Enable assertions in integration tests

When starting a standalone cluster, we do not able assertions. This is
problematic because it means that we miss opportunities to catch
bugs. This commit enables assertions for standalone integration tests,
and fixes a couple bugs that were uncovered by enabling these.

Relates #22334

buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy

@@ -72,10 +72,12 @@ class ClusterConfiguration {
     boolean useMinimumMasterNodes = true
 
     @Input
-    String jvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +
+    String jvmArgs = "-ea" +
+        " " + "-Xms" + System.getProperty('tests.heap.size', '512m') +
         " " + "-Xmx" + System.getProperty('tests.heap.size', '512m') +
         " " + System.getProperty('tests.jvm.argline', '')
 
+
     /**
      * A closure to call which returns the unicast host to connect to for cluster formation.
      *

core/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -592,15 +592,14 @@ public class InternalEngine extends Engine {
     private boolean assertSequenceNumber(final Engine.Operation.Origin origin, final long seqNo) {
         if (engineConfig.getIndexSettings().getIndexVersionCreated().before(Version.V_6_0_0_alpha1_UNRELEASED) && origin == Operation.Origin.LOCAL_TRANSLOG_RECOVERY) {
             // legacy support
-            assert seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO : "old op recovering but it already has a seq no." +
+            assert seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO : "old op recovering but it already has a seq no.;" +
-                " index version: " + engineConfig.getIndexSettings().getIndexVersionCreated() + ". seq no: " + seqNo;
+                " index version: " + engineConfig.getIndexSettings().getIndexVersionCreated() + ", seqNo: " + seqNo;
         } else if (origin == Operation.Origin.PRIMARY) {
             // sequence number should not be set when operation origin is primary
-            assert seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO : "primary ops should never have an assigned seq no. got: " + seqNo;
+            assert seqNo == SequenceNumbersService.UNASSIGNED_SEQ_NO : "primary ops should never have an assigned seq no.; seqNo: " + seqNo;
-        } else {
+        } else if (engineConfig.getIndexSettings().getIndexVersionCreated().onOrAfter(Version.V_6_0_0_alpha1_UNRELEASED)) {
             // sequence number should be set when operation origin is not primary
-            assert seqNo >= 0 : "recovery or replica ops should have an assigned seq no. origin: " + origin +
+            assert seqNo >= 0 : "recovery or replica ops should have an assigned seq no.; origin: " + origin;
-                " index version: " + engineConfig.getIndexSettings().getIndexVersionCreated();
         }
         return true;
     }

core/src/main/java/org/elasticsearch/rest/BytesRestResponse.java

@@ -89,10 +89,11 @@ public class BytesRestResponse extends RestResponse {
             this.content = BytesArray.EMPTY;
             this.contentType = TEXT_CONTENT_TYPE;
         } else {
-            XContentBuilder builder = convert(channel, status, e);
+            try (final XContentBuilder builder = convert(channel, status, e)) {
                 this.content = builder.bytes();
                 this.contentType = builder.contentType().mediaType();
             }
+        }
         if (e instanceof ElasticsearchException) {
             copyHeaders(((ElasticsearchException) e));
         }

core/src/main/java/org/elasticsearch/rest/action/admin/indices/RestGetMappingAction.java

@@ -72,8 +72,10 @@ public class RestGetMappingAction extends BaseRestHandler {
                     if (indices.length != 0 && types.length != 0) {
                         return new BytesRestResponse(OK, builder.startObject().endObject());
                     } else if (indices.length != 0) {
+                        builder.close();
                         return new BytesRestResponse(channel, new IndexNotFoundException(indices[0]));
                     } else if (types.length != 0) {
+                        builder.close();
                         return new BytesRestResponse(channel, new TypeMissingException("_all", types[0]));
                     } else {
                         return new BytesRestResponse(OK, builder.startObject().endObject());