X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/ccf679adb3790903849f7d85b970b67582220952..1a33201461b96128740a4334928745c11d3dfe3e:/src/part13.rs diff --git a/src/part13.rs b/src/part13.rs index 811411c..31f629b 100644 --- a/src/part13.rs +++ b/src/part13.rs @@ -1,161 +1,212 @@ -// Rust-101, Part 13: Slices, Arrays, External Dependencies -// ======================================================== - -//@ To complete rgrep, there are two pieces we still need to implement: Sorting, and taking the job options -//@ as argument to the program, rather than hard-coding them. Let's start with sorting. - -// ## Slices -//@ Again, we first have to think about the type we want to give to our sorting function. We may be inclined to -//@ pass it a `Vec`. Of course, sorting does not actually consume the argument, so we should make that a `&mut Vec`. -//@ But there's a problem with that: If we want to implement some divide-and-conquer sorting algorithm (say, -//@ Quicksort), then we will have to *split* our argument at some point, and operate recursively on the two parts. -//@ But we can't split a `Vec`! We could now extend the function signature to also take some indices, marking the -//@ part of the vector we are supposed to sort, but that's all rather clumsy. Rust offers a nicer solution. - -//@ `[T]` is the type of an (unsized) *array*, with elements of type `T`. All this means is that there's a contiguous -//@ region of memory, where a bunch of `T` are stored. How many? We can't tell! This is an unsized type. Just like for -//@ trait objects, this means we can only operate on pointers to that type, and these pointers will carry the missing -//@ information - namely, the length. Such a pointer is called a *slice*. As we will see, a slice can be split. -//@ Our function can thus take a borrowed slice, and promise to sort all elements in there. -pub fn sort(data: &mut [T]) { - if data.len() < 2 { return; } - - // We decide that the element at 0 is our pivot, and then we move our cursors through the rest of the slice, - // making sure that everything on the left is no larger than the pivot, and everything on the right is no smaller. - let mut lpos = 1; - let mut rpos = data.len(); - /* 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 - // test function for `sort`. - unimplemented!() - } - - // Once our cursors met, we need to put the pivot in the right place. - data.swap(0, lpos-1); - - // Finally, we split our slice to sort the two halves. The nice part about slices is that splitting them is cheap: - //@ They are just a pointer to a start address, and a length. We can thus get two pointers, one at the beginning and - //@ one in the middle, and set the lengths appropriately such that they don't overlap. This is what `split_at_mut` does. - //@ Since the two slices don't overlap, there is no aliasing and we can have them both mutably borrowed. - let (part1, part2) = data.split_at_mut(lpos); - //@ The index operation can not only be used to address certain elements, it can also be used for *slicing*: Giving a range - //@ of indices, and obtaining an appropriate part of the slice we started with. Here, we remove the last element from - //@ `part1`, which is the pivot. This makes sure both recursive calls work on strictly smaller slices. - sort(&mut part1[..lpos-1]); /*@*/ - sort(part2); /*@*/ +// 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::*; -// **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 -// only, not by filename or line number! - -// Now, we can sort, e.g., an vector of numbers. -fn sort_nums(data: &mut Vec) { - //@ Vectors support slicing, just like slices do. Here, `..` denotes the full range, which means we want to slice the entire vector. - //@ It is then passed to the `sort` function, which doesn't even know that it is working on data inside a vector. - sort(&mut data[..]); +pub struct Options { + pub files: Vec, + pub pattern: String, + pub output_mode: OutputMode, } -// ## Arrays -//@ An *array* in Rust is given be the type `[T; n]`, where `n` is some *fixed* number. So, `[f64; 10]` is an array of 10 floating-point -//@ numbers, all one right next to the other in memory. Arrays are sized, and hence can be used like any other type. But we can also -//@ borrow them as slices, e.g., to sort them. -fn sort_array() { - let mut array_of_data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1]; - sort(&mut array_of_data); +//@ 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` 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, out_channel: SyncSender) { + 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. } -// ## 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/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. -//@ -//@ For sure, we are not the first to equip a Rust program with support for command-line arguments. Someone must have written a library -//@ for the job, right? Indeed, someone has. Rust has a central repository of published libraries, called [crates.io](https://crates.io/). -//@ It's a bit like [PyPI](https://pypi.python.org/pypi) or the [Ruby Gems](https://rubygems.org/): Everybody can upload their code, -//@ and there's tooling for importing that code into your project. This tooling is provided by `cargo`, the tool we are already using to -//@ build this tutorial. (`cargo` also has support for *publishing* your crate on crates.io, I refer you to [the documentation](http://doc.crates.io/crates-io.html) for more details.) -//@ In this case, we are going to use the [`docopt` crate](https://crates.io/crates/docopt), which creates a parser for command-line -//@ 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, -//@ 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). - -// I disabled the following module (using a rather bad hack), because it only compiles if `docopt` is linked. However, before enabling it, -// you still have get the external library into the global namespace. This is done with `extern crate docopt`, and that statement *has* to be -// in `main.rs`. So please go there, and enable this commented-out line. Then remove the attribute of the `rgrep` module. -#[cfg(feature = "disabled")] -pub mod rgrep { - // Now that `docopt` is linked and declared in `main.rs`, we can import it with `use`. We also import some other pieces that we will need. - use docopt::Docopt; - use part12::{run, Options, OutputMode}; - use std::process; - - // The `USAGE` string documents how the program is to be called. It's written in a format that `docopt` can parse. - static USAGE: &'static str = " -Usage: rgrep [-c] [-s] ... - -Options: - -c, --count Count number of matching lines (rather than printing them). - -s, --sort Sort the lines before printing. -"; - - // 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/). - //@ The function `and_then` takes a closure from `T` to `Result`, and uses it to transform a `Result` to a - //@ `Result`. 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. - //@ There's a similar function for `Option`. `unwrap_or_else` is a bit like `unwrap`, but rather than panicking in - //@ case of an `Err`, it calls the closure. - 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 sort = args.get_bool("-s"); - let pattern = args.get_str(""); - let files = args.get_vec(""); - if count && sort { - println!("Setting both '-c' and '-s' at the same time does not make any sense."); - process::exit(1); +// 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, + in_channel: Receiver, + out_channel: SyncSender) { + // 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(); /*@*/ } + } +} - // We need to make the strings owned to construct the `Options` instance. - //@ If you check all the types carefully, you will notice that `pattern` above is of type `&str`. `str` is the type of a UTF-8 - //@ encoded string, that is, a bunch of bytes in memory (`[u8]`) that are valid according of UTF-8. `str` is unsized. `&str` - //@ stores the address of the character data, and their length. String literals like "this one" are - //@ of type `&'static str`: They point right to the constant section of the binary, so - //@ However, the borrow is valid for as long as the program runs, hence it has lifetime `'static`. Calling - //@ `to_string` will copy the string data into an owned buffer on the heap, and thus convert it to `String`. - let mode = if count { - OutputMode::Count - } else if sort { - OutputMode::SortAndPrint - } else { - OutputMode::Print - }; - Options { - files: files.iter().map(|file| file.to_string()).collect(), - pattern: pattern.to_string(), - output_mode: mode, +// The third function performs the output operations, receiving the relevant lines on its +// `in_channel`. +fn output_lines(options: Arc, in_channel: Receiver) { + 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 = in_channel.iter().collect(); + // ...and implement the actual sorting later. + unimplemented!() } } +} - // Finally, we can call the `run` function from the previous part on the options extracted using `get_options`. Edit `main.rs` to call this function. - // You can now use `cargo run -- ` to call your program, and see the argument parser and the threads we wrote previously in action! - pub fn main() { - run(get_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); + + // 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(); } -// **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 -// 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.) +// 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); +} -//@ [index](main.html) | [previous](part12.html) | [next](main.html) +// **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.
+//@ 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. + +//@ [index](main.html) | [previous](part12.html) | [raw source](workspace/src/part13.rs) | +//@ [next](part14.html)