifted apart, and the canonical-form invariant was enforced in several overlapping places for BITMAP but only one for VARIANT.

Concrete divergences:

- Conversion-class validation used a per-type static whitelist set (INPUT_OUTPUT_CONVERSION). BitmapType listed both the interface and the concrete RoaringBitmapData, while VariantType listed only the interface, so the two produced asymmetric extraction and bridging results.
- DataStructureConverters registered VARIANT as a plain identity converter but BITMAP through a bespoke BitmapBitmapConverter that additionally re-validated the runtime value.
- In DataFormatConverters, BitmapConverter carried a per-record instanceof RoaringBitmapData guard that VariantConverter did not have.
- BitmapSerializer.serialize did not enforce the canonical form, whereas VariantSerializer.serialize does via toBinaryVariant.

This change makes the two types follow the same paths and enforces the canonical form once per type, in the serializer:

- BitmapType/VariantType: supportsInputConversion/supportsOutputConversion use Class.isAssignableFrom, dropping the static whitelist sets.
- ClassDataTypeConverter/ValueDataTypeConverter: both types resolve to their interface conversion class uniformly, removing the duplicated per-value branches.
- DataStructureConverters: VARIANT and BITMAP both return a shared, stateless IdentityConverter.INSTANCE; the redundant switch guard is dropped. IdentityConverter gains that singleton, reused for all identity conversions.
- DataFormatConverters: BitmapConverter becomes a plain identity converter, matching VariantConverter.
- BitmapSerializer.serialize routes through RoaringBitmapData.toRoaringBitmapData, matching VariantSerializer.
- The now-unused BitmapBitmapConverter is removed.

Note on behavior: There is no user-facing change for the supported implementations, RoaringBitmapData and BinaryVariant. Custom implementations remain unsupported; the Bitmap/Variant Javadoc contract is unchanged. The only difference is that a non-canonical conversion class is now rejected at the serialization boundary instead of by a plan-time whitelist.



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