[branch-53] fix: preserve None projection semantics across FFI boundary in ForeignTableProvider::scan (#20393)#20895
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…nTableProvider::scan (apache#20393) ## 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.
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alamb
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Mar 12, 2026
| provider: &Self, | ||
| session: FFI_SessionRef, | ||
| projections: RVec<usize>, | ||
| projections: ROption<RVec<usize>>, |
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this seems like an API change to me -- are we ok porting to 53?
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53.0.0(Feb 2026 / Mar 2026) #19692This PR: