优雅的关闭和清理
列表 21-20 中的代码通过使用线程池异步响应请求，正如我们所期望的那样。我们得到了一些关于 workers、id 和 thread 字段的警告，这些字段我们没有直接使用，这提醒我们没有清理任何内容。当我们使用不太优雅的 ctrl-c 方法来停止主线程时，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，以便在池中的每个线程上调用join，使它们能够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通知线程它们应该停止接受新请求并关闭。为了演示这段代码，我们将修改我们的服务器，使其仅接受两个请求后优雅地关闭其线程池。
我们需要注意的一点是：这不会影响处理执行闭包的代码部分，因此即使我们使用线程池为异步运行时，这里的一切也会完全相同。
在ThreadPool上实现Drop特性
让我们从在我们的线程池上实现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。
清单 21-22 显示了Drop实现的第一次尝试；这段代码还不能完全工作。
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 }
}
}首先，我们遍历线程池中的每个 workers。我们使用 &mut，因为 self 是一个可变引用，我们还需要能够修改 worker。对于每个工作线程，我们打印一条消息，说明该工作线程正在关闭，然后调用该工作线程的 join 方法。如果 join 调用失败，我们使用 unwrap 使 Rust 进入非正常关闭状态。
当我们编译这段代码时，会出现以下错误：
$ 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`
--> file:///home/.rustup/toolchains/1.82/lib/rustlib/src/rust/library/std/src/thread/mod.rs:1763:17
|
1763 | pub fn join(self) -> Result<T> {
| ^^^^
For more information about this error, try `rustc --explain E0507`.
error: could not compile `hello` (lib) due to 1 previous error
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，而join需要拥有其参数。为了解决这个问题，我们需要将线程从拥有thread的Worker实例中移出，以便join可以消耗线程。我们在清单17-15中做到了这一点：如果Worker持有一个Option<thread::JoinHandle<()>>，我们就可以调用Option的take方法，将值从Some变体中移出，并在其位置留下一个None变体。换句话说，正在运行的Worker将在thread中有一个Some变体，而当我们想要清理一个Worker时，我们将用None替换Some，这样Worker就没有线程可以运行。
所以我们知道我们想这样更新 Worker 的定义：
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 }
}
}现在让我们依赖编译器来找到其他需要更改的地方。 检查这段代码，我们得到了两个错误：
$ 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<()>`
--> file:///home/.rustup/toolchains/1.82/lib/rustlib/src/rust/library/std/src/thread/mod.rs:1763:5
|
1763 | pub fn join(self) -> Result<T> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Worker::new末尾的代码；我们需要在创建新的Worker时将thread值包装在Some中。进行以下更改以修复此错误：
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，打算对 Option 值调用 take 以将 thread 从 worker 中移出。以下更改将实现这一点：
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 18 章所述，Option 上的 take 方法取出 Some 变体并在其位置留下 None。我们使用 if let 来解构 Some 并获取线程；然后我们在该线程上调用 join。如果某个工作线程已经是 None，我们知道该工作线程已经被清理，因此在这种情况下不会发生任何事情。
向线程发出停止监听任务的信号
随着我们所做的所有更改，我们的代码编译时没有任何警告。
然而，坏消息是这段代码还没有按我们希望的方式工作。
关键在于由Worker实例的线程运行的闭包中的逻辑：目前，我们调用join，但这不会关闭线程
因为它们会无限循环地寻找任务。如果我们尝试使用当前实现的drop来删除我们的ThreadPool，主线程将
永远阻塞等待第一个线程完成。
为了解决这个问题，我们需要更改 ThreadPool 的 drop 实现，然后更改 Worker 循环。
首先，我们将更改 ThreadPool 的 drop 实现，以在等待线程完成之前显式地释放 sender。列表 21-23 显示了对 ThreadPool 的更改，以显式地释放 sender。我们使用与处理线程时相同的 Option 和 take 技术，以便能够将 sender 从 ThreadPool 中移出：
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),
}
}
}sender before joining the worker threads释放 sender 会关闭通道，这表示不会再发送更多消息。当这种情况发生时，工作线程在无限循环中对 recv 的所有调用都将返回错误。在示例 21-24 中，我们更改了 Worker 循环，使其在这种情况下优雅地退出循环，这意味着当 ThreadPool 的 drop 实现调用 join 时，线程将结束。
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),
}
}
}recv returns an error要查看此代码的运行情况，让我们修改 main 以仅接受两个请求，然后优雅地关闭服务器，如示例 21-25 所示。
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();
}你不会希望一个现实世界的Web服务器在仅服务了两个请求后就关闭。这段代码只是证明了优雅的关闭和清理机制是有效的。
take 方法在 Iterator 特性中定义，将迭代限制为最多前两个项目。ThreadPool 将在 main 结束时超出作用域，drop 实现将运行。
使用 cargo run 启动服务器，并发出三个请求。第三个请求应该出错，并且在您的终端中应看到类似的输出：
$ cargo run
Compiling hello v0.1.0 (file:///projects/hello)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.41s
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
你可能会看到不同的工作线程和消息打印顺序。我们可以通过这些消息了解代码的工作原理：工作线程 0 和 3 获取了前两个请求。服务器在第二个连接后停止接受连接，而 ThreadPool 上的 Drop 实现甚至在工作线程 3 开始其任务之前就开始执行。释放 sender 会断开所有工作线程的连接并告诉它们关闭。每个工作线程在断开连接时都会打印一条消息，然后线程池调用 join 等待每个工作线程完成。
注意这个特定执行的一个有趣方面：ThreadPool 丢弃了 sender，并且在任何工作线程收到错误之前，我们尝试加入工作线程 0。工作线程 0 尚未从 recv 收到错误，因此主线程阻塞等待工作线程 0 完成。在此期间，工作线程 3 收到了一个任务，然后所有线程都收到了错误。当工作线程 0 完成时，主线程等待其余工作线程完成。那时，它们都已经退出了循环并停止。
恭喜！我们现在完成了项目；我们有一个使用线程池异步响应的基本Web服务器。我们能够执行服务器的优雅关闭，清理池中的所有线程。
以下是完整的代码供参考：
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();
}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),
}
}
}我们还可以做得更多！如果您想继续增强这个项目，这里有一些想法：
- 为
ThreadPool及其公共方法添加更多文档。 - 添加测试以检验库的功能。
- 将
unwrap的调用更改为更健壮的错误处理。 - 使用
ThreadPool执行除处理网页请求之外的某些任务。 - 在crates.io上找到一个线程池库，并使用该库实现一个类似的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。
摘要
干得好！你已经到达了这本书的结尾！我们要感谢你加入我们这次的Rust之旅。你现在可以开始实现自己的Rust项目，并帮助其他人的项目。请记住，有一个欢迎的Rustaceans社区，他们很乐意在你的Rust之旅中遇到的任何挑战时帮助你。