Returning a RWLockReadGuard independently from a method

Question

I have an object of type

Arc<RwLock<SessionData>>

And I have a method that is supposed to take some kind of reference to SessionData

fn some_method(session: ...)

I'm using Rocket (a web-framework for Rust), and I can't directly invoke the method, because it is invoked by Rocket. However, I can provide it with an implementation that creates an object that will be passed to the handler. It looks a bit like this:

impl<'a, 'r> request::FromRequest<'a, 'r> for SomeType {
    type Error = ();

    fn from_request(request: &'a request::Request<'r>) -> request::Outcome<Self, Self::Error> {
        // return object here
    }
}

I want to avoid returning an RwLock directly, because I want the handler to have an already-locked object passed to it. However, I can't return a reference or a RwLockReadGuard, because both of them depend on the RwLock, which would go out of scope.

Instead, I am trying to create some kind of self-sufficient type that would contain an Arc<RwLock<SessionData>>, contain the lock guard to this lock, and deref to a SessionData object.

So far, I have tried some combinations of the following:

• A Session object that contains an Arc<RwLock<SessionData>> and a RwLockReadGuard<SessionData>
• An object that contains an Arc<RwLock<SessionData>> and a RwLockReadGuardRef<SessionData> from the owning-ref library.
• An object that would use the OwnedHandle type from the owning-ref library.

However, I haven't been able to do what I want to do, running into various lifetime borrowing issues and whatnot.

Is it at all possible to create a sort of a self-contained 'Handle'-like object that would contain both the lock and the lock guard to the object that it points to?

This is a similar, but slightly different situation than described in How to return reference to a sub-value of a value that is under a mutex?. In there, the MutexGuardRef internally depends on Mutex, and cannot exist if the Mutex (or MyStruct) goes out of scope. In order to achieve similar behaviour, I'd have to pass a struct that contains my RwLock and then do the locking inside the method. This is fine, but I'm wondering if I can go another step further, and pass a struct that is both independent and serves as a RwLockGuard, avoiding the need to lock manually.

Basically, I want to move the locking of the RwLock from the client to the provider of the value.

Tobytobye

Answer 1

As described in Why can't I store a value and a reference to that value in the same struct?, the Rental crate allows for self-referential structs in certain cases.

#[macro_use]
extern crate rental;

use std::sync::{Arc, RwLock};

struct SessionData;
impl SessionData {
    fn hello(&self) -> u8 { 42 }
}

rental! {
    mod owning_lock {
        use std::sync::{Arc, RwLock, RwLockReadGuard};

        #[rental(deref_suffix)]
        pub struct OwningReadGuard<T>
        where
            T: 'static,
        {
            lock: Arc<RwLock<T>>,
            guard: RwLockReadGuard<'lock, T>,
        }
    }
}

use owning_lock::OwningReadGuard;

fn owning_lock(session: Arc<RwLock<SessionData>>) -> OwningReadGuard<SessionData> {
    OwningReadGuard::new(session, |s| s.read().unwrap())
}

fn main() {
    let session = Arc::new(RwLock::new(SessionData));

    let lock = owning_lock(session.clone());
    println!("{}", lock.hello());

    assert!(session.try_read().is_ok());
    assert!(session.try_write().is_err());

    drop(lock);

    assert!(session.try_write().is_ok());
}

See also:

• How to return a reference to a sub-value of a value that is under a mutex?
• Returning the T borrowed from RefCell<T>
• Why can't I store a value and a reference to that value in the same struct?

Jamiejamieson

Woah, this is amazing. I'd drown you in all my upvotes if I could. This is exactly what I need. Huge thanks! – Tobytobye

Is there any newer recommendation now that rental's repo is read only? – Dulcie

Answer 2

In case someone needs another solution, then here is the example from @Shelpmaster rewritten using ouroboros:

use ouroboros::self_referencing;
use std::sync::{Arc, RwLock, RwLockReadGuard};

struct SessionData;
impl SessionData {
    fn hello(&self) -> u8 {
        42
    }
}

#[self_referencing]
pub struct OwningReadGuard<T>
where
    T: 'static,
{
    lock: Arc<RwLock<T>>,
    #[covariant]
    #[borrows(lock)]
    guard: RwLockReadGuard<'this, T>,
}

fn owning_lock(lock: Arc<RwLock<SessionData>>) -> OwningReadGuard<SessionData> {
    OwningReadGuardBuilder {
        lock,
        guard_builder: |l: &Arc<RwLock<SessionData>>| l.read().unwrap(),
    }
    .build()
}

fn main() {
    let session = Arc::new(RwLock::new(SessionData));

    let lock = owning_lock(session.clone());
    println!("{}", lock.borrow_guard().hello());

    assert!(session.try_read().is_ok());
    assert!(session.try_write().is_err());

    drop(lock);

    assert!(session.try_write().is_ok());
}

Gusman