Server: Problem with consuming behaviour of `Server::run_server_on_runtime`

[cbusoft]

Hi, currently the function Server::run_server_on_runtime consumes the tokio::runtime::Runtime. This works fine with block set to true, but when block is set to false, the runtime gets dropped after Some(runtime.spawn(server_task)).

opcua/lib/src/server/server.rs

Lines 250 to 266 in fcc89d8

	pub fn run_server_on_runtime<F>(
	runtime: tokio::runtime::Runtime,
	server_task: F,
	block: bool,
	) -> Option<tokio::task::JoinHandle<<F as futures::Future>::Output>>
	where
	F: std::future::Future + Send + 'static,
	F::Output: Send + 'static,
	{
	if block {
	runtime.block_on(server_task);
	info!("Server has finished");
	None
	} else {
	Some(runtime.spawn(server_task))
	}
	}

This is a problem, because "Shutting down the runtime is done by dropping the value" (see https://docs.rs/tokio/1.41.1/tokio/runtime/struct.Runtime.html#shutdown). I don't see a reason why the run_server_on_runtime method would need to consume the value, so passing in the runtime by reference would most likely work, but I think it would even be better to use a tokio::runtime::Handle, since this can also be obtained via Handle::current() when the runtime is created by tokio through #[tokio::main] instead of explicitly by the user.