Fix list equal for empty offset list array (#1818)

* Fix list equal for empty offset list array

* For review

Author: viirya

arrow/src/array/equal/list.rs

@@ -73,6 +73,11 @@ pub(super) fn list_equal<T: OffsetSizeTrait>(
     // however, one is more likely to slice into a list array and get a region that has 0
     // child values.
     // The test that triggered this behaviour had [4, 4] as a slice of 1 value slot.
+    // For the edge case that zero length list arrays are always equal.
+    if len == 0 {
+        return true;
+    }
+
     let lhs_child_length = lhs_offsets[lhs_start + len].to_usize().unwrap()
         - lhs_offsets[lhs_start].to_usize().unwrap();
 

arrow/src/array/equal/mod.rs

@@ -629,6 +629,57 @@ mod tests {
         test_equal(&a, &b, false);
     }
 
+    #[test]
+    fn test_empty_offsets_list_equal() {
+        let empty: Vec<i32> = vec![];
+        let values = Int32Array::from(empty);
+        let empty_offsets: [u8; 0] = [];
+
+        let a = ArrayDataBuilder::new(DataType::List(Box::new(Field::new(
+            "item",
+            DataType::Int32,
+            true,
+        ))))
+        .len(0)
+        .add_buffer(Buffer::from(&empty_offsets))
+        .add_child_data(values.data().clone())
+        .null_bit_buffer(Some(Buffer::from(&empty_offsets)))
+        .build()
+        .unwrap();
+
+        let b = ArrayDataBuilder::new(DataType::List(Box::new(Field::new(
+            "item",
+            DataType::Int32,
+            true,
+        ))))
+        .len(0)
+        .add_buffer(Buffer::from(&empty_offsets))
+        .add_child_data(values.data().clone())
+        .null_bit_buffer(Some(Buffer::from(&empty_offsets)))
+        .build()
+        .unwrap();
+
+        test_equal(&a, &b, true);
+
+        let c = ArrayDataBuilder::new(DataType::List(Box::new(Field::new(
+            "item",
+            DataType::Int32,
+            true,
+        ))))
+        .len(0)
+        .add_buffer(Buffer::from(vec![0i32, 2, 3, 4, 6, 7, 8].to_byte_slice()))
+        .add_child_data(
+            Int32Array::from(vec![1, 2, -1, -2, 3, 4, -3, -4])
+                .data()
+                .clone(),
+        )
+        .null_bit_buffer(Some(Buffer::from(vec![0b00001001])))
+        .build()
+        .unwrap();
+
+        test_equal(&a, &c, true);
+    }
+
     // Test the case where null_count > 0
     #[test]
     fn test_list_null() {