wording

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

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

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

//@ 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 borrow 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_ms(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_ms(20);
            counter2.increment(3);
        }
    });

    // Now we watch the threads working on the counter.
    for _ in 0..50 {
        thread::sleep_ms(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_innter` 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 borrows. 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 borrow 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 mutably borrowed values. So in terms of concurrency, sending a mutable borrow 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 borrow? 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 borrow of 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 borrow is good enough
//@ for cloning, and we don't want other threads to clone our local `Rc`, 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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