Merge pull request #38 from bzindovic/part04-variable-type-patch

[rust-101.git] / src / part13.rs

diff --git a/src/part13.rs b/src/part13.rs
index 8edcc9912db61ce1ec85b0bab10382772788c61f..31f629bb32e88660b9e49a4ffbb5f1df6d8ca4d2 100644 (file)
--- a/src/part13.rs
+++ b/src/part13.rs
@@ -6,15 +6,18 @@ use std::{io, fs, thread};
 use std::sync::mpsc::{sync_channel, SyncSender, Receiver};
 use std::sync::Arc;
 
 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",
-//@ 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
-//@ 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/>
-//@ Besides just printing all the matching lines, we will also offer to count them, or alternatively to sort them.
+//@ 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 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 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.
+//@ Besides just printing all the matching lines, we will also offer to count them, or
+//@ alternatively to sort them.

 #[derive(Clone,Copy)]
 pub enum OutputMode {
     Print,
 #[derive(Clone,Copy)]
 pub enum OutputMode {
     Print,


@@ -29,17 +32,19 @@ pub struct Options {
     pub output_mode: OutputMode,
 }
 
     pub output_mode: OutputMode,
 }
 

-//@ Now we can write three functions to do the actual job of reading, matching, and printing, respectively.
-//@ To get the data from one thread to the next, we will use *message passing*: We will establish communication
-//@ channels between the threads, with one thread *sending* data, and the other one *receiving* it. `SyncSender<T>`
-//@ is the type of the sending end of a synchronous channel transmitting data of type `T`. *Synchronous* here
-//@ means that the `send` operation could block, waiting for the other side to make progress. We don't want to
-//@ end up with the entire file being stored in the buffer of the channels, and the output not being fast enough
-//@ to keep up with the speed of input.
+//@ Now we can write three functions to do the actual job of reading, matching, and printing,
+//@ respectively. To get the data from one thread to the next, we will use *message passing*: We
+//@ will establish communication channels between the threads, with one thread *sending* data, and
+//@ the other one *receiving* it. `SyncSender<T>` is the type of the sending end of a synchronous
+//@ channel transmitting data of type `T`.
+//@ *Synchronous* here means that the `send` operation could block, waiting for the other side to
+//@ make progress. We don't want to end up with the entire file being stored in the buffer of the
+//@ channels, and the output not being fast enough to keep up with the speed of input.

 //@
 //@

-//@ We also need all the threads to have access to the options of the job they are supposed to do. Since it would
-//@ be rather unnecessary to actually copy these options around, we will use reference-counting to share them between
-//@ all threads. `Arc` is the thread-safe version of `Rc`, using atomic operations to keep the reference count up-to-date.
+//@ We also need all the threads to have access to the options of the job they are supposed to do.
+//@ Since it would be rather unnecessary to actually copy these options around, we will use
+//@ reference-counting to share them between all threads. `Arc` is the thread-safe version of `Rc`,
+//@ using atomic operations to keep the reference count up-to-date.

 
 // The first function reads the files, and sends every line over the `out_channel`.
 fn read_files(options: Arc<Options>, out_channel: SyncSender<String>) {
 
 // The first function reads the files, and sends every line over the `out_channel`.
 fn read_files(options: Arc<Options>, out_channel: SyncSender<String>) {


@@ -64,15 +69,17 @@ fn filter_lines(options: Arc<Options>,
                 out_channel: SyncSender<String>) {
     // We can simply iterate over the channel, which will stop when the channel is closed.
     for line in in_channel.iter() {
                 out_channel: SyncSender<String>) {
     // We can simply iterate over the channel, which will stop when the channel is closed.
     for line in in_channel.iter() {

-        // `contains` works on lots of types of patterns, but in particular, we can use it to test whether
-        // one string is contained in another. This is another example of Rust using traits as substitute for overloading.
+        // `contains` works on lots of types of patterns, but in particular, we can use it to test
+        // whether one string is contained in another. This is another example of Rust using traits
+        // as substitute for overloading.

         if line.contains(&options.pattern) {
             out_channel.send(line).unwrap();                        /*@*/
         }
     }
 }
 
         if line.contains(&options.pattern) {
             out_channel.send(line).unwrap();                        /*@*/
         }
     }
 }
 

-// The third function performs the output operations, receiving the relevant lines on its `in_channel`.
+// The third function performs the output operations, receiving the relevant lines on its
+// `in_channel`.

 fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
     match options.output_mode {
         Print => {
 fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
     match options.output_mode {
         Print => {


@@ -82,13 +89,14 @@ fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
             }
         },
         Count => {
             }
         },
         Count => {

-            // We are supposed to count the number of matching lines. There's a convenient iterator adapter that
-            // we can use for this job.
+            // We are supposed to count the number of matching lines. There's a convenient iterator
+            // adapter that we can use for this job.

             let count = in_channel.iter().count();                  /*@*/
             println!("{} hits for {}.", count, options.pattern);    /*@*/
         },
         SortAndPrint => {
             let count = in_channel.iter().count();                  /*@*/
             println!("{} hits for {}.", count, options.pattern);    /*@*/
         },
         SortAndPrint => {

-            // We are asked to sort the matching lines before printing. So let's collect them all in a local vector...
+            // We are asked to sort the matching lines before printing. So let's collect them all
+            // in a local vector...

             let mut data: Vec<String> = in_channel.iter().collect();
             // ...and implement the actual sorting later.
             unimplemented!()
             let mut data: Vec<String> = in_channel.iter().collect();
             // ...and implement the actual sorting later.
             unimplemented!()


@@ -96,20 +104,21 @@ fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
     }
 }
 
     }
 }
 

-// With the operations of the three threads defined, we can now implement a function that performs grepping according
-// to some given options.
+// With the operations of the three threads defined, we can now implement a function that performs
+// grepping according to some given options.

 pub fn run(options: Options) {
     // We move the `options` into an `Arc`, as that's what the thread workers expect.
     let options = Arc::new(options);
 
 pub fn run(options: Options) {
     // We move the `options` into an `Arc`, as that's what the thread workers expect.
     let options = Arc::new(options);
 

-    // This sets up the channels. We use a `sync_channel` with buffer-size of 16 to avoid needlessly filling RAM.
+    // This sets up the channels. We use a `sync_channel` with buffer-size of 16 to avoid needlessly
+    // filling RAM.

     let (line_sender, line_receiver) = sync_channel(16);
     let (filtered_sender, filtered_receiver) = sync_channel(16);
 
     // Spawn the read thread: `thread::spawn` takes a closure that is run in a new thread.
     let (line_sender, line_receiver) = sync_channel(16);
     let (filtered_sender, filtered_receiver) = sync_channel(16);
 
     // Spawn the read thread: `thread::spawn` takes a closure that is run in a new thread.

-    //@ The `move` keyword again tells Rust that we want ownership of captured variables to be moved into the
-    //@ closure. This means we need to do the `clone` *first*, otherwise we would lose our `options` to the
-    //@ new thread!
+    //@ The `move` keyword again tells Rust that we want ownership of captured variables to be
+    //@ moved into the closure. This means we need to do the `clone` *first*, otherwise we would
+    //@ lose our `options` to the new thread!

     let options1 = options.clone();
     let handle1 = thread::spawn(move || read_files(options1, line_sender));
 
     let options1 = options.clone();
     let handle1 = thread::spawn(move || read_files(options1, line_sender));
 


@@ -124,9 +133,9 @@ 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.
+    //@ 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();


@@ -145,48 +154,59 @@ pub fn main() {
     run(options);
 }
 
     run(options);
 }
 

-// **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.
+// **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.

 
 //@ ## Ownership, Borrowing, and Concurrency
 
 //@ ## Ownership, Borrowing, and Concurrency

-//@ The little demo above showed that concurrency in Rust has a fairly simple API. Considering Rust has closures,
-//@ that should not be entirely surprising. However, as it turns out, Rust goes well beyond this and actually ensures
-//@ the absence of data races. <br/>
-//@ A data race is typically defined as having two concurrent, unsynchronized
-//@ accesses to the same memory location, at least one of which is a write. In other words, a data race is mutation in
-//@ the presence of aliasing, which Rust reliably rules out! It turns out that the same mechanism that makes our single-threaded
-//@ programs memory safe, and that prevents us from invalidating iterators, also helps secure our multi-threaded code against
-//@ data races. For example, notice how `read_files` sends a `String` to `filter_lines`. At run-time, only the pointer to
-//@ the character data will actually be moved around (just like when a `String` is passed to a function with full ownership). However,
-//@ `read_files` has to *give up* ownership of the string to perform `send`, to it is impossible for the string to still be borrowed.
-//@ 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.
+//@ The little demo above showed that concurrency in Rust has a fairly simple API. Considering Rust
+//@ has closures, that should not be entirely surprising. However, as it turns out, Rust goes well
+//@ beyond this and actually ensures the absence of data races. <br/>
+//@ A data race is typically defined as having two concurrent, unsynchronized accesses to the same
+//@ memory location, at least one of which is a write. In other words, a data race is mutation in
+//@ the presence of aliasing, which Rust reliably rules out! It turns out that the same mechanism
+//@ that makes our single-threaded programs memory safe, and that prevents us from invalidating
+//@ iterators, also helps secure our multi-threaded code against data races. For example, notice
+//@ how `read_files` sends a `String` to `filter_lines`.
+//@ At run-time, only the pointer to the character data will actually be moved around (just like
+//@ when a `String` is passed to a function with full ownership). However, `read_files` has to
+//@ *give up* ownership of the string to perform `send`, to it is impossible for the string to
+//@ still be borrowed. 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 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`
-//@ to be valid for lifetime `'static`, as you can see in [its documentation](https://doc.rust-lang.org/stable/std/thread/fn.spawn.html).
-//@ This avoids another kind of data race, where the thread's access races with the callee deallocating its stack frame.
-//@ It is only thanks to the concept of lifetimes that this can be expressed as part of the type of `spawn`.
+//@ 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` to be
+//@ valid for lifetime `'static`, as you can see in
+//@ [its documentation](https://doc.rust-lang.org/stable/std/thread/fn.spawn.html). This avoids
+//@ another kind of data race, where the thread's access races with the callee deallocating its
+//@ stack frame. It is only thanks to the concept of lifetimes that this can be expressed as part
+//@ of the type of `spawn`.

 
 //@ ## Send
 
 //@ ## Send

-//@ However, the story goes even further. I said above that `Arc` is a thread-safe version of `Rc`, which uses atomic operations
-//@ to manipulate the reference count. It is thus crucial that we don't use `Rc` across multiple threads, or the reference count may
-//@ become invalid. And indeed, if you replace `Arc` by `Rc` (and add the appropriate imports), Rust will tell you that something
-//@ is wrong. That's great, of course, but how did it do that?
+//@ However, the story goes even further. I said above that `Arc` is a thread-safe version of `Rc`,
+//@ which uses atomic operations to manipulate the reference count. It is thus crucial that we
+//@ don't use `Rc` across multiple threads, or the reference count may become invalid. And indeed,
+//@ if you replace `Arc` by `Rc` (and add the appropriate imports), Rust will tell you that
+//@ something is wrong. That's great, of course, but how did it do that?

 //@ 
 //@ 

-//@ 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
-//@ 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! 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.
+//@ 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 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! 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.
-//@ So if the environment of your closure contains an `Rc`, it won't be `Send`, preventing it from causing trouble. If however every
-//@ captured variable *is* `Send`, then so is the entire environment, and you are good.
-
-//@ [index](main.html) | [previous](part12.html) | [raw source](workspace/src/part13.rs) | [next](part14.html)
+//@ 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. So if the environment
+//@ of your closure contains an `Rc`, it won't be `Send`, preventing it from causing trouble. If
+//@ however every captured variable *is* `Send`, then so is the entire environment, and you are
+//@ good.
+
+//@ [index](main.html) | [previous](part12.html) | [raw source](workspace/src/part13.rs) |
+//@ [next](part14.html)