tweak parts 11-15 again

diff --git a/solutions/src/counter.rs b/solutions/src/counter.rs
index afea9d04654d5e85452f7233318b93c588645879..319058e51796f2e7fd34bf9686687276137a5861 100644 (file)
--- a/solutions/src/counter.rs
+++ b/solutions/src/counter.rs
@@ -11,19 +11,19 @@ impl ConcurrentCounter {
     }
 
     pub fn increment(&self, by: usize) {
     }
 
     pub fn increment(&self, by: usize) {

-        let mut counter = self.0.write().unwrap();
+        let mut counter = self.0.write().unwrap_or_else(|e| e.into_inner());

         *counter = *counter + by;
     }
 
     pub fn compare_and_inc(&self, test: usize, by: usize) {
         *counter = *counter + by;
     }
 
     pub fn compare_and_inc(&self, test: usize, by: usize) {

-        let mut counter = self.0.write().unwrap();
+        let mut counter = self.0.write().unwrap_or_else(|e| e.into_inner());

         if *counter == test {
             *counter += by;
         }
     }
 
     pub fn get(&self) -> usize {
         if *counter == test {
             *counter += by;
         }
     }
 
     pub fn get(&self) -> usize {

-        let counter = self.0.read().unwrap();
+        let counter = self.0.read().unwrap_or_else(|e| e.into_inner());

         *counter
     }
 }
         *counter
     }
 }

diff --git a/src/main.rs b/src/main.rs
index 1111443b89b04b19c00c13ec861e4899626f25b2..1317856fb397a8224da1a3bd0636bc57b140340c 100644 (file)
--- a/src/main.rs
+++ b/src/main.rs
@@ -84,7 +84,7 @@
 // * [Part 12: Rc, Interior Mutability, Cell, RefCell](part12.html)
 // * [Part 13: Concurrency, Arc, Send](part13.html)
 // * [Part 14: Slices, Arrays, External Dependencies](part14.html)
 // * [Part 12: Rc, Interior Mutability, Cell, RefCell](part12.html)
 // * [Part 13: Concurrency, Arc, Send](part13.html)
 // * [Part 14: Slices, Arrays, External Dependencies](part14.html)

-// * [Part 15: Mutex, Interior Mutability (cont.), Sync](part15.html)
+// * [Part 15: Mutex, Interior Mutability (cont.), RwLock, Sync](part15.html)

 // * (to be continued)
 // 
 #![allow(dead_code, unused_imports, unused_variables, unused_mut, unreachable_code)]
 // * (to be continued)
 // 
 #![allow(dead_code, unused_imports, unused_variables, unused_mut, unreachable_code)]

diff --git a/src/part11.rs b/src/part11.rs
index 5cc14622651e51e6006435141ad942725296e036..9d093e38163296786605f76b1e2af38a6cc2592a 100644 (file)
--- a/src/part11.rs
+++ b/src/part11.rs
@@ -42,7 +42,7 @@ impl Callbacks {
 
     // Registration simply stores the callback.
     pub fn register(&mut self, callback: Box<FnMut(i32)>) {
 
     // Registration simply stores the callback.
     pub fn register(&mut self, callback: Box<FnMut(i32)>) {

-        self.callbacks.push(callback);                              /*@*/
+        self.callbacks.push(callback);

     }
 
     // We can also write a generic version of `register`, such that it will be instantiated with some concrete closure type `F`
     }
 
     // We can also write a generic version of `register`, such that it will be instantiated with some concrete closure type `F`

diff --git a/src/part12.rs b/src/part12.rs
index c749865dcae7cf366d9b8f40b0c7884043a9dfe3..40d9ad52a0a1f7f0648f64239c2f8adb65bada10 100644 (file)
--- a/src/part12.rs
+++ b/src/part12.rs
@@ -26,7 +26,7 @@ struct Callbacks {
 
 impl Callbacks {
     pub fn new() -> Self {
 
 impl Callbacks {
     pub fn new() -> Self {

-        Callbacks { callbacks: Vec::new() }                         /*@*/
+        Callbacks { callbacks: Vec::new() }

     }
 
     // Registration works just like last time, except that we are creating an `Rc` now.
     }
 
     // Registration works just like last time, except that we are creating an `Rc` now.


@@ -37,7 +37,7 @@ impl Callbacks {
     pub fn call(&self, val: i32) {
         // We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
         for callback in self.callbacks.iter() {
     pub fn call(&self, val: i32) {
         // We only need a shared iterator here. Since `Rc` is a smart pointer, we can directly call the callback.
         for callback in self.callbacks.iter() {

-            callback(val);                                      /*@*/
+            callback(val);                                          /*@*/

         }
     }
 }
         }
     }
 }


@@ -108,11 +108,11 @@ struct CallbacksMut {
 
 impl CallbacksMut {
     pub fn new() -> Self {
 
 impl CallbacksMut {
     pub fn new() -> Self {

-        CallbacksMut { callbacks: Vec::new() }                      /*@*/
+        CallbacksMut { callbacks: Vec::new() }

     }
 
     pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
     }
 
     pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {

-        let cell = Rc::new(RefCell::new(callback));
+        let cell = Rc::new(RefCell::new(callback));                 /*@*/

         self.callbacks.push(cell);                                  /*@*/
     }
 
         self.callbacks.push(cell);                                  /*@*/
     }
 

diff --git a/src/part13.rs b/src/part13.rs
index 76d7154b8d62dd3b44a39d17a01c5892cdef0cce..9f42b8223771ee7a9c5d2d4bed804fdee30cf8f9 100644 (file)
--- a/src/part13.rs
+++ b/src/part13.rs
@@ -7,10 +7,10 @@ use std::sync::mpsc::{sync_channel, SyncSender, Receiver};
 use std::sync::Arc;
 
 //@ Our next stop are the concurrency features of Rust. We are going to write our own small version of "grep",
 use std::sync::Arc;
 
 //@ Our next stop are the concurrency features of Rust. We are going to write our own small version of "grep",

-//@ called *rgrep*, and it is going to make use of concurrency: One thread reads the input files, one thread does
+//@ called *rgrep*, and it is going to perform three jobs concurrently: One thread reads the input files, one thread does

 //@ the actual matching, and one thread writes the output. I already mentioned in the beginning of the course that
 //@ Rust's type system (more precisely, the discipline of ownership and borrowing) will help us to avoid a common
 //@ the actual matching, and one thread writes the output. I already mentioned in the beginning of the course that
 //@ Rust's type system (more precisely, the discipline of ownership and borrowing) will help us to avoid a common

-//@ pitfall of concurrent programming: data races.
+//@ pitfall of concurrent programming: data races. We will see how that works concretely.

 
 // Before we come to the actual code, we define a data-structure `Options` to store all the information we need
 // to complete the job: Which files to work on, which pattern to look for, and how to output. <br/>
 
 // Before we come to the actual code, we define a data-structure `Options` to store all the information we need
 // to complete the job: Which files to work on, which pattern to look for, and how to output. <br/>


@@ -145,7 +145,7 @@ pub fn main() {
     run(options);
 }
 
     run(options);
 }
 

-// **Exercise 12.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file
+// **Exercise 13.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file

 // and the number of the line in the file. You will have to change the type of the channels from `String` to something
 // that records this extra information.
 
 // and the number of the line in the file. You will have to change the type of the channels from `String` to something
 // that records this extra information.
 


@@ -163,7 +163,7 @@ pub fn main() {
 //@ still be around. After it sent the string to the other side, `read_files` has no pointer into the string content
 //@ anymore, and hence no way to race on the data with someone else.
 //@ 
 //@ still be around. After it sent the string to the other side, `read_files` has no pointer into the string content
 //@ anymore, and hence no way to race on the data with someone else.
 //@ 

-//@ There is a little more to this. Remember the `'static` bound we had to add to `register` in the previous part, to make
+//@ There is a little more to this. Remember the `'static` bound we had to add to `register` in the previous parts, to make

 //@ sure that the callbacks do not reference any pointers that might become invalid? This is just as crucial for spawning
 //@ a thread: In general, that thread could last for much longer than the current stack frame. Thus, it must not use
 //@ any pointers to data in that stack frame. This is achieved by requiring the `FnOnce` closure passed to `thread::spawn`
 //@ sure that the callbacks do not reference any pointers that might become invalid? This is just as crucial for spawning
 //@ a thread: In general, that thread could last for much longer than the current stack frame. Thus, it must not use
 //@ any pointers to data in that stack frame. This is achieved by requiring the `FnOnce` closure passed to `thread::spawn`


@@ -179,9 +179,10 @@ pub fn main() {
 //@ 
 //@ The answer is already hinted at in the error: It will say something about `Send`. You may have noticed that the closure in
 //@ `thread::spawn` does not just have a `'static` bound, but also has to satisfy `Send`. `Send` is a trait, and just like `Copy`,
 //@ 
 //@ The answer is already hinted at in the error: It will say something about `Send`. You may have noticed that the closure in
 //@ `thread::spawn` does not just have a `'static` bound, but also has to satisfy `Send`. `Send` is a trait, and just like `Copy`,

-//@ it's just a marker - there are no functions provided by `Send`. What the trait says is that types which are `Send`, can be
+//@ it's just a marker - there are no functions provided by `Send`. What the trait says is that types which are `Send` can be

 //@ safely sent to another thread without causing trouble. Of course, all the primitive data-types are `Send`. So is `Arc`,
 //@ safely sent to another thread without causing trouble. Of course, all the primitive data-types are `Send`. So is `Arc`,

-//@ which is why Rust accepted our code. But `Rc` is not `Send`, and for a good reason!
+//@ which is why Rust accepted our code. But `Rc` is not `Send`, and for a good reason! If had two `Rc` to the same data, and
+//@ sent one of them to another thread, things could go havoc due to the lack of synchronization.

 //@ 
 //@ Now, `Send` as a trait is fairly special. It has a so-called *default implementation*. This means that *every type* implements
 //@ `Send`, unless it opts out. Opting out is viral: If your type contains a type that opted out, then you don't have `Send`, either.
 //@ 
 //@ Now, `Send` as a trait is fairly special. It has a so-called *default implementation*. This means that *every type* implements
 //@ `Send`, unless it opts out. Opting out is viral: If your type contains a type that opted out, then you don't have `Send`, either.

diff --git a/src/part14.rs b/src/part14.rs
index 6550fe52d2ed45c9e225bd76b79c9a9731585ac2..eb2011a958f5377f2a36642aa79794427a42fce8 100644 (file)
--- a/src/part14.rs
+++ b/src/part14.rs
@@ -27,7 +27,7 @@ pub fn sort<T: PartialOrd>(data: &mut [T]) {
     /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
        [rpos,len) is >= pivot; lpos < rpos */
     loop {
     /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
        [rpos,len) is >= pivot; lpos < rpos */
     loop {

-        // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a
+        // **Exercise 14.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a

         // test function for `sort`.
         unimplemented!()
     }
         // test function for `sort`.
         unimplemented!()
     }


@@ -47,8 +47,8 @@ pub fn sort<T: PartialOrd>(data: &mut [T]) {
     sort(part2);                                                    /*@*/
 }
 
     sort(part2);                                                    /*@*/
 }
 

-// **Exercise 13.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
-// to call the sort function above. If you did exercise 12.1, you will have slightly more work. Make sure you sort by the matched line
+// **Exercise 14.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
+// to call the sort function above. If you did exercise 13.1, you will have slightly more work. Make sure you sort by the matched line

 // only, not by filename or line number!
 
 // Now, we can sort, e.g., an vector of numbers.
 // only, not by filename or line number!
 
 // Now, we can sort, e.g., an vector of numbers.


@@ -82,7 +82,7 @@ fn sort_array() {
 //@ arguments based on the usage string. External dependencies are declared in the `Cargo.toml` file.
 
 //@ I already prepared that file, but the declaration of the dependency is still commented out. So please open `Cargo.toml` of your workspace
 //@ arguments based on the usage string. External dependencies are declared in the `Cargo.toml` file.
 
 //@ I already prepared that file, but the declaration of the dependency is still commented out. So please open `Cargo.toml` of your workspace

-//@ now, and enabled the two commented-out lines. Then do `cargo build`. Cargo will now download the crate from crates.io, compile it,
+//@ now, and enable the two commented-out lines. Then do `cargo build`. Cargo will now download the crate from crates.io, compile it,

 //@ and link it to your program. In the future, you can do `cargo update` to make it download new versions of crates you depend on.
 //@ Note that crates.io is only the default location for dependencies, you can also give it the URL of a git repository or some local
 //@ path. All of this is explained in the [Cargo Guide](http://doc.crates.io/guide.html).
 //@ and link it to your program. In the future, you can do `cargo update` to make it download new versions of crates you depend on.
 //@ Note that crates.io is only the default location for dependencies, you can also give it the URL of a git repository or some local
 //@ path. All of this is explained in the [Cargo Guide](http://doc.crates.io/guide.html).


@@ -91,7 +91,8 @@ fn sort_array() {
 // Remove the attribute of the `rgrep` module to enable compilation.
 #[cfg(feature = "disabled")]
 pub mod rgrep {
 // Remove the attribute of the `rgrep` module to enable compilation.
 #[cfg(feature = "disabled")]
 pub mod rgrep {

-    // Now that `docopt` is linked, we can first add it to the namespace and then import shorter names with `use`. We also import some other pieces that we will need.
+    // Now that `docopt` is linked, we can first add it to the namespace with `extern crate` and then import shorter names with `use`.
+    // We also import some other pieces that we will need.

     extern crate docopt;
     use self::docopt::Docopt;
     use part12::{run, Options, OutputMode};
     extern crate docopt;
     use self::docopt::Docopt;
     use part12::{run, Options, OutputMode};


@@ -108,7 +109,7 @@ Options:
 
     // This function extracts the rgrep options from the command-line arguments.
     fn get_options() -> Options {
 
     // This function extracts the rgrep options from the command-line arguments.
     fn get_options() -> Options {

-        // Parse `argv` and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/).
+        // This parses `argv` and exit the program with an error message if it fails. The code is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/). <br/>

         //@ The function `and_then` takes a closure from `T` to `Result<U, E>`, and uses it to transform a `Result<T, E>` to a
         //@ `Result<U, E>`. This way, we can chain computations that only happen if the previous one succeeded (and the error
         //@ type has to stay the same). In case you know about monads, this style of programming will be familiar to you.
         //@ The function `and_then` takes a closure from `T` to `Result<U, E>`, and uses it to transform a `Result<T, E>` to a
         //@ `Result<U, E>`. This way, we can chain computations that only happen if the previous one succeeded (and the error
         //@ type has to stay the same). In case you know about monads, this style of programming will be familiar to you.


@@ -153,7 +154,7 @@ Options:
     }
 }
 
     }
 }
 

-// **Exercise 13.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular
+// **Exercise 14.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular

 // expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
 // the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
 // (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)
 // expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
 // the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
 // (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)

diff --git a/src/part15.rs b/src/part15.rs
index a7836898bf1348576b1962712d8b2304cbebf50f..9ae5aaf93dd62c9508bc5119a33381490a7c3d19 100644 (file)
--- a/src/part15.rs
+++ b/src/part15.rs
@@ -1,20 +1,23 @@
-// Rust-101, Part 15: Mutex, Interior Mutability (cont.), Sync
-// ===========================================================
+// 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*
 
 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. 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 looking at 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*.
+//@ 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.

 //@ 
 //@ 

-//@ As an example, let us write a concurrent counter. As usual in Rust, we first have to think about our data layout.
-//@ 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 - and it is impossible to access the data in any other way. This is generally considered
-//@ good style, but other languages typically lack the ability to actually enforce this.
-//@ Of course, we want multiple threads to have access to this `Mutex`, so we wrap it in an `Arc`.
+//@ The most basic type for interior mutability that supports concurrency is [`Mutex<T>`](http://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
 //@ 
 //@ 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


@@ -32,26 +35,12 @@ impl ConcurrentCounter {
         ConcurrentCounter(Arc::new(Mutex::new(val)))                /*@*/
     }
 
         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` permits 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.
+    // The core operation is, of course, `increment`.

     pub fn increment(&self, by: usize) {
     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.
+        // `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();
         let mut counter = self.0.lock().unwrap();

-        //@ The guard is another example of a smart pointer, and it can be used as if it were a pointer to the data protected
-        //@ by the lock.
+        //@ 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
         *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


@@ -106,16 +95,28 @@ pub fn main() {
     println!("Final value: {}", counter.get());
 }
 
     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.
-// 
-// **Exercise 14.2**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
+// **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`.
 // `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`](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. 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
 
 //@ ## Sync

-//@ In part 12, we talked about types that are marked `Send` and thus can be moved to another thread. However, we did *not*
+//@ 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`](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
 //@ 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


@@ -124,43 +125,22 @@ pub fn main() {
 //@ 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
 //@ 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 `&T` is `Send`. Just like `Send`, `Sync` has a default implementation
+//@ 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.
 //@ 
 //@ 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`.
 //@ 
 //@ 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`.
 //@ 

-//@ In the next part, we will learn about a type called `RefCell` that is `Send`, but not `Sync`.
-//@ 

 //@ 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.
 
 //@ 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.
 

-// FIXME TODO some old outdated explanation FIXME TODO
-
-//@ [`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)
 //@ [index](main.html) | [previous](part14.html) | [next](main.html)

diff --git a/workspace/src/part11.rs b/workspace/src/part11.rs
index ac19371db6d69594d87526d5e46d7267f4631de9..746aed9e96398b6b03cc8f033cdeec25764919a2 100644 (file)
--- a/workspace/src/part11.rs
+++ b/workspace/src/part11.rs
@@ -23,7 +23,7 @@ impl Callbacks {
 
     // Registration simply stores the callback.
     pub fn register(&mut self, callback: Box<FnMut(i32)>) {
 
     // Registration simply stores the callback.
     pub fn register(&mut self, callback: Box<FnMut(i32)>) {

-        unimplemented!()
+        self.callbacks.push(callback);

     }
 
     // We can also write a generic version of `register`, such that it will be instantiated with some concrete closure type `F`
     }
 
     // We can also write a generic version of `register`, such that it will be instantiated with some concrete closure type `F`

diff --git a/workspace/src/part12.rs b/workspace/src/part12.rs
index a3519957ca7dbfcf0d207f6004c3fa3df909ff1e..23db4f60e586faa8eca2eb1951b93ef1a30bef69 100644 (file)
--- a/workspace/src/part12.rs
+++ b/workspace/src/part12.rs
@@ -13,7 +13,7 @@ struct Callbacks {
 
 impl Callbacks {
     pub fn new() -> Self {
 
 impl Callbacks {
     pub fn new() -> Self {

-        unimplemented!()
+        Callbacks { callbacks: Vec::new() }

     }
 
     // Registration works just like last time, except that we are creating an `Rc` now.
     }
 
     // Registration works just like last time, except that we are creating an `Rc` now.


@@ -69,11 +69,10 @@ struct CallbacksMut {
 
 impl CallbacksMut {
     pub fn new() -> Self {
 
 impl CallbacksMut {
     pub fn new() -> Self {

-        unimplemented!()
+        CallbacksMut { callbacks: Vec::new() }

     }
 
     pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {
     }
 
     pub fn register<F: FnMut(i32)+'static>(&mut self, callback: F) {

-        let cell = Rc::new(RefCell::new(callback));

         unimplemented!()
     }
 
         unimplemented!()
     }
 

diff --git a/workspace/src/part13.rs b/workspace/src/part13.rs
index 501fb7ddd4cb777407f7869f8188b6fed9f82c6b..ae12cd112cb02e480951937c3bc947cfc919b1d5 100644 (file)
--- a/workspace/src/part13.rs
+++ b/workspace/src/part13.rs
@@ -120,7 +120,7 @@ pub fn main() {
     run(options);
 }
 
     run(options);
 }
 

-// **Exercise 12.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file
+// **Exercise 13.1**: Change rgrep such that it prints not only the matching lines, but also the name of the file

 // and the number of the line in the file. You will have to change the type of the channels from `String` to something
 // that records this extra information.
 
 // and the number of the line in the file. You will have to change the type of the channels from `String` to something
 // that records this extra information.
 

diff --git a/workspace/src/part14.rs b/workspace/src/part14.rs
index 6e007aab82ec3258cd1a5a379943f701f425a701..fb580f9c379d3e15fdd795b281f099454a9e00e9 100644 (file)
--- a/workspace/src/part14.rs
+++ b/workspace/src/part14.rs
@@ -14,7 +14,7 @@ pub fn sort<T: PartialOrd>(data: &mut [T]) {
     /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
        [rpos,len) is >= pivot; lpos < rpos */
     loop {
     /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot;
        [rpos,len) is >= pivot; lpos < rpos */
     loop {

-        // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a
+        // **Exercise 14.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a

         // test function for `sort`.
         unimplemented!()
     }
         // test function for `sort`.
         unimplemented!()
     }


@@ -27,8 +27,8 @@ pub fn sort<T: PartialOrd>(data: &mut [T]) {
     unimplemented!()
 }
 
     unimplemented!()
 }
 

-// **Exercise 13.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
-// to call the sort function above. If you did exercise 12.1, you will have slightly more work. Make sure you sort by the matched line
+// **Exercise 14.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part
+// to call the sort function above. If you did exercise 13.1, you will have slightly more work. Make sure you sort by the matched line

 // only, not by filename or line number!
 
 // Now, we can sort, e.g., an vector of numbers.
 // only, not by filename or line number!
 
 // Now, we can sort, e.g., an vector of numbers.


@@ -49,7 +49,8 @@ fn sort_array() {
 // Remove the attribute of the `rgrep` module to enable compilation.
 #[cfg(feature = "disabled")]
 pub mod rgrep {
 // Remove the attribute of the `rgrep` module to enable compilation.
 #[cfg(feature = "disabled")]
 pub mod rgrep {

-    // Now that `docopt` is linked, we can first add it to the namespace and then import shorter names with `use`. We also import some other pieces that we will need.
+    // Now that `docopt` is linked, we can first add it to the namespace with `extern crate` and then import shorter names with `use`.
+    // We also import some other pieces that we will need.

     extern crate docopt;
     use self::docopt::Docopt;
     use part12::{run, Options, OutputMode};
     extern crate docopt;
     use self::docopt::Docopt;
     use part12::{run, Options, OutputMode};


@@ -66,7 +67,7 @@ Options:
 
     // This function extracts the rgrep options from the command-line arguments.
     fn get_options() -> Options {
 
     // This function extracts the rgrep options from the command-line arguments.
     fn get_options() -> Options {

-        // Parse `argv` and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/).
+        // This parses `argv` and exit the program with an error message if it fails. The code is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/). <br/>

         let args = Docopt::new(USAGE).and_then(|d| d.parse()).unwrap_or_else(|e| e.exit());
         // Now we can get all the values out.
         let count = args.get_bool("-c");
         let args = Docopt::new(USAGE).and_then(|d| d.parse()).unwrap_or_else(|e| e.exit());
         // Now we can get all the values out.
         let count = args.get_bool("-c");


@@ -100,7 +101,7 @@ Options:
     }
 }
 
     }
 }
 

-// **Exercise 13.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular
+// **Exercise 14.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular

 // expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
 // the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
 // (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)
 // expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch
 // the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site.
 // (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.)

diff --git a/workspace/src/part15.rs b/workspace/src/part15.rs
index d006b23ae92f220c3c27ba5fc122139a0b64e04d..1a6873e2009931dfeeba396768646e1f8cafb961 100644 (file)
--- a/workspace/src/part15.rs
+++ b/workspace/src/part15.rs
@@ -1,5 +1,5 @@
-// Rust-101, Part 15: Mutex, Interior Mutability (cont.), Sync
-// ===========================================================
+// Rust-101, Part 15: Mutex, Interior Mutability (cont.), RwLock, Sync
+// ===================================================================

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


@@ -15,8 +15,9 @@ impl ConcurrentCounter {
         unimplemented!()
     }
 
         unimplemented!()
     }
 

+    // The core operation is, of course, `increment`.

     pub fn increment(&self, by: usize) {
     pub fn increment(&self, by: usize) {

-        // `lock` on a mutex returns a *guard*, giving access to the data contained in the mutex.
+        // `lock` on a mutex returns a guard, very much like `RefCell`. The guard gives access to the data contained in the mutex.

         let mut counter = self.0.lock().unwrap();
         *counter = *counter + by;
     }
         let mut counter = self.0.lock().unwrap();
         *counter = *counter + by;
     }


@@ -61,16 +62,13 @@ pub fn main() {
     println!("Final value: {}", counter.get());
 }
 
     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.
-// 
-// **Exercise 14.2**: Add an operation `compare_and_inc(&self, test: usize, by: usize)` that increments the counter by
+// **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`.
 // `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.

 
 
 
 

-// FIXME TODO some old outdated explanation FIXME TODO
-
+// **Exercise 15.3**:  Change the code above to use `RwLock`, such that multiple calls to `get` can be executed at the same time.