Add first version of part 14

diff --git a/solutions/src/counter.rs b/solutions/src/counter.rs
new file mode 100644 (file)
index 0000000..265fb99
--- /dev/null
+++ b/solutions/src/counter.rs
@@ -0,0 +1,56 @@
+use std::sync::{Arc, RwLock};
+use std::thread;
+
+#[derive(Clone)]
+struct ConcurrentCounter(Arc<RwLock<usize>>);
+
+impl ConcurrentCounter {
+    // The constructor should not be surprising.
+    pub fn new(val: usize) -> Self {
+        ConcurrentCounter(Arc::new(RwLock::new(val)))
+    }
+
+    pub fn increment(&self, by: usize) {
+        let mut counter = self.0.write().unwrap();
+        *counter = *counter + by;
+    }
+
+    pub fn get(&self) -> usize {
+        let counter = self.0.read().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 want to watch the threads working on the counter.
+    for _ in 0..50 {
+        thread::sleep_ms(5);
+        println!("Current value: {}", counter.get());
+    }
+
+    // Finally, 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());
+}

diff --git a/solutions/src/main.rs b/solutions/src/main.rs
index a0e3f72c85dc58e842311717dfda09636b556584..be6e3d59b36a2bcb209797da2843d668febc8133 100644 (file)
--- a/solutions/src/main.rs
+++ b/solutions/src/main.rs
@@ -8,6 +8,7 @@ extern crate docopt;
 pub mod bigint;
 pub mod vec;
 pub mod rgrep;
 pub mod bigint;
 pub mod vec;
 pub mod rgrep;

+pub mod counter;

 
 pub fn main() {
     rgrep::main();
 
 pub fn main() {
     rgrep::main();

diff --git a/src/main.rs b/src/main.rs
index 3a42ec0ff4c239ec0bcd8542f50ea3c9daa0d994..55a8aa9ceb9661986605ff0d660bb092a9f6b4e7 100644 (file)
--- a/src/main.rs
+++ b/src/main.rs
@@ -105,6 +105,7 @@ mod part10;
 mod part11;
 mod part12;
 mod part13;
 mod part11;
 mod part12;
 mod part13;

+mod part14;

 
 // To actually run the code of some part (after filling in the blanks, if necessary), simply edit the `main`
 // function.
 
 // To actually run the code of some part (after filling in the blanks, if necessary), simply edit the `main`
 // function.

diff --git a/src/part12.rs b/src/part12.rs
index 3e959f97b4449f20ba1499f79db21c9dea8684b0..dc0da61df5ae952c90fc243128e825e32537f1ea 100644 (file)
--- a/src/part12.rs
+++ b/src/part12.rs
@@ -124,6 +124,9 @@ pub fn run(options: Options) {
     let handle3 = thread::spawn(move || output_lines(options3, filtered_receiver));
 
     // Finally, wait until all three threads did their job.
     let handle3 = thread::spawn(move || output_lines(options3, filtered_receiver));
 
     // Finally, wait until all three threads did their job.

+    //@ Joining a thread waits for its termination. This can fail if that thread panicked: In this case, we could get
+    //@ access to the data that it provided to `panic!`. Here, we just assert that they did not panic - so we will panic ourselves
+    //@ if that happened.

     handle1.join().unwrap();
     handle2.join().unwrap();
     handle3.join().unwrap();
     handle1.join().unwrap();
     handle2.join().unwrap();
     handle3.join().unwrap();

diff --git a/src/part13.rs b/src/part13.rs
index bd1fca70ef80055ab8b9b48acfcf69918199b109..811411cc25c27113588cec5c5624bba17ae7394c 100644 (file)
--- a/src/part13.rs
+++ b/src/part13.rs
@@ -69,7 +69,7 @@ fn sort_array() {
 
 // ## External Dependencies
 //@ This leaves us with just one more piece to complete rgrep: Taking arguments from the command-line. We could now directly work on
 
 // ## External Dependencies
 //@ This leaves us with just one more piece to complete rgrep: Taking arguments from the command-line. We could now directly work on

-//@ [`std::env::args`](http://doc.rust-lang.org/beta/std/env/fn.args.html) to gain access to those arguments, and this would become
+//@ [`std::env::args`](http://doc.rust-lang.org/stable/std/env/fn.args.html) to gain access to those arguments, and this would become

 //@ a pretty boring lesson in string manipulation. Instead, I want to use this opportunity to show how easy it is to benefit from
 //@ other people's work in your program.
 //@ 
 //@ a pretty boring lesson in string manipulation. Instead, I want to use this opportunity to show how easy it is to benefit from
 //@ other people's work in your program.
 //@ 

diff --git a/src/part14.rs b/src/part14.rs
new file mode 100644 (file)
index 0000000..32f0fcd
--- /dev/null
+++ b/src/part14.rs
@@ -0,0 +1,143 @@
+// Rust-101, Part 14: Mutex, Sync (WIP)
+// ==============================
+
+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 everybody
+//@ with *read-only* to memory: Since there is aliasing, Rust cannot, in general, permit mutation. If however,
+//@ some care would be taken at run-time, then mutation would still be all right: We have to ensure that whenever
+//@ someone changes the data, nobody else is working on it.  In other words, we need a *critical section* or (as it
+//@ is called in Rust) a [`Mutex`](http://doc.rust-lang.org/stable/std/sync/struct.Mutex.html). Some other languages also call this a *lock*.
+//@ 
+//@ As an example, let us write a concurrent counter. As usual, we first have to think about our data-structure in Rust.
+//@ In case of the mutex, this means we have to declare the type of the data that we want to be protected. In Rust,
+//@ a `Mutex` protects data, not code. This is generally considered good style, but other languages typically lack
+//@ the ability to actually enforce this. As we will see, it is impossible to forget to acquire the mutex in Rust.
+//@ 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 should not be surprising.
+    pub fn new(val: usize) -> Self {
+        ConcurrentCounter(Arc::new(Mutex::new(val)))
+    }
+
+    //@ The core operation is, of course, `increment`. The type may be surprising at first: A shared borrow?
+    //@ How can this be, since `increment` definitely modifies the counter? We already discussed above that `Mutex` is
+    //@ a way to get around this restriction in Rust. This phenomenon of data that can be mutated through a shared
+    //@ borrow is called *interior mutability*: We are changing the inner parts of the object, but seen from the outside,
+    //@ this does not count as "mutation". This stands in contrast to *exterior mutability*, which is the kind of
+    //@ mutability we saw so far, where one piece of data is replaced by something else of the same type. If you are familiar
+    //@ with languages like ML, you can compare this to how something of type `ref` permit mutation, even though it is
+    //@ itself a functional value (more precisely, a location) like all the others.
+    //@ 
+    //@ Interior mutability breaks the rules of Rust that I outlined earlier: There is aliasing (a shared borrow) and mutation.
+    //@ The reason that this still works is careful programming of the primitives for interior mutability - in this case, that's
+    //@ `Mutex`. It has to ensure with dynamic checks, at run-time, that things don't fall apart. In particular, it has to ensure
+    //@ that the data covered by the mutex can only ever be accessed from inside a critical section. This is where Rust's type
+    //@ system comes into play: With its discipline of ownership and borrowing, it can enforce such rules. Let's see how this goes.
+    pub fn increment(&self, by: usize) {
+        // `lock` on a mutex returns a *guard*, giving 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();
+        *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 thus 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.
+    }
+
+    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 want to watch the threads working on the counter.
+    for _ in 0..50 {
+        thread::sleep_ms(5);
+        println!("Current value: {}", counter.get());
+    }
+
+    // Finally, 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 14.1**: Besides `Mutex`, there's also [`RwLock`](http://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.) Change
+// the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
+
+//@ ## Sync
+//@ In part 12, 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`](http://doc.rust-lang.org/beta/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.
+//@ 
+//@ But what about `&T`, a shared borrow? Without interior mutability, it would always be all-right to send such values.
+//@ After one, 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 `&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.
+//@ 
+//@ 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`.
+//@ 
+//@ There's also an example of a type that's `Send`, but not `Sync`: [`RefCell`](http://doc.rust-lang.org/beta/std/cell/struct.RefCell.html).
+//@ This type 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.
+//@ 
+//@ 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](part13.html) | [next](main.html)

diff --git a/workspace/src/main.rs b/workspace/src/main.rs
index 7e7c2005bdf031502c00b10486bb8a94345a2081..26e9eed38c1cbe29df83791a5458ff7beaa59393 100644 (file)
--- a/workspace/src/main.rs
+++ b/workspace/src/main.rs
@@ -15,6 +15,7 @@ mod part10;
 mod part11;
 mod part12;
 mod part13;
 mod part11;
 mod part12;
 mod part13;

+mod part14;

 
 // This decides which part is actually run.
 fn main() {
 
 // This decides which part is actually run.
 fn main() {

diff --git a/workspace/src/part14.rs b/workspace/src/part14.rs
new file mode 100644 (file)
index 0000000..37afcbc
--- /dev/null
+++ b/workspace/src/part14.rs
@@ -0,0 +1,69 @@
+// Rust-101, Part 14: Mutex, Sync (WIP)
+// ==============================
+
+use std::sync::{Arc, Mutex};
+use std::thread;
+
+
+// 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 should not be surprising.
+    pub fn new(val: usize) -> Self {
+        ConcurrentCounter(Arc::new(Mutex::new(val)))
+    }
+
+    pub fn increment(&self, by: usize) {
+        // `lock` on a mutex returns a *guard*, giving access to the data contained in the mutex.
+        let mut counter = self.0.lock().unwrap();
+        *counter = *counter + by;
+    }
+
+    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 want to watch the threads working on the counter.
+    for _ in 0..50 {
+        thread::sleep_ms(5);
+        println!("Current value: {}", counter.get());
+    }
+
+    // Finally, 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 14.1**: Besides `Mutex`, there's also [`RwLock`](http://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.) Change
+// the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.
+
+