From 6fc84cfed9866682c9d00e8d4d41271705990aa8 Mon Sep 17 00:00:00 2001 From: sunface Date: Wed, 1 Mar 2023 11:22:41 +0800 Subject: [PATCH] add web server chapter in multi thread section --- src/advance-practice1/graceful-shutdown.md | 395 +++++++++++++++++++++ 1 file changed, 395 insertions(+) diff --git a/src/advance-practice1/graceful-shutdown.md b/src/advance-practice1/graceful-shutdown.md index e245c17f..2c426211 100644 --- a/src/advance-practice1/graceful-shutdown.md +++ b/src/advance-practice1/graceful-shutdown.md @@ -1,8 +1,403 @@ # 优雅关闭和资源清理 +之前的程序，如果使用 `ctrl-c` 的方法来关闭，所有的线程都会立即停止，这会造成正在请求的用户感知到一个明显的错误。 +因此我们需要添加一些优雅关闭( Graceful Shutdown )，以更好的完成资源清理等收尾工作。 +## 为线程池实现 Drop + +当线程池被 drop 时，需要等待所有的子线程完成它们的工作，然后再退出，下面是一个初步尝试: + +```rust +impl Drop for ThreadPool { + fn drop(&mut self) { + for worker in &mut self.workers { + println!("Shutting down worker {}", worker.id); + + worker.thread.join().unwrap(); + } + } +} +``` + +这里通过实现 `Drop` 特征来为线程池添加资源收尾工作，代码比较简单，就是依次调用每个线程的 `join` 方法。编译下试试： + +```rust +$ cargo check + Checking hello v0.1.0 (file:///projects/hello) +error[E0507]: cannot move out of `worker.thread` which is behind a mutable reference + --> src/lib.rs:52:13 + | +52 | worker.thread.join().unwrap(); + | ^^^^^^^^^^^^^ ------ `worker.thread` moved due to this method call + | | + | move occurs because `worker.thread` has type `JoinHandle<()>`, which does not implement the `Copy` trait + | +note: this function takes ownership of the receiver `self`, which moves `worker.thread` + +For more information about this error, try `rustc --explain E0507`. +error: could not compile `hello` due to previous error +``` + +这里的报错很明显，`worker.thread` 试图拿走所有权，但是 `worker` 仅仅是一个可变借用，显然是不可行的。 + +目前来看，只能将 `thread` 从 `worker` 中移动出来，一个可行的尝试: + +```rust +struct Worker { + id: usize, + thread: Option>, +} +``` + +对于 `Option` 类型，可以使用 `take` 方法拿走内部值的所有权，同时留下一个 `None` 在风中孤独凌乱。继续尝试编译驱动开发模式: + +```shell +$ cargo check + Checking hello v0.1.0 (file:///projects/hello) +error[E0599]: no method named `join` found for enum `Option` in the current scope + --> src/lib.rs:52:27 + | +52 | worker.thread.join().unwrap(); + | ^^^^ method not found in `Option>` + | +note: the method `join` exists on the type `JoinHandle<()>` +help: consider using `Option::expect` to unwrap the `JoinHandle<()>` value, panicking if the value is an `Option::None` + | +52 | worker.thread.expect("REASON").join().unwrap(); + | +++++++++++++++++ + +error[E0308]: mismatched types + --> src/lib.rs:72:22 + | +72 | Worker { id, thread } + | ^^^^^^ expected enum `Option`, found struct `JoinHandle` + | + = note: expected enum `Option>` + found struct `JoinHandle<_>` +help: try wrapping the expression in `Some` + | +72 | Worker { id, thread: Some(thread) } + | +++++++++++++ + +``` + +先来解决第二个类型不匹配的错误: + +```rust +impl Worker { + fn new(id: usize, receiver: Arc>>) -> Worker { + // --snip-- + + Worker { + id, + thread: Some(thread), + } + } +} +``` + +简单搞定，回头看看第一个错误，既然换了 `Option`，就可以用 `take` 拿走所有权: + +```rust +impl Drop for ThreadPool { + fn drop(&mut self) { + for worker in &mut self.workers { + println!("Shutting down worker {}", worker.id); + + if let Some(thread) = worker.thread.take() { + thread.join().unwrap(); + } + } + } +} +``` + +注意这种 `if let` 的写法，若 `worker.thread` 已经是 `None`，什么都不会发生，符合我们的预期; 若包含一个线程，那就拿走其所有权，然后调用 `join`。 + +## 停止工作线程 + +虽然调用了 `join` ，但是目标线程依然不会停止，原因在于它们在无限的 `loop` 循环等待，看起来需要借用 `channel` 的 `drop` 机制：释放 `sender`发送端后，`receiver` 接收端会收到报错，然后再退出即可。 + +```rust +pub struct ThreadPool { + workers: Vec, + sender: Option>, +} +// --snip-- +impl ThreadPool { + pub fn new(size: usize) -> ThreadPool { + // --snip-- + + ThreadPool { + workers, + sender: Some(sender), + } + } + + pub fn execute(&self, f: F) + where + F: FnOnce() + Send + 'static, + { + let job = Box::new(f); + + self.sender.as_ref().unwrap().send(job).unwrap(); + } +} + +impl Drop for ThreadPool { + fn drop(&mut self) { + drop(self.sender.take()); + + for worker in &mut self.workers { + println!("Shutting down worker {}", worker.id); + + if let Some(thread) = worker.thread.take() { + thread.join().unwrap(); + } + } + } +} +``` + +上面做了两处改变: + +1. 为 `sender` 增加 `Option` 封装，这样可以用 `take` 拿走所有权，跟之前的 `thread` 一样 +2. 主动调用 `drop` 关闭发送端 `sender` + +关闭 `sender` 后，将关闭对应的 `channel`，意味着不会再有任何消息被发送。随后，所有的处于无限 `loop` 的接收端将收到一个错误，我们根据错误再进行进一步的处理。 + + +```rust +impl Worker { + fn new(id: usize, receiver: Arc>>) -> Worker { + let thread = thread::spawn(move || loop { + let message = receiver.lock().unwrap().recv(); + + match message { + Ok(job) => { + println!("Worker {id} got a job; executing."); + + job(); + } + Err(_) => { + println!("Worker {id} disconnected; shutting down."); + break; + } + } + }); + + Worker { + id, + thread: Some(thread), + } + } +} +``` + +为了快速验证代码是否正确，修改 `main` 函数，让其只接收前两个请求: + +```rust +fn main() { + let listener = TcpListener::bind("127.0.0.1:7878").unwrap(); + let pool = ThreadPool::new(4); + + for stream in listener.incoming().take(2) { + let stream = stream.unwrap(); + + pool.execute(|| { + handle_connection(stream); + }); + } + + println!("Shutting down."); +} +``` + +`take` 是迭代器 `Iterator` 上的方法，会限制后续的迭代进行最多两次，然后就结束监听，随后 `ThreadPool` 也将超出作用域并自动触发 `drop`。 + +```shell +$ cargo run + Compiling hello v0.1.0 (file:///projects/hello) + Finished dev [unoptimized + debuginfo] target(s) in 1.0s + Running `target/debug/hello` +Worker 0 got a job; executing. +Shutting down. +Shutting down worker 0 +Worker 3 got a job; executing. +Worker 1 disconnected; shutting down. +Worker 2 disconnected; shutting down. +Worker 3 disconnected; shutting down. +Worker 0 disconnected; shutting down. +Shutting down worker 1 +Shutting down worker 2 +Shutting down worker 3 +``` + +可以看到，代码按照我们的设想如期运行，至此，一个基于线程池的简单 Web 服务器已经完成，下面是完整的代码: + + +## 完整代码 +```rust +// src/main.rs +use hello::ThreadPool; +use std::fs; +use std::io::prelude::*; +use std::net::TcpListener; +use std::net::TcpStream; +use std::thread; +use std::time::Duration; + +fn main() { + let listener = TcpListener::bind("127.0.0.1:7878").unwrap(); + let pool = ThreadPool::new(4); + + for stream in listener.incoming().take(2) { + let stream = stream.unwrap(); + + pool.execute(|| { + handle_connection(stream); + }); + } + + println!("Shutting down."); +} + +fn handle_connection(mut stream: TcpStream) { + let mut buffer = [0; 1024]; + stream.read(&mut buffer).unwrap(); + + let get = b"GET / HTTP/1.1\r\n"; + let sleep = b"GET /sleep HTTP/1.1\r\n"; + + let (status_line, filename) = if buffer.starts_with(get) { + ("HTTP/1.1 200 OK", "hello.html") + } else if buffer.starts_with(sleep) { + thread::sleep(Duration::from_secs(5)); + ("HTTP/1.1 200 OK", "hello.html") + } else { + ("HTTP/1.1 404 NOT FOUND", "404.html") + }; + + let contents = fs::read_to_string(filename).unwrap(); + + let response = format!( + "{}\r\nContent-Length: {}\r\n\r\n{}", + status_line, + contents.len(), + contents + ); + + stream.write_all(response.as_bytes()).unwrap(); + stream.flush().unwrap(); +} +``` + +```rust +// src/lib.rs +use std::{ + sync::{mpsc, Arc, Mutex}, + thread, +}; + +pub struct ThreadPool { + workers: Vec, + sender: Option>, +} + +type Job = Box; + +impl ThreadPool { + /// Create a new ThreadPool. + /// + /// The size is the number of threads in the pool. + /// + /// # Panics + /// + /// The `new` function will panic if the size is zero. + pub fn new(size: usize) -> ThreadPool { + assert!(size > 0); + + let (sender, receiver) = mpsc::channel(); + + let receiver = Arc::new(Mutex::new(receiver)); + + let mut workers = Vec::with_capacity(size); + + for id in 0..size { + workers.push(Worker::new(id, Arc::clone(&receiver))); + } + + ThreadPool { + workers, + sender: Some(sender), + } + } + + pub fn execute(&self, f: F) + where + F: FnOnce() + Send + 'static, + { + let job = Box::new(f); + + self.sender.as_ref().unwrap().send(job).unwrap(); + } +} + +impl Drop for ThreadPool { + fn drop(&mut self) { + drop(self.sender.take()); + + for worker in &mut self.workers { + println!("Shutting down worker {}", worker.id); + + if let Some(thread) = worker.thread.take() { + thread.join().unwrap(); + } + } + } +} + +struct Worker { + id: usize, + thread: Option>, +} + +impl Worker { + fn new(id: usize, receiver: Arc>>) -> Worker { + let thread = thread::spawn(move || loop { + let message = receiver.lock().unwrap().recv(); + + match message { + Ok(job) => { + println!("Worker {id} got a job; executing."); + + job(); + } + Err(_) => { + println!("Worker {id} disconnected; shutting down."); + break; + } + } + }); + + Worker { + id, + thread: Some(thread), + } + } +} +``` + +### 可以做的更多 + +事实上，我们还可以做更多，但是受制于篇幅，就不再展开，感兴趣的同学可以自行完成。 + +- 增加更多的文档 +- 为线程池增加测试 +- 尽可能移除 `unwrap`，替换为错误处理 +- 使用线程池完成其它类型的工作，而不仅仅是本章的 Web 服务器 +- 在 `crates.io` 上找到一个线程池实现，然后使用该包实现一个类似的 Web 服务器 ## 上一章节的遗留问题