[TASK-87] Optimize copying in CPP bindings

[fresh-borzoni]

Summary

closes #87

Replace value-copying FFI layer with opaque Rust-backed types for zero-copy reads and cheaper writes in CPP bindings.

Changes

• Opaque types: replace Datum/FfiDatum/FfiGenericRow/FfiScanRecord copying infrastructure with three opaque Rust types — GenericRowInner (writes), ScanResultInner (scan reads), LookupResultInner (lookup reads)
• Zero-copy reads: new RowView class borrows directly from Rust scan results; LookupResult class replaces (bool found, GenericRow out) pattern

Benchmark Results

Environment: Apple M1 Max, 32 GB RAM, macOS 15.7.1, local fluss cluster
Workload: 1M rows per run, averaged over 100 runs

Two scan configurations:

• 8 mixed fields: int, string, float, int, date, time, timestamp, timestamp_ltz
• 4×100B strings: int + 4 string fields (100 bytes each, ~381 MB raw data)

Each scan polls all 1M records from the server, accumulates them in memory,
then reads every field. Reported time covers both phases (poll + field access).

Memory is measured as C++ heap delta (malloc_zone_statistics size_in_use)
before and after accumulating all records.

Throughput

Metric	main	opaque types	Change
Append fire-and-forget	1,634 ms	1,176 ms	-28%
Append with ack	1,662 ms	1,109 ms	-33%
Scan (8 mixed fields)	639 ms	222 ms	-65%
Scan (4×100B strings)	1,115 ms	349 ms	-69%

Memory (held during scan)

Metric	main	opaque types	Change
Scan (8 mixed fields)	915 MB	119 MB	-87%
Scan (4×100B strings)	1,344 MB	471 MB	-65%

Memory figures reflect worst-case accumulation. In typical streaming usage memory stays flat, but per-batch overhead is still reduced since the intermediate FfiDatum copy layer is eliminated entirely.

[fresh-borzoni]

@zhaohaidao @luoyuxia PTAL 🙏

[luoyuxia]

cc @leekeiabstraction

[luoyuxia]

@fresh-borzoni Thanks for the pr. Maybe you can share the performance improvement benchmark in here.

[Copilot]

Pull request overview

This PR refactors the C++ bindings FFI layer to remove per-row value copying and instead use opaque Rust-backed types for writes and reads, enabling zero-copy scan reads and a more efficient lookup API.

Changes:

• Replace FfiDatum/FfiGenericRow/FfiScanRecords copying with opaque Rust types (GenericRowInner, ScanResultInner, LookupResultInner) exposed via cxx.
• Add zero-copy read APIs in C++ (RowView, ScanRecords iterator) and replace lookup (found, out_row) with LookupResult.
• Add AppendArrowBatch to write Arrow RecordBatches via the Arrow C Data Interface.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
bindings/cpp/src/types.rs	Updates type-id mappings (adds Char/Binary) and introduces resolve_row_types / owned conversion helpers for the new opaque-row write/read paths.
bindings/cpp/src/table.cpp	Implements C++ wrappers for the new opaque types (GenericRow, RowView, ScanRecords, LookupResult) and updates writers/scanners to use them.
bindings/cpp/src/lib.rs	Replaces row/scan/lookup copy-based FFI structs with opaque Rust types and accessor methods; adds Arrow batch import for append.
bindings/cpp/src/ffi_converter.hpp	Removes old GenericRow/ScanRecords conversions and adds support for custom_properties in table descriptors/info.
bindings/cpp/include/fluss.hpp	Updates the public C++ API: write-only GenericRow, new RowView/ScanRecords/LookupResult, new ChangeType, and custom_properties.
bindings/cpp/examples/kv_example.cpp	Migrates lookup example to LookupResult and demonstrates name-based getters.
bindings/cpp/examples/example.cpp	Migrates scan iteration to the new ScanRecords API and adds an AppendArrowBatch example.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[fresh-borzoni]

Addressed comments.
Added simple benchmark table to demonstrate the difference. It's with my local fluss cluster, so it's just for demonstration.

[leekeiabstraction]

Thank you for the PR! Not used to C++ so a bit out of water here but left some comments.

[zhaohaidao]

@fresh-borzoni Thanks for your pr. left minor comments.

[fresh-borzoni]

@leekeiabstraction @zhaohaidao
Ty for the review. Addressed comments.
PTAL 🙏

[leekeiabstraction]

Thank you for the revision! Left a couple of comments

[fresh-borzoni]

@leekeiabstraction @zhaohaidao Ty for the review!

Addressed comments.
PTAL 🙏

[zhaohaidao]

@fresh-borzoni Thank you. Only left one comment.

[zhaohaidao]

Can u add field < row.get_field_count() to avoid panic paths in InternalRow::is_null_at / get_* when schema and row length diverge