Fix flaky DecommissionControllerTests.testTimesOut

This test fails pretty reliably if I run it on repeat. I believe the
problem is that the test assumes the function will take longer than 2ms,
which is likely not a valid assumption in all cases. Fortunately, I can
pass in a zero duration which is guaranteed to timeout even if the
system clock does not advance at all.

Also moved the assertions out of the callback into the main test method,
otherwise the assertion error messages would get buried and the test
report would just show a timeout error.

server/src/test/java/org/opensearch/cluster/decommission/DecommissionControllerTests.java

@@ -8,6 +8,7 @@
 
 package org.opensearch.cluster.decommission;
 
+import org.hamcrest.MatcherAssert;
 import org.junit.After;
 import org.junit.Before;
 import org.opensearch.OpenSearchTimeoutException;
@@ -45,6 +46,7 @@
 import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.stream.Collectors;
 import java.util.stream.StreamSupport;
 
@@ -53,6 +55,7 @@
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.empty;
 import static org.hamcrest.Matchers.instanceOf;
+import static org.hamcrest.Matchers.notNullValue;
 import static org.hamcrest.Matchers.sameInstance;
 import static org.opensearch.cluster.ClusterState.builder;
 import static org.opensearch.cluster.OpenSearchAllocationTestCase.createAllocationService;
@@ -213,24 +216,25 @@ public void testTimesOut() throws InterruptedException {
         nodesToBeRemoved.add(clusterService.state().nodes().get("node13"));
         nodesToBeRemoved.add(clusterService.state().nodes().get("node14"));
         nodesToBeRemoved.add(clusterService.state().nodes().get("node15"));
+        final AtomicReference<Exception> exceptionReference = new AtomicReference<>();
         decommissionController.removeDecommissionedNodes(
             nodesToBeRemoved,
             "unit-test-timeout",
-            TimeValue.timeValueMillis(2),
-            new ActionListener<Void>() {
+            TimeValue.timeValueMillis(0),
+            new ActionListener<>() {
                 @Override
-                public void onResponse(Void unused) {
-                    fail("response shouldn't have been called");
-                }
+                public void onResponse(Void unused) {}
 
                 @Override
                 public void onFailure(Exception e) {
-                    assertThat(e, instanceOf(OpenSearchTimeoutException.class));
-                    assertThat(e.getMessage(), containsString("waiting for removal of decommissioned nodes"));
+                    exceptionReference.set(e);
                     countDownLatch.countDown();
                 }
             }
         );
+        MatcherAssert.assertThat("Expected onFailure to be called", exceptionReference.get(), notNullValue());
+        MatcherAssert.assertThat(exceptionReference.get(), instanceOf(OpenSearchTimeoutException.class));
+        MatcherAssert.assertThat(exceptionReference.get().getMessage(), containsString("waiting for removal of decommissioned nodes"));
         assertTrue(countDownLatch.await(30, TimeUnit.SECONDS));
     }