#
actix_rt/
arbiter.rs

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    use std::{
        cell::RefCell,
        fmt,
        future::Future,
        pin::Pin,
        sync::atomic::{AtomicUsize, Ordering},
        task::{Context, Poll},
        thread,
    };

    use futures_core::ready;
    use tokio::sync::mpsc;

    use crate::system::{System, SystemCommand};

    pub(crate) static COUNT: AtomicUsize = AtomicUsize::new(0);

    thread_local!(
        static HANDLE: RefCell<Option<ArbiterHandle>> = const { RefCell::new(None) };
    );

    pub(crate) enum ArbiterCommand {
        Stop,
        Execute(Pin<Box<dyn Future<Output = ()> + Send>>),
    }

    impl fmt::Debug for ArbiterCommand {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self {
                ArbiterCommand::Stop => write!(f, "ArbiterCommand::Stop"),
                ArbiterCommand::Execute(_) => write!(f, "ArbiterCommand::Execute"),
            }
        }
    }

    /// A handle for sending spawn and stop messages to an [Arbiter].
    #[derive(Debug, Clone)]
    pub struct ArbiterHandle {
        tx: mpsc::UnboundedSender<ArbiterCommand>,
    }

    impl ArbiterHandle {
        pub(crate) fn new(tx: mpsc::UnboundedSender<ArbiterCommand>) -> Self {
            Self { tx }
        }

        /// Send a future to the [Arbiter]'s thread and spawn it.
        ///
        /// If you require a result, include a response channel in the future.
        ///
        /// Returns true if future was sent successfully and false if the [Arbiter] has died.
        pub fn spawn<Fut>(&self, future: Fut) -> bool
        where
            Fut: Future<Output = ()> + Send + 'static,
        {
            self.tx
                .send(ArbiterCommand::Execute(Box::pin(future)))
                .is_ok()
        }

        /// Send a function to the [Arbiter]'s thread and execute it.
        ///
        /// Any result from the function is discarded. If you require a result, include a response
        /// channel in the function.
        ///
        /// Returns true if function was sent successfully and false if the [Arbiter] has died.
        pub fn spawn_fn<F>(&self, f: F) -> bool
        where
            F: FnOnce() + Send + 'static,
        {
            self.spawn(async { f() })
        }

        /// Instruct [Arbiter] to stop processing it's event loop.
        ///
        /// Returns true if stop message was sent successfully and false if the [Arbiter] has
        /// been dropped.
        pub fn stop(&self) -> bool {
            self.tx.send(ArbiterCommand::Stop).is_ok()
        }
    }

    /// An Arbiter represents a thread that provides an asynchronous execution environment for futures
    /// and functions.
    ///
    /// When an arbiter is created, it spawns a new [OS thread](thread), and hosts an event loop.
    #[derive(Debug)]
    pub struct Arbiter {
        tx: mpsc::UnboundedSender<ArbiterCommand>,
        thread_handle: thread::JoinHandle<()>,
    }

    impl Arbiter {
        /// Spawn a new Arbiter thread and start its event loop.
        ///
        /// # Panics
        /// Panics if a [System] is not registered on the current thread.
        #[cfg(not(all(target_os = "linux", feature = "io-uring")))]
        #[allow(clippy::new_without_default)]
        pub fn new() -> Arbiter {
            Self::with_tokio_rt(|| {
                crate::runtime::default_tokio_runtime().expect("Cannot create new Arbiter's Runtime.")
            })
        }

        /// Spawn a new Arbiter using the [Tokio Runtime](tokio-runtime) returned from a closure.
        ///
        /// [tokio-runtime]: tokio::runtime::Runtime
        #[cfg(not(all(target_os = "linux", feature = "io-uring")))]
        pub fn with_tokio_rt<F>(runtime_factory: F) -> Arbiter
        where
            F: FnOnce() -> tokio::runtime::Runtime + Send + 'static,
        {
            let sys = System::current();
            let system_id = sys.id();
            let arb_id = COUNT.fetch_add(1, Ordering::Relaxed);

            let name = format!("actix-rt|system:{}|arbiter:{}", system_id, arb_id);
            let (tx, rx) = mpsc::unbounded_channel();

            let (ready_tx, ready_rx) = std::sync::mpsc::channel::<()>();

            let thread_handle = thread::Builder::new()
                .name(name.clone())
                .spawn({
                    let tx = tx.clone();
                    move || {
                        let rt = crate::runtime::Runtime::from(runtime_factory());
                        let hnd = ArbiterHandle::new(tx);

                        System::set_current(sys);

                        HANDLE.with(|cell| *cell.borrow_mut() = Some(hnd.clone()));

                        // register arbiter
                        let _ = System::current()
                            .tx()
                            .send(SystemCommand::RegisterArbiter(arb_id, hnd));

                        ready_tx.send(()).unwrap();

                        // run arbiter event processing loop
                        rt.block_on(ArbiterRunner { rx });

                        // deregister arbiter
                        let _ = System::current()
                            .tx()
                            .send(SystemCommand::DeregisterArbiter(arb_id));
                    }
                })
                .unwrap_or_else(|err| panic!("Cannot spawn Arbiter's thread: {name:?}: {err:?}"));

            ready_rx.recv().unwrap();

            Arbiter { tx, thread_handle }
        }

        /// Spawn a new Arbiter thread and start its event loop with `tokio-uring` runtime.
        ///
        /// # Panics
        /// Panics if a [System] is not registered on the current thread.
        #[cfg(all(target_os = "linux", feature = "io-uring"))]
        #[allow(clippy::new_without_default)]
        pub fn new() -> Arbiter {
            let sys = System::current();
            let system_id = sys.id();
            let arb_id = COUNT.fetch_add(1, Ordering::Relaxed);

            let name = format!("actix-rt|system:{}|arbiter:{}", system_id, arb_id);
            let (tx, rx) = mpsc::unbounded_channel();

            let (ready_tx, ready_rx) = std::sync::mpsc::channel::<()>();

            let thread_handle = thread::Builder::new()
                .name(name.clone())
                .spawn({
                    let tx = tx.clone();
                    move || {
                        let hnd = ArbiterHandle::new(tx);

                        System::set_current(sys);

                        HANDLE.with(|cell| *cell.borrow_mut() = Some(hnd.clone()));

                        // register arbiter
                        let _ = System::current()
                            .tx()
                            .send(SystemCommand::RegisterArbiter(arb_id, hnd));

                        ready_tx.send(()).unwrap();

                        // run arbiter event processing loop
                        tokio_uring::start(ArbiterRunner { rx });

                        // deregister arbiter
                        let _ = System::current()
                            .tx()
                            .send(SystemCommand::DeregisterArbiter(arb_id));
                    }
                })
                .unwrap_or_else(|err| panic!("Cannot spawn Arbiter's thread: {name:?}: {err:?}"));

            ready_rx.recv().unwrap();

            Arbiter { tx, thread_handle }
        }

        /// Sets up an Arbiter runner in a new System using the environment's local set.
        pub(crate) fn in_new_system() -> ArbiterHandle {
            let (tx, rx) = mpsc::unbounded_channel();

            let hnd = ArbiterHandle::new(tx);

            HANDLE.with(|cell| *cell.borrow_mut() = Some(hnd.clone()));

            crate::spawn(ArbiterRunner { rx });

            hnd
        }

        /// Return a handle to the this Arbiter's message sender.
        pub fn handle(&self) -> ArbiterHandle {
            ArbiterHandle::new(self.tx.clone())
        }

        /// Return a handle to the current thread's Arbiter's message sender.
        ///
        /// # Panics
        /// Panics if no Arbiter is running on the current thread.
        pub fn current() -> ArbiterHandle {
            HANDLE.with(|cell| match *cell.borrow() {
                Some(ref hnd) => hnd.clone(),
                None => panic!("Arbiter is not running."),
            })
        }

        /// Try to get current running arbiter handle.
        ///
        /// Returns `None` if no Arbiter has been started.
        ///
        /// Unlike [`current`](Self::current), this never panics.
        pub fn try_current() -> Option<ArbiterHandle> {
            HANDLE.with(|cell| cell.borrow().clone())
        }

        /// Stop Arbiter from continuing it's event loop.
        ///
        /// Returns true if stop message was sent successfully and false if the Arbiter has been dropped.
        pub fn stop(&self) -> bool {
            self.tx.send(ArbiterCommand::Stop).is_ok()
        }

        /// Send a future to the Arbiter's thread and spawn it.
        ///
        /// If you require a result, include a response channel in the future.
        ///
        /// Returns true if future was sent successfully and false if the Arbiter has died.
        #[track_caller]
        pub fn spawn<Fut>(&self, future: Fut) -> bool
        where
            Fut: Future<Output = ()> + Send + 'static,
        {
            self.tx
                .send(ArbiterCommand::Execute(Box::pin(future)))
                .is_ok()
        }

        /// Send a function to the Arbiter's thread and execute it.
        ///
        /// Any result from the function is discarded. If you require a result, include a response
        /// channel in the function.
        ///
        /// Returns true if function was sent successfully and false if the Arbiter has died.
        #[track_caller]
        pub fn spawn_fn<F>(&self, f: F) -> bool
        where
            F: FnOnce() + Send + 'static,
        {
            self.spawn(async { f() })
        }

        /// Wait for Arbiter's event loop to complete.
        ///
        /// Joins the underlying OS thread handle. See [`JoinHandle::join`](thread::JoinHandle::join).
        pub fn join(self) -> thread::Result<()> {
            self.thread_handle.join()
        }
    }

    /// A persistent future that processes [Arbiter] commands.
    struct ArbiterRunner {
        rx: mpsc::UnboundedReceiver<ArbiterCommand>,
    }

    impl Future for ArbiterRunner {
        type Output = ();

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            // process all items currently buffered in channel
            loop {
                match ready!(self.rx.poll_recv(cx)) {
                    // channel closed; no more messages can be received
                    None => return Poll::Ready(()),

                    // process arbiter command
                    Some(item) => match item {
                        ArbiterCommand::Stop => {
                            return Poll::Ready(());
                        }
                        ArbiterCommand::Execute(task_fut) => {
                            tokio::task::spawn_local(task_fut);
                        }
                    },
                }
            }
        }
    }