rt: Allow concurrent block_on's with basic_scheduler (#2804)

tokio/src/runtime/basic_scheduler.rs

@@ -1,22 +1,35 @@
+use crate::future::poll_fn;
+use crate::loom::sync::Mutex;
 use crate::park::{Park, Unpark};
-use crate::runtime;
 use crate::runtime::task::{self, JoinHandle, Schedule, Task};
+use crate::sync::Notify;
 use crate::util::linked_list::{Link, LinkedList};
-use crate::util::{waker_ref, Wake};
+use crate::util::{waker_ref, Wake, WakerRef};
 
 use std::cell::RefCell;
 use std::collections::VecDeque;
 use std::fmt;
 use std::future::Future;
-use std::sync::{Arc, Mutex};
-use std::task::Poll::Ready;
+use std::sync::{Arc, PoisonError};
+use std::task::Poll::{Pending, Ready};
 use std::time::Duration;
 
 /// Executes tasks on the current thread
-pub(crate) struct BasicScheduler<P>
-where
-    P: Park,
-{
+pub(crate) struct BasicScheduler<P: Park> {
+    /// Inner state guarded by a mutex that is shared
+    /// between all `block_on` calls.
+    inner: Mutex<Option<Inner<P>>>,
+
+    /// Notifier for waking up other threads to steal the
+    /// parker.
+    notify: Notify,
+
+    /// Sendable task spawner
+    spawner: Spawner,
+}
+
+/// The inner scheduler that owns the task queue and the main parker P.
+struct Inner<P: Park> {
     /// Scheduler run queue
     ///
     /// When the scheduler is executed, the queue is removed from `self` and
@@ -59,7 +72,7 @@ struct Shared {
     unpark: Box<dyn Unpark>,
 }
 
-/// Thread-local context
+/// Thread-local context.
 struct Context {
     /// Shared scheduler state
     shared: Arc<Shared>,
@@ -68,38 +81,43 @@ struct Context {
     tasks: RefCell<Tasks>,
 }
 
-/// Initial queue capacity
+/// Initial queue capacity.
 const INITIAL_CAPACITY: usize = 64;
 
 /// Max number of tasks to poll per tick.
 const MAX_TASKS_PER_TICK: usize = 61;
 
-/// How often ot check the remote queue first
+/// How often to check the remote queue first.
 const REMOTE_FIRST_INTERVAL: u8 = 31;
 
-// Tracks the current BasicScheduler
+// Tracks the current BasicScheduler.
 scoped_thread_local!(static CURRENT: Context);
 
-impl<P> BasicScheduler<P>
-where
-    P: Park,
-{
+impl<P: Park> BasicScheduler<P> {
     pub(crate) fn new(park: P) -> BasicScheduler<P> {
         let unpark = Box::new(park.unpark());
 
-        BasicScheduler {
+        let spawner = Spawner {
+            shared: Arc::new(Shared {
+                queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
+                unpark: unpark as Box<dyn Unpark>,
+            }),
+        };
+
+        let inner = Mutex::new(Some(Inner {
             tasks: Some(Tasks {
                 owned: LinkedList::new(),
                 queue: VecDeque::with_capacity(INITIAL_CAPACITY),
             }),
-            spawner: Spawner {
-                shared: Arc::new(Shared {
-                    queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
-                    unpark: unpark as Box<dyn Unpark>,
-                }),
-            },
+            spawner: spawner.clone(),
             tick: 0,
             park,
+        }));
+
+        BasicScheduler {
+            inner,
+            notify: Notify::new(),
+            spawner,
         }
     }
 
@@ -108,7 +126,6 @@ where
     }
 
     /// Spawns a future onto the thread pool
-    #[allow(dead_code)]
     pub(crate) fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
     where
         F: Future + Send + 'static,
@@ -117,13 +134,57 @@ where
         self.spawner.spawn(future)
     }
 
-    pub(crate) fn block_on<F>(&mut self, future: F) -> F::Output
-    where
-        F: Future,
-    {
+    pub(crate) fn block_on<F: Future>(&self, future: F) -> F::Output {
+        pin!(future);
+
+        // Attempt to steal the dedicated parker and block_on the future if we can there,
+        // othwerwise, lets select on a notification that the parker is available
+        // or the future is complete.
+        loop {
+            if let Some(inner) = &mut self.take_inner() {
+                return inner.block_on(future);
+            } else {
+                let mut enter = crate::runtime::enter(false);
+
+                let notified = self.notify.notified();
+                pin!(notified);
+
+                if let Some(out) = enter
+                    .block_on(poll_fn(|cx| {
+                        if notified.as_mut().poll(cx).is_ready() {
+                            return Ready(None);
+                        }
+
+                        if let Ready(out) = future.as_mut().poll(cx) {
+                            return Ready(Some(out));
+                        }
+
+                        Pending
+                    }))
+                    .expect("Failed to `Enter::block_on`")
+                {
+                    return out;
+                }
+            }
+        }
+    }
+
+    fn take_inner(&self) -> Option<InnerGuard<'_, P>> {
+        let inner = self.inner.lock().unwrap().take()?;
+
+        Some(InnerGuard {
+            inner: Some(inner),
+            basic_scheduler: &self,
+        })
+    }
+}
+
+impl<P: Park> Inner<P> {
+    /// Block on the future provided and drive the runtime's driver.
+    fn block_on<F: Future>(&mut self, future: F) -> F::Output {
         enter(self, |scheduler, context| {
-            let _enter = runtime::enter(false);
-            let waker = waker_ref(&scheduler.spawner.shared);
+            let _enter = crate::runtime::enter(false);
+            let waker = scheduler.spawner.waker_ref();
             let mut cx = std::task::Context::from_waker(&waker);
 
             pin!(future);
@@ -178,16 +239,16 @@ where
 
 /// Enter the scheduler context. This sets the queue and other necessary
 /// scheduler state in the thread-local
-fn enter<F, R, P>(scheduler: &mut BasicScheduler<P>, f: F) -> R
+fn enter<F, R, P>(scheduler: &mut Inner<P>, f: F) -> R
 where
-    F: FnOnce(&mut BasicScheduler<P>, &Context) -> R,
+    F: FnOnce(&mut Inner<P>, &Context) -> R,
     P: Park,
 {
     // Ensures the run queue is placed back in the `BasicScheduler` instance
     // once `block_on` returns.`
     struct Guard<'a, P: Park> {
         context: Option<Context>,
-        scheduler: &'a mut BasicScheduler<P>,
+        scheduler: &'a mut Inner<P>,
     }
 
     impl<P: Park> Drop for Guard<'_, P> {
@@ -214,12 +275,23 @@ where
     CURRENT.set(context, || f(scheduler, context))
 }
 
-impl<P> Drop for BasicScheduler<P>
-where
-    P: Park,
-{
+impl<P: Park> Drop for BasicScheduler<P> {
     fn drop(&mut self) {
-        enter(self, |scheduler, context| {
+        // Avoid a double panic if we are currently panicking and
+        // the lock may be poisoned.
+
+        let mut inner = match self
+            .inner
+            .lock()
+            .unwrap_or_else(PoisonError::into_inner)
+            .take()
+        {
+            Some(inner) => inner,
+            None if std::thread::panicking() => return,
+            None => panic!("Oh no! We never placed the Inner state back, this is a bug!"),
+        };
+
+        enter(&mut inner, |scheduler, context| {
             // Loop required here to ensure borrow is dropped between iterations
             #[allow(clippy::while_let_loop)]
             loop {
@@ -269,6 +341,10 @@ impl Spawner {
     fn pop(&self) -> Option<task::Notified<Arc<Shared>>> {
         self.shared.queue.lock().unwrap().pop_front()
     }
+
+    fn waker_ref(&self) -> WakerRef<'_> {
+        waker_ref(&self.shared)
+    }
 }
 
 impl fmt::Debug for Spawner {
@@ -325,3 +401,43 @@ impl Wake for Shared {
         arc_self.unpark.unpark();
     }
 }
+
+// ===== InnerGuard =====
+
+/// Used to ensure we always place the Inner value
+/// back into its slot in `BasicScheduler`, even if the
+/// future panics.
+struct InnerGuard<'a, P: Park> {
+    inner: Option<Inner<P>>,
+    basic_scheduler: &'a BasicScheduler<P>,
+}
+
+impl<P: Park> InnerGuard<'_, P> {
+    fn block_on<F: Future>(&mut self, future: F) -> F::Output {
+        // The only time inner gets set to `None` is if we have dropped
+        // already so this unwrap is safe.
+        self.inner.as_mut().unwrap().block_on(future)
+    }
+}
+
+impl<P: Park> Drop for InnerGuard<'_, P> {
+    fn drop(&mut self) {
+        if let Some(scheduler) = self.inner.take() {
+            // We can ignore the poison error here since we are
+            // just replacing the state.
+            let mut lock = self
+                .basic_scheduler
+                .inner
+                .lock()
+                .unwrap_or_else(PoisonError::into_inner);
+
+            // Replace old scheduler back into the state to allow
+            // other threads to pick it up and drive it.
+            lock.replace(scheduler);
+
+            // Wake up other possible threads that could steal
+            // the dedicated parker P.
+            self.basic_scheduler.notify.notify_one()
+        }
+    }
+}

tokio/src/runtime/builder.rs

@@ -496,7 +496,7 @@ cfg_rt_core! {
             let blocking_spawner = blocking_pool.spawner().clone();
 
             Ok(Runtime {
-                kind: Kind::Basic(Mutex::new(Some(scheduler))),
+                kind: Kind::Basic(scheduler),
                 handle: Handle {
                     spawner,
                     io_handle: resources.io_handle,

tokio/src/runtime/mod.rs

@@ -296,7 +296,7 @@ enum Kind {
 
     /// Execute all tasks on the current-thread.
     #[cfg(feature = "rt-core")]
-    Basic(Mutex<Option<BasicScheduler<driver::Driver>>>),
+    Basic(BasicScheduler<driver::Driver>),
 
     /// Execute tasks across multiple threads.
     #[cfg(feature = "rt-threaded")]
@@ -401,7 +401,7 @@ impl Runtime {
             Kind::Shell(_) => panic!("task execution disabled"),
             #[cfg(feature = "rt-threaded")]
             Kind::ThreadPool(exec) => exec.spawn(future),
-            Kind::Basic(_exec) => self.handle.spawner.spawn(future),
+            Kind::Basic(exec) => exec.spawn(future),
         }
     }
 
@@ -461,24 +461,7 @@ impl Runtime {
                 }
             }
             #[cfg(feature = "rt-core")]
-            Kind::Basic(exec) => {
-                // TODO(lucio): clean this up and move this impl into
-                // `basic_scheduler.rs`, this is hacky and bad but will work for
-                // now.
-                let exec_temp = {
-                    let mut lock = exec.lock().unwrap();
-                    lock.take()
-                };
-
-                if let Some(mut exec_temp) = exec_temp {
-                    let res = exec_temp.block_on(future);
-                    exec.lock().unwrap().replace(exec_temp);
-                    res
-                } else {
-                    let mut enter = crate::runtime::enter(true);
-                    enter.block_on(future).unwrap()
-                }
-            }
+            Kind::Basic(exec) => exec.block_on(future),
             #[cfg(feature = "rt-threaded")]
             Kind::ThreadPool(exec) => exec.block_on(future),
         })

tokio/src/sync/mod.rs

@@ -456,6 +456,13 @@ cfg_sync! {
     pub mod watch;
 }
 
+cfg_not_sync! {
+    cfg_rt_core! {
+        mod notify;
+        pub(crate) use notify::Notify;
+    }
+}
+
 cfg_not_sync! {
     cfg_atomic_waker_impl! {
         mod task;

tokio/src/sync/notify.rs

@@ -1,3 +1,10 @@
+// Allow `unreachable_pub` warnings when sync is not enabled
+// due to the usage of `Notify` within the `rt-core` feature set.
+// When this module is compiled with `sync` enabled we will warn on
+// this lint. When `rt-core` is enabled we use `pub(crate)` which
+// triggers this warning but it is safe to ignore in this case.
+#![cfg_attr(not(feature = "sync"), allow(unreachable_pub, dead_code))]
+
 use crate::loom::sync::atomic::AtomicU8;
 use crate::loom::sync::Mutex;
 use crate::util::linked_list::{self, LinkedList};

tokio/src/util/mod.rs

@@ -12,6 +12,10 @@ mod rand;
 mod wake;
 pub(crate) use wake::{waker_ref, Wake};
 
+cfg_rt_core! {
+    pub(crate) use wake::WakerRef;
+}
+
 cfg_rt_threaded! {
     pub(crate) use rand::FastRand;