Add the ability to store objects with a ScrollContext (#24777)

This commit adds the ability to store and retrieve data that should be associated with a
ScrollContext. Additionally the ScrollContext was made final as we should only have a single
implementation of this concept.

core/src/main/java/org/elasticsearch/search/internal/ScrollContext.java

@@ -22,11 +22,35 @@ package org.elasticsearch.search.internal;
 import org.apache.lucene.search.ScoreDoc;
 import org.elasticsearch.search.Scroll;
 
+import java.util.HashMap;
+import java.util.Map;
+
 /** Wrapper around information that needs to stay around when scrolling. */
-public class ScrollContext {
+public final class ScrollContext {
+
+    private Map<String, Object> context = null;
 
     public int totalHits = -1;
     public float maxScore;
     public ScoreDoc lastEmittedDoc;
     public Scroll scroll;
+
+    /**
+     * Returns the object or <code>null</code> if the given key does not have a
+     * value in the context
+     */
+    @SuppressWarnings("unchecked") // (T)object
+    public <T> T getFromContext(String key) {
+        return context != null ? (T) context.get(key) : null;
+    }
+
+    /**
+     * Puts the object into the context
+     */
+    public void putInContext(String key, Object value) {
+        if (context == null) {
+            context = new HashMap<>();
+        }
+        context.put(key, value);
+    }
 }

core/src/test/java/org/elasticsearch/search/internal/ScrollContextTests.java

@@ -0,0 +1,36 @@
+/*
+ * 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.search.internal;
+
+import org.elasticsearch.test.ESTestCase;
+
+public class ScrollContextTests extends ESTestCase {
+
+    public void testStoringObjectsInScrollContext() {
+        final ScrollContext scrollContext = new ScrollContext();
+        final String key = randomAlphaOfLengthBetween(1, 16);
+        assertNull(scrollContext.getFromContext(key));
+
+        final String value = randomAlphaOfLength(6);
+        scrollContext.putInContext(key, value);
+
+        assertEquals(value, scrollContext.getFromContext(key));
+    }
+}