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    #![cfg_attr(not(feature = "full"), allow(dead_code))]

    //! Opt-in yield points for improved cooperative scheduling.
    //!
    //! A single call to [`poll`] on a top-level task may potentially do a lot of
    //! work before it returns `Poll::Pending`. If a task runs for a long period of
    //! time without yielding back to the executor, it can starve other tasks
    //! waiting on that executor to execute them, or drive underlying resources.
    //! Since Rust does not have a runtime, it is difficult to forcibly preempt a
    //! long-running task. Instead, this module provides an opt-in mechanism for
    //! futures to collaborate with the executor to avoid starvation.
    //!
    //! Consider a future like this one:
    //!
    //! ```
    //! # use tokio_stream::{Stream, StreamExt};
    //! async fn drop_all<I: Stream + Unpin>(mut input: I) {
    //!     while let Some(_) = input.next().await {}
    //! }
    //! ```
    //!
    //! It may look harmless, but consider what happens under heavy load if the
    //! input stream is _always_ ready. If we spawn `drop_all`, the task will never
    //! yield, and will starve other tasks and resources on the same executor. With
    //! opt-in yield points, this problem is alleviated:
    //!
    //! ```ignore
    //! # use tokio_stream::{Stream, StreamExt};
    //! async fn drop_all<I: Stream + Unpin>(mut input: I) {
    //!     while let Some(_) = input.next().await {
    //!         tokio::coop::proceed().await;
    //!     }
    //! }
    //! ```
    //!
    //! The `proceed` future will coordinate with the executor to make sure that
    //! every so often control is yielded back to the executor so it can run other
    //! tasks.
    //!
    //! # Placing yield points
    //!
    //! Voluntary yield points should be placed _after_ at least some work has been
    //! done. If they are not, a future sufficiently deep in the task hierarchy may
    //! end up _never_ getting to run because of the number of yield points that
    //! inevitably appear before it is reached. In general, you will want yield
    //! points to only appear in "leaf" futures -- those that do not themselves poll
    //! other futures. By doing this, you avoid double-counting each iteration of
    //! the outer future against the cooperating budget.
    //!
    //! [`poll`]: method@std::future::Future::poll

    // NOTE: The doctests in this module are ignored since the whole module is (currently) private.

    use std::cell::Cell;

    thread_local! {
        static CURRENT: Cell<Budget> = Cell::new(Budget::unconstrained());
    }

    /// Opaque type tracking the amount of "work" a task may still do before
    /// yielding back to the scheduler.
    #[derive(Debug, Copy, Clone)]
    pub(crate) struct Budget(Option<u8>);

    impl Budget {
        /// Budget assigned to a task on each poll.
        ///
        /// The value itself is chosen somewhat arbitrarily. It needs to be high
        /// enough to amortize wakeup and scheduling costs, but low enough that we
        /// do not starve other tasks for too long. The value also needs to be high
        /// enough that particularly deep tasks are able to do at least some useful
        /// work at all.
        ///
        /// Note that as more yield points are added in the ecosystem, this value
        /// will probably also have to be raised.
        const fn initial() -> Budget {
            Budget(Some(128))
        }

        /// Returns an unconstrained budget. Operations will not be limited.
        const fn unconstrained() -> Budget {
            Budget(None)
        }
    }

    cfg_rt_multi_thread! {
        impl Budget {
            fn has_remaining(self) -> bool {
                self.0.map(|budget| budget > 0).unwrap_or(true)
            }
        }
    }

    /// Run the given closure with a cooperative task budget. When the function
    /// returns, the budget is reset to the value prior to calling the function.
    #[inline(always)]
    pub(crate) fn budget<R>(f: impl FnOnce() -> R) -> R {
        with_budget(Budget::initial(), f)
    }

    #[inline(always)]
    fn with_budget<R>(budget: Budget, f: impl FnOnce() -> R) -> R {
        struct ResetGuard<'a> {
            cell: &'a Cell<Budget>,
            prev: Budget,
        }

        impl<'a> Drop for ResetGuard<'a> {
            fn drop(&mut self) {
                self.cell.set(self.prev);
            }
        }

        CURRENT.with(move |cell| {
            let prev = cell.get();

            cell.set(budget);

            let _guard = ResetGuard { cell, prev };

            f()
        })
    }

    cfg_rt_multi_thread! {
        /// Set the current task's budget
        pub(crate) fn set(budget: Budget) {
            CURRENT.with(|cell| cell.set(budget))
        }

        #[inline(always)]
        pub(crate) fn has_budget_remaining() -> bool {
            CURRENT.with(|cell| cell.get().has_remaining())
        }
    }

    cfg_rt! {
        /// Forcibly remove the budgeting constraints early.
        ///
        /// Returns the remaining budget
        pub(crate) fn stop() -> Budget {
            CURRENT.with(|cell| {
                let prev = cell.get();
                cell.set(Budget::unconstrained());
                prev
            })
        }
    }

    cfg_coop! {
        use std::task::{Context, Poll};

        #[must_use]
        pub(crate) struct RestoreOnPending(Cell<Budget>);

        impl RestoreOnPending {
            pub(crate) fn made_progress(&self) {
                self.0.set(Budget::unconstrained());
            }
        }

        impl Drop for RestoreOnPending {
            fn drop(&mut self) {
                // Don't reset if budget was unconstrained or if we made progress.
                // They are both represented as the remembered budget being unconstrained.
                let budget = self.0.get();
                if !budget.is_unconstrained() {
                    CURRENT.with(|cell| {
                        cell.set(budget);
                    });
                }
            }
        }

        /// Returns `Poll::Pending` if the current task has exceeded its budget and should yield.
        ///
        /// When you call this method, the current budget is decremented. However, to ensure that
        /// progress is made every time a task is polled, the budget is automatically restored to its
        /// former value if the returned `RestoreOnPending` is dropped. It is the caller's
        /// responsibility to call `RestoreOnPending::made_progress` if it made progress, to ensure
        /// that the budget empties appropriately.
        ///
        /// Note that `RestoreOnPending` restores the budget **as it was before `poll_proceed`**.
        /// Therefore, if the budget is _further_ adjusted between when `poll_proceed` returns and
        /// `RestRestoreOnPending` is dropped, those adjustments are erased unless the caller indicates
        /// that progress was made.
        #[inline]
        pub(crate) fn poll_proceed(cx: &mut Context<'_>) -> Poll<RestoreOnPending> {
            CURRENT.with(|cell| {
                let mut budget = cell.get();

                if budget.decrement() {
                    let restore = RestoreOnPending(Cell::new(cell.get()));
                    cell.set(budget);
                    Poll::Ready(restore)
                } else {
                    cx.waker().wake_by_ref();
                    Poll::Pending
                }
            })
        }

        impl Budget {
            /// Decrement the budget. Returns `true` if successful. Decrementing fails
            /// when there is not enough remaining budget.
            fn decrement(&mut self) -> bool {
                if let Some(num) = &mut self.0 {
                    if *num > 0 {
                        *num -= 1;
                        true
                    } else {
                        false
                    }
                } else {
                    true
                }
            }

            fn is_unconstrained(self) -> bool {
                self.0.is_none()
            }
        }
    }

    #[cfg(all(test, not(loom)))]
    mod test {
        use super::*;

        fn get() -> Budget {
            CURRENT.with(|cell| cell.get())
        }

        #[test]
        fn bugeting() {
            use futures::future::poll_fn;
            use tokio_test::*;

            assert!(get().0.is_none());

            let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));

            assert!(get().0.is_none());
            drop(coop);
            assert!(get().0.is_none());

            budget(|| {
                assert_eq!(get().0, Budget::initial().0);

                let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
                assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
                drop(coop);
                // we didn't make progress
                assert_eq!(get().0, Budget::initial().0);

                let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
                assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
                coop.made_progress();
                drop(coop);
                // we _did_ make progress
                assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);

                let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
                assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 2);
                coop.made_progress();
                drop(coop);
                assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 2);

                budget(|| {
                    assert_eq!(get().0, Budget::initial().0);

                    let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
                    assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
                    coop.made_progress();
                    drop(coop);
                    assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 1);
                });

                assert_eq!(get().0.unwrap(), Budget::initial().0.unwrap() - 2);
            });

            assert!(get().0.is_none());

            budget(|| {
                let n = get().0.unwrap();

                for _ in 0..n {
                    let coop = assert_ready!(task::spawn(()).enter(|cx, _| poll_proceed(cx)));
                    coop.made_progress();
                }

                let mut task = task::spawn(poll_fn(|cx| {
                    let coop = ready!(poll_proceed(cx));
                    coop.made_progress();
                    Poll::Ready(())
                }));

                assert_pending!(task.poll());
            });
        }
    }