Fix a race in thread wakeup (#459)

tokio-threadpool/src/park/default_park.rs

@@ -133,8 +133,8 @@ impl Inner {
             None => self.condvar.wait(m).unwrap(),
         };
 
-        // Transition back to idle. If the state has transitions dto `NOTIFY`,
-        // this will consume that notification
+        // Transition back to idle. If the state has transitioned to `NOTIFY`,
+        // this will consume that notification.
         self.state.store(IDLE, SeqCst);
 
         // Explicitly drop the mutex guard. There is no real point in doing it
@@ -155,10 +155,12 @@ impl Inner {
         // The other half is sleeping, this requires a lock
         let _m = self.mutex.lock().unwrap();
 
-        // Transition from SLEEP -> NOTIFY
-        match self.state.compare_and_swap(SLEEP, NOTIFY, SeqCst) {
+        // Transition to NOTIFY
+        match self.state.swap(NOTIFY, SeqCst) {
             SLEEP => {}
-            _ => return,
+            NOTIFY => return,
+            IDLE => return,
+            _ => unreachable!(),
         }
 
         // Wakeup the sleeper