chore: preempt single block downloading (#11647)

crates/net/p2p/src/full_block.rs

@@ -177,6 +177,9 @@ where
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         let this = self.get_mut();
 
+        // preemptive yield point
+        let mut budget = 4;
+
         loop {
             match ready!(this.request.poll(cx)) {
                 ResponseResult::Header(res) => {
@@ -232,6 +235,14 @@ where
             if let Some(res) = this.take_block() {
                 return Poll::Ready(res)
             }
+
+            // ensure we still have enough budget for another iteration
+            budget -= 1;
+            if budget == 0 {
+                // make sure we're woken up again
+                cx.waker().wake_by_ref();
+                return Poll::Pending
+            }
         }
     }
 }