Implement Future for SpawnedTask.

It allows polling a SpawnedTask, instead of just joining it.

The implementation is changed from `JoinSet` to a `JoinHandle` to simplify
the code, as `JoinSet` doesn't provide any additional benefits.

Author: ashdnazg

datafusion/common-runtime/src/common.rs

@@ -15,18 +15,25 @@
 // specific language governing permissions and limitations
 // under the License.
 
-use std::future::Future;
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
 
-use crate::JoinSet;
-use tokio::task::JoinError;
+use tokio::task::{JoinError, JoinHandle};
+
+use crate::trace_utils::{trace_block, trace_future};
 
 /// Helper that  provides a simple API to spawn a single task and join it.
 /// Provides guarantees of aborting on `Drop` to keep it cancel-safe.
+/// Note that if the task was spawned with `spawn_blocking`, it will only be
+/// aborted if it hasn't started yet.
 ///
-/// Technically, it's just a wrapper of `JoinSet` (with size=1).
+/// Technically, it's just a wrapper of a `JoinHandle` overriding drop.
 #[derive(Debug)]
 pub struct SpawnedTask<R> {
-    inner: JoinSet<R>,
+    inner: JoinHandle<R>,
 }
 
 impl<R: 'static> SpawnedTask<R> {
@@ -36,8 +43,9 @@ impl<R: 'static> SpawnedTask<R> {
         T: Send + 'static,
         R: Send,
     {
-        let mut inner = JoinSet::new();
-        inner.spawn(task);
+        // Ok to use spawn here as SpawnedTask handles aborting/cancelling the task on Drop
+        #[allow(clippy::disallowed_methods)]
+        let inner = tokio::task::spawn(trace_future(task));
         Self { inner }
     }
 
@@ -47,22 +55,21 @@ impl<R: 'static> SpawnedTask<R> {
         T: Send + 'static,
         R: Send,
     {
-        let mut inner = JoinSet::new();
-        inner.spawn_blocking(task);
+        // Ok to use spawn_blocking here as SpawnedTask handles aborting/cancelling the task on Drop
+        #[allow(clippy::disallowed_methods)]
+        let inner = tokio::task::spawn_blocking(trace_block(task));
         Self { inner }
     }
 
     /// Joins the task, returning the result of join (`Result<R, JoinError>`).
-    pub async fn join(mut self) -> Result<R, JoinError> {
-        self.inner
-            .join_next()
-            .await
-            .expect("`SpawnedTask` instance always contains exactly 1 task")
+    /// Same as awaiting the spawned task, but left for backwards compatibility.
+    pub async fn join(self) -> Result<R, JoinError> {
+        self.await
     }
 
     /// Joins the task and unwinds the panic if it happens.
     pub async fn join_unwind(self) -> Result<R, JoinError> {
-        self.join().await.map_err(|e| {
+        self.await.map_err(|e| {
             // `JoinError` can be caused either by panic or cancellation. We have to handle panics:
             if e.is_panic() {
                 std::panic::resume_unwind(e.into_panic());
@@ -77,17 +84,32 @@ impl<R: 'static> SpawnedTask<R> {
     }
 }
 
+impl<R> Future for SpawnedTask<R> {
+    type Output = Result<R, JoinError>;
+
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        Pin::new(&mut self.inner).poll(cx)
+    }
+}
+
+impl<R> Drop for SpawnedTask<R> {
+    fn drop(&mut self) {
+        self.inner.abort();
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
     use std::future::{pending, Pending};
 
-    use tokio::runtime::Runtime;
+    use tokio::{runtime::Runtime, sync::oneshot};
 
     #[tokio::test]
     async fn runtime_shutdown() {
         let rt = Runtime::new().unwrap();
+        #[allow(clippy::async_yields_async)]
         let task = rt
             .spawn(async {
                 SpawnedTask::spawn(async {
@@ -119,4 +141,36 @@ mod tests {
             .await
             .ok();
     }
+
+    #[tokio::test]
+    async fn cancel_not_started_task() {
+        let (sender, receiver) = oneshot::channel::<i32>();
+        let task = SpawnedTask::spawn(async {
+            // Shouldn't be reached.
+            sender.send(42).unwrap();
+        });
+
+        drop(task);
+
+        // If the task was cancelled, the sender was also dropped,
+        // and awaiting the receiver should result in an error.
+        assert!(receiver.await.is_err());
+    }
+
+    #[tokio::test]
+    async fn cancel_ongoing_task() {
+        let (sender, mut receiver) = tokio::sync::mpsc::channel(1);
+        let task = SpawnedTask::spawn(async move {
+            sender.send(1).await.unwrap();
+            // This line will never be reached because the channel has a buffer
+            // of 1.
+            sender.send(2).await.unwrap();
+        });
+        // Let the task start.
+        assert_eq!(receiver.recv().await.unwrap(), 1);
+        drop(task);
+
+        // The sender was dropped so we receive `None`.
+        assert!(receiver.recv().await.is_none());
+    }
 }