Fix a bug where gossip loops forever while splitting messages (#7032)

* Fix a bug where gossip loops forever while splitting messages

* Get rid of while loop

* Minor clean up and rename

core/src/cluster_info.rs

@@ -913,27 +913,37 @@ impl ClusterInfo {
     /// each Vec is no larger than `PROTOCOL_PAYLOAD_SIZE`
     /// Note: some messages cannot be contained within that size so in the worst case this returns
     /// N nested Vecs with 1 item each.
-    fn split_gossip_messages(mut msgs: Vec<CrdsValue>) -> Vec<Vec<CrdsValue>> {
+    fn split_gossip_messages(msgs: Vec<CrdsValue>) -> Vec<Vec<CrdsValue>> {
         let mut messages = vec![];
-        while !msgs.is_empty() {
-            let mut payload = vec![];
-            let mut size = serialized_size(&payload).expect("Couldn't check size");
-            while let Some(msg) = msgs.pop() {
-                let msg_size = msg.size();
-                if size + msg_size > MAX_PROTOCOL_PAYLOAD_SIZE as u64 {
-                    if msg_size < MAX_PROTOCOL_PAYLOAD_SIZE as u64 {
-                        msgs.push(msg);
-                    } else {
-                        debug!(
-                            "dropping message larger than the maximum payload size {:?}",
-                            msg
-                        );
+        let mut payload = vec![];
+        let base_size = serialized_size(&payload).expect("Couldn't check size");
+        let max_payload_size = MAX_PROTOCOL_PAYLOAD_SIZE - base_size;
+        let mut payload_size = 0;
+        for msg in msgs {
+            let msg_size = msg.size();
+            // If the message is too big to fit in this batch
+            if payload_size + msg_size > max_payload_size as u64 {
+                // See if it can fit in the next batch
+                if msg_size <= max_payload_size as u64 {
+                    if !payload.is_empty() {
+                        // Flush the  current payload
+                        messages.push(payload);
+                        // Init the next payload
+                        payload = vec![msg];
+                        payload_size = msg_size;
                     }
-                    break;
+                } else {
+                    debug!(
+                        "dropping message larger than the maximum payload size {:?}",
+                        msg
+                    );
                 }
-                size += msg_size;
-                payload.push(msg);
+                continue;
             }
+            payload_size += msg_size;
+            payload.push(msg);
+        }
+        if !payload.is_empty() {
             messages.push(payload);
         }
         messages
@@ -2442,6 +2452,42 @@ mod tests {
         test_split_messages(value);
     }
 
+    #[test]
+    fn test_split_messages_packet_size() {
+        // Test that if a value is smaller than payload size but too large to be wrappe in a vec
+        // that it is still dropped
+        let payload: Vec<CrdsValue> = vec![];
+        let vec_size = serialized_size(&payload).unwrap();
+        let desired_size = MAX_PROTOCOL_PAYLOAD_SIZE - vec_size;
+        let mut value = CrdsValue::new_unsigned(CrdsData::EpochSlots(EpochSlots {
+            from: Pubkey::default(),
+            root: 0,
+            slots: BTreeSet::new(),
+            wallclock: 0,
+        }));
+
+        let mut i = 0;
+        while value.size() < desired_size {
+            let slots = (0..i).collect::<BTreeSet<_>>();
+            if slots.len() > 200 {
+                panic!(
+                    "impossible to match size: last {:?} vs desired {:?}",
+                    serialized_size(&value).unwrap(),
+                    desired_size
+                );
+            }
+            value.data = CrdsData::EpochSlots(EpochSlots {
+                from: Pubkey::default(),
+                root: 0,
+                slots,
+                wallclock: 0,
+            });
+            i += 1;
+        }
+        let split = ClusterInfo::split_gossip_messages(vec![value.clone()]);
+        assert_eq!(split.len(), 0);
+    }
+
     fn test_split_messages(value: CrdsValue) {
         const NUM_VALUES: usize = 30;
         let value_size = value.size();