Fix panic when dropping sync 'ConnectionPool'.

Fixes #2375.

contrib/sync_db_pools/lib/src/connection.rs

@@ -195,7 +195,11 @@ impl<K, C: Poolable> Drop for Connection<K, C> {
 impl<K, C: Poolable> Drop for ConnectionPool<K, C> {
     fn drop(&mut self) {
         let pool = self.pool.take();
-        tokio::task::spawn_blocking(move || drop(pool));
+        // Only use spawn_blocking if the Tokio runtime is still available
+        if let Ok(handle) = tokio::runtime::Handle::try_current() {
+            handle.spawn_blocking(move || drop(pool));
+        }
+        // Otherwise the pool will be dropped on the current thread
     }
 }
 

contrib/sync_db_pools/lib/tests/shutdown.rs

@@ -0,0 +1,32 @@
+#[cfg(all(feature = "diesel_sqlite_pool"))]
+#[cfg(test)]
+mod sqlite_shutdown_test {
+    use rocket::{async_test, Build, Rocket};
+    use rocket_sync_db_pools::database;
+
+    #[database("test")]
+    struct Pool(diesel::SqliteConnection);
+
+    async fn rocket() -> Rocket<Build> {
+        use rocket::figment::{util::map, Figment};
+
+        let options = map!["url" => ":memory:"];
+        let config = Figment::from(rocket::Config::debug_default())
+            .merge(("databases", map!["test" => &options]));
+
+        rocket::custom(config).attach(Pool::fairing())
+    }
+
+    #[test]
+    fn test_shutdown() {
+        let _rocket = async_test(
+            async {
+                let rocket = rocket().await.ignite().await.expect("unable to ignite");
+                // request shutdown
+                rocket.shutdown().notify();
+                rocket.launch().await.expect("unable to launch")
+            }
+        );
+        // _rocket is dropped here after the runtime is dropped
+    }
+}