rt: avoid early task shutdown (#3870)

Tokio 1.7.0 introduced a change intended to eagerly shutdown newly
spawned tasks if the runtime is in the process of shutting down.
However, it introduced a bug where already spawned tasks could be
shutdown too early, resulting in the potential introduction of deadlocks
if tasks acquired mutexes in drop handlers.

Fixes #3869

tokio/CHANGELOG.md

@@ -1,3 +1,11 @@
+# 1.7.1 (June 18, 2021)
+
+### Fixed
+
+- runtime: fix early task shutdown during runtime shutdown ([#3870])
+
+[#3870]: https://github.com/tokio-rs/tokio/pull/3870
+
 # 1.7.0 (June 15, 2021)
 
 ### Added

tokio/Cargo.toml

@@ -7,12 +7,12 @@ name = "tokio"
 #   - README.md
 # - Update CHANGELOG.md.
 # - Create "v1.0.x" git tag.
-version = "1.7.0"
+version = "1.7.1"
 edition = "2018"
 authors = ["Tokio Contributors <team@tokio.rs>"]
 license = "MIT"
 readme = "README.md"
-documentation = "https://docs.rs/tokio/1.7.0/tokio/"
+documentation = "https://docs.rs/tokio/1.7.1/tokio/"
 repository = "https://github.com/tokio-rs/tokio"
 homepage = "https://tokio.rs"
 description = """

tokio/src/lib.rs

@@ -9,6 +9,7 @@
     rust_2018_idioms,
     unreachable_pub
 )]
+#![deny(unused_must_use)]
 #![cfg_attr(docsrs, deny(broken_intra_doc_links))]
 #![doc(test(
     no_crate_inject,

tokio/src/runtime/queue.rs

@@ -124,9 +124,14 @@ impl<T> Local<T> {
                 // There is capacity for the task
                 break tail;
             } else if steal != real {
-                // Concurrently stealing, this will free up capacity, so
-                // only push the new task onto the inject queue
-                inject.push(task);
+                // Concurrently stealing, this will free up capacity, so only
+                // push the new task onto the inject queue
+                //
+                // If the task failes to be pushed on the injection queue, there
+                // is nothing to be done at this point as the task cannot be a
+                // newly spawned task. Shutting down this task is handled by the
+                // worker shutdown process.
+                let _ = inject.push(task);
                 return;
             } else {
                 // Push the current task and half of the queue into the
@@ -507,19 +512,19 @@ impl<T: 'static> Inject<T> {
     }
 
     /// Pushes a value into the queue.
-    pub(super) fn push(&self, task: task::Notified<T>)
+    ///
+    /// Returns `Err(task)` if pushing fails due to the queue being shutdown.
+    /// The caller is expected to call `shutdown()` on the task **if and only
+    /// if** it is a newly spawned task.
+    pub(super) fn push(&self, task: task::Notified<T>) -> Result<(), task::Notified<T>>
     where
         T: crate::runtime::task::Schedule,
     {
         // Acquire queue lock
         let mut p = self.pointers.lock();
 
         if p.is_closed {
-            // Drop the mutex to avoid a potential deadlock when
-            // re-entering.
-            drop(p);
-            task.shutdown();
-            return;
+            return Err(task);
         }
 
         // safety: only mutated with the lock held
@@ -538,6 +543,7 @@ impl<T: 'static> Inject<T> {
         p.tail = Some(task);
 
         self.len.store(len + 1, Release);
+        Ok(())
     }
 
     pub(super) fn push_batch(

tokio/src/runtime/thread_pool/mod.rs

@@ -94,7 +94,13 @@ impl Spawner {
         F::Output: Send + 'static,
     {
         let (task, handle) = task::joinable(future);
-        self.shared.schedule(task, false);
+
+        if let Err(task) = self.shared.schedule(task, false) {
+            // The newly spawned task could not be scheduled because the runtime
+            // is shutting down. The task must be explicitly shutdown at this point.
+            task.shutdown();
+        }
+
         handle
     }
 

tokio/src/runtime/thread_pool/worker.rs

@@ -709,32 +709,39 @@ impl task::Schedule for Arc<Worker> {
     }
 
     fn schedule(&self, task: Notified) {
-        self.shared.schedule(task, false);
+        // Because this is not a newly spawned task, if scheduling fails due to
+        // the runtime shutting down, there is no special work that must happen
+        // here.
+        let _ = self.shared.schedule(task, false);
     }
 
     fn yield_now(&self, task: Notified) {
-        self.shared.schedule(task, true);
+        // Because this is not a newly spawned task, if scheduling fails due to
+        // the runtime shutting down, there is no special work that must happen
+        // here.
+        let _ = self.shared.schedule(task, true);
     }
 }
 
 impl Shared {
-    pub(super) fn schedule(&self, task: Notified, is_yield: bool) {
+    pub(super) fn schedule(&self, task: Notified, is_yield: bool) -> Result<(), Notified> {
         CURRENT.with(|maybe_cx| {
             if let Some(cx) = maybe_cx {
                 // Make sure the task is part of the **current** scheduler.
                 if self.ptr_eq(&cx.worker.shared) {
                     // And the current thread still holds a core
                     if let Some(core) = cx.core.borrow_mut().as_mut() {
                         self.schedule_local(core, task, is_yield);
-                        return;
+                        return Ok(());
                     }
                 }
             }
 
             // Otherwise, use the inject queue
-            self.inject.push(task);
+            self.inject.push(task)?;
             self.notify_parked();
-        });
+            Ok(())
+        })
     }
 
     fn schedule_local(&self, core: &mut Core, task: Notified, is_yield: bool) {

tokio/tests/rt_threaded.rs

@@ -12,8 +12,8 @@ use std::future::Future;
 use std::pin::Pin;
 use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::Relaxed;
-use std::sync::{mpsc, Arc};
-use std::task::{Context, Poll};
+use std::sync::{mpsc, Arc, Mutex};
+use std::task::{Context, Poll, Waker};
 
 #[test]
 fn single_thread() {
@@ -405,6 +405,74 @@ async fn hang_on_shutdown() {
     tokio::time::sleep(std::time::Duration::from_secs(1)).await;
 }
 
+/// Demonstrates tokio-rs/tokio#3869
+#[test]
+fn wake_during_shutdown() {
+    struct Shared {
+        waker: Option<Waker>,
+    }
+
+    struct MyFuture {
+        shared: Arc<Mutex<Shared>>,
+        put_waker: bool,
+    }
+
+    impl MyFuture {
+        fn new() -> (Self, Self) {
+            let shared = Arc::new(Mutex::new(Shared { waker: None }));
+            let f1 = MyFuture {
+                shared: shared.clone(),
+                put_waker: true,
+            };
+            let f2 = MyFuture {
+                shared,
+                put_waker: false,
+            };
+            (f1, f2)
+        }
+    }
+
+    impl Future for MyFuture {
+        type Output = ();
+
+        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
+            let me = Pin::into_inner(self);
+            let mut lock = me.shared.lock().unwrap();
+            println!("poll {}", me.put_waker);
+            if me.put_waker {
+                println!("putting");
+                lock.waker = Some(cx.waker().clone());
+            }
+            Poll::Pending
+        }
+    }
+
+    impl Drop for MyFuture {
+        fn drop(&mut self) {
+            println!("drop {} start", self.put_waker);
+            let mut lock = self.shared.lock().unwrap();
+            if !self.put_waker {
+                lock.waker.take().unwrap().wake();
+            }
+            drop(lock);
+            println!("drop {} stop", self.put_waker);
+        }
+    }
+
+    let rt = tokio::runtime::Builder::new_multi_thread()
+        .worker_threads(1)
+        .enable_all()
+        .build()
+        .unwrap();
+
+    let (f1, f2) = MyFuture::new();
+
+    rt.spawn(f1);
+    rt.spawn(f2);
+
+    rt.block_on(async { tokio::time::sleep(tokio::time::Duration::from_millis(20)).await });
+}
+
 fn rt() -> Runtime {
     Runtime::new().unwrap()
 }