Add Test

Author: lukewhiting

x-pack/qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/DataStreamsUpgradeIT.java

@@ -605,6 +605,11 @@ private void upgradeDataStream(String dataStreamName, int numRolloversOnOldClust
                     assertThat(statusResponseString, ((List<Object>) statusResponseMap.get("errors")).size(), equalTo(expectedErrorCount));
                 }
             }, 60, TimeUnit.SECONDS);
+
+            // Verify it's possible to reindex again after a successful reindex
+            reindexResponse = upgradeUserClient.performRequest(reindexRequest);
+            assertOK(reindexResponse);
+
             Request cancelRequest = new Request("POST", "_migration/reindex/" + dataStreamName + "/_cancel");
             Response cancelResponse = upgradeUserClient.performRequest(cancelRequest);
             assertOK(cancelResponse);