Fix memory leak in runtime interface (#4837)

* Fix memory leak in runtime interface

We used `slice::from_raw_parts` in runtime-interface which did not free
the memory afterwards. This pr changes it to `Vec::from_raw_parts` to
make sure `drop` is called properly and the values are freed.

* Check that `len` is non-zero

* Adds comment

substrate/primitives/runtime-interface/test-wasm/src/lib.rs

@@ -39,6 +39,17 @@ pub trait TestApi {
 		data
 	}
 
+	/// Returns 16kb data.
+	///
+	/// # Note
+	///
+	/// We return a `Vec<u32>` because this will use the code path that uses SCALE
+	/// to pass the data between native/wasm. (Vec<u8> is passed without encoding the
+	/// data)
+	fn return_16kb() -> Vec<u32> {
+		vec![0; 4 * 1024]
+	}
+
 	/// Set the storage at key with value.
 	fn set_storage(&mut self, key: &[u8], data: &[u8]) {
 		self.place_storage(key.to_vec(), Some(data.to_vec()));
@@ -211,4 +222,28 @@ wasm_export_functions! {
 			assert_eq!(*val, test_api::get_and_return_i128(*val));
 		}
 	}
+
+	fn test_vec_return_value_memory_is_freed() {
+		let mut len = 0;
+		for _ in 0..1024 {
+			len += test_api::return_16kb().len();
+		}
+		assert_eq!(1024 * 1024 * 4, len);
+	}
+
+	fn test_encoded_return_value_memory_is_freed() {
+		let mut len = 0;
+		for _ in 0..1024 {
+			len += test_api::return_option_input(vec![0; 16 * 1024]).map(|v| v.len()).unwrap();
+		}
+		assert_eq!(1024 * 1024 * 16, len);
+	}
+
+	fn test_array_return_value_memory_is_freed() {
+		let mut len = 0;
+		for _ in 0..1024 * 1024 {
+			len += test_api::get_and_return_array([0; 34])[1];
+		}
+		assert_eq!(0, len);
+	}
 }