[ffi] Fix arrow-array null_count error during conversion from C to Ru…

…st (apache#6674)

* Fix arrow-array null_count error during conversion from C to Rust

* add ffi_array.null_count_opt

* add Safety to follow clippy

* add Safety to follow clippy

arrow-array/src/ffi.rs

@@ -302,8 +302,8 @@ impl ImportedArrowArray<'_> {
         let len = self.array.len();
         let offset = self.array.offset();
         let null_count = match &self.data_type {
-            DataType::Null => 0,
-            _ => self.array.null_count(),
+            DataType::Null => Some(0),
+            _ => self.array.null_count_opt(),
         };
 
         let data_layout = layout(&self.data_type);
@@ -329,7 +329,7 @@ impl ImportedArrowArray<'_> {
             ArrayData::new_unchecked(
                 self.data_type,
                 len,
-                Some(null_count),
+                null_count,
                 null_bit_buffer,
                 offset,
                 buffers,
@@ -635,6 +635,38 @@ mod tests_to_then_from_ffi {
     }
     // case with nulls is tested in the docs, through the example on this module.
 
+    #[test]
+    fn test_null_count_handling() {
+        let int32_data = ArrayData::builder(DataType::Int32)
+            .len(10)
+            .add_buffer(Buffer::from_slice_ref([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]))
+            .null_bit_buffer(Some(Buffer::from([0b01011111, 0b00000001])))
+            .build()
+            .unwrap();
+        let mut ffi_array = FFI_ArrowArray::new(&int32_data);
+        assert_eq!(3, ffi_array.null_count());
+        assert_eq!(Some(3), ffi_array.null_count_opt());
+        // Simulating uninitialized state
+        unsafe {
+            ffi_array.set_null_count(-1);
+        }
+        assert_eq!(None, ffi_array.null_count_opt());
+        let int32_data = unsafe { from_ffi_and_data_type(ffi_array, DataType::Int32) }.unwrap();
+        assert_eq!(3, int32_data.null_count());
+
+        let null_data = &ArrayData::new_null(&DataType::Null, 10);
+        let mut ffi_array = FFI_ArrowArray::new(null_data);
+        assert_eq!(10, ffi_array.null_count());
+        assert_eq!(Some(10), ffi_array.null_count_opt());
+        // Simulating uninitialized state
+        unsafe {
+            ffi_array.set_null_count(-1);
+        }
+        assert_eq!(None, ffi_array.null_count_opt());
+        let null_data = unsafe { from_ffi_and_data_type(ffi_array, DataType::Null) }.unwrap();
+        assert_eq!(0, null_data.null_count());
+    }
+
     fn test_generic_string<Offset: OffsetSizeTrait>() -> Result<()> {
         // create an array natively
         let array = GenericStringArray::<Offset>::from(vec![Some("a"), None, Some("aaa")]);

arrow-data/src/ffi.rs

@@ -271,6 +271,21 @@ impl FFI_ArrowArray {
         self.null_count as usize
     }
 
+    /// Returns the null count, checking for validity
+    #[inline]
+    pub fn null_count_opt(&self) -> Option<usize> {
+        usize::try_from(self.null_count).ok()
+    }
+
+    /// Set the null count of the array
+    ///
+    /// # Safety
+    /// Null count must match that of null buffer
+    #[inline]
+    pub unsafe fn set_null_count(&mut self, null_count: i64) {
+        self.null_count = null_count;
+    }
+
     /// Returns the buffer at the provided index
     ///
     /// # Panic