Fix #2119 and failing state assertion (#2212)

Add a test for #2119 and failing state assertion,
and a fix to go with it.

Co-authored-by: Carl Lerche <[email protected]>

tokio/src/runtime/blocking/pool.rs

@@ -65,10 +65,21 @@ where
     let rt = Handle::current();
 
     let (task, handle) = task::joinable(BlockingTask::new(func));
-    rt.blocking_spawner.spawn(task, &rt);
+    let _ = rt.blocking_spawner.spawn(task, &rt);
     handle
 }
 
+#[allow(dead_code)]
+pub(crate) fn try_spawn_blocking<F, R>(func: F) -> Result<(), ()>
+where
+    F: FnOnce() -> R + Send + 'static,
+{
+    let rt = Handle::current();
+
+    let (task, _handle) = task::joinable(BlockingTask::new(func));
+    rt.blocking_spawner.spawn(task, &rt)
+}
+
 // ===== impl BlockingPool =====
 
 impl BlockingPool {
@@ -137,7 +148,7 @@ impl fmt::Debug for BlockingPool {
 // ===== impl Spawner =====
 
 impl Spawner {
-    fn spawn(&self, task: Task, rt: &Handle) {
+    fn spawn(&self, task: Task, rt: &Handle) -> Result<(), ()> {
         let shutdown_tx = {
             let mut shared = self.inner.shared.lock().unwrap();
 
@@ -146,7 +157,7 @@ impl Spawner {
                 task.shutdown();
 
                 // no need to even push this task; it would never get picked up
-                return;
+                return Err(());
             }
 
             shared.queue.push_back(task);
@@ -178,6 +189,8 @@ impl Spawner {
         if let Some(shutdown_tx) = shutdown_tx {
             self.spawn_thread(shutdown_tx, rt);
         }
+
+        Ok(())
     }
 
     fn spawn_thread(&self, shutdown_tx: shutdown::Sender, rt: &Handle) {
@@ -217,9 +230,6 @@ impl Inner {
                 run_task(task);
 
                 shared = self.shared.lock().unwrap();
-                if shared.shutdown {
-                    break; // Need to increment idle before we exit
-                }
             }
 
             // IDLE