Cannot share Arc variables when spawning threads - multithreading

Question

I have the following struct to represent the server object:
pub struct Server {
client_managers: Arc<ClientManager>,
listener: Option<TcpListener>,
}
Here is the code that receives a client's connection and handles it in a new thread:
fn serve(&self) {
for stream in self.listener.as_ref().unwrap().incoming() {
match stream {
Ok(stream) => {
let client_manager = &mut self.client_managers.clone();
// let client_manager = Arc.new(self.client_managers);
thread::spawn(move || {
client_manager.do_something();
});
}
Err(e) => {
println!("connection error: {}", e);
}
}
}
}
However, I get the following error when compiling:
error[E0716]: temporary value dropped while borrowed
--> server/src/server.rs:37:47
|
37 | let client_manager = &mut self.client_managers.clone();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ creates a temporary which is freed while still in use
38 | // let client_manager = Arc.new(self.client_managers);
39 | / thread::spawn(move || {
40 | | client_manager.nothing();
41 | | });
| |______________________- argument requires that borrow lasts for `'static`
42 | }
| - temporary value is freed at the end of this statement
I understood why this error happened. My question is:
1) I use Arc by following some tutorials online. (Example) But why do their examples work but not mine?
2) How can I fix this error in my situation? (I still want to share the object client_manager).

Answer 1

thread::spawn takes a closure that is 'static, meaning that it cannot borrow data from outside the thread. However, this line will clone the Arc and borrow it, and passing the borrowed reference into the thread:
let client_manager = &mut self.client_managers.clone();
thread::spawn(move || {
client_manager.do_something();
// ^-- client_manager is a `&mut Arc<_>` borrowed from outside the thread
});
Instead, what you want is to just clone the Arc, not borrow it in any way before its passed into the thread:
let client_manager = self.client_managers.clone();
thread::spawn(move || {
client_manager.do_something();
// ^-- client_manager is a `Arc<_>` owned by the new thread
});
The Arc has shared ownership over the value, so it is only destroyed once all Arc pointers referring to it falls out of scope, even across threads.