perf: omit per-message select branch to await PubSubClient if no listeners

pgdog/src/backend/pool/connection/mod.rs

@@ -316,7 +316,9 @@ impl Connection {
     ///
     pub(crate) async fn read(&mut self) -> Result<Message, Error> {
         select! {
-            notification = self.pub_sub.recv() => {
+            // If we're listening on a NOTIFY channel, wait for a message, and if
+            // one is available, then we return it here.
+            notification = self.pub_sub.recv(), if self.pub_sub.is_listening() => {
                 Ok(notification.ok_or_else(|| Error::ProtocolOutOfSync)?.message())
             }
 

pgdog/src/backend/pub_sub/client.rs

@@ -31,6 +31,14 @@ impl PubSubClient {
         }
     }
 
+    // Do we have an active `listen` on anything
+    // This is solely used to determine if we should have an extra select! branch
+    // within our `read()` method. If we're not listening on anything,
+    // no point in adding the extra overhead within the branch.
+    pub(crate) fn is_listening(&self) -> bool {
+        !self.unlisten.is_empty()
+    }
+
     /// Listen on a channel.
     pub(crate) fn listen(&mut self, channel: &str, mut rx: Listener) {
         let tx = self.tx.clone();