Fix possible NPE in put mapping validators (#43000)

When applying put mapping validators, we apply all the validators in the
collection. If a failure occurs, we collect that as a top-level
exception, and suppress any additional failures into the top-level
exception. However, if a request passes the validator after a top-level
exception has been collected, we would try to suppress a null exception
into the top-level exception. This is a violation of the
Throwable#addSuppressed API. This commit addresses this, and adds test
to cover the logic of collecting the failures when validating a put
mapping request.

server/src/main/java/org/elasticsearch/action/admin/indices/mapping/put/TransportPutMappingAction.java

@@ -126,10 +126,11 @@ public class TransportPutMappingAction extends TransportMasterNodeAction<PutMapp
             this.validators = validators;
         }
 
-        private Exception validateRequest(PutMappingRequest request, ClusterState state, Index[] indices) {
+        Exception validateRequest(PutMappingRequest request, ClusterState state, Index[] indices) {
             Exception firstException = null;
             for (MappingRequestValidator validator : validators) {
                 final Exception e = validator.validateRequest(request, state, indices);
+                if (e == null) continue;
                 if (firstException == null) {
                     firstException = e;
                 } else {

server/src/test/java/org/elasticsearch/action/admin/indices/mapping/put/TransportPutMappingRequestValidatorsTests.java

@@ -0,0 +1,88 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.elasticsearch.action.admin.indices.mapping.put;
+
+import org.elasticsearch.common.Randomness;
+import org.elasticsearch.test.ESTestCase;
+import org.hamcrest.Matchers;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class TransportPutMappingRequestValidatorsTests extends ESTestCase {
+
+    private final MappingRequestValidator EMPTY = (request, state, indices) -> null;
+    private final MappingRequestValidator FAIL = (request, state, indices) -> new Exception("failure");
+
+    public void testValidates() {
+        final int numberOfValidations = randomIntBetween(0, 8);
+        final List<MappingRequestValidator> validators = new ArrayList<>(numberOfValidations);
+        for (int i = 0; i < numberOfValidations; i++) {
+            validators.add(EMPTY);
+        }
+        final TransportPutMappingAction.RequestValidators requestValidators = new TransportPutMappingAction.RequestValidators(validators);
+        assertNull(requestValidators.validateRequest(null, null, null));
+    }
+
+    public void testFailure() {
+        final TransportPutMappingAction.RequestValidators validators = new TransportPutMappingAction.RequestValidators(List.of(FAIL));
+        assertNotNull(validators.validateRequest(null, null, null));
+    }
+
+    public void testValidatesAfterFailure() {
+        final TransportPutMappingAction.RequestValidators validators =
+                new TransportPutMappingAction.RequestValidators(List.of(FAIL, EMPTY));
+        assertNotNull(validators.validateRequest(null, null, null));
+    }
+
+    public void testMultipleFailures() {
+        final int numberOfFailures = randomIntBetween(2, 8);
+        final List<MappingRequestValidator> validators = new ArrayList<>(numberOfFailures);
+        for (int i = 0; i < numberOfFailures; i++) {
+            validators.add(FAIL);
+        }
+        final TransportPutMappingAction.RequestValidators requestValidators = new TransportPutMappingAction.RequestValidators(validators);
+        final Exception e = requestValidators.validateRequest(null, null, null);
+        assertNotNull(e);
+        assertThat(e.getSuppressed(), Matchers.arrayWithSize(numberOfFailures - 1));
+    }
+
+    public void testRandom() {
+        final int numberOfValidations = randomIntBetween(0, 8);
+        final int numberOfFailures = randomIntBetween(0, 8);
+        final List<MappingRequestValidator> validators = new ArrayList<>(numberOfValidations + numberOfFailures);
+        for (int i = 0; i < numberOfValidations; i++) {
+            validators.add(EMPTY);
+        }
+        for (int i = 0; i < numberOfFailures; i++) {
+            validators.add(FAIL);
+        }
+        Randomness.shuffle(validators);
+        final TransportPutMappingAction.RequestValidators requestValidators = new TransportPutMappingAction.RequestValidators(validators);
+        final Exception e = requestValidators.validateRequest(null, null, null);
+        if (numberOfFailures == 0) {
+            assertNull(e);
+        } else {
+            assertNotNull(e);
+        }
+        assertThat(e.getSuppressed(), Matchers.arrayWithSize(numberOfFailures - 1));
+    }
+
+}