X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/6b347ac8c0f8710bd0ca42d20d32511fcb53f188..e50539ecd5ce8becea63c915568192264c595d84:/src/part13.rs?ds=sidebyside diff --git a/src/part13.rs b/src/part13.rs index 861ec2d..31f629b 100644 --- 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; -//@ 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. +//@ 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, @@ -29,17 +32,19 @@ pub struct Options { 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` -//@ 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` 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, out_channel: SyncSender) { @@ -64,15 +69,17 @@ fn filter_lines(options: Arc, 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. + // `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`. +// 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 => { @@ -82,13 +89,14 @@ fn output_lines(options: Arc, in_channel: Receiver) { } }, 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 => { - // 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 = in_channel.iter().collect(); // ...and implement the actual sorting later. unimplemented!() @@ -96,20 +104,21 @@ fn output_lines(options: Arc, in_channel: Receiver) { } } -// 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); - // 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. - //@ 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)); @@ -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. - //@ 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(); @@ -145,48 +154,59 @@ pub fn main() { 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 -//@ 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 an outstanding borrow to -//@ 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. +//@ 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`. +//@ 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? +//@ 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) | [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)