Avoid unneeded refresh with concurrent realtime gets (elastic#47895)

This change should reduce refreshes for a use-case where we perform 
multiple realtime gets at the same time on an active index. Currently,
we only call refresh if the index operation is still on the versionMap.
However, at the time we call refresh, that operation might be already or
will be included in the latest reader. Hence, we do not need to refresh.
Adding another lock here is not an issue as the refresh is already
sequential.

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

@@ -671,7 +671,8 @@ public GetResult get(Get get, BiFunction<String, SearcherScope, Engine.Searcher>
                             trackTranslogLocation.set(true);
                         }
                     }
-                    refresh("realtime_get", SearcherScope.INTERNAL, true);
+                    assert versionValue.seqNo >= 0 : versionValue;
+                    refreshIfNeeded("realtime_get", versionValue.seqNo);
                 }
                 scope = SearcherScope.INTERNAL;
             } else {

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -165,6 +165,7 @@
 import java.util.concurrent.BrokenBarrierException;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.CyclicBarrier;
+import java.util.concurrent.Phaser;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
@@ -6102,4 +6103,58 @@ private void runTestDeleteFailure(
         }
     }
 
+    public void testRealtimeGetOnlyRefreshIfNeeded() throws Exception {
+        final AtomicInteger refreshCount = new AtomicInteger();
+        final ReferenceManager.RefreshListener refreshListener = new ReferenceManager.RefreshListener() {
+            @Override
+            public void beforeRefresh() {
+
+            }
+
+            @Override
+            public void afterRefresh(boolean didRefresh) {
+                if (didRefresh) {
+                    refreshCount.incrementAndGet();
+                }
+            }
+        };
+        try (Store store = createStore()) {
+            final EngineConfig config = config(defaultSettings, store, createTempDir(), newMergePolicy(), null,
+                refreshListener, null, null, engine.config().getCircuitBreakerService());
+            try (InternalEngine engine = createEngine(config)) {
+                int numDocs = randomIntBetween(10, 100);
+                Set<String> ids = new HashSet<>();
+                for (int i = 0; i < numDocs; i++) {
+                    String id = Integer.toString(i);
+                    engine.index(indexForDoc(createParsedDoc(id, null)));
+                    ids.add(id);
+                }
+                final int refreshCountBeforeGet = refreshCount.get();
+                Thread[] getters = new Thread[randomIntBetween(1, 4)];
+                Phaser phaser = new Phaser(getters.length + 1);
+                for (int t = 0; t < getters.length; t++) {
+                    getters[t] = new Thread(() -> {
+                        phaser.arriveAndAwaitAdvance();
+                        int iters = randomIntBetween(1, 10);
+                        for (int i = 0; i < iters; i++) {
+                            ParsedDocument doc = createParsedDoc(randomFrom(ids), null);
+                            try (Engine.GetResult getResult = engine.get(newGet(true, doc), engine::acquireSearcher)) {
+                                assertThat(getResult.exists(), equalTo(true));
+                                assertThat(getResult.docIdAndVersion(), notNullValue());
+                            }
+                        }
+                    });
+                    getters[t].start();
+                }
+                phaser.arriveAndAwaitAdvance();
+                for (int i = 0; i < numDocs; i++) {
+                    engine.index(indexForDoc(createParsedDoc("more-" + i, null)));
+                }
+                for (Thread getter : getters) {
+                    getter.join();
+                }
+                assertThat(refreshCount.get(), lessThanOrEqualTo(refreshCountBeforeGet + 1));
+            }
+        }
+    }
 }