BulkProcessor flush runnable preserves the thread context from creation time (#26718)

When using a bulk processor, the thread context was not preserved for the flush runnable which is
executed in another thread in the thread pool. This change wraps the flush runnable in a context
preserving runnable so that the headers and transients from the creation time of the bulk processor
are available during the execution of the flush.

Closes #26596

core/src/main/java/org/elasticsearch/action/bulk/BulkProcessor.java

@@ -302,7 +302,8 @@ public class BulkProcessor implements Closeable {
             };
         }
 
-        return threadPool.scheduleWithFixedDelay(new Flush(), flushInterval, ThreadPool.Names.GENERIC);
+        final Runnable flushRunnable = threadPool.getThreadContext().preserveContext(new Flush());
+        return threadPool.scheduleWithFixedDelay(flushRunnable, flushInterval, ThreadPool.Names.GENERIC);
     }
 
     private void executeIfNeeded() {

core/src/test/java/org/elasticsearch/action/bulk/BulkProcessorTests.java

@@ -0,0 +1,100 @@
+/*
+ * 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.bulk;
+
+import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.index.IndexRequest;
+import org.elasticsearch.common.unit.ByteSizeUnit;
+import org.elasticsearch.common.unit.ByteSizeValue;
+import org.elasticsearch.common.unit.TimeValue;
+import org.elasticsearch.common.util.concurrent.ThreadContext;
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.threadpool.TestThreadPool;
+import org.elasticsearch.threadpool.ThreadPool;
+import org.junit.After;
+import org.junit.Before;
+
+import java.util.concurrent.CountDownLatch;
+import java.util.function.BiConsumer;
+
+public class BulkProcessorTests extends ESTestCase {
+
+    private ThreadPool threadPool;
+
+    @Before
+    public void startThreadPool() {
+        threadPool = new TestThreadPool("BulkProcessorTests");
+    }
+
+    @After
+    public void stopThreadPool() throws InterruptedException {
+        terminate(threadPool);
+    }
+
+    public void testBulkProcessorFlushPreservesContext() throws InterruptedException {
+        final CountDownLatch latch = new CountDownLatch(1);
+        final String headerKey = randomAlphaOfLengthBetween(1, 8);
+        final String transientKey = randomAlphaOfLengthBetween(1, 8);
+        final String headerValue = randomAlphaOfLengthBetween(1, 32);
+        final Object transientValue = new Object();
+
+        BiConsumer<BulkRequest, ActionListener<BulkResponse>> consumer = (request, listener) -> {
+            ThreadContext threadContext = threadPool.getThreadContext();
+            assertEquals(headerValue, threadContext.getHeader(headerKey));
+            assertSame(transientValue, threadContext.getTransient(transientKey));
+            latch.countDown();
+        };
+
+        final int bulkSize = randomIntBetween(2, 32);
+        final TimeValue flushInterval = TimeValue.timeValueSeconds(1L);
+        final BulkProcessor bulkProcessor;
+        assertNull(threadPool.getThreadContext().getHeader(headerKey));
+        assertNull(threadPool.getThreadContext().getTransient(transientKey));
+        try (ThreadContext.StoredContext ctx = threadPool.getThreadContext().stashContext()) {
+            threadPool.getThreadContext().putHeader(headerKey, headerValue);
+            threadPool.getThreadContext().putTransient(transientKey, transientValue);
+            bulkProcessor = new BulkProcessor(consumer, BackoffPolicy.noBackoff(), new BulkProcessor.Listener() {
+                @Override
+                public void beforeBulk(long executionId, BulkRequest request) {
+                }
+
+                @Override
+                public void afterBulk(long executionId, BulkRequest request, BulkResponse response) {
+                }
+
+                @Override
+                public void afterBulk(long executionId, BulkRequest request, Throwable failure) {
+                }
+            }, 1, bulkSize, new ByteSizeValue(5, ByteSizeUnit.MB), flushInterval, threadPool);
+        }
+        assertNull(threadPool.getThreadContext().getHeader(headerKey));
+        assertNull(threadPool.getThreadContext().getTransient(transientKey));
+
+        // add a single item which won't be over the size or number of items
+        bulkProcessor.add(new IndexRequest());
+
+        // wait for flush to execute
+        latch.await();
+
+        assertNull(threadPool.getThreadContext().getHeader(headerKey));
+        assertNull(threadPool.getThreadContext().getTransient(transientKey));
+        bulkProcessor.close();
+    }
+}