Is this because the mutex was not released? - rust

Question

I've read this Turning Our Single-Threaded Server into a Multithreaded Server.
And tried to implement it.
I wrote this:
use std::sync::mpsc::{channel, Receiver, Sender};
use std::sync::{Arc, Mutex};
use std::thread;
type task = dyn FnOnce() + Send + 'static;
pub struct Threadpool {
threads: Vec<thread::JoinHandle<()>>,
rx: Arc<Mutex<Receiver<Box<task>>>>,
tx: Sender<Box<task>>,
}
impl Threadpool {
pub fn new(size: usize) -> Threadpool {
let mut tasks = Vec::with_capacity(size);
let (tx, rx): (Sender<Box<task>>, Receiver<Box<task>>) = channel();
let rx = Arc::new(Mutex::new(rx));
for _ in 0..size {
let rx = rx.clone();
let task = thread::spawn(move || {
loop {
let job= rx.lock().unwrap().recv().unwrap();
job();
}
});
tasks.push(task);
}
Threadpool {
threads: tasks,
rx,
tx,
}
}
pub fn execute<F>(&self, f: F)
where
F: FnOnce() + Send + 'static,
{
self.tx.send(Box::new(f)).unwrap();
}
}
It works.
But when I change
let job= rx.lock().unwrap().recv().unwrap();
job();
to
rx.lock().unwrap().recv().unwrap()();
When I open localhost:port/sleep, and then open localhost:port, it will takes 5 seconds.
I set this in main
"GET /sleep HTTP/1.1" => {
thread::sleep(Duration::from_secs(5));
("HTTP/1.1 200 OK", "hello.html")
}
I already knew that while let will cause that.
But I can't figure out why my code above will also lead to that.
Can anybody give me the answer.

Answer 1

In Rust temporary objects are dropped at the end of the expression that contains them (with a few caveats not relevant here).
And the temporary we are interested in is the guard of the mutex, whose drop is responsible of releasing the mutex lock.
So, writingh the drop explicitly, your first code:
let job = rx.lock().unwrap().recv().unwrap();
job();
Is equivalent to:
let mut guard = rx.lock().unwrap();
let job = guard.recv().unwrap();
drop(guard);
job();
And your second code:
rx.lock().unwrap().recv().unwrap()();
Is equivalent to:
let mut guard = rx.lock().unwrap();
let job = guard.recv().unwrap()
job();
drop(guard);
As you can see, now you are calling the job() function with the mutex still locked.

Answer 2

Is this because the mutex was not released?
Yes, you are basically doing this
{
let rx = rx.lock().unwrap(); // got the lock
let job = rx.recv().unwrap(); // got the job
// going to sleep while still holding mutex lock
job(); // std::thread::sleep(Duration::from_secs(5))
drop(rx); // lock is released
}
As all the threads share the mutex, and will try to acquire lock, they are effectively blocked until sleeping thread with lock wakes up. Which is why after requesting sleep endpoint, other threads are unable to perform any other job.
There's another issue however. Even if it didn't sleep, it still calls Receiver::recv() while still holding the lock, which blocks the current thread (goes to sleep) until something is sent down the channel. But considering that one thread will only block others if there's no jobs on the channel, I guess that is by design.