Field eventsToDeliver is a LinkedList, i.e., not thread-safe. Accesses to field eventsToDeliver are protected by synchronization on itself, but not in 1 location.

Author: adriannistor

.changes/next-release/bugfix-AWSSDKforJavav2-685db17.json

@@ -0,0 +1,5 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java v2",
+    "description": "Fixing race condition in EventStreamAsyncResponseTransformer.  Field eventsToDeliver is a LinkedList, i.e., not thread-safe.  Accesses to field eventsToDeliver are protected by synchronization on itself, but not in 1 location."
+}

core/aws-core/src/main/java/software/amazon/awssdk/awscore/eventstream/EventStreamAsyncResponseTransformer.java

@@ -394,7 +394,9 @@ public void onError(Throwable throwable) {
         @Override
         public void onComplete() {
             // Add the special on complete event to signal drainEvents to complete the subscriber
-            eventsToDeliver.add(ON_COMPLETE_EVENT);
+            synchronized (eventsToDeliver) {
+                eventsToDeliver.add(ON_COMPLETE_EVENT);
+            }
             drainEventsIfNotAlready();
             transformFuture.complete(null);
         }