blocking: fix spawn_blocking after shutdown (#1875)

The task handle needs to be shutdown explicitly and not dropped.

Closes #1853

tokio/src/runtime/blocking/pool.rs

@@ -197,6 +197,9 @@ impl Spawner {
             let mut shared = self.inner.shared.lock().unwrap();
 
             if shared.shutdown {
+                // Shutdown the task
+                task.shutdown();
+
                 // no need to even push this task; it would never get picked up
                 return;
             }

tokio/tests/rt_common.rs

@@ -44,7 +44,7 @@ rt_test! {
     use tokio::prelude::*;
     use tokio::runtime::Runtime;
     use tokio::sync::oneshot;
-    use tokio::time;
+    use tokio::{task, time};
     use tokio_test::{assert_err, assert_ok};
 
     use futures::future::poll_fn;
@@ -438,6 +438,23 @@ rt_test! {
         });
     }
 
+    #[test]
+    fn spawn_blocking_after_shutdown() {
+        let rt = rt();
+        let handle = rt.handle().clone();
+
+        // Shutdown
+        drop(rt);
+
+        handle.enter(|| {
+            let res = task::spawn_blocking(|| unreachable!());
+
+            // Avoid using a tokio runtime
+            let out = futures::executor::block_on(res);
+            assert!(out.is_err());
+        });
+    }
+
     #[test]
     fn io_driver_called_when_under_load() {
         let mut rt = rt();