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    //! Thread pool for blocking operations

    use crate::loom::sync::{Arc, Condvar, Mutex};
    use crate::loom::thread;
    use crate::runtime::blocking::schedule::NoopSchedule;
    use crate::runtime::blocking::shutdown;
    use crate::runtime::blocking::task::BlockingTask;
    use crate::runtime::builder::ThreadNameFn;
    use crate::runtime::context;
    use crate::runtime::task::{self, JoinHandle};
    use crate::runtime::{Builder, Callback, Handle};

    use std::collections::{HashMap, VecDeque};
    use std::fmt;
    use std::time::Duration;

    pub(crate) struct BlockingPool {
        spawner: Spawner,
        shutdown_rx: shutdown::Receiver,
    }

    #[derive(Clone)]
    pub(crate) struct Spawner {
        inner: Arc<Inner>,
    }

    struct Inner {
        /// State shared between worker threads
        shared: Mutex<Shared>,

        /// Pool threads wait on this.
        condvar: Condvar,

        /// Spawned threads use this name
        thread_name: ThreadNameFn,

        /// Spawned thread stack size
        stack_size: Option<usize>,

        /// Call after a thread starts
        after_start: Option<Callback>,

        /// Call before a thread stops
        before_stop: Option<Callback>,

        // Maximum number of threads
        thread_cap: usize,

        // Customizable wait timeout
        keep_alive: Duration,
    }

    struct Shared {
        queue: VecDeque<Task>,
        num_th: usize,
        num_idle: u32,
        num_notify: u32,
        shutdown: bool,
        shutdown_tx: Option<shutdown::Sender>,
        /// Prior to shutdown, we clean up JoinHandles by having each timed-out
        /// thread join on the previous timed-out thread. This is not strictly
        /// necessary but helps avoid Valgrind false positives, see
        /// https://github.com/tokio-rs/tokio/commit/646fbae76535e397ef79dbcaacb945d4c829f666
        /// for more information.
        last_exiting_thread: Option<thread::JoinHandle<()>>,
        /// This holds the JoinHandles for all running threads; on shutdown, the thread
        /// calling shutdown handles joining on these.
        worker_threads: HashMap<usize, thread::JoinHandle<()>>,
        /// This is a counter used to iterate worker_threads in a consistent order (for loom's
        /// benefit)
        worker_thread_index: usize,
    }

    type Task = task::Notified<NoopSchedule>;

    const KEEP_ALIVE: Duration = Duration::from_secs(10);

    /// Run the provided function on an executor dedicated to blocking operations.
    pub(crate) fn spawn_blocking<F, R>(func: F) -> JoinHandle<R>
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        let rt = context::current().expect("not currently running on the Tokio runtime.");
        rt.spawn_blocking(func)
    }

    #[allow(dead_code)]
    pub(crate) fn try_spawn_blocking<F, R>(func: F) -> Result<(), ()>
    where
        F: FnOnce() -> R + Send + 'static,
        R: Send + 'static,
    {
        let rt = context::current().expect("not currently running on the Tokio runtime.");

        let (task, _handle) = task::joinable(BlockingTask::new(func));
        rt.blocking_spawner.spawn(task, &rt)
    }

    // ===== impl BlockingPool =====

    impl BlockingPool {
        pub(crate) fn new(builder: &Builder, thread_cap: usize) -> BlockingPool {
            let (shutdown_tx, shutdown_rx) = shutdown::channel();
            let keep_alive = builder.keep_alive.unwrap_or(KEEP_ALIVE);

            BlockingPool {
                spawner: Spawner {
                    inner: Arc::new(Inner {
                        shared: Mutex::new(Shared {
                            queue: VecDeque::new(),
                            num_th: 0,
                            num_idle: 0,
                            num_notify: 0,
                            shutdown: false,
                            shutdown_tx: Some(shutdown_tx),
                            last_exiting_thread: None,
                            worker_threads: HashMap::new(),
                            worker_thread_index: 0,
                        }),
                        condvar: Condvar::new(),
                        thread_name: builder.thread_name.clone(),
                        stack_size: builder.thread_stack_size,
                        after_start: builder.after_start.clone(),
                        before_stop: builder.before_stop.clone(),
                        thread_cap,
                        keep_alive,
                    }),
                },
                shutdown_rx,
            }
        }

        pub(crate) fn spawner(&self) -> &Spawner {
            &self.spawner
        }

        pub(crate) fn shutdown(&mut self, timeout: Option<Duration>) {
            let mut shared = self.spawner.inner.shared.lock();

            // The function can be called multiple times. First, by explicitly
            // calling `shutdown` then by the drop handler calling `shutdown`. This
            // prevents shutting down twice.
            if shared.shutdown {
                return;
            }

            shared.shutdown = true;
            shared.shutdown_tx = None;
            self.spawner.inner.condvar.notify_all();

            let last_exited_thread = std::mem::take(&mut shared.last_exiting_thread);
            let workers = std::mem::replace(&mut shared.worker_threads, HashMap::new());

            drop(shared);

            if self.shutdown_rx.wait(timeout) {
                let _ = last_exited_thread.map(|th| th.join());

                // Loom requires that execution be deterministic, so sort by thread ID before joining.
                // (HashMaps use a randomly-seeded hash function, so the order is nondeterministic)
                let mut workers: Vec<(usize, thread::JoinHandle<()>)> = workers.into_iter().collect();
                workers.sort_by_key(|(id, _)| *id);

                for (_id, handle) in workers.into_iter() {
                    let _ = handle.join();
                }
            }
        }
    }

    impl Drop for BlockingPool {
        fn drop(&mut self) {
            self.shutdown(None);
        }
    }

    impl fmt::Debug for BlockingPool {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("BlockingPool").finish()
        }
    }

    // ===== impl Spawner =====

    impl Spawner {
        pub(crate) fn spawn(&self, task: Task, rt: &Handle) -> Result<(), ()> {
            let shutdown_tx = {
                let mut shared = self.inner.shared.lock();

                if shared.shutdown {
                    // Shutdown the task
                    task.shutdown();

                    // no need to even push this task; it would never get picked up
                    return Err(());
                }

                shared.queue.push_back(task);

                if shared.num_idle == 0 {
                    // No threads are able to process the task.

                    if shared.num_th == self.inner.thread_cap {
                        // At max number of threads
                        None
                    } else {
                        shared.num_th += 1;
                        assert!(shared.shutdown_tx.is_some());
                        shared.shutdown_tx.clone()
                    }
                } else {
                    // Notify an idle worker thread. The notification counter
                    // is used to count the needed amount of notifications
                    // exactly. Thread libraries may generate spurious
                    // wakeups, this counter is used to keep us in a
                    // consistent state.
                    shared.num_idle -= 1;
                    shared.num_notify += 1;
                    self.inner.condvar.notify_one();
                    None
                }
            };

            if let Some(shutdown_tx) = shutdown_tx {
                let mut shared = self.inner.shared.lock();

                let id = shared.worker_thread_index;
                shared.worker_thread_index += 1;

                let handle = self.spawn_thread(shutdown_tx, rt, id);

                shared.worker_threads.insert(id, handle);
            }

            Ok(())
        }

        fn spawn_thread(
            &self,
            shutdown_tx: shutdown::Sender,
            rt: &Handle,
            id: usize,
        ) -> thread::JoinHandle<()> {
            let mut builder = thread::Builder::new().name((self.inner.thread_name)());

            if let Some(stack_size) = self.inner.stack_size {
                builder = builder.stack_size(stack_size);
            }

            let rt = rt.clone();

            builder
                .spawn(move || {
                    // Only the reference should be moved into the closure
                    let _enter = crate::runtime::context::enter(rt.clone());
                    rt.blocking_spawner.inner.run(id);
                    drop(shutdown_tx);
                })
                .unwrap()
        }
    }

    impl Inner {
        fn run(&self, worker_thread_id: usize) {
            if let Some(f) = &self.after_start {
                f()
            }

            let mut shared = self.shared.lock();
            let mut join_on_thread = None;

            'main: loop {
                // BUSY
                while let Some(task) = shared.queue.pop_front() {
                    drop(shared);
                    task.run();

                    shared = self.shared.lock();
                }

                // IDLE
                shared.num_idle += 1;

                while !shared.shutdown {
                    let lock_result = self.condvar.wait_timeout(shared, self.keep_alive).unwrap();

                    shared = lock_result.0;
                    let timeout_result = lock_result.1;

                    if shared.num_notify != 0 {
                        // We have received a legitimate wakeup,
                        // acknowledge it by decrementing the counter
                        // and transition to the BUSY state.
                        shared.num_notify -= 1;
                        break;
                    }

                    // Even if the condvar "timed out", if the pool is entering the
                    // shutdown phase, we want to perform the cleanup logic.
                    if !shared.shutdown && timeout_result.timed_out() {
                        // We'll join the prior timed-out thread's JoinHandle after dropping the lock.
                        // This isn't done when shutting down, because the thread calling shutdown will
                        // handle joining everything.
                        let my_handle = shared.worker_threads.remove(&worker_thread_id);
                        join_on_thread = std::mem::replace(&mut shared.last_exiting_thread, my_handle);

                        break 'main;
                    }

                    // Spurious wakeup detected, go back to sleep.
                }

                if shared.shutdown {
                    // Drain the queue
                    while let Some(task) = shared.queue.pop_front() {
                        drop(shared);
                        task.shutdown();

                        shared = self.shared.lock();
                    }

                    // Work was produced, and we "took" it (by decrementing num_notify).
                    // This means that num_idle was decremented once for our wakeup.
                    // But, since we are exiting, we need to "undo" that, as we'll stay idle.
                    shared.num_idle += 1;
                    // NOTE: Technically we should also do num_notify++ and notify again,
                    // but since we're shutting down anyway, that won't be necessary.
                    break;
                }
            }

            // Thread exit
            shared.num_th -= 1;

            // num_idle should now be tracked exactly, panic
            // with a descriptive message if it is not the
            // case.
            shared.num_idle = shared
                .num_idle
                .checked_sub(1)
                .expect("num_idle underflowed on thread exit");

            if shared.shutdown && shared.num_th == 0 {
                self.condvar.notify_one();
            }

            drop(shared);

            if let Some(f) = &self.before_stop {
                f()
            }

            if let Some(handle) = join_on_thread {
                let _ = handle.join();
            }
        }
    }

    impl fmt::Debug for Spawner {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("blocking::Spawner").finish()
        }
    }