#
actix_rt/
system.rs

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

    use futures_core::ready;
    use tokio::sync::{mpsc, oneshot};

    use crate::{arbiter::ArbiterHandle, Arbiter};

    static SYSTEM_COUNT: AtomicUsize = AtomicUsize::new(0);

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

    /// A manager for a per-thread distributed async runtime.
    #[derive(Clone, Debug)]
    pub struct System {
        id: usize,
        sys_tx: mpsc::UnboundedSender<SystemCommand>,

        /// Handle to the first [Arbiter] that is created with the System.
        arbiter_handle: ArbiterHandle,
    }

    #[cfg(not(feature = "io-uring"))]
    impl System {
        /// Create a new system.
        ///
        /// # Panics
        /// Panics if underlying Tokio runtime can not be created.
        #[allow(clippy::new_ret_no_self)]
        pub fn new() -> SystemRunner {
            Self::with_tokio_rt(|| {
                crate::runtime::default_tokio_runtime()
                    .expect("Default Actix (Tokio) runtime could not be created.")
            })
        }

        /// Create a new System using the [Tokio Runtime](tokio-runtime) returned from a closure.
        ///
        /// [tokio-runtime]: tokio::runtime::Runtime
        pub fn with_tokio_rt<F>(runtime_factory: F) -> SystemRunner
        where
            F: FnOnce() -> tokio::runtime::Runtime,
        {
            let (stop_tx, stop_rx) = oneshot::channel();
            let (sys_tx, sys_rx) = mpsc::unbounded_channel();

            let rt = crate::runtime::Runtime::from(runtime_factory());
            let sys_arbiter = rt.block_on(async { Arbiter::in_new_system() });
            let system = System::construct(sys_tx, sys_arbiter.clone());

            system
                .tx()
                .send(SystemCommand::RegisterArbiter(usize::MAX, sys_arbiter))
                .unwrap();

            // init background system arbiter
            let sys_ctrl = SystemController::new(sys_rx, stop_tx);
            rt.spawn(sys_ctrl);

            SystemRunner { rt, stop_rx }
        }
    }

    #[cfg(feature = "io-uring")]
    impl System {
        /// Create a new system.
        ///
        /// # Panics
        /// Panics if underlying Tokio runtime can not be created.
        #[allow(clippy::new_ret_no_self)]
        pub fn new() -> SystemRunner {
            SystemRunner
        }

        /// Create a new System using the [Tokio Runtime](tokio-runtime) returned from a closure.
        ///
        /// [tokio-runtime]: tokio::runtime::Runtime
        #[doc(hidden)]
        pub fn with_tokio_rt<F>(_: F) -> SystemRunner
        where
            F: FnOnce() -> tokio::runtime::Runtime,
        {
            unimplemented!("System::with_tokio_rt is not implemented for io-uring feature yet")
        }
    }

    impl System {
        /// Constructs new system and registers it on the current thread.
        pub(crate) fn construct(
            sys_tx: mpsc::UnboundedSender<SystemCommand>,
            arbiter_handle: ArbiterHandle,
        ) -> Self {
            let sys = System {
                sys_tx,
                arbiter_handle,
                id: SYSTEM_COUNT.fetch_add(1, Ordering::SeqCst),
            };

            System::set_current(sys.clone());

            sys
        }

        /// Get current running system.
        ///
        /// # Panics
        /// Panics if no system is registered on the current thread.
        pub fn current() -> System {
            CURRENT.with(|cell| match *cell.borrow() {
                Some(ref sys) => sys.clone(),
                None => panic!("System is not running"),
            })
        }

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

        /// Get handle to a the System's initial [Arbiter].
        pub fn arbiter(&self) -> &ArbiterHandle {
            &self.arbiter_handle
        }

        /// Check if there is a System registered on the current thread.
        pub fn is_registered() -> bool {
            CURRENT.with(|sys| sys.borrow().is_some())
        }

        /// Register given system on current thread.
        #[doc(hidden)]
        pub fn set_current(sys: System) {
            CURRENT.with(|cell| {
                *cell.borrow_mut() = Some(sys);
            })
        }

        /// Numeric system identifier.
        ///
        /// Useful when using multiple Systems.
        pub fn id(&self) -> usize {
            self.id
        }

        /// Stop the system (with code 0).
        pub fn stop(&self) {
            self.stop_with_code(0)
        }

        /// Stop the system with a given exit code.
        pub fn stop_with_code(&self, code: i32) {
            let _ = self.sys_tx.send(SystemCommand::Exit(code));
        }

        pub(crate) fn tx(&self) -> &mpsc::UnboundedSender<SystemCommand> {
            &self.sys_tx
        }
    }

    /// Runner that keeps a [System]'s event loop alive until stop message is received.
    #[cfg(not(feature = "io-uring"))]
    #[must_use = "A SystemRunner does nothing unless `run` is called."]
    #[derive(Debug)]
    pub struct SystemRunner {
        rt: crate::runtime::Runtime,
        stop_rx: oneshot::Receiver<i32>,
    }

    #[cfg(not(feature = "io-uring"))]
    impl SystemRunner {
        /// Starts event loop and will return once [System] is [stopped](System::stop).
        pub fn run(self) -> io::Result<()> {
            let exit_code = self.run_with_code()?;

            match exit_code {
                0 => Ok(()),
                nonzero => Err(io::Error::new(
                    io::ErrorKind::Other,
                    format!("Non-zero exit code: {}", nonzero),
                )),
            }
        }

        /// Runs the event loop until [stopped](System::stop_with_code), returning the exit code.
        pub fn run_with_code(self) -> io::Result<i32> {
            let SystemRunner { rt, stop_rx, .. } = self;

            // run loop
            rt.block_on(stop_rx)
                .map_err(|err| io::Error::new(io::ErrorKind::Other, err))
        }

        /// Retrieves a reference to the underlying [Actix runtime](crate::Runtime) associated with this
        /// `SystemRunner` instance.
        ///
        /// The Actix runtime is responsible for managing the event loop for an Actix system and
        /// executing asynchronous tasks. This method provides access to the runtime, allowing direct
        /// interaction with its features.
        ///
        /// In a typical use case, you might need to share the same runtime between different
        /// parts of your project. For example, some components might require a [`Runtime`] to spawn
        /// tasks on the same runtime.
        ///
        /// Read more in the documentation for [`Runtime`].
        ///
        /// # Examples
        ///
        /// ```
        /// let system_runner = actix_rt::System::new();
        /// let actix_runtime = system_runner.runtime();
        ///
        /// // Use the runtime to spawn an async task or perform other operations
        /// ```
        ///
        /// # Note
        ///
        /// While this method provides an immutable reference to the Actix runtime, which is safe to
        /// share across threads, be aware that spawning blocking tasks on the Actix runtime could
        /// potentially impact system performance. This is because the Actix runtime is responsible for
        /// driving the system, and blocking tasks could delay other tasks in the run loop.
        ///
        /// [`Runtime`]: crate::Runtime
        pub fn runtime(&self) -> &crate::runtime::Runtime {
            &self.rt
        }

        /// Runs the provided future, blocking the current thread until the future completes.
        #[track_caller]
        #[inline]
        pub fn block_on<F: Future>(&self, fut: F) -> F::Output {
            self.rt.block_on(fut)
        }
    }

    /// Runner that keeps a [System]'s event loop alive until stop message is received.
    #[cfg(feature = "io-uring")]
    #[must_use = "A SystemRunner does nothing unless `run` is called."]
    #[derive(Debug)]
    pub struct SystemRunner;

    #[cfg(feature = "io-uring")]
    impl SystemRunner {
        /// Starts event loop and will return once [System] is [stopped](System::stop).
        pub fn run(self) -> io::Result<()> {
            unimplemented!("SystemRunner::run is not implemented for io-uring feature yet");
        }

        /// Runs the event loop until [stopped](System::stop_with_code), returning the exit code.
        pub fn run_with_code(self) -> io::Result<i32> {
            unimplemented!("SystemRunner::run_with_code is not implemented for io-uring feature yet");
        }

        /// Runs the provided future, blocking the current thread until the future completes.
        #[inline]
        pub fn block_on<F: Future>(&self, fut: F) -> F::Output {
            tokio_uring::start(async move {
                let (stop_tx, stop_rx) = oneshot::channel();
                let (sys_tx, sys_rx) = mpsc::unbounded_channel();

                let sys_arbiter = Arbiter::in_new_system();
                let system = System::construct(sys_tx, sys_arbiter.clone());

                system
                    .tx()
                    .send(SystemCommand::RegisterArbiter(usize::MAX, sys_arbiter))
                    .unwrap();

                // init background system arbiter
                let sys_ctrl = SystemController::new(sys_rx, stop_tx);
                tokio_uring::spawn(sys_ctrl);

                let res = fut.await;
                drop(stop_rx);
                res
            })
        }
    }

    #[derive(Debug)]
    pub(crate) enum SystemCommand {
        Exit(i32),
        RegisterArbiter(usize, ArbiterHandle),
        DeregisterArbiter(usize),
    }

    /// There is one `SystemController` per [System]. It runs in the background, keeping track of
    /// [Arbiter]s and is able to distribute a system-wide stop command.
    #[derive(Debug)]
    pub(crate) struct SystemController {
        stop_tx: Option<oneshot::Sender<i32>>,
        cmd_rx: mpsc::UnboundedReceiver<SystemCommand>,
        arbiters: HashMap<usize, ArbiterHandle>,
    }

    impl SystemController {
        pub(crate) fn new(
            cmd_rx: mpsc::UnboundedReceiver<SystemCommand>,
            stop_tx: oneshot::Sender<i32>,
        ) -> Self {
            SystemController {
                cmd_rx,
                stop_tx: Some(stop_tx),
                arbiters: HashMap::with_capacity(4),
            }
        }
    }

    impl Future for SystemController {
        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.cmd_rx.poll_recv(cx)) {
                    // channel closed; no more messages can be received
                    None => return Poll::Ready(()),

                    // process system command
                    Some(cmd) => match cmd {
                        SystemCommand::Exit(code) => {
                            // stop all arbiters
                            for arb in self.arbiters.values() {
                                arb.stop();
                            }

                            // stop event loop
                            // will only fire once
                            if let Some(stop_tx) = self.stop_tx.take() {
                                let _ = stop_tx.send(code);
                            }
                        }

                        SystemCommand::RegisterArbiter(id, arb) => {
                            self.arbiters.insert(id, arb);
                        }

                        SystemCommand::DeregisterArbiter(id) => {
                            self.arbiters.remove(&id);
                        }
                    },
                }
            }
        }
    }