Sending Requests to Threads Via Channels
The next problem to tackle is that our closures do absolutely nothing. We’ve
been working around the problem that we get the actual closure we want to
execute in the execute method, but it feels like we need to know the actual
closures when we create the ThreadPool.
Let’s think about what we really want to do though: we want the Worker
structs that we just created to fetch jobs from a queue that the ThreadPool
holds, and run those jobs in a thread.
In Chapter 16, we learned about channels. Channels are a great way to
communicate between two threads, and they’re perfect for this use-case. The
channel will function as the queue of jobs, and execute will send a job from
the ThreadPool to the Worker instances that are checking for jobs in the
thread they’ve spawned. Here’s the plan:
ThreadPoolwill create a channel and hold on to the sending side.- Each
Workerwill hold on to the receiving side of the channel. - A new
Jobstruct will hold the closures we want to send down the channel. - The
executemethod ofThreadPoolwill send the job it wants to execute down the sending side of the channel. - In a thread, the
Workerwill loop over its receiving side of the channel and execute the closures of any jobs it receives.
Let’s start by creating a channel in ThreadPool::new and holding the sending
side in the ThreadPool instance, as shown in Listing 20-16. Job is the type
of item we’re going to be sending down the channel; it’s a struct that doesn’t
hold anything for now:
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { # use std::thread; // --snip-- use std::sync::mpsc; pub struct ThreadPool { workers: Vec<Worker>, sender: mpsc::Sender<Job>, } struct Job; impl ThreadPool { // --snip-- pub fn new(size: usize) -> ThreadPool { assert!(size > 0); let (sender, receiver) = mpsc::channel(); let mut workers = Vec::with_capacity(size); for id in 0..size { workers.push(Worker::new(id)); } ThreadPool { workers, sender, } } // --snip-- } # # struct Worker { # id: usize, # thread: thread::JoinHandle<()>, # } # # impl Worker { # fn new(id: usize) -> Worker { # let thread = thread::spawn(|| {}); # # Worker { # id, # thread, # } # } # } #}
Listing 20-16: Modifying ThreadPool to store the
sending end of a channel that sends Job instances
In ThreadPool::new, we create our new channel, and then have the pool hang on
to the sending end. This will successfully compile, still with warnings.
Let’s try passing a receiving end of the channel into each worker when the
thread pool creates them. We know we want to use the receiving end of the
channel in the thread that the workers spawn, so we’re going to reference the
receiver parameter in the closure. The code shown here in Listing 20-17
won’t quite compile yet:
Filename: src/lib.rs
impl ThreadPool {
// --snip--
pub fn new(size: usize) -> ThreadPool {
assert!(size > 0);
let (sender, receiver) = mpsc::channel();
let mut workers = Vec::with_capacity(size);
for id in 0..size {
workers.push(Worker::new(id, receiver));
}
ThreadPool {
workers,
sender,
}
}
// --snip--
}
// --snip--
impl Worker {
fn new(id: usize, receiver: mpsc::Receiver<Job>) -> Worker {
let thread = thread::spawn(|| {
receiver;
});
Worker {
id,
thread,
}
}
}
Listing 20-17: Passing the receiving end of the channel to the workers
These are small and straightforward changes: we pass in the receiving end of
the channel into Worker::new, and then we use it inside of the closure.
If we try to check this, we get this error:
$ cargo check
Compiling hello v0.1.0 (file:///projects/hello)
error[E0382]: use of moved value: `receiver`
--> src/lib.rs:27:42
|
27 | workers.push(Worker::new(id, receiver));
| ^^^^^^^^ value moved here in
previous iteration of loop
|
= note: move occurs because `receiver` has type
`std::sync::mpsc::Receiver<Job>`, which does not implement the `Copy` trait
The code as written won’t quite work since it’s trying to pass receiver to
multiple Worker instances. Recall from Chapter 16 that the channel
implementation provided by Rust is multiple producer, single consumer, so
we can’t just clone the consuming end of the channel to fix this. We also don’t
want to clone the consuming end even if we wanted to; sharing the single
receiver between all of the workers is the mechanism by which we’d like to
distribute the jobs across the threads.
Additionally, taking a job off the channel queue involves mutating receiver,
so the threads need a safe way to share receiver and be allowed to modify it.
If the modifications weren’t thread-safe, we might get race conditions such as
two threads executing the same job if they both take the same job off the queue
at the same time.
So remembering the thread-safe smart pointers that we discussed in Chapter 16,
in order to share ownership across multiple threads and allow the threads to
mutate the value, we need to use Arc<Mutex<T>>. Arc will let multiple
workers own the receiver, and Mutex will make sure that only one worker is
getting a job from the receiver at a time. Listing 20-18 shows the changes we
need to make:
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { # use std::thread; # use std::sync::mpsc; use std::sync::Arc; use std::sync::Mutex; // --snip-- # pub struct ThreadPool { # workers: Vec<Worker>, # sender: mpsc::Sender<Job>, # } # struct Job; # impl ThreadPool { // --snip-- 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, } } // --snip-- } # struct Worker { # id: usize, # thread: thread::JoinHandle<()>, # } # impl Worker { fn new(id: usize, receiver: Arc<Mutex<mpsc::Receiver<Job>>>) -> Worker { // --snip-- # let thread = thread::spawn(|| { # receiver; # }); # # Worker { # id, # thread, # } } } #}
Listing 20-18: Sharing the receiving end of the channel
between the workers by using Arc and Mutex
In ThreadPool::new, we put the receiving end of the channel in an Arc and a
Mutex. For each new worker, we clone the Arc to bump the reference count so
the workers can share ownership of the receiving end.
With these changes, the code compiles! We’re getting there!
Let’s finally implement the execute method on ThreadPool. We’re also going
to change the Job struct: instead of being a struct, Job is going to be a
type alias for a trait object that holds the type of closure that execute
receives. We discussed how type aliases can help make long types shorter, and
this is such a case! Take a look at Listing 20-19:
Filename: src/lib.rs
# #![allow(unused_variables)] #fn main() { // --snip-- # pub struct ThreadPool { # workers: Vec<Worker>, # sender: mpsc::Sender<Job>, # } # use std::sync::mpsc; # struct Worker {} type Job = Box<FnOnce() + Send + 'static>; impl ThreadPool { // --snip-- pub fn execute<F>(&self, f: F) where F: FnOnce() + Send + 'static { let job = Box::new(f); self.sender.send(job).unwrap(); } } // --snip-- #}
Listing 20-19: Creating a Job type alias for a Box
that holds each closure, then sending the job down the channel
After creating a new Job instance using the closure we get in
execute, we send that job down the sending end of the channel. We’re calling
unwrap on send since sending may fail if the receiving end has stopped
receiving new messages, which would happen if we stop all of our threads from
executing. This isn’t possible right now, though, since our threads continue
executing as long as the pool exists. We use unwrap since we know the failure
case won’t happen even though the compiler can’t tell that, which is an
appropriate use of unwrap as we discussed in Chapter 9.
Are we done yet? Not quite! In the worker, we’ve still got a closure being
passed to thread::spawn that only references the receiving end of the
channel. Instead, we need the closure to loop forever, asking the receiving end
of the channel for a job, and running the job when it gets one. Let’s make the
change shown in Listing 20-20 to Worker::new:
Filename: src/lib.rs
// --snip--
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 {} got a job; executing.", id);
(*job)();
}
});
Worker {
id,
thread,
}
}
}
Listing 20-20: Receiving and executing the jobs in the worker’s thread
Here, we first call lock on the receiver to acquire the mutex, then
unwrap to panic on any errors. Acquiring a lock might fail if the mutex is in
a state called poisoned, which can happen if some other thread panicked while
holding the lock rather than releasing it. If this thread can’t get the lock
for that reason, calling unwrap to have this thread panic is the correct
action to take as well. Feel free to change this unwrap to an expect with
an error message that is meaningful to you if you’d like.
If we get the lock on the mutex, then we call recv to receive a Job from
the channel. A final unwrap moves past those errors as well. recv will
return Err if the thread holding the sending side of the channel has shut
down, similar to how the send method returns Err if the receiving side
shuts down.
The call to recv blocks; that is, if there’s no job yet, this thread will sit
here until a job becomes available. The Mutex<T> makes sure that only one
Worker thread at a time is trying to request a job.
Theoretically, this code should compile. Unfortunately, the Rust compiler isn’t perfect yet, and we get this error:
error[E0161]: cannot move a value of type std::ops::FnOnce() +
std::marker::Send: the size of std::ops::FnOnce() + std::marker::Send cannot be
statically determined
--> src/lib.rs:63:17
|
63 | (*job)();
| ^^^^^^
This error is fairly cryptic, and that’s because the problem is fairly cryptic.
In order to call a FnOnce closure that is stored in a Box<T> (which is what
our Job type alias is), the closure needs to be able to move itself out of
the Box<T> since when we call the closure, it takes ownership of self. In
general, moving a value out of a Box<T> isn’t allowed since Rust doesn’t know
how big the value inside the Box<T> is going to be; recall in Chapter 15 that
we used Box<T> precisely because we had something of an unknown size that we
wanted to store in a Box<T> to get a value of a known size.
We saw in Chapter 17, Listing 17-15 that we can write methods that use the
syntax self: Box<Self> so that the method takes ownership of a Self value
that is stored in a Box<T>. That’s what we want to do here, but unfortunately
the part of Rust that implements what happens when we call a closure isn’t
implemented using self: Box<Self>. So Rust doesn’t yet understand that it
could use self: Box<Self> in this situation in order to take ownership of the
closure and move the closure out of the Box<T>.
In the future, the code in Listing 20-20 should work just fine. Rust is still a work in progress with places that the compiler could be improved. There are people just like you working to fix this and other issues! Once you’ve finished the book, we would love for you to join in.
But for now, let’s work around this problem. Luckily, there’s a trick that
involves telling Rust explicitly that we’re in a case where we can take
ownership of the value inside the Box<T> using self: Box<Self>, and once we
have ownership of the closure, we can call it. This involves defining a new
trait that has a method call_box that uses self: Box<Self> in its
signature, defining that trait for any type that implements FnOnce(),
changing our type alias to use the new trait, and changing Worker to use the
call_box method. These changes are shown in Listing 20-21:
Filename: src/lib.rs
trait FnBox {
fn call_box(self: Box<Self>);
}
impl<F: FnOnce()> FnBox for F {
fn call_box(self: Box<F>) {
(*self)()
}
}
type Job = Box<FnBox + Send + 'static>;
// --snip--
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 {} got a job; executing.", id);
job.call_box();
}
});
Worker {
id,
thread,
}
}
}
Listing 20-21: Adding a new trait FnBox to work around
the current limitations of Box<FnOnce()>
First, we create a new trait named FnBox. This trait has one method,
call_box, similar to the call methods on the other Fn* traits, except
this method takes self: Box<Self> in order to take ownership of self and
move the value out of the Box<T>.
Next, we implement the FnBox trait for any type F that implements the
FnOnce() trait. Effectively, this means that any FnOnce() closures can use
our call_box method. The implementation of call_box uses (*self)() to
move the closure out of the Box<T> and call the closure.
Instead of FnOnce(), we now want our Job type alias to be a Box of
anything that implements our new trait FnBox. This will allow us to use
call_box in Worker when we get a Job value. Because we implemented the
FnBox trait for any FnOnce() closure, we don’t have to change anything
about the actual values we’re sending down the channel.
Finally, in the closure run in the thread in Worker::new, we use call_box
instead of invoking the closure directly. Now Rust is able to understand that
what we want to do is fine.
This is a very sneaky, complicated trick. Don’t worry too much if it doesn’t make perfect sense; someday, it will be completely unnecessary.
With this trick, our thread pool is in a working state! Give it a cargo run,
and make some requests:
$ cargo run
Compiling hello v0.1.0 (file:///projects/hello)
warning: field is never used: `workers`
--> src/lib.rs:7:5
|
7 | workers: Vec<Worker>,
| ^^^^^^^^^^^^^^^^^^^^
|
= note: #[warn(dead_code)] on by default
warning: field is never used: `id`
--> src/lib.rs:61:5
|
61 | id: usize,
| ^^^^^^^^^
|
= note: #[warn(dead_code)] on by default
warning: field is never used: `thread`
--> src/lib.rs:62:5
|
62 | thread: thread::JoinHandle<()>,
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: #[warn(dead_code)] on by default
Finished dev [unoptimized + debuginfo] target(s) in 0.99 secs
Running `target/debug/hello`
Worker 0 got a job; executing.
Worker 2 got a job; executing.
Worker 1 got a job; executing.
Worker 3 got a job; executing.
Worker 0 got a job; executing.
Worker 2 got a job; executing.
Worker 1 got a job; executing.
Worker 3 got a job; executing.
Worker 0 got a job; executing.
Worker 2 got a job; executing.
Success! We now have a thread pool executing connections asynchronously. We
never create more than four threads, so our system won’t get overloaded if the
server gets a lot of requests. If we make a request to /sleep, the server
will be able to serve other requests by having another thread run them.
What about those warnings, though? Don’t we use the workers, id, and
thread fields? Well, right now, we’re using all three of these fields to hold
onto some data, but we don’t actually do anything with the data once we’ve
set up the thread pool and started running the code that sends jobs down the
channel to the threads. If we didn’t hold onto these values, though, they’d go
out of scope: for example, if we didn’t return the Vec<Worker> value as part
of the ThreadPool, the vector would get cleaned up at the end of
ThreadPool::new.
So are these warnings wrong? In one sense yes, the warnings are wrong, since we are using the fields to store data we need to keep around. In another sense, no, the warnings aren’t wrong, and they’re telling us that we’ve forgotten to do something: we never do anything to clean up our thread pool once it’s done being used, we just use ctrl-C to stop the program and let the operating system clean up after us. Let’s implement a graceful shutdown that cleans up everything we’ve created instead.