Optimize use of tokio::select!: Use biased selection (#580)

as there is no need to randomize the first one to be polled.

For `cancel_on_user_sig_term` and `StreamReadable::fill_buf`, the
cancellation future should always to be polled first so that user would
feel responsive.

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

crates/binstalk-downloader/src/download/stream_readable.rs

@@ -122,10 +122,12 @@ where
             let option = self.handle.block_on(async {
                 if let Some(cancellation_future) = self.cancellation_future.as_mut() {
                     tokio::select! {
-                        res = next_stream(&mut self.stream) => res,
+                        biased;
+
                         res = cancellation_future => {
                             Err(res.err().unwrap_or_else(|| io::Error::from(DownloadError::UserAbort)))
                         },
+                        res = next_stream(&mut self.stream) => res,
                     }
                 } else {
                     next_stream(&mut self.stream).await

crates/binstalk/src/helpers/signal.rs

@@ -20,11 +20,13 @@ pub async fn cancel_on_user_sig_term<T>(
     ignore_signals()?;
 
     tokio::select! {
-        res = handle => res,
+        biased;
+
         res = wait_on_cancellation_signal() => {
             res.map_err(BinstallError::Io)
                 .and(Err(BinstallError::UserAbort))
         }
+        res = handle => res,
     }
 }
 
@@ -59,6 +61,8 @@ async fn wait_on_cancellation_signal_inner() -> Result<(), io::Error> {
     }
 
     tokio::select! {
+        biased;
+
         res = signal::ctrl_c() => res,
         res = inner() => res,
     }
@@ -72,6 +76,8 @@ mod unix {
     /// Same as [`wait_on_cancellation_signal`] but is only available on unix.
     pub async fn wait_on_cancellation_signal_unix() -> Result<(), io::Error> {
         tokio::select! {
+            biased;
+
             res = wait_for_signal_unix(SignalKind::interrupt()) => res,
             res = wait_for_signal_unix(SignalKind::hangup()) => res,
             res = wait_for_signal_unix(SignalKind::terminate()) => res,