Why we immediately replace back the new value with the old one?

1

The source code I am referring to is located here: https://github.com/tokio-rs/tokio/blob/master/tokio/src/runtime/context.rs

    thread_local! {
        static CONTEXT: RefCell<Option<Handle>> = RefCell::new(None)
    }
    /// Set this [`Handle`] as the current active [`Handle`].
    ///
    /// [`Handle`]: Handle
    pub(crate) fn enter<F, R>(new: Handle, f: F) -> R
    where 
        F: FnOnce() -> R,
    {
        struct DropGuard(Option<Handle>);

        impl Drop for DropGuard {
            fn drop(&mut self) {
                CONTEXT.with(|ctx| {
                    *ctx.borrow_mut() = self.0.take();
                })
            }
        }

        let _guard = CONTEXT.with(|ctx| {
            let old = ctx.borrow_mut().replace(new);
            DropGuard(old)
        });

        f()
    }   // <------------ fn drop()  method gets called here?

I am not able to understand why we immediately replace back the wrapped Option with the value old, as soon as we return from this function.

I might be lacking understanding of how the drop method works, but it seems that it gets invoked when '_guard' variable goes out of scope. It means that the handle is only active during the execution of function 'f'.

I apologize if this answer to this question is straightforward.

2

Doing it this way will cause the handle to be restored to the old value by the drop impl of DropGuard even when there is a panic.

3 Likes
3

Thanks. Can you please explain the part about panic or maybe give an example?

4

If it was written like

pub(crate) fn enter<F, R>(new: Handle, f: F) -> R
where
    F: FnOnce() -> R,
{
    let old = CONTEXT.with(|ctx| {
        ctx.borrow_mut().replace(new)
    });

    let val = f();
    
    CONTEXT.with(|ctx| {
        *ctx.borrow_mut() = old
    });
    
    val
}

Then if f() panics, CONTEXT.with(|ctx| { *ctx.borrow_mut() = old }); will never be executed. With the code written as is the drop impl of DropGuard will run when either returning normally or when unwinding the stack.

2 Likes
5

Thanks it is clear now.

6

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