[rust-101.git] / src / part15.rs

// Rust-101, Part 15: Mutex, Interior Mutability (cont.), RwLock, Sync
// ===================================================================

use std::sync::{Arc, Mutex};
use std::thread;
use std::time::Duration;

//@ We already saw that we can use `Arc` to share memory between threads. However, `Arc` can only
//@ provide *read-only* access to memory: Since there is aliasing, Rust cannot, in general, permit
//@ mutation. To implement shared-memory concurrency, we need to have aliasing and permutation -
//@ following, of course, some strict rules to make sure there are no data races. In Rust, shared-
//@ memory concurrency is obtained through *interior mutability*, which we already discussed in a
//@ single-threaded context in part 12.
//@ 
//@ ## `Mutex`
//@ The most basic type for interior mutability that supports concurrency is
//@ [`Mutex<T>`](https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html).
//@ This type implements *critical sections* (or *locks*), but in a data-driven way: One has to
//@ specify the type of the data that's protected by the mutex, and Rust ensures that the data is
//@ *only* accessed through the mutex. In other words, "lock data, not code" is actually enforced
//@ by the type system, which becomes possible because of the discipline of ownership and
//@ borrowing.
//@ 
//@ As an example, let us write a concurrent counter. As usual in Rust, we first have to think
//@ about our data layout: That will be `Mutex<usize>`. Of course, we want multiple threads to have
//@ access to this `Mutex`, so we wrap it in an `Arc`.
//@ 
//@ Rather than giving every field a name, a struct can also be defined by just giving a sequence
//@ of types (similar to how a variant of an `enum` is defined). This is called a *tuple struct*.
//@ It is often used when constructing a *newtype*, as we do here: `ConcurrentCounter` is
//@ essentially just a new name for `Arc<Mutex<usize>>`. However, is is a locally declared types,
//@ so we can give it an inherent implementation and implement traits for it. Since the field is
//@ private, nobody outside this module can even know the type we are wrapping.

// The derived `Clone` implementation will clone the `Arc`, so all clones will actually talk about
// the same counter.
#[derive(Clone)]
struct ConcurrentCounter(Arc<Mutex<usize>>);

impl ConcurrentCounter {
    // The constructor just wraps the constructors of `Arc` and `Mutex`.
    pub fn new(val: usize) -> Self {
        ConcurrentCounter(Arc::new(Mutex::new(val)))                /*@*/
    }

    // The core operation is, of course, `increment`.
    pub fn increment(&self, by: usize) {
        // `lock` on a mutex returns a guard, very much like `RefCell`. The guard gives access to
        // the data contained in the mutex.
        //@ (We will discuss the `unwrap` soon.) `.0` is how we access the first component of a
        //@ tuple or a struct.
        let mut counter = self.0.lock().unwrap();
        //@ The guard is a smart pointer to the content.
        *counter = *counter + by;
        //@ At the end of the function, `counter` is dropped and the mutex is available again.
        //@ This can only happen when full ownership of the guard is given up. In particular, it is
        //@ impossible for us to take a reference to some of its content, release the lock of the
        //@ mutex, and subsequently access the protected data without holding the lock. Enforcing
        //@ the locking discipline is expressible in the Rust type system, so we don't have to
        //@ worry about data races *even though* we are mutating shared memory!
        //@ 
        //@ One of the subtle aspects of locking is *poisoning*. If a thread panics while it holds
        //@ a lock, it could leave the data-structure in a bad state. The lock is hence considered
        //@ *poisoned*. Future attempts to `lock` it will fail.
        //@ Above, we simply assert via `unwrap` that this will never happen. Alternatively, we
        //@ could have a look at the poisoned state and attempt to recover from it.
    }

    // The function `get` returns the current value of the counter.
    pub fn get(&self) -> usize {
        let counter = self.0.lock().unwrap();                       /*@*/
        *counter                                                    /*@*/
    }
}

// Now our counter is ready for action.
pub fn main() {
    let counter = ConcurrentCounter::new(0);

    // We clone the counter for the first thread, which increments it by 2 every 15ms.
    let counter1 = counter.clone();
    let handle1 = thread::spawn(move || {
        for _ in 0..10 {
            thread::sleep(Duration::from_millis(15));
            counter1.increment(2);
        }
    });

    // The second thread increments the counter by 3 every 20ms.
    let counter2 = counter.clone();
    let handle2 = thread::spawn(move || {
        for _ in 0..10 {
            thread::sleep(Duration::from_millis(20));
            counter2.increment(3);
        }
    });

    // Now we watch the threads working on the counter.
    for _ in 0..50 {
        thread::sleep(Duration::from_millis(5));
        println!("Current value: {}", counter.get());
    }

    // Finally, we wait for all the threads to finish to be sure we can catch the counter's final
    // value.
    handle1.join().unwrap();
    handle2.join().unwrap();
    println!("Final value: {}", counter.get());
}

// **Exercise 15.1**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that
// increments the counter by `by` *only if* the current value is `test`.
// 
// **Exercise 15.2**: Rather than panicking in case the lock is poisoned, we can use `into_inner`
// on the error to recover the data inside the lock. Change the code above to do that. Try using
// `unwrap_or_else` for this job.

//@ ## `RwLock`

//@ Besides `Mutex`, there's also [`RwLock`](https://doc.rust-
//@ lang.org/stable/std/sync/struct.RwLock.html), which provides two ways of locking: One that
//@ grants only read-only access, to any number of concurrent readers, and another one for
//@ exclusive write access. Notice that this is the same pattern we already saw with shared vs.
//@ mutable references. Hence another way of explaining `RwLock` is to say that it is like
//@ `RefCell`, but works even for concurrent access. Rather than panicking when the data is already
//@ borrowed, `RwLock` will of course block the current thread until the lock is available.
//@ In this view, `Mutex` is a stripped-down version of `RwLock` that does not distinguish readers
//@ and writers.

// **Exercise 15.3**:  Change the code above to use `RwLock`, such that multiple calls to `get` can
// be executed at the same time.

//@ ## `Sync`
//@ Clearly, if we had used `RefCell` rather than `Mutex`, the code above could not work: `RefCell`
//@ is not prepared for multiple threads trying to access the data at the same time. How does Rust
//@ make sure that we don't accidentally use `RefCell` across multiple threads?
//@ 
//@ In part 13, we talked about types that are marked `Send` and thus can be moved to another
//@ thread. However, we did *not* talk about the question whether a reference is `Send`. For `&mut
//@ T`, the answer is: It is `Send` whenever `T` is send.
//@ `&mut` allows moving values back and forth, it is even possible to
//@ [`swap`](https://doc.rust-lang.org/stable/std/mem/fn.swap.html) the contents of two mutable
//@ references. So in terms of concurrency, sending a mutable, unique reference is very much like
//@ sending full ownership, in the sense that it can be used to move the object to another thread.
//@ 
//@ But what about `&T`, a shared reference? Without interior mutability, it would always be all-
//@ right to send such values. After all, no mutation can be performed, so there can be as many
//@ threads accessing the data as we like. In the presence of interior mutability though, the story
//@ gets more complicated. Rust introduces another marker trait for this purpose: `Sync`. A type
//@ `T` is `Sync` if and only if `&T` is `Send`. Just like `Send`, `Sync` has a default
//@ implementation and is thus automatically implemented for a data-structure *if* all its members
//@ implement it.
//@ 
//@ Since `Arc` provides multiple threads with a shared reference to its content, `Arc<T>` is only
//@ `Send` if `T` is `Sync`. So if we had used `RefCell` above, which is *not* `Sync`, Rust would
//@ have caught that mistake. Notice however that `RefCell` *is* `Send`: If ownership of the entire
//@ cell is moved to another thread, it is still not possible for several threads to try to access
//@ the data at the same time.
//@ 
//@ Almost all the types we saw so far are `Sync`, with the exception of `Rc`. Remember that a
//@ shared reference is good enough for cloning, and we don't want other threads to clone our local
//@ `Rc` (they would race for updating the reference count), so it must not be `Sync`. The rule of
//@ `Mutex` is to enforce synchronization, so it should not be entirely surprising that `Mutex<T>`
//@ is `Send` *and* `Sync` provided that `T` is `Send`.
//@ 
//@ You may be curious whether there is a type that's `Sync`, but not `Send`. There are indeed
//@ rather esoteric examples of such types, but that's not a topic I want to go into. In case you
//@ are curious, there's a
//@ [Rust RFC](https://github.com/rust-lang/rfcs/blob/master/text/0458-send-//@ improvements.md),
//@ which contains a type `RcMut` that would be `Sync` and not `Send`.
//@ You may also be interested in this
//@ [blog post](https://huonw.github.io/blog/2015/02/some-notes-on-send-and-sync/) on the topic.

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//@ [next](part16.html)