[Test] Wait all results to be gathered before assertion

muted-tests.yml

@@ -519,9 +519,6 @@ tests:
 - class: org.elasticsearch.xpack.esql.qa.single_node.GenerativeForkIT
   method: test {csv-spec:spatial_shapes.ConvertFromStringParseError}
   issue: https://github.com/elastic/elasticsearch/issues/134254
-- class: org.elasticsearch.action.support.nodes.TransportNodesActionTests
-  method: testConcurrentlyCompletionAndCancellation
-  issue: https://github.com/elastic/elasticsearch/issues/134277
 
 # Examples:
 #

server/src/test/java/org/elasticsearch/action/support/nodes/TransportNodesActionTests.java

@@ -397,9 +397,11 @@ protected void onCancelled() {
         }
 
         final var raceBarrier = new CyclicBarrier(3);
+        final var completedLatch = new CountDownLatch(1);
         final Thread completeThread = new Thread(() -> {
             safeAwait(raceBarrier);
             nodeResponses.add(completeOneRequest(capturedRequests[capturedRequests.length - 1]));
+            completedLatch.countDown();
         });
         final Thread cancelThread = new Thread(() -> {
             safeAwait(raceBarrier);
@@ -420,6 +422,7 @@ protected void onCancelled() {
             assertThat(e.getMessage(), containsString("task cancelled [simulated]"));
             assertTrue(cancellableTask.isCancelled());
             safeAwait(onCancelledLatch); // wait for the latch, the listener for releasing node responses is called before it
+            safeAwait(completedLatch); // Wait till all responses are gathered
             assertTrue(nodeResponses.stream().allMatch(r -> r.hasReferences() == false));
         }