thread pool

src/bin/server.rs

 use std::{
-    io::{prelude::*, BufReader},
+    io::{prelude::*, BufReader, Lines},
     net::{TcpListener, TcpStream},
 };
-use std::io::Lines;
+use std::thread::available_parallelism;
+use crate::thread_pool::ThreadPool;
+
+mod thread_pool;
+
 
 fn main() {
+    // start server on localhost (for remote access use: 0.0.0.0)
     let listener = TcpListener::bind("127.0.0.1:7878").unwrap();
+    
+    // create thread pool (size = no. of cpu cores)
+    let pool = ThreadPool::new(available_parallelism().unwrap().get());
 
+    // handling function for incoming requests
     for stream in listener.incoming() {
         let stream = stream.unwrap();
 
-        handle_connection(stream);
+        // pass request to thread pool
+        pool.execute(|| {
+            handle_connection(stream);
+        });
     }
+    println!("Shutting down.");
 }
 
+
 fn handle_connection(mut stream: TcpStream) {
+    // clone TcpStream for simultaneous receiving and sending
     let mut stream2: TcpStream = stream.try_clone().unwrap();
+
+    // initialize reading buffer (for better performance!)
     let buf_reader: BufReader<&mut TcpStream> = BufReader::new(&mut stream);
+    // request: read received text line by line (continuous until connection closed)
     let request: Lines<BufReader<&mut TcpStream>> = buf_reader.lines();
 
     println!("Request: {:#?}", request);
 
+    // processing function for every text line received by request (terminates when conn closed)
     for elem in request
     {
+        // store received line in variable s
         let s = elem.unwrap();
         println!("{:?}", s);
-        stream2.write(s.as_bytes()).unwrap();
+
+        // send received line back to sender (append LF)
+        stream2.write_all(s.as_bytes()).unwrap();
         stream2.write_all(b"\n").unwrap();
     }
 }

src/bin/thread_pool.rs

+use std::{
+    sync::{mpsc, Arc, Mutex},
+    thread
+};
+
+
+pub struct ThreadPool {
+    workers: Vec<Worker>,
+    sender: Option<mpsc::Sender<Job>>,
+}
+
+// Box: pointer for heap allocation (smart pointer); dyn: unknown type to compile time
+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);
+
+        // create asynchronous channel with sender/receiver pair (for jobs!)
+        let (sender, receiver) = mpsc::channel();
+
+        // protect receiver with thread-safe shared (Arc) mutex to avoid multiple access
+        let receiver = Arc::new(Mutex::new(receiver));
+
+        // pre(!)allocate space for workers
+        let mut workers = Vec::with_capacity(size);
+
+        // initialize all workers...
+        for id in 0..size {
+            // create worker with unique id and pointer to the shared mutex
+            workers.push(Worker::new(id, Arc::clone(&receiver)));
+        }
+
+        // set thread pool variables
+        ThreadPool {
+            workers,
+            // Some --> enum variant (Option): indicates that sender may not exist (=be null)
+            sender: Some(sender),
+        }
+    }
+
+    pub fn execute<F>(&self, f: F)
+        where
+            // FnOnce() --> only callable once; Send --> transfer to another thread; 'static --> lifetime
+            F: FnOnce() + Send + 'static,
+    {
+        let job = Box::new(f);
+
+        // commit job to sender object --> pass to threads for execution
+        self.sender.as_ref().unwrap().send(job).unwrap();
+    }
+}
+
+impl Drop for ThreadPool {
+    fn drop(&mut self) {
+        // drop sender first
+        drop(self.sender.take());
+
+        // then drop workers
+        for worker in &mut self.workers {
+            println!("Shutting down worker {}", worker.id);
+
+            // join worker thread
+            if let Some(thread) = worker.thread.take() {
+                thread.join().unwrap();
+            }
+        }
+    }
+}
+
+
+struct Worker {
+    id: usize,
+    thread: Option<thread::JoinHandle<()>>,
+}
+
+impl Worker {
+    /// Create a new Worker..
+    ///
+    /// The id is a unique identifier.
+    /// The receiver is a shared pointer to a receiver protected by Mutex.
+    fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker {
+        // spawn thread and enter infinite loop
+        let thread = thread::spawn(move || loop {
+            // lock mutex and wait for job
+            let message = receiver.lock().unwrap().recv();
+
+            match message {
+                // normal operation --> execute job
+                Ok(job) => {
+                    println!("Worker {id} got a job; executing.");
+
+                    job();
+                }
+                // exit gracefully when pool is closed --> recv will return error then
+                Err(_) => {
+                    println!("Worker {id} disconnected; shutting down.");
+                    break;
+                }
+            }
+        });
+
+        Worker {
+            id,
+            thread: Some(thread),
+        }
+    }
+}