feat(rt): implement `stop_future`/`into_parts` (#808)

actix-rt/CHANGES.md

@@ -3,6 +3,7 @@
 ## Unreleased
 
 - Minimum supported Rust version (MSRV) is now 1.88.
+- Add `SystemRunner::stop_future` and `SystemRunner::into_parts` for awaiting system stop inside `block_on`.
 
 ## 2.11.0
 

actix-rt/src/lib.rs

@@ -68,7 +68,7 @@ use tokio::task::JoinHandle;
 pub use self::{
     arbiter::{Arbiter, ArbiterHandle},
     runtime::Runtime,
-    system::{System, SystemRunner},
+    system::{System, SystemRunner, SystemStop},
 };
 
 pub mod signal {

actix-rt/src/system.rs

@@ -9,7 +9,7 @@ use std::{
 };
 
 use futures_core::ready;
-use tokio::sync::{mpsc, oneshot};
+use tokio::sync::{mpsc, watch};
 
 use crate::{arbiter::ArbiterHandle, Arbiter};
 
@@ -50,7 +50,7 @@ impl System {
     where
         F: FnOnce() -> tokio::runtime::Runtime,
     {
-        let (stop_tx, stop_rx) = oneshot::channel();
+        let (stop_tx, stop_rx) = watch::channel(None);
         let (sys_tx, sys_rx) = mpsc::unbounded_channel();
 
         let rt = crate::runtime::Runtime::from(runtime_factory());
@@ -176,7 +176,7 @@ impl System {
 #[derive(Debug)]
 pub struct SystemRunner {
     rt: crate::runtime::Runtime,
-    stop_rx: oneshot::Receiver<i32>,
+    stop_rx: watch::Receiver<Option<i32>>,
 }
 
 #[cfg(not(feature = "io-uring"))]
@@ -196,7 +196,7 @@ impl SystemRunner {
         let SystemRunner { rt, stop_rx, .. } = self;
 
         // run loop
-        rt.block_on(stop_rx).map_err(io::Error::other)
+        rt.block_on(wait_for_stop(stop_rx))
     }
 
     /// Retrieves a reference to the underlying [Actix runtime](crate::Runtime) associated with this
@@ -233,6 +233,43 @@ impl SystemRunner {
         &self.rt
     }
 
+    /// Returns a future that resolves with the system's exit code when it is stopped.
+    ///
+    /// This can be used to react to a system stop signal while running a future with
+    /// [`SystemRunner::block_on`], such as when coordinating shutdown with `tokio::select!`.
+    ///
+    /// # Examples
+    /// ```no_run
+    /// use std::process::ExitCode;
+    /// use actix_rt::System;
+    ///
+    /// let sys = System::new();
+    /// let stop = sys.stop_future();
+    ///
+    /// let exit = sys.block_on(async move {
+    ///     actix_rt::spawn(async {
+    ///         System::current().stop_with_code(0);
+    ///     });
+    ///
+    ///     let code = stop.await.unwrap_or(1);
+    ///     ExitCode::from(code as u8)
+    /// });
+    ///
+    /// # drop(exit);
+    /// ```
+    pub fn stop_future(&self) -> SystemStop {
+        SystemStop::new(self.stop_rx.clone())
+    }
+
+    /// Splits this runner into its runtime and a future that resolves when the system stops.
+    ///
+    /// After calling this method, [`SystemRunner::run`] and [`SystemRunner::run_with_code`] can no
+    /// longer be used.
+    pub fn into_parts(self) -> (crate::runtime::Runtime, SystemStop) {
+        let SystemRunner { rt, stop_rx } = self;
+        (rt, SystemStop::new(stop_rx))
+    }
+
     /// Runs the provided future, blocking the current thread until the future completes.
     #[track_caller]
     #[inline]
@@ -259,11 +296,21 @@ impl SystemRunner {
         unimplemented!("SystemRunner::run_with_code is not implemented for io-uring feature yet");
     }
 
+    /// Returns a future that resolves with the system's exit code when it is stopped.
+    pub fn stop_future(&self) -> SystemStop {
+        unimplemented!("SystemRunner::stop_future is not implemented for io-uring feature yet");
+    }
+
+    /// Splits this runner into its runtime and a future that resolves when the system stops.
+    pub fn into_parts(self) -> (crate::runtime::Runtime, SystemStop) {
+        unimplemented!("SystemRunner::into_parts 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 (stop_tx, stop_rx) = watch::channel(None);
             let (sys_tx, sys_rx) = mpsc::unbounded_channel();
 
             let sys_arbiter = Arbiter::in_new_system();
@@ -285,6 +332,40 @@ impl SystemRunner {
     }
 }
 
+/// Future that resolves with the exit code when a [`System`] is stopped.
+#[must_use = "SystemStop does nothing unless polled or awaited."]
+pub struct SystemStop {
+    inner: Pin<Box<dyn Future<Output = io::Result<i32>> + 'static>>,
+}
+
+impl SystemStop {
+    #[cfg_attr(feature = "io-uring", allow(dead_code))]
+    fn new(stop_rx: watch::Receiver<Option<i32>>) -> Self {
+        Self {
+            inner: Box::pin(wait_for_stop(stop_rx)),
+        }
+    }
+}
+
+impl Future for SystemStop {
+    type Output = io::Result<i32>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        self.inner.as_mut().poll(cx)
+    }
+}
+
+#[cfg_attr(feature = "io-uring", allow(dead_code))]
+async fn wait_for_stop(mut stop_rx: watch::Receiver<Option<i32>>) -> io::Result<i32> {
+    loop {
+        if let Some(code) = *stop_rx.borrow() {
+            return Ok(code);
+        }
+
+        stop_rx.changed().await.map_err(io::Error::other)?;
+    }
+}
+
 #[derive(Debug)]
 pub(crate) enum SystemCommand {
     Exit(i32),
@@ -296,7 +377,7 @@ pub(crate) enum SystemCommand {
 /// [Arbiter]s and is able to distribute a system-wide stop command.
 #[derive(Debug)]
 pub(crate) struct SystemController {
-    stop_tx: Option<oneshot::Sender<i32>>,
+    stop_tx: Option<watch::Sender<Option<i32>>>,
     cmd_rx: mpsc::UnboundedReceiver<SystemCommand>,
     arbiters: HashMap<usize, ArbiterHandle>,
 }
@@ -304,7 +385,7 @@ pub(crate) struct SystemController {
 impl SystemController {
     pub(crate) fn new(
         cmd_rx: mpsc::UnboundedReceiver<SystemCommand>,
-        stop_tx: oneshot::Sender<i32>,
+        stop_tx: watch::Sender<Option<i32>>,
     ) -> Self {
         SystemController {
             cmd_rx,
@@ -335,7 +416,7 @@ impl Future for SystemController {
                         // stop event loop
                         // will only fire once
                         if let Some(stop_tx) = self.stop_tx.take() {
-                            let _ = stop_tx.send(code);
+                            let _ = stop_tx.send(Some(code));
                         }
                     }
 

actix-rt/tests/tests.rs

@@ -34,6 +34,34 @@ fn run_with_code() {
     assert_eq!(exit_code, 42);
 }
 
+#[cfg(not(feature = "io-uring"))]
+#[test]
+fn stop_future_resolves() {
+    let sys = System::new();
+    let stop = sys.stop_future();
+
+    let exit_code = sys.block_on(async move {
+        System::current().stop_with_code(7);
+        stop.await.expect("stop future should resolve")
+    });
+
+    assert_eq!(exit_code, 7);
+}
+
+#[cfg(not(feature = "io-uring"))]
+#[test]
+fn into_parts_stop_future_resolves() {
+    let sys = System::new();
+    let (rt, stop) = sys.into_parts();
+
+    let exit_code = rt.block_on(async move {
+        System::current().stop_with_code(9);
+        stop.await.expect("stop future should resolve")
+    });
+
+    assert_eq!(exit_code, 9);
+}
+
 #[test]
 fn join_another_arbiter() {
     let time = Duration::from_secs(1);