Box<dyn> and Arc<dyn> in multiple threads situation - rust

Question

i have code below, it is a threadpool implementation, type Job is my problem:
pub struct ThreadPool {
workers:Vec<Worker>,
sender: mpsc::Sender<Job>
}
type Job = Box<dyn FnOnce() + Send + 'static>;
impl ThreadPool {
pub fn new (size: usize) -> ThreadPool {
let mut workers = Vec::with_capacity(size);
let (sender, receiver) = mpsc::channel();
let receiver = Arc::new(Mutex::new(receiver));
for id in 0..size {
workers.push(Worker::new(id, Arc::clone(&receiver)));
}
ThreadPool{ workers, sender }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce()->() + Send + 'static,
{
&self.sender.send(Box::new(f)).unwrap();
}
}
struct Worker {
id:usize,
thread: thread::JoinHandle<Arc<Mutex<Receiver<Job>>>>
}
impl Worker {
fn new(id:usize, reciever:Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn( move || loop {
let job = reciever.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job()
});
Worker { id, thread }
}
}
the code above can work well and type Job is defined by Box.
then i tried to define type Job by Arc,
type Job = Arc<dyn FnOnce() + Send + 'static>;
the code shows error below:
error[E0277]: `(dyn FnOnce() + Send + 'static)` cannot be shared between threads safely
--> src\lib.rs:36:22
|
36 | let thread = thread::spawn( move || loop {
| ^^^^^^^^^^^^^ `(dyn FnOnce() + Send + 'static)` cannot be shared between threads safely
|
= help: the trait `Sync` is not implemented for `(dyn FnOnce() + Send + 'static)`
= note: required for `Arc<(dyn FnOnce() + Send + 'static)>` to implement `Send`
= note: 1 redundant requirement hidden
= note: required for `std::sync::mpsc::Receiver<Arc<(dyn FnOnce() + Send + 'static)>>` to implement `Send`
= note: required for `Mutex<std::sync::mpsc::Receiver<Arc<(dyn FnOnce() + Send + 'static)>>>` to implement `Sync`
= note: 1 redundant requirement hidden
= note: required for `Arc<Mutex<std::sync::mpsc::Receiver<Arc<(dyn FnOnce() + Send + 'static)>>>>` to implement `Send`
note: required by a bound in `spawn`
i think Arc is a thread safe smart pointer, why i can not replace Arc with Box?

Answer 1

Arc is not always thread-safe, it is only Send and Sync if the type inside is Send and Sync. This is because it can be used to access data from threads other than the thread created it. dyn FnOnce() + Send is obviously Send, but not Sync. Replace it with dyn FnOnce() + Send + Sync.