add: clean shutdown

src/lib.rs

@@ -5,7 +5,7 @@ use std::{
 
 pub struct ThreadPool {
     workers: Vec<Worker>,
-    sender: mpsc::Sender<Job>,
+    sender: mpsc::Sender<Message>,
 }
 
 impl ThreadPool {
@@ -37,31 +37,68 @@ impl ThreadPool {
     {
         let job = Box::new(f);
 
-        self.sender.send(job).unwrap();
+        self.sender.send(Message::NewJob(job)).unwrap();
+    }
+}
+
+impl Drop for ThreadPool {
+    fn drop(&mut self) {
+        println!("Sending terminate message to all workers.");
+        for _ in &self.workers {
+            // Panic is an acceptable, if we cannot shut down cleanly
+            self.sender.send(Message::Terminate).unwrap();
+        }
+
+        println!("Shutting down all workers.");
+
+        for worker in &mut self.workers {
+            println!("Shutting down worker {}", worker.id);
+
+            if let Some(thread) = worker.thread.take() {
+                // Panic is an acceptable, if we cannot shut down cleanly
+                thread.join().unwrap();
+            }
+        }
     }
 }
 
 type Job = Box<dyn FnOnce() + Send + 'static>;
 
+enum Message {
+    NewJob(Job),
+    Terminate,
+}
+
 struct Worker {
     id: usize,
-    thread: thread::JoinHandle<()>,
+    thread: Option<thread::JoinHandle<()>>,
 }
 
 impl Worker {
-    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
+    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Message>>>) -> Worker {
         let thread = thread::spawn(move || loop {
-            let job = receiver
+            let message = receiver
                 .lock()
                 .expect("Mutex poisoned; bailing out.")
                 .recv()
-                .expect("Failed to receive job.");
+                .expect("Failed to receive message.");
 
-            println!("Worker {} got a job; executing.", id);
+            match message {
+                Message::NewJob(job) => {
+                    println!("Worker {} got a job; executing.", id);
 
-            job();
+                    job();
+                }
+                Message::Terminate => {
+                    println!("Worker {} was told to terminate.", id);
+                    break;
+                }
+            }
         });
 
-        Worker { id, thread }
+        Worker {
+            id,
+            thread: Some(thread),
+        }
     }
 }