From 54f894ef8f8b4e4f03092d08c7d50a5da07dc673 Mon Sep 17 00:00:00 2001
From: Yifan Chen <30335308+emecii@users.noreply.github.com>
Date: Wed, 9 Sep 2026 05:31:27 -0700
Subject: [PATCH] fix: preserve scalar UDF strictness across FFI

Forward ScalarUDFImpl::is_strict through FFI_ScalarUDF and cover both forced-foreign and cross-library round trips.

Generated-by: Codex (GPT-5)
---
 datafusion/ffi/src/tests/udf_udaf_udwf.rs |  4 ++++
 datafusion/ffi/src/udf/mod.rs             | 17 +++++++++++++++++
 datafusion/ffi/tests/ffi_udf.rs           |  2 ++
 3 files changed, 23 insertions(+)

diff --git a/datafusion/ffi/src/tests/udf_udaf_udwf.rs b/datafusion/ffi/src/tests/udf_udaf_udwf.rs
index 830c639c743d6..c5ca9056bce4e 100644
--- a/datafusion/ffi/src/tests/udf_udaf_udwf.rs
+++ b/datafusion/ffi/src/tests/udf_udaf_udwf.rs
@@ -150,6 +150,10 @@ impl ScalarUDFImpl for PlacementUDF {
         datafusion_common::internal_err!("placement_udf is not meant to be invoked")
     }
 
+    fn is_strict(&self) -> bool {
+        true
+    }
+
     fn placement(&self, args: &[ExpressionPlacement]) -> ExpressionPlacement {
         // Push to the leaves only for a (Column, Literal) pairing, so the
         // test catches dropped, reordered, or truncated arguments.
diff --git a/datafusion/ffi/src/udf/mod.rs b/datafusion/ffi/src/udf/mod.rs
index d14614f1474a3..1d70b678645b8 100644
--- a/datafusion/ffi/src/udf/mod.rs
+++ b/datafusion/ffi/src/udf/mod.rs
@@ -130,6 +130,9 @@ pub struct FFI_ScalarUDF {
             udf: &Self,
             config: FFI_ConfigOptions,
         ) -> FFI_Result<FFI_Option<FFI_ScalarUDF>>,
+
+    /// FFI equivalent to [`ScalarUDFImpl::is_strict`].
+    pub is_strict: unsafe extern "C" fn(udf: &Self) -> bool,
 }
 
 unsafe impl Send for FFI_ScalarUDF {}
@@ -193,6 +196,10 @@ unsafe extern "C" fn placement_fn_wrapper(
     udf.inner().placement(&args).into()
 }
 
+unsafe extern "C" fn is_strict_fn_wrapper(udf: &FFI_ScalarUDF) -> bool {
+    udf.inner().is_strict()
+}
+
 unsafe extern "C" fn preserves_lex_ordering_fn_wrapper(
     udf: &FFI_ScalarUDF,
     inputs: SVec<FFI_ExprProperties>,
@@ -321,6 +328,7 @@ impl From<Arc<ScalarUDF>> for FFI_ScalarUDF {
             library_marker_id: crate::get_library_marker_id,
             preserves_lex_ordering: preserves_lex_ordering_fn_wrapper,
             with_updated_config: with_updated_config_fn_wrapper,
+            is_strict: is_strict_fn_wrapper,
         }
     }
 }
@@ -502,6 +510,10 @@ impl ScalarUDFImpl for ForeignScalarUDF {
         self.udf.short_circuits
     }
 
+    fn is_strict(&self) -> bool {
+        unsafe { (self.udf.is_strict)(&self.udf) }
+    }
+
     fn coerce_types(&self, arg_types: &[DataType]) -> Result<Vec<DataType>> {
         unsafe {
             let arg_types = vec_datatype_to_rvec_wrapped(arg_types)?;
@@ -576,6 +588,10 @@ mod tests {
             internal_err!("placement_udf is not meant to be invoked")
         }
 
+        fn is_strict(&self) -> bool {
+            true
+        }
+
         fn placement(&self, args: &[ExpressionPlacement]) -> ExpressionPlacement {
             // Push to the leaves only for a (Column, Literal) pairing, so the
             // test catches dropped, reordered, or truncated arguments.
@@ -663,6 +679,7 @@ mod tests {
         ffi_udf.library_marker_id = crate::mock_foreign_marker_id;
         let foreign_udf: Arc<dyn ScalarUDFImpl> = (&ffi_udf).into();
         assert!(foreign_udf.is::<ForeignScalarUDF>());
+        assert!(foreign_udf.is_strict());
 
         // Without the plumbing the override is dropped and every call is
         // KeepInPlace. The three cases also check the arguments survive the
diff --git a/datafusion/ffi/tests/ffi_udf.rs b/datafusion/ffi/tests/ffi_udf.rs
index 73fafef13415b..3ecf7043bf91a 100644
--- a/datafusion/ffi/tests/ffi_udf.rs
+++ b/datafusion/ffi/tests/ffi_udf.rs
@@ -100,6 +100,8 @@ mod tests {
         let ffi_placement_func = (module.create_placement_udf)();
         let foreign_func: Arc<dyn ScalarUDFImpl> = (&ffi_placement_func).into();
 
+        assert!(foreign_func.is_strict());
+
         // The override pushes to the leaves only for (Column, Literal), so these
         // also check the arguments cross the boundary in order.
         assert_eq!(