Extensible Concurrency with the
Interestingly, the Rust language itself knows very little about concurrency. Almost everything we’ve talked about so far in this chapter has been part of the standard library, not the language. Our concurrency options are not limited to the language or the standard library, meaning we can write our own concurrency options or use ones others have written.
There are two concurrency concepts embedded in the language, however: the
Allowing Transference of Ownership Between Threads with
Send marker trait indicates that ownership of the type implementing
Send may be transferred between threads. Almost every Rust type is
but there are some exceptions, including
Rc<T>: this cannot be
if we cloned an
Rc<T> value and tried to transfer ownership of the clone to
another thread, both threads might update the reference count at the same time.
For this reason,
Rc<T> is implemented for use in single-threaded situations
where you don’t want to pay the threadsafe performance penalty.
In this way Rust’s type system and trait bounds ensure we can never
accidentally send an
Rc<T> value across threads unsafely. When we tried to do
this in Listing 16-14, we got an error that said
the trait Send is not implemented for Rc<Mutex<i32>>. When we switched to
Arc<T>, which is
the code compiled.
Any type composed entirely of
Send types is automatically marked as
well. Almost all primitive types are
Send, aside from raw pointers, which
we’ll discuss in Chapter 19.
Allowing Access from Multiple Threads with
Sync marker trait indicates that it is safe for the type implementing
Sync to be referenced from multiple threads. Another way to say this is that
&T (a reference to
Send, meaning the
reference can be sent safely to another thread. In a similar manner as
primitive types are
Sync and types composed entirely of types that are
Rc<T> is also not
Sync, for the same reasons that it’s not
RefCell<T> (which we talked about in Chapter 15) and the family of related
Cell<T> types are not
Sync. The implementation of borrow checking that
RefCell<T> does at runtime is not threadsafe.
Sync, and can
be used to share access with multiple threads as we saw in the previous section.
Sync Manually is Unsafe
Because types that are made up of
Sync traits are automatically
Sync, we don’t have to implement those traits ourselves. As
marker traits, they don’t even have any methods to implement. They’re just
useful for enforcing concurrency-related invariants.
Manually implementing these traits involves implementing unsafe Rust code.
We’re going to be talking about using unsafe Rust code in Chapter 19; for now,
the important information is that building new concurrent types not made up of
Sync parts requires careful thought, in order to uphold the safety
guarantees. The Nomicon has more information about these guarantees and how
to uphold them.
This isn’t the last we’ll see of concurrency in this book; the project in Chapter 20 will use these concepts in a more realistic situation than the smaller examples discussed here.
As we mentioned, since very little of how Rust deals with concurrency is part of the language, many concurrency solutions are implemented as crates. These evolve more quickly than the standard library; search online for the current state-of-the-art crates to use in multithreaded situations.
Rust provides channels for message passing and smart pointer types like
Arc<T> that are safe to use in concurrent contexts. The type
system and the borrow checker will make sure the code using these solutions
won’t end up with data races or invalid references. Once we get our code
compiling, we can rest assured that it will happily run on multiple threads
without the kinds of hard-to-track-down bugs common in other languages.
Concurrent programming is no longer something to be afraid of: go forth and
make your programs concurrent, fearlessly!
Next, let’s talk about idiomatic ways to model problems and structure solutions as your Rust programs get bigger, and how Rust’s idioms relate to those you might be familiar with from Object Oriented Programming.