-// Rust-101, Part 15: Interior Mutability (cont.), RefCell, Cell, Drop
+// Rust-101, Part 15: Mutex, Interior Mutability (cont.), RwLock, Sync
// ===================================================================
-//@ [`RefCell`](http://doc.rust-lang.org/beta/std/cell/struct.RefCell.html)
-//@ [`is very much like `RwLock`, but it's not thread-safe: "Locking" is done without atomic operations.
-//@ One can also see it as a dynamically checked version of Rust's usual borrowing rules. You have to explicitly say
-//@ when you want to borrow the data in there shared, or mutably, and Rust will complain at run-time if you have
-//@ a mutable borrow while any other borrow is active. You can then write programs that Rust may otherwise not
-//@ accept. Sending a shared borrow to this to another thread is dangerous, as the checks are not performed in
-//@ a thread-safe manner. However, sending the *entire* `RefCell` is okay, because there's only ever one owner, and all
-//@ we need to ensure is that everybody attempting to borrow is in the same thread as the owner. <br/>
-//@ [`Cell<T>`](http://doc.rust-lang.org/beta/std/cell/struct.Cell.html) is like a stripped-down version of `RefCell<T>`: It doesn't allow
-//@ you to borrow its content. Instead, it has a methods `get` and `set` to change the value stored in the cell, and to copy it out.
-//@ For obvious reasons, this requires `T` to be `Copy`.
-//@
-//@ You can also think about all these types coming from the other end: Starting with `Cell`, we have a primitive for
-//@ interior mutability that provides `get` and `set`, both just requiring a shared borrow. Think of these functions as
-//@ mutating the *content* of the cell, but not the cell itself, the container. (Just like in ML, where assignment to a
-//@ `ref` changes the content, not the location.) However, due to the ownership discipline, `Cell` only works for types
-//@ that are `Copy`. Hence we also have `RefCell`, which allows working with the data right in the cell, rather than
-//@ having to copy it out. `RefCell` uses non-atomic operations for this purpose, so for the multi-threaded setting, there's
-//@ the thread-safe `RwLock`. And finally, in case a distinction between readers and writers is not helpful, one can use the
-//@ more efficient `Mutex`.
-
-//@ [index](main.html) | [previous](part14.html) | [next](main.html)
+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, it 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.
+
+//@ [index](main.html) | [previous](part14.html) | [raw source](workspace/src/part15.rs) |
+//@ [next](part16.html)
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