feat!(ffi): new visit_schema FFI and rename old visit_schema to visit_snapshot_schema

ffi/src/schema.rs

@@ -1,6 +1,6 @@
 use std::os::raw::c_void;
 
-use crate::scan::CStringMap;
+use crate::scan::{CStringMap, SharedSchema};
 use crate::{handle::Handle, kernel_string_slice, KernelStringSlice, SharedSnapshot};
 use delta_kernel::schema::{ArrayType, DataType, MapType, PrimitiveType, StructType};
 
@@ -201,11 +201,32 @@ pub struct EngineSchemaVisitor {
 ///
 /// Caller is responsible for passing a valid snapshot handle and schema visitor.
 #[no_mangle]
-pub unsafe extern "C" fn visit_schema(
+pub unsafe extern "C" fn visit_snapshot_schema(
     snapshot: Handle<SharedSnapshot>,
     visitor: &mut EngineSchemaVisitor,
 ) -> usize {
     let snapshot = unsafe { snapshot.as_ref() };
+    visit_schema_impl(snapshot.schema(), visitor)
+}
+
+/// Visit the given `schema` using the provided `visitor`. See the documentation of
+/// [`EngineSchemaVisitor`] for a description of how this visitor works.
+///
+/// This method returns the id of the list allocated to hold the top level schema columns.
+///
+/// # Safety
+///
+/// Caller is responsible for passing a valid schema handle and schema visitor.
+#[no_mangle]
+pub unsafe extern "C" fn visit_schema(
+    schema: Handle<SharedSchema>,
+    visitor: &mut EngineSchemaVisitor,
+) -> usize {
+    let schema = unsafe { schema.as_ref() };
+    visit_schema_impl(schema, visitor)
+}
+
+fn visit_schema_impl(schema: &StructType, visitor: &mut EngineSchemaVisitor) -> usize {
     // Visit all the fields of a struct and return the list of children
     fn visit_struct_fields(visitor: &EngineSchemaVisitor, s: &StructType) -> usize {
         let child_list_id = (visitor.make_field_list)(visitor.data, s.fields.len());
@@ -316,5 +337,5 @@ pub unsafe extern "C" fn visit_schema(
         }
     }
 
-    visit_struct_fields(visitor, snapshot.schema())
+    visit_struct_fields(visitor, schema)
 }