fix: preserve None projection semantics across FFI boundary in ForeignTableProvider::scan

[Kontinuation]

Which issue does this PR close?

N/A (newly discovered bug)

This is originally found in apache/sedona-db when working on a custom plan node: apache/sedona-db#611 (comment)

Rationale for this change

ForeignTableProvider::scan() converts a None projection (meaning "return all columns") into an empty RVec<usize> before passing it across the FFI boundary. On the receiving side, scan_fn_wrapper always wraps the received RVec in Some(...), passing Some(&vec![]) to the inner TableProvider::scan(). This means "project zero columns" — the exact opposite of the intended "project all columns."

The root cause is that the FFI_TableProvider::scan function signature uses RVec<usize> for the projections parameter. Since RVec<usize> cannot represent None, the None vs. empty-vec distinction is lost at the FFI boundary.

What changes are included in this PR?

Three coordinated changes in datafusion/ffi/src/table_provider.rs:

1. FFI struct definition: Changed scan function pointer signature from RVec<usize> to ROption<RVec<usize>> for the projections parameter, matching how limit already uses ROption<usize> for the same None-vs-value distinction.

2. Receiver side (scan_fn_wrapper): Converts ROption<RVec<usize>> via .into_option().map(...) and passes projections.as_ref() to the inner provider, preserving None semantics.

3. Sender side (ForeignTableProvider::scan): Converts Option<&Vec<usize>> to ROption<RVec<usize>> via .into() instead of using unwrap_or_default().

Plus a new unit test test_scan_with_none_projection_returns_all_columns that directly exercises the FFI round-trip with projection=None and verifies all 3 columns are returned.

Also fixed the existing test_aggregation test to set library_marker_id = mock_foreign_marker_id so it actually exercises the FFI path instead of taking the local bypass.

How are these changes tested?

• New test test_scan_with_none_projection_returns_all_columns: creates a 3-column MemTable, wraps it through FFI with the foreign marker set, calls scan(None), and asserts 3 columns are returned (previously returned 0).

Are these changes safe?

This is a breaking FFI ABI change to the FFI_TableProvider::scan function pointer signature. However:

• The abi_stable crate's #[derive(StableAbi)] generates layout checks at dylib load time, so mismatched dylibs will be caught at load rather than causing silent corruption.
• All existing providers construct FFI_TableProvider via ::new() or ::new_with_ffi_codec(), which internally wire up scan_fn_wrapper — nobody constructs the scan function pointer manually.

[paleolimbot]

cc @timsaucer !

[timsaucer]

Thank you for a very clear and concise solution!

One alternative we could do that would not be breaking for the FFI boundary would be on the Foreign side to check for Some([]) and return an empty batch executor and then on the local side (wrapped fn) change an empty projections into None.

[alamb]

What shall we do with this PR? Is it ready to merge? Should we try @timsaucer 's alternate suggestion?

[Kontinuation]

I had a hard time figuring out how to construct an execution plan producing RecordBatches with zero columns while matching the total number of records in the foreign table. This can be done but in a cumbersome way. I don't think we can simply return an empty batch executor for such cases, since we still need to produce batches with no columns.

Another approach without breaking the ABI is to expand None projection into Some([0, 1, .. n - 1]). The relationship between local projection and FFI projection is as follows:

Semantics     Local                 FFI                Local (scan_fn_wrapper)
Project all   None           -->   [0,1,..n-1]   -->  Some([0,1,..n-1])
Project none  Some([])       -->   []            -->  Some([])
Project       Some([a,b,..]) -->   [a,b,..]      -->  Some([a,b,..])

The "select all" projection represented by None will become Some([0,1,...n-1]) after the roundtrip, but the semantics should be the same.

I think we should either stick to the current approach to make the FFI APIs look the same as local APIs for better consistency and less mental burden, or expand None to Some([0, 1, .. n-1]) when transferring the projection from local side to FFI.