Support Native Iceberg MOR write

[unikdahal]

What is the problem?

Comet supports native Iceberg scans and data-file writes, but Iceberg V2 merge-on-read DELETE, UPDATE, and MERGE still execute the position-delta write through Iceberg's JVM WriteDelta path.

The goal of this issue is to make the executor-side Iceberg position-delta write native while keeping Iceberg planning, validation, and the final table commit on the Spark/Iceberg JVM side.

MergeRowsExec / CometMergeRowsExec
        |
        | operation + _file + _pos + _spec_id + _partition
        v
CometIcebergDeltaWriteExec
        |
        +-- data rows -----> iceberg-rust data writer
        |
        +-- deletes -------> iceberg-rust position-delete writer
        |
        v
Iceberg WriterCommitMessage
        |
        v
existing Iceberg JVM commit

The native path should support:

• Iceberg format v2
• DELETE, UPDATE, and MERGE
• Parquet position deletes
• PARTITION and FILE delete granularity
• partition evolution / historical specs
• replacement of existing file-scoped position deletes
• JVM fallback whenever the native path cannot safely reproduce Iceberg behavior

Prerequisites

iceberg-rust

• feat: add file-scoped position delete index loader iceberg-rust#3291 — load existing file-scoped position deletes
• feat: add sorting position-only delete writer iceberg-rust#3329 — sort/de-duplicate unordered position-delete input

Tracked under apache/iceberg-rust#3287.

These keep generic Iceberg delete-file loading/rewrite behavior in iceberg-rust rather than reimplementing it in Comet.

Comet writer/runtime foundations

• Arrow struct writer should respect projected schema width #6576 / fix: write columnar structs to Arrow schema width #6578 — support projected Spark structs when writing the narrower Arrow schema required by metadata projections
• Native Iceberg Write: close cleanup ownership gap during JVM handoff #6581 / fix: close Iceberg cleanup ownership gap during JVM handoff #6582 — close the native → JVM cleanup ownership gap before extending the writer to position-delete files

General native Iceberg writer productionization remains tracked by #5649.

Comet implementation

Proposed incremental split:

• WriteDelta planning and JVM fallback

  • recognize Iceberg WriteDelta
  • extract operation/data/row-id/metadata projections
  • preserve existing Iceberg JVM execution as the fallback path

• Native position-delta engine

  • route data rows and position deletes
  • load/rewrite existing file-scoped deletes
  • handle historical partition specs
  • produce native data/delete file metadata

• Native WriteDelta integration

  • serde / task payload
  • CometIcebergDeltaWriteExec
  • reconstruct Iceberg writer commit messages
  • preserve JVM-side table commit

• Spark-version integration

  • integrate with native MergeRowsExec where its Spark contract can be preserved
  • preserve JVM fallback for unsupported row-level execution shapes

MERGE compatibility

#5318 provides native MergeRowsExec independently of native Iceberg writes.

Its current compatibility boundary is intentionally:

• Spark 3.5.x / 4.0.x — native MergeRowsExec
• Spark 4.1+ — JVM fallback until action metrics and MergeSummary / V2 writer commit semantics are preserved end-to-end
• Spark 4.2 InsertOnlyMergeExec — separate operator/follow-up

DELETE and UPDATE do not depend on native MergeRowsExec and can use the native delta writer independently.

Scope

This issue is limited to Iceberg V2 merge-on-read position-delta writes.

Out of scope:

• equality-delete generation
• Iceberg V3 deletion vectors
• reimplementing Iceberg snapshot/transaction commit logic
• general native Iceberg writer productionization already tracked by [EPIC] Production-quality native Iceberg writes #5649

Related

• Accelerate row-level MERGE / UPDATE / DELETE plans (MergeRowsExec, ReplaceDataExec, WriteDeltaExec) #5122 — row-level MERGE / UPDATE / DELETE
• feat: add native support for MergeRowsExec (row-level MERGE INTO) #5318 — native MergeRowsExec
• [EPIC] Production-quality native Iceberg writes #5649 — production-quality native Iceberg writes
• Arrow struct writer should respect projected schema width #6576 / fix: write columnar structs to Arrow schema width #6578 — Arrow struct projection support
• Native Iceberg Write: close cleanup ownership gap during JVM handoff #6581 / fix: close Iceberg cleanup ownership gap during JVM handoff #6582 — native/JVM cleanup ownership handoff
• Support loading and rewriting file-scoped position deletes iceberg-rust#3287
• feat: add file-scoped position delete index loader iceberg-rust#3291
• feat: add sorting position-only delete writer iceberg-rust#3329

[unikdahal]

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