From 3ad4794aa006c239fb60923449d96bb8ae70d891 Mon Sep 17 00:00:00 2001
From: Stefan Miklosovic <smiklosovic@apache.org>
Date: Wed, 4 Mar 2026 09:07:17 +0100
Subject: [PATCH] Node does not send multiple inflight echos

patch by Cameron Zemek; reviewed by Stefan Miklosovic, Brandon Williams for CASSANDRA-18866
---
 CHANGES.txt                                   |  1 +
 .../org/apache/cassandra/gms/Gossiper.java    | 30 ++++++++++++++-----
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/CHANGES.txt b/CHANGES.txt
index 271fb4ec6e..6c5b443133 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,4 +1,5 @@
 4.0.20
+ * Node does not send multiple inflight echos (CASSANDRA-18866)
  * Obsolete expired SSTables before compaction starts (CASSANDRA-19776)
  * No need to evict already prepared statements, as it creates a race condition between multiple threads (CASSANDRA-17401)
  * Switch lz4-java to at.yawk.lz4 version due to CVE (CASSANDRA-21052)
diff --git a/src/java/org/apache/cassandra/gms/Gossiper.java b/src/java/org/apache/cassandra/gms/Gossiper.java
index 12c532b162..867bcbbca7 100644
--- a/src/java/org/apache/cassandra/gms/Gossiper.java
+++ b/src/java/org/apache/cassandra/gms/Gossiper.java
@@ -140,6 +140,9 @@ public class Gossiper implements IFailureDetectionEventListener, GossiperMBean
     @VisibleForTesting
     final Set<InetAddressAndPort> liveEndpoints = new ConcurrentSkipListSet<>();
 
+    /* Inflight echo requests. */
+    private final Map<InetAddressAndPort, EndpointState> inflightEcho = new ConcurrentHashMap<>();
+
     /* unreachable member set */
     private final Map<InetAddressAndPort, Long> unreachableEndpoints = new ConcurrentHashMap<>();
 
@@ -180,6 +183,7 @@ public class Gossiper implements IFailureDetectionEventListener, GossiperMBean
     {
         unreachableEndpoints.clear();
         liveEndpoints.clear();
+        inflightEcho.clear();
         justRemovedEndpoints.clear();
         expireTimeEndpointMap.clear();
         endpointStateMap.clear();
@@ -659,6 +663,7 @@ public class Gossiper implements IFailureDetectionEventListener, GossiperMBean
         }
 
         liveEndpoints.remove(endpoint);
+        inflightEcho.remove(endpoint);
         unreachableEndpoints.remove(endpoint);
         MessagingService.instance().versions.reset(endpoint);
         quarantineEndpoint(endpoint);
@@ -1328,14 +1333,24 @@ public class Gossiper implements IFailureDetectionEventListener, GossiperMBean
     {
         localState.markDead();
 
-        Message<NoPayload> echoMessage = Message.out(ECHO_REQ, noPayload);
-        logger.trace("Sending ECHO_REQ to {}", addr);
-        RequestCallback echoHandler = msg ->
+        EndpointState prevState = inflightEcho.put(addr, localState);
+        boolean sendEcho = !localState.equals(prevState);
+        if (sendEcho)
         {
-            runInGossipStageBlocking(() -> realMarkAlive(addr, localState));
-        };
-
-        MessagingService.instance().sendWithCallback(echoMessage, addr, echoHandler);
+            Message<NoPayload> echoMessage = Message.out(ECHO_REQ, noPayload);
+            logger.trace("Sending ECHO_REQ to {}", addr);
+            RequestCallback echoHandler = msg ->
+            {
+                runInGossipStageBlocking(() -> {
+                    EndpointState epState = inflightEcho.remove(addr);
+                    if (epState != null)
+                        realMarkAlive(addr, epState);
+                });
+            };
+            MessagingService.instance().sendWithCallback(echoMessage, addr, echoHandler);
+        }
+        else
+            logger.trace("Skipping ECHO_REQ to {} since it is already inflight", addr);
 
         GossiperDiagnostics.markedAlive(this, addr, localState);
     }
@@ -1386,6 +1401,7 @@ public class Gossiper implements IFailureDetectionEventListener, GossiperMBean
     {
         localState.markDead();
         liveEndpoints.remove(addr);
+        inflightEcho.remove(addr);
         unreachableEndpoints.put(addr, System.nanoTime());
     }