Subscription requests now take priority over notifications (#7031)

Author: steveluscher

CHANGELOG.md

@@ -15,6 +15,11 @@ Release channels have their own copy of this changelog:
 <a name="edge-channel"></a>
 ## 3.0.0 - Unreleased
 
+### RPC
+
+#### Changes
+* The subscription server now prioritizes processing received messages before sending out responses. This ensures that new subscription requests and time-sensitive messages like `PING` opcodes take priority over notifications.
+
 ### Validator
 
 #### Breaking

rpc/src/rpc_pubsub_service.rs

@@ -401,6 +401,12 @@ async fn handle_connection(
             pin!(receive_future);
             loop {
                 select! {
+                    biased; // See [prioritization] note below.
+
+                    // [prioritization]
+                    // This block must come FIRST in the `select!` macro. This prioritizes
+                    // processing received messages over sending messages. This ensures the timely
+                    // processing of new subscriptions and time-sensitive opcodes like `PING`.
                     result = &mut receive_future => match result {
                         Ok(_) => break,
                         Err(soketto::connection::Error::Closed) => return Ok(()),