正常关闭和清理
示例 20-20 中的代码通过使用线程池异步响应请求，正如我们所希望的那样。我们收到了一些关于 worker 的警告，
id 和 thread 字段，我们没有以直接的方式使用它们来提醒我们
我们没有清理任何东西。当我们使用不太优雅的
ctrl - c 方法来停止主线程，所有其他线程也会立即停止，即使它们正在处理请求。
接下来，我们将实现 Drop trait 以在池中的每个线程上调用 join，以便它们可以在关闭之前完成正在处理的请求。然后，我们将实现一种方法来告诉线程它们应该停止接受新请求并关闭。为了查看此代码的实际效果，我们将修改服务器以仅接受两个请求，然后正常关闭其线程池。
在 ThreadPool 上实现 Drop 特征
让我们从在线程池上实现 Drop 开始。当池被丢弃时，我们的线程应该全部加入以确保它们完成工作。示例 20-22 显示了 Drop 实现的第一次尝试;这段代码还不能完全工作。
文件名： src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
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 }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
worker.thread.join().unwrap();
}
}
}
struct Worker {
id: usize,
thread: thread::JoinHandle<()>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker { id, thread }
}
}
示例 20-22：当线程池超出范围时加入每个线程
首先，我们遍历每个线程池 worker。我们为此使用 &mut 是因为 self 是一个可变引用，我们还需要能够改变 worker。对于每个 worker，我们打印一条消息，说明这个特定的 worker 正在关闭，然后在该 worker 的线程上调用 join。如果对 join 的调用失败，我们使用 unwrap 使 Rust panic 并进入非体面关闭。
以下是我们在编译此代码时遇到的错误：
$ 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: `JoinHandle::<T>::join` takes ownership of the receiver `self`, which moves `worker.thread`
--> /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/thread/mod.rs:1778:17
For more information about this error, try `rustc --explain E0507`.
error: could not compile `hello` (lib) due to 1 previous error
该错误告诉我们，我们不能调用 join，因为我们只有每个 worker 的可变借用，并且 join 获得了其参数的所有权。为了解决这个问题，我们需要将线程移出拥有
thread 因此 join 可以使用该线程。我们在示例 17-15 中是这样做的：如果
Worker 持有一个 Option<thread::JoinHandle<()>> ，我们可以调用
take 方法将值移出 Some 变体，并在其位置保留 None 变体。换句话说，正在运行的 Worker 在 thread 中将具有 Some 变体，当我们想要清理
Worker，我们将 Some 替换为 None，这样 Worker 就没有要运行的线程。
所以我们知道我们想要更新 Worker 的定义，如下所示：
文件名： src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
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 }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
worker.thread.join().unwrap();
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker { id, thread }
}
}
现在，让我们依靠编译器来查找其他需要更改的地方。检查此代码，我们得到两个错误：
$ 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<JoinHandle<()>>`
|
note: the method `join` exists on the type `JoinHandle<()>`
--> /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/std/src/thread/mod.rs:1778:5
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 `Option<JoinHandle<()>>`, found `JoinHandle<_>`
|
= note: expected enum `Option<JoinHandle<()>>`
found struct `JoinHandle<_>`
help: try wrapping the expression in `Some`
|
72 | Worker { id, thread: Some(thread) }
| +++++++++++++ +
Some errors have detailed explanations: E0308, E0599.
For more information about an error, try `rustc --explain E0308`.
error: could not compile `hello` (lib) due to 2 previous errors
让我们解决第二个错误，它指向
工人：：新;当我们创建一个新的 Worker 时，我们需要将 thread 值包装在 Some 中。进行以下更改以修复此错误：
文件名： src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
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 }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
impl Drop for ThreadPool {
fn drop(&mut self) {
for worker in &mut self.workers {
println!("Shutting down worker {}", worker.id);
worker.thread.join().unwrap();
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
// --snip--
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker {
id,
thread: Some(thread),
}
}
}
第一个错误在我们的 Drop 实现中。我们之前提到过，我们打算调用 Option 值的 take 来将线程移出 worker。以下更改将执行此作：
文件名： src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: mpsc::Sender<Job>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
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 }
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
let job = Box::new(f);
self.sender.send(job).unwrap();
}
}
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();
}
}
}
}
struct Worker {
id: usize,
thread: Option<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker {
id,
thread: Some(thread),
}
}
}
如第 17 章所述，Option 上的 take 方法接受 Some
variant 退出，并在其位置上留下 None。我们使用 if let 来解构 Some 并获取线程;然后我们在线程上调用 join。如果 worker 的线程已经是 None，则我们知道 worker 已经清理了它的线程，因此在这种情况下不会发生任何事情。
向线程发出信号以停止侦听作业
通过我们所做的所有更改，我们的代码可以编译而没有任何警告。但是，坏消息是此代码尚未按我们想要的方式运行。关键是由 Worker 的线程运行的闭包中的逻辑
实例：目前，我们调用 join，但这不会关闭线程，因为它们会永远循环寻找作业。如果我们尝试将
ThreadPool 使用我们当前的 drop 实现，主线程将永远阻塞，等待第一个线程完成。
要解决此问题，我们需要更改 ThreadPooldrop
实现，然后在 Worker 循环中进行更改。
首先，我们将更改 ThreadPooldrop 实现，以便在等待线程完成之前显式删除发送方。示例 20-23 显示了对 ThreadPool 的更改，以显式删除 sender。我们使用相同的选项
并采用我们对线程所做的技术，以便能够将 sender 移出 ThreadPool：
文件名： src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: Option<mpsc::Sender<Job>>,
}
// --snip--
type Job = Box<dyn FnOnce() + Send + 'static>;
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 {
// --snip--
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<F>(&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<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
let thread = thread::spawn(move || loop {
let job = receiver.lock().unwrap().recv().unwrap();
println!("Worker {id} got a job; executing.");
job();
});
Worker {
id,
thread: Some(thread),
}
}
}
示例 20-23：在加入 worker 线程之前显式删除 sender
删除 sender 将关闭通道，这表示不会再发送任何消息。发生这种情况时，worker 在无限循环中对 recv 的所有调用都将返回错误。在示例 20-24 中，我们将 Worker
loop 来正常退出循环，这意味着线程将
finish 当 ThreadPooldrop 实现调用 join 时。
文件名： src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: Option<mpsc::Sender<Job>>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
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<F>(&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<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> 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),
}
}
}
示例 20-24：显式跳出循环
recv 返回错误
为了查看这段代码的实际效果，让我们修改 main 只接受两个请求，然后优雅地关闭服务器，如示例 20-25 所示。
文件名： src/main.rs
use hello::ThreadPool;
use std::{
fs,
io::{prelude::*, BufReader},
net::{TcpListener, TcpStream},
thread,
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 buf_reader = BufReader::new(&stream);
let request_line = buf_reader.lines().next().unwrap().unwrap();
let (status_line, filename) = match &request_line[..] {
"GET / HTTP/1.1" => ("HTTP/1.1 200 OK", "hello.html"),
"GET /sleep HTTP/1.1" => {
thread::sleep(Duration::from_secs(5));
("HTTP/1.1 200 OK", "hello.html")
}
_ => ("HTTP/1.1 404 NOT FOUND", "404.html"),
};
let contents = fs::read_to_string(filename).unwrap();
let length = contents.len();
let response =
format!("{status_line}\r\nContent-Length: {length}\r\n\r\n{contents}");
stream.write_all(response.as_bytes()).unwrap();
}
示例 20-25：在提供两个请求后退出循环关闭服务器
您不希望实际的 Web 服务器在仅提供两个请求后关闭。此代码仅演示正常关闭和清理处于正常工作状态。
take 方法在 Iterator trait 中定义，并将迭代限制为最多前两项。ThreadPool 将在 main 结束时超出范围，并且 drop 实现将运行。
使用 cargo run 启动服务器，并发出 3 个请求。第三个请求应该出错，在您的终端中，您应该会看到类似于以下内容的输出：
$ 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
您可能会看到打印的工作程序和消息的顺序不同。我们可以从消息中看到这段代码是如何工作的：worker 0 和 3 收到了前两个请求。服务器在第二个连接后停止接受连接，并且 ThreadPool 上的 Drop 实现甚至在工作程序 3 启动其作业之前就开始执行。删除发送方会断开所有工作程序的连接，并告诉它们关闭。每个工作线程在断开连接时打印一条消息，然后线程池调用 join 以等待每个工作线程完成。
请注意此特定执行的一个有趣方面：ThreadPool
删除了发件人，在任何 worker 收到错误之前，我们尝试加入 worker 0。Worker 0 尚未从 recv 收到错误，因此主线程阻塞等待 worker 0 完成。与此同时，工作线程 3 收到了一个作业，然后所有线程都收到了错误。当 worker 0 完成时，主线程等待其余 worker 完成。那时，他们都退出了他们的循环并停了下来。
恭喜！我们现在已经完成了我们的项目;我们有一个基本的 Web 服务器，它使用线程池来异步响应。我们能够正常关闭服务器，从而清理池中的所有线程。
以下是供参考的完整代码：
文件名： src/main.rs
use hello::ThreadPool;
use std::{
fs,
io::{prelude::*, BufReader},
net::{TcpListener, TcpStream},
thread,
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 buf_reader = BufReader::new(&stream);
let request_line = buf_reader.lines().next().unwrap().unwrap();
let (status_line, filename) = match &request_line[..] {
"GET / HTTP/1.1" => ("HTTP/1.1 200 OK", "hello.html"),
"GET /sleep HTTP/1.1" => {
thread::sleep(Duration::from_secs(5));
("HTTP/1.1 200 OK", "hello.html")
}
_ => ("HTTP/1.1 404 NOT FOUND", "404.html"),
};
let contents = fs::read_to_string(filename).unwrap();
let length = contents.len();
let response =
format!("{status_line}\r\nContent-Length: {length}\r\n\r\n{contents}");
stream.write_all(response.as_bytes()).unwrap();
}
文件名： src/lib.rs
use std::{
sync::{mpsc, Arc, Mutex},
thread,
};
pub struct ThreadPool {
workers: Vec<Worker>,
sender: Option<mpsc::Sender<Job>>,
}
type Job = Box<dyn FnOnce() + Send + 'static>;
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<F>(&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<thread::JoinHandle<()>>,
}
impl Worker {
fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> 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),
}
}
}
我们可以在这里做得更多！如果您想继续增强此项目，这里有一些想法：
向ThreadPool及其公共方法添加更多文档。
添加对库功能的测试。
将对unwrap的调用更改为更可靠的错误处理。
使用ThreadPool执行除提供 Web 请求之外的其他任务。
在 crates.io 上找到一个线程池 crate，并使用该 crate 实现类似的 Web 服务器。然后将其 API 和健壮性与我们实现的线程池进行比较。
总结
干的好！你已经读到书的结尾了！我们要感谢你加入我们的 Rust 之旅。你现在已经准备好实现自己的 Rust 项目并帮助其他人的项目了。请记住，有一个由其他 Rustacean 组成的热情社区，他们很乐意帮助你解决你在 Rust 之旅中遇到的任何挑战。