[manuzhang]

Add the refs metadata table so callers can inspect a table's branches and tags through Table::Inspect(MetadataTableKind::kRefs). Rows are sorted by reference name and expose six columns: name, type (BRANCH or TAG), snapshot_id, max_reference_age_in_ms, min_snapshots_to_keep, and max_snapshot_age_in_ms. Unset retention values are null.

RefsTable and RefsScan use the current metadata scan builder, static tasks, and shared batch helper. They support column selection and projection, configurable batch sizes, and tasks and streams that retain their inputs. Time-travel options return kNotSupported. Null source tables return kInvalidArgument; null snapshot references return kInvalid when opening a task, with errors preserved through repeated Arrow C stream reads.

Move the existing metadata tests and shared fixture into src/iceberg/test/inspect. Refs coverage includes format versions 1, 2, and 3, branch and tag values, null retention fields, empty tables, batch boundaries, projection and selection, task and stream lifetimes, and invalid inputs.

Validation: all 55 tests in metadata_table_test passed, including 25 refs tests, using a Debug build with system Arrow 23.0.1. pre-commit and git diff --check passed.

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Copilot review overview

4 open findings

• This introduces C++20-only features in implementation code: abbreviated function templates (`const… · New
• AppendOptional always appends via AppendInt(... int64_t ...), but column 4 in this table schema… · New
• This introduces C++20-only features in implementation code: abbreviated function templates (`const… · New
• The new stream implementation adds batching logic via MetadataTable::kBatchSize, but the added… · New

What changed in this PR

This PR adds an inspect metadata table (RefsTable) that exposes Iceberg branch/tag snapshot references as an Arrow stream, along with a shared internal helper for emitting bounded Arrow record batches.

Changes:

• Introduces RefsTable (schema + scan implementation) and wires it into the metadata table kind enum / build system.
• Adds a shared internal Arrow row-stream helper to generate bounded Arrow batches from row iterators.
• Adds RefsTable unit tests (format v2/v3) and integrates them into the test target.

File	Description
src/​iceberg/​type_fwd.h	Adds forward declaration for RefsTable.
src/​iceberg/​inspect/​metadata_table.h	Adds kRefs metadata table kind.
src/​iceberg/​inspect/​refs_table.h	Declares the new RefsTable metadata table API.
src/​iceberg/​inspect/​refs_table.cc	Implements RefsTable schema + scanning over snapshot refs; sorts results.
src/​iceberg/​inspect/​metadata_table_stream_internal.h	Adds internal Arrow stream implementation for iterating rows in bounded batches.
src/​iceberg/​test/​inspect/​refs_table_test.cc	Adds tests validating schema, sorting, and retention-field projection.
src/​iceberg/​test/​CMakeLists.txt	Registers the new refs table tests in the test target.
src/​iceberg/​cpp20_compatibility_internal.h	Includes the new header for compatibility aggregation.
src/​iceberg/​CMakeLists.txt	Adds inspect/refs_table.cc to library sources.

🧠 Review effort: Lite

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Warning

Copilot couldn't run its full agentic review because it didn't start before the timeout. Make sure your repository has a runner available, or add a copilot-code-review.yml file specifying one with the runs-on attribute. See the docs for more details.

Copilot review overview

3 open findings

• This CMakeLists change points the metadata_table_test target at inspect/*.cc source paths, but… · New
• AppendOptional() always appends via AppendInt(... int64_t ...), but column 4… · New
• The new stream implementation adds batching logic via MetadataTable::kBatchSize, but the added…

3 resolved since last review

• This introduces C++20-only features in implementation code: abbreviated function templates (`const…
• AppendOptional always appends via AppendInt(... int64_t ...), but column 4 in this table schema…
• This introduces C++20-only features in implementation code: abbreviated function templates (`const…

🧠 Review effort: Lite

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