rt: add Handle::spawn_blocking method (#2501)

This follows a similar pattern to `Handle::spawn` to add the
blocking spawn capabilities to `Handle`.

tokio/src/runtime/blocking/mod.rs

@@ -9,7 +9,7 @@ cfg_blocking_impl! {
 
     mod schedule;
     mod shutdown;
-    mod task;
+    pub(crate) mod task;
 
     use crate::runtime::Builder;
 

tokio/src/runtime/blocking/pool.rs

@@ -148,7 +148,7 @@ impl fmt::Debug for BlockingPool {
 // ===== impl Spawner =====
 
 impl Spawner {
-    fn spawn(&self, task: Task, rt: &Handle) -> Result<(), ()> {
+    pub(crate) fn spawn(&self, task: Task, rt: &Handle) -> Result<(), ()> {
         let shutdown_tx = {
             let mut shared = self.inner.shared.lock().unwrap();
 

tokio/src/runtime/blocking/schedule.rs

@@ -6,7 +6,7 @@ use crate::runtime::task::{self, Task};
 ///
 /// We avoid storing the task by forgetting it in `bind` and re-materializing it
 /// in `release.
-pub(super) struct NoopSchedule;
+pub(crate) struct NoopSchedule;
 
 impl task::Schedule for NoopSchedule {
     fn bind(_task: Task<Self>) -> NoopSchedule {

tokio/src/runtime/blocking/task.rs

@@ -3,13 +3,13 @@ use std::pin::Pin;
 use std::task::{Context, Poll};
 
 /// Converts a function to a future that completes on poll
-pub(super) struct BlockingTask<T> {
+pub(crate) struct BlockingTask<T> {
     func: Option<T>,
 }
 
 impl<T> BlockingTask<T> {
     /// Initializes a new blocking task from the given function
-    pub(super) fn new(func: T) -> BlockingTask<T> {
+    pub(crate) fn new(func: T) -> BlockingTask<T> {
         BlockingTask { func: Some(func) }
     }
 }

tokio/src/runtime/handle.rs

@@ -1,6 +1,11 @@
 use crate::runtime::{blocking, context, io, time, Spawner};
 use std::{error, fmt};
 
+cfg_blocking! {
+    use crate::runtime::task;
+    use crate::runtime::blocking::task::BlockingTask;
+}
+
 cfg_rt_core! {
     use crate::task::JoinHandle;
 
@@ -263,6 +268,79 @@ cfg_rt_core! {
     }
 }
 
+cfg_blocking! {
+    impl Handle {
+        /// Runs the provided closure on a thread where blocking is acceptable.
+        ///
+        /// In general, issuing a blocking call or performing a lot of compute in a
+        /// future without yielding is not okay, as it may prevent the executor from
+        /// driving other futures forward. This function runs the provided closure
+        /// on a thread dedicated to blocking operations. See the [CPU-bound tasks
+        /// and blocking code][blocking] section for more information.
+        ///
+        /// Tokio will spawn more blocking threads when they are requested through
+        /// this function until the upper limit configured on the [`Builder`] is
+        /// reached.  This limit is very large by default, because `spawn_blocking` is
+        /// often used for various kinds of IO operations that cannot be performed
+        /// asynchronously. When you run CPU-bound code using `spawn_blocking`, you
+        /// should keep this large upper limit in mind; to run your CPU-bound
+        /// computations on only a few threads, you should use a separate thread
+        /// pool such as [rayon] rather than configuring the number of blocking
+        /// threads.
+        ///
+        /// This function is intended for non-async operations that eventually
+        /// finish on their own. If you want to spawn an ordinary thread, you should
+        /// use [`thread::spawn`] instead.
+        ///
+        /// Closures spawned using `spawn_blocking` cannot be cancelled. When you
+        /// shut down the executor, it will wait indefinitely for all blocking
+        /// operations to finish. You can use [`shutdown_timeout`] to stop waiting
+        /// for them after a certain timeout. Be aware that this will still not
+        /// cancel the tasks — they are simply allowed to keep running after the
+        /// method returns.
+        ///
+        /// Note that if you are using the [basic scheduler], this function will
+        /// still spawn additional threads for blocking operations. The basic
+        /// scheduler's single thread is only used for asynchronous code.
+        ///
+        /// [`Builder`]: struct@crate::runtime::Builder
+        /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code
+        /// [rayon]: https://docs.rs/rayon
+        /// [basic scheduler]: fn@crate::runtime::Builder::basic_scheduler
+        /// [`thread::spawn`]: fn@std::thread::spawn
+        /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime::Runtime;
+        ///
+        /// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{
+        /// // Create the runtime
+        /// let rt = Runtime::new().unwrap();
+        /// let handle = rt.handle();
+        ///
+        /// let res = handle.spawn_blocking(move || {
+        ///     // do some compute-heavy work or call synchronous code
+        ///     "done computing"
+        /// }).await?;
+        ///
+        /// assert_eq!(res, "done computing");
+        /// # Ok(())
+        /// # }
+        /// ```
+        pub fn spawn_blocking<F, R>(&self, f: F) -> JoinHandle<R>
+        where
+            F: FnOnce() -> R + Send + 'static,
+            R: Send + 'static,
+        {
+            let (task, handle) = task::joinable(BlockingTask::new(f));
+            let _ = self.blocking_spawner.spawn(task, self);
+            handle
+        }
+    }
+}
+
 /// Error returned by `try_current` when no Runtime has been started
 pub struct TryCurrentError(());