How do I share a mutable object between threads using Arc? - multithreading

Question

I'm trying to share a mutable object between threads in Rust using Arc, but I get this error:
error[E0596]: cannot borrow data in a `&` reference as mutable
--> src/main.rs:11:13
|
11 | shared_stats_clone.add_stats();
| ^^^^^^^^^^^^^^^^^^ cannot borrow as mutable
This is the sample code:
use std::{sync::Arc, thread};
fn main() {
let total_stats = Stats::new();
let shared_stats = Arc::new(total_stats);
let threads = 5;
for _ in 0..threads {
let mut shared_stats_clone = shared_stats.clone();
thread::spawn(move || {
shared_stats_clone.add_stats();
});
}
}
struct Stats {
hello: u32,
}
impl Stats {
pub fn new() -> Stats {
Stats { hello: 0 }
}
pub fn add_stats(&mut self) {
self.hello += 1;
}
}
What can I do?

Answer 1

Arc's documentation says:
Shared references in Rust disallow mutation by default, and Arc is no exception: you cannot generally obtain a mutable reference to something inside an Arc. If you need to mutate through an Arc, use Mutex, RwLock, or one of the Atomic types.
You will likely want a Mutex combined with an Arc:
use std::{
sync::{Arc, Mutex},
thread,
};
struct Stats;
impl Stats {
fn add_stats(&mut self, _other: &Stats) {}
}
fn main() {
let shared_stats = Arc::new(Mutex::new(Stats));
let threads = 5;
for _ in 0..threads {
let my_stats = shared_stats.clone();
thread::spawn(move || {
let mut shared = my_stats.lock().unwrap();
shared.add_stats(&Stats);
});
// Note: Immediately joining, no multithreading happening!
// THIS WAS A LIE, see below
}
}
This is largely cribbed from the Mutex documentation.
How can I use shared_stats after the for? (I'm talking about the Stats object). It seems that the shared_stats cannot be easily converted to Stats.
As of Rust 1.15, it's possible to get the value back. See my additional answer for another solution as well.
[A comment in the example] says that there is no multithreading. Why?
Because I got confused! :-)
In the example code, the result of thread::spawn (a JoinHandle) is immediately dropped because it's not stored anywhere. When the handle is dropped, the thread is detached and may or may not ever finish. I was confusing it with JoinGuard, a old, removed API that joined when it is dropped. Sorry for the confusion!
For a bit of editorial, I suggest avoiding mutability completely:
use std::{ops::Add, thread};
#[derive(Debug)]
struct Stats(u64);
// Implement addition on our type
impl Add for Stats {
type Output = Stats;
fn add(self, other: Stats) -> Stats {
Stats(self.0 + other.0)
}
}
fn main() {
let threads = 5;
// Start threads to do computation
let threads: Vec<_> = (0..threads).map(|_| thread::spawn(|| Stats(4))).collect();
// Join all the threads, fail if any of them failed
let result: Result<Vec<_>, _> = threads.into_iter().map(|t| t.join()).collect();
let result = result.unwrap();
// Add up all the results
let sum = result.into_iter().fold(Stats(0), |i, sum| sum + i);
println!("{:?}", sum);
}
Here, we keep a reference to the JoinHandle and then wait for all the threads to finish. We then collect the results and add them all up. This is the common map-reduce pattern. Note that no thread needs any mutability, it all happens in the master thread.