Add exercise 10.2 (#24)

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

// Rust-101, Part 13: Concurrency, Arc, Send
// =========================================

use std::io::prelude::*;
use std::{io, fs, thread};
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.
//@ 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,
    SortAndPrint,
    Count,
}
use self::OutputMode::*;

pub struct Options {
    pub files: Vec<String>,
    pub pattern: String,
    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.
//@
//@ 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>) {
    for file in options.files.iter() {
        // First, we open the file, ignoring any errors.
        let file = fs::File::open(file).unwrap();
        // Then we obtain a `BufReader` for it, which provides the `lines` function.
        let file = io::BufReader::new(file);
        for line in file.lines() {
            let line = line.unwrap();
            // Now we send the line over the channel, ignoring the possibility of `send` failing.
            out_channel.send(line).unwrap();
        }
    }
    // When we drop the `out_channel`, it will be closed, which the other end can notice.
}

// The second function filters the lines it receives through `in_channel` with the pattern, and sends
// matches via `out_channel`.
fn filter_lines(options: Arc<Options>,
                in_channel: Receiver<String>,
                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.
        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`.
fn output_lines(options: Arc<Options>, in_channel: Receiver<String>) {
    match options.output_mode {
        Print => {
            // Here, we just print every line we see.
            for line in in_channel.iter() {
                println!("{}", line);                               /*@*/
            }
        },
        Count => {
            // 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 => {
            // 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!()
        }
    }
}

// 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);

    // 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.
    //@ 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));

    // Same with the filter thread.
    let options2 = options.clone();
    let handle2 = thread::spawn(move || {
        filter_lines(options2, line_receiver, filtered_sender)
    });

    // And the output thread.
    let options3 = options.clone();
    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();
}

// Now we have all the pieces together for testing our rgrep with some hard-coded options.
//@ We need to call `to_string` on string literals to convert them to a fully-owned `String`.
pub fn main() {
    let options = Options {
        files: vec!["src/part10.rs".to_string(),
                    "src/part11.rs".to_string(),
                    "src/part12.rs".to_string()],
        pattern: "let".to_string(),
        output_mode: Print
    };
    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.

//@ ## 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.
//@ 
//@ 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
//@ 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.
//@ 
//@ 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.

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