rt: add Runtime::shutdown_timeout (#2186)

Provides an API for forcing a runtime to shutdown even if there are
still running tasks.

tokio/src/park/thread.rs

@@ -10,13 +10,7 @@ pub(crate) struct ParkThread {
     inner: Arc<Inner>,
 }
 
-/// Error returned by `ParkThread`
-///
-/// This currently is never returned, but might at some point in the future.
-#[derive(Debug)]
-pub(crate) struct ParkError {
-    _p: (),
-}
+pub(crate) type ParkError = ();
 
 /// Unblocks a thread that was blocked by `ParkThread`.
 #[derive(Clone, Debug)]
@@ -240,7 +234,7 @@ cfg_blocking_impl! {
             F: FnOnce(&ParkThread) -> R,
         {
             CURRENT_PARKER.try_with(|inner| f(inner))
-                .map_err(|_| ParkError { _p: () })
+                .map_err(|_| ())
         }
     }
 

tokio/src/runtime/blocking/mod.rs

@@ -21,6 +21,7 @@ cfg_blocking_impl! {
 
 cfg_not_blocking_impl! {
     use crate::runtime::Builder;
+    use std::time::Duration;
 
     #[derive(Debug, Clone)]
     pub(crate) struct BlockingPool {}
@@ -35,5 +36,8 @@ cfg_not_blocking_impl! {
         pub(crate) fn spawner(&self) -> &BlockingPool {
             self
         }
+
+        pub(crate) fn shutdown(&mut self, _duration: Option<Duration>) {
+        }
     }
 }

tokio/src/runtime/blocking/pool.rs

@@ -101,19 +101,30 @@ impl BlockingPool {
     pub(crate) fn spawner(&self) -> &Spawner {
         &self.spawner
     }
-}
 
-impl Drop for BlockingPool {
-    fn drop(&mut self) {
+    pub(crate) fn shutdown(&mut self, timeout: Option<Duration>) {
         let mut shared = self.spawner.inner.shared.lock().unwrap();
 
+        // 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();
 
         drop(shared);
 
-        self.shutdown_rx.wait();
+        self.shutdown_rx.wait(timeout);
+    }
+}
+
+impl Drop for BlockingPool {
+    fn drop(&mut self) {
+        self.shutdown(None);
     }
 }
 

tokio/src/runtime/blocking/shutdown.rs

@@ -6,6 +6,8 @@
 use crate::loom::sync::Arc;
 use crate::sync::oneshot;
 
+use std::time::Duration;
+
 #[derive(Debug, Clone)]
 pub(super) struct Sender {
     tx: Arc<oneshot::Sender<()>>,
@@ -26,7 +28,11 @@ pub(super) fn channel() -> (Sender, Receiver) {
 
 impl Receiver {
     /// Blocks the current thread until all `Sender` handles drop.
-    pub(crate) fn wait(&mut self) {
+    ///
+    /// If `timeout` is `Some`, the thread is blocked for **at most** `timeout`
+    /// duration. If `timeout` is `None`, then the thread is blocked until the
+    /// shutdown signal is received.
+    pub(crate) fn wait(&mut self, timeout: Option<Duration>) {
         use crate::runtime::enter::{enter, try_enter};
 
         let mut e = if std::thread::panicking() {
@@ -43,6 +49,10 @@ impl Receiver {
         // If blocking fails to wait, this indicates a problem parking the
         // current thread (usually, shutting down a runtime stored in a
         // thread-local).
-        let _ = e.block_on(&mut self.rx);
+        if let Some(timeout) = timeout {
+            let _ = e.block_on_timeout(&mut self.rx, timeout);
+        } else {
+            let _ = e.block_on(&mut self.rx);
+        }
     }
 }

tokio/src/runtime/enter.rs

@@ -75,6 +75,7 @@ pub(crate) fn exit<F: FnOnce() -> R, R>(f: F) -> R {
 
 cfg_blocking_impl! {
     use crate::park::ParkError;
+    use std::time::Duration;
 
     impl Enter {
         /// Blocks the thread on the specified future, returning the value with
@@ -104,6 +105,44 @@ cfg_blocking_impl! {
                 park.park()?;
             }
         }
+
+        /// Blocks the thread on the specified future for **at most** `timeout`
+        ///
+        /// If the future completes before `timeout`, the result is returned. If
+        /// `timeout` elapses, then `Err` is returned.
+        pub(crate) fn block_on_timeout<F>(&mut self, mut f: F, timeout: Duration) -> Result<F::Output, ParkError>
+        where
+            F: std::future::Future,
+        {
+            use crate::park::{CachedParkThread, Park};
+            use std::pin::Pin;
+            use std::task::Context;
+            use std::task::Poll::Ready;
+            use std::time::Instant;
+
+            let mut park = CachedParkThread::new();
+            let waker = park.get_unpark()?.into_waker();
+            let mut cx = Context::from_waker(&waker);
+
+            // `block_on` takes ownership of `f`. Once it is pinned here, the original `f` binding can
+            // no longer be accessed, making the pinning safe.
+            let mut f = unsafe { Pin::new_unchecked(&mut f) };
+            let when = Instant::now() + timeout;
+
+            loop {
+                if let Ready(v) = f.as_mut().poll(&mut cx) {
+                    return Ok(v);
+                }
+
+                let now = Instant::now();
+
+                if now >= when {
+                    return Err(());
+                }
+
+                park.park_timeout(when - now)?;
+            }
+        }
     }
 }
 

tokio/src/runtime/mod.rs

@@ -235,6 +235,7 @@ cfg_rt_core! {
 }
 
 use std::future::Future;
+use std::time::Duration;
 
 /// The Tokio runtime.
 ///
@@ -441,4 +442,46 @@ impl Runtime {
     pub fn handle(&self) -> &Handle {
         &self.handle
     }
+
+    /// Shutdown the runtime, waiting for at most `duration` for all spawned
+    /// task to shutdown.
+    ///
+    /// Usually, dropping a `Runtime` handle is sufficient as tasks are able to
+    /// shutdown in a timely fashion. However, dropping a `Runtime` will wait
+    /// indefinitely for all tasks to terminate, and there are cases where a long
+    /// blocking task has been spawned which can block dropping `Runtime`.
+    ///
+    /// In this case, calling `shutdown_timeout` with an explicit wait timeout
+    /// can work. The `shutdown_timeout` will signal all tasks to shutdown and
+    /// will wait for at most `duration` for all spawned tasks to terminate. If
+    /// `timeout` elapses before all tasks are dropped, the function returns and
+    /// outstanding tasks are potentially leaked.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::runtime::Runtime;
+    /// use tokio::task;
+    ///
+    /// use std::thread;
+    /// use std::time::Duration;
+    ///
+    /// fn main() {
+    ///    let mut runtime = Runtime::new().unwrap();
+    ///
+    ///    runtime.block_on(async move {
+    ///        task::spawn_blocking(move || {
+    ///            thread::sleep(Duration::from_secs(10_000));
+    ///        });
+    ///    });
+    ///
+    ///    runtime.shutdown_timeout(Duration::from_millis(100));
+    /// }
+    /// ```
+    pub fn shutdown_timeout(self, duration: Duration) {
+        let Runtime {
+            mut blocking_pool, ..
+        } = self;
+        blocking_pool.shutdown(Some(duration));
+    }
 }

tokio/tests/rt_common.rs

@@ -709,6 +709,23 @@ rt_test! {
         }
     }
 
+    #[test]
+    fn shutdown_timeout() {
+        let (tx, rx) = oneshot::channel();
+        let mut runtime = rt();
+
+        runtime.block_on(async move {
+            task::spawn_blocking(move || {
+                tx.send(()).unwrap();
+                thread::sleep(Duration::from_secs(10_000));
+            });
+
+            rx.await.unwrap();
+        });
+
+        runtime.shutdown_timeout(Duration::from_millis(100));
+    }
+
     #[test]
     fn runtime_in_thread_local() {
         use std::cell::RefCell;