Speed up Zebra shutdown: skip waiting for the tokio runtime

zebrad/src/components/tokio.rs

@@ -46,16 +46,22 @@ pub(crate) trait RuntimeRun {
 impl RuntimeRun for Runtime {
     fn run(&mut self, fut: impl Future<Output = Result<(), Report>>) {
         let result = self.block_on(async move {
-            // If the run task and shutdown are both ready, select! chooses
-            // one of them at random.
+            // Always poll the shutdown future first.
+            //
+            // Otherwise, a busy Zebra instance could starve the shutdown future,
+            // and delay shutting down.
             tokio::select! {
-                result = fut => result,
+                biased;
                 _ = shutdown() => Ok(()),
+                result = fut => result,
             }
         });
 
         match result {
-            Ok(()) => {}
+            Ok(()) => {
+                // Don't wait for the runtime to shut down all the tasks.
+                app_writer().shutdown(Shutdown::Graceful);
+            }
             Err(e) => {
                 eprintln!("Error: {:?}", e);
                 app_writer().shutdown(Shutdown::Forced);