Does mpsc::Receiver need to be in a Mutex to be used with Arc?

Question

I am reading https://doc.rust-lang.org/book/ch20-02-multithreaded.html and it states the following:

...taking a job off the channel queue involves mutating the receiver, so the threads need a safe way to share and modify receiver; otherwise, we might get race conditions...

However when I look at the Receiver::recv docs it shows that the method takes an immutable reference, so why is the book implying that: Receiving from the Receiver mutates it and thus should be Mutex'd?

Would it not work correctly just behind an Arc (no mutex)?

Answer 1

Yes.

We know because in the Trait Implementation section of the documentation we see that it implements:

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> !Sync for Receiver<T> {}

This means it can not be shared safely without some form of synchronization structure such as a mutex.

That being said, you probably wont want to use a mutex. mpsc stands for Multiple Producer, Single Consumer. You likely want mpmc (Multiple Producer, Multiple Consumer) channels instead. The crossbeam-channel provides this type of functionality and is the most popular crate (that I know of) for channels. You can create multiple receivers for a single channel by cloning the first receiver.