$ diff -U3 elasticsearch-{master,7.x}/server/src/main/java/org/elasticsearch/index/engine/CompletionStatsCache.java
--- elasticsearch-master/server/src/main/java/org/elasticsearch/index/engine/CompletionStatsCache.java	2020-02-27 08:47:44.000000000 +0000
+++ elasticsearch-7.x/server/src/main/java/org/elasticsearch/index/engine/CompletionStatsCache.java	2020-02-27 07:55:08.000000000 +0000
@@ -29,10 +29,10 @@
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.support.PlainActionFuture;
 import org.elasticsearch.common.FieldMemoryStats;
+import org.elasticsearch.common.Nullable;
 import org.elasticsearch.common.regex.Regex;
 import org.elasticsearch.search.suggest.completion.CompletionStats;

-import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Supplier;

 class CompletionStatsCache implements ReferenceManager.RefreshListener {
@@ -45,7 +45,13 @@
      * Futures are eventually completed with stats that include all fields, requiring further filtering (see
      * {@link CompletionStatsCache#filterCompletionStatsByFieldName}).
      */
-    private final AtomicReference<PlainActionFuture<CompletionStats>> completionStatsFutureRef = new AtomicReference<>();
+    @Nullable
+    private PlainActionFuture<CompletionStats> completionStatsFuture;
+
+    /**
+     * Protects accesses to {@code completionStatsFuture} since we can't use {@link java.util.concurrent.atomic.AtomicReference} in JDK8.
+     */
+    private final Object completionStatsFutureMutex = new Object();

     CompletionStatsCache(Supplier<Engine.Searcher> searcherSupplier) {
         this.searcherSupplier = searcherSupplier;
@@ -53,7 +59,18 @@

     CompletionStats get(String... fieldNamePatterns) {
         final PlainActionFuture<CompletionStats> newFuture = new PlainActionFuture<>();
-        final PlainActionFuture<CompletionStats> oldFuture = completionStatsFutureRef.compareAndExchange(null, newFuture);
+
+        // final PlainActionFuture<CompletionStats> oldFuture = completionStatsFutureRef.compareAndExchange(null, newFuture);
+        // except JDK8 doesn't have compareAndExchange so we emulate it:
+        final PlainActionFuture<CompletionStats> oldFuture;
+        synchronized (completionStatsFutureMutex) {
+            if (completionStatsFuture == null) {
+                completionStatsFuture = newFuture;
+                oldFuture = null;
+            } else {
+                oldFuture = completionStatsFuture;
+            }
+        }

         if (oldFuture != null) {
             // we lost the race, someone else is already computing stats, so we wait for that to finish
@@ -91,7 +108,13 @@
         } finally {
             if (success == false) {
                 // invalidate the cache (if not already invalidated) so that future calls will retry
-                completionStatsFutureRef.compareAndSet(newFuture, null);
+
+                // completionStatsFutureRef.compareAndSet(newFuture, null); except we're not using AtomicReference in JDK8
+                synchronized (completionStatsFutureMutex) {
+                    if (completionStatsFuture == newFuture) {
+                        completionStatsFuture = null;
+                    }
+                }
             }
         }

@@ -121,7 +144,10 @@
     @Override
     public void afterRefresh(boolean didRefresh) {
         if (didRefresh) {
-            completionStatsFutureRef.set(null);
+            // completionStatsFutureRef.set(null); except we're not using AtomicReference in JDK8
+            synchronized (completionStatsFutureMutex) {
+                completionStatsFuture = null;
+            }
         }
     }
 }