//!
//! Blocking run of the async processes
//!

use crossbeam_utils::sync::{Parker, Unparker};
use std::cell::Cell;
use std::future::Future;
use std::mem::ManuallyDrop;
use std::ops::Deref;
use std::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};

pub(crate) fn block<F, T>(f: F) -> T
where
    F: Future<Output = T>,
{
    thread_local! {
        // May hold a pre-allocated parker that can be reused for efficiency.
        //
        // Note that each invocation of `block` needs its own parker. In particular, if `block`
        // recursively calls itself, we must make sure that each recursive call uses a distinct
        // parker instance.
        static CACHE: Cell<Option<Parker>> = Cell::new(None);
    }

    pin_utils::pin_mut!(f);

    CACHE.with(|cache| {
        // Reuse a cached parker or create a new one for this invocation of `block`.
        let parker: Parker = cache.take().unwrap_or_else(|| Parker::new());

        let ptr = Unparker::into_raw(parker.unparker().clone());
        let vt = vtable();

        // Waker must not be dropped until it's no longer required. We also happen to know that a
        // Parker contains at least one reference to `Unparker` so the relevant `Unparker` will not
        // be dropped at least until the `Parker` is.
        let waker = unsafe { Waker::from_raw(RawWaker::new(ptr, vt)) };
        let cx = &mut Context::from_waker(&waker);

        loop {
            if let Poll::Ready(t) = f.as_mut().poll(cx) {
                // Save the parker for the next invocation of `block`.
                cache.set(Some(parker));
                return t;
            }
            parker.park();
        }
    })
}

fn vtable() -> &'static RawWakerVTable {
    /// This function will be called when the RawWaker gets cloned, e.g. when the Waker in which
    /// the RawWaker is stored gets cloned.
    //
    /// The implementation of this function must retain all resources that are required for this
    /// additional instance of a RawWaker and associated task. Calling wake on the resulting
    /// RawWaker should result in a wakeup of the same task that would have been awoken by the
    /// original RawWaker.
    unsafe fn clone_raw(ptr: *const ()) -> RawWaker {
        // [`Unparker`] implements `Clone` and upholds the contract stated above. The current
        // Implementation is simply an Arc over the actual inner values. However clone takes the
        // original value by reference so we need to make sure to not drop it.
        let unparker = ManuallyDrop::new(Unparker::from_raw(ptr));
        RawWaker::new(Unparker::into_raw(unparker.deref().clone()), vtable())
    }

    /// This function will be called when wake is called on the Waker. It must wake up the task
    /// associated with this RawWaker.
    ///
    /// The implementation of this function must make sure to release any resources that are
    /// associated with this instance of a RawWaker and associated task.
    unsafe fn wake_raw(ptr: *const ()) {
        // We reconstruct the Unparker from the pointer here thus ensuring it is dropped at the
        // end of this function call.
        Unparker::from_raw(ptr).unpark();
    }

    /// This function will be called when wake_by_ref is called on the Waker. It must wake up the
    /// task associated with this RawWaker.
    ///
    /// This function is similar to wake, but must not consume the provided data pointer.
    unsafe fn wake_by_ref_raw(ptr: *const ()) {
        // We **must not** drop the resulting Unparker so we wrap it in `ManuallyDrop`.
        let unparker = ManuallyDrop::new(Unparker::from_raw(ptr));
        unparker.unpark();
    }

    /// This function gets called when a RawWaker gets dropped.
    ///
    /// The implementation of this function must make sure to release any resources that are
    /// associated with this instance of a RawWaker and associated task.
    unsafe fn drop_raw(ptr: *const ()) {
        drop(Unparker::from_raw(ptr))
    }

    &RawWakerVTable::new(clone_raw, wake_raw, wake_by_ref_raw, drop_raw)
}