Trait object cannot be shared between threads safely - rust

Question

When I am writing a zookeeper client in RUST.
code like this :
pub trait ZookeeperHandler {
fn on_child_change(&self, path : &str, value : &String);
fn on_valude_change(&self, path : &str, value: String);
fn reload_all_child(&self, path : &str, children : HashMap<String, String>);
}
pub struct WatchNode {
path : String,
node_type: i32,
handler : Box<dyn ZookeeperHandler>,
}
pub struct WatchNodeManager {
watch_nodes : HashMap<String, WatchNode>,
}
impl WatchNodeManager {
//do something
}
pub struct ZKProxy {
init : bool,
address: String,
zk : Option<ZooKeeper>,
watch_nodes : WatchNodeManager,
}
lazy_static! {
pub static ref ZK_PROXY : Box<ZKProxy> = Box::new(ZKProxy::new(String::from("127.0.0.1:2081")));
}
This fails with the following message:
|
59 | / lazy_static! {
60 | | pub static ref ZK_PROXY : Box<ZKProxy> = Box::new(ZKProxy::new(String::from("127.0.0.1:2081")));
61 | | }
| |_^ `(dyn ZookeeperHandler + 'static)` cannot be shared between threads safely
|
As a beginner of RUST, I can't figure out what this error means and how to fix it.

Answer 1

As global statics can be accessed by multiple threads at the same time, they have to be thread safe. Objects can have the auto-trait Sync that indicates that they are thread-safe.
Unless something prevents that, all types are Send + Sync. In your case, though, your type contains a Box<dyn Zookeeper>, which is neither Send nor Sync, because it could be anything that is dyn Zookeeper. You need to tell Rust that this only accepts Sync objects:
pub struct WatchNode {
path: String,
node_type: i32,
handler: Box<dyn ZookeeperHandler + Sync>,
}