Peer recovery should flush at the end (#41660)

Flushing at the end of a peer recovery (if needed) can bring these
benefits:

1. Closing an index won't end up with the red state for a recovering
replica should always be ready for closing whether it performs the
verifying-before-close step or not.

2. Good opportunities to compact store (i.e., flushing and merging
Lucene, and trimming translog)

Closes #40024
Closes #39588

server/src/main/java/org/elasticsearch/indices/recovery/RecoveryTarget.java

@@ -27,6 +27,7 @@
 import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.action.ActionListener;
+import org.elasticsearch.action.admin.indices.flush.FlushRequest;
 import org.elasticsearch.cluster.node.DiscoveryNode;
 import org.elasticsearch.common.UUIDs;
 import org.elasticsearch.common.bytes.BytesReference;
@@ -38,6 +39,7 @@
 import org.elasticsearch.index.mapper.MapperException;
 import org.elasticsearch.index.seqno.ReplicationTracker;
 import org.elasticsearch.index.seqno.RetentionLeases;
+import org.elasticsearch.index.seqno.SequenceNumbers;
 import org.elasticsearch.index.shard.IndexShard;
 import org.elasticsearch.index.shard.IndexShardNotRecoveringException;
 import org.elasticsearch.index.shard.IndexShardState;
@@ -298,11 +300,19 @@ public void finalizeRecovery(final long globalCheckpoint, ActionListener<Void> l
             // Persist the global checkpoint.
             indexShard.sync();
             indexShard.persistRetentionLeases();
+            if (hasUncommittedOperations()) {
+                indexShard.flush(new FlushRequest().force(true).waitIfOngoing(true));
+            }
             indexShard.finalizeRecovery();
             return null;
         });
     }
 
+    private boolean hasUncommittedOperations() throws IOException {
+        long localCheckpointOfCommit = Long.parseLong(indexShard.commitStats().getUserData().get(SequenceNumbers.LOCAL_CHECKPOINT_KEY));
+        return indexShard.estimateNumberOfHistoryOperations("peer-recovery", localCheckpointOfCommit + 1) > 0;
+    }
+
     @Override
     public void handoffPrimaryContext(final ReplicationTracker.PrimaryContext primaryContext) {
         indexShard.activateWithPrimaryContext(primaryContext);

server/src/test/java/org/elasticsearch/indices/recovery/IndexRecoveryIT.java

@@ -31,6 +31,7 @@
 import org.elasticsearch.action.admin.indices.recovery.RecoveryResponse;
 import org.elasticsearch.action.admin.indices.stats.CommonStatsFlags;
 import org.elasticsearch.action.admin.indices.stats.IndicesStatsResponse;
+import org.elasticsearch.action.admin.indices.stats.ShardStats;
 import org.elasticsearch.action.index.IndexRequestBuilder;
 import org.elasticsearch.action.search.SearchResponse;
 import org.elasticsearch.action.support.WriteRequest.RefreshPolicy;
@@ -52,9 +53,12 @@
 import org.elasticsearch.index.analysis.TokenFilterFactory;
 import org.elasticsearch.index.mapper.MapperParsingException;
 import org.elasticsearch.index.recovery.RecoveryStats;
+import org.elasticsearch.index.seqno.SequenceNumbers;
+import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.index.store.Store;
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.indices.analysis.AnalysisModule;
+import org.elasticsearch.indices.flush.SyncedFlushUtil;
 import org.elasticsearch.indices.recovery.RecoveryState.Stage;
 import org.elasticsearch.node.RecoverySettingsChunkSizePlugin;
 import org.elasticsearch.plugins.AnalysisPlugin;
@@ -84,14 +88,19 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Semaphore;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Consumer;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
+import java.util.stream.Stream;
 
 import static java.util.Collections.singletonMap;
+import static java.util.stream.Collectors.toList;
 import static org.elasticsearch.node.RecoverySettingsChunkSizePlugin.CHUNK_SIZE_SETTING;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertHitCount;
@@ -910,6 +919,39 @@ public void testDoNotInfinitelyWaitForMapping() {
         assertHitCount(client().prepareSearch().get(), numDocs);
     }
 
+    public void testRecoveryFlushReplica() throws Exception {
+        internalCluster().ensureAtLeastNumDataNodes(3);
+        String indexName = "test-index";
+        createIndex(indexName, Settings.builder().put("index.number_of_replicas", 0).put("index.number_of_shards", 1).build());
+        int numDocs = randomIntBetween(0, 10);
+        indexRandom(randomBoolean(), false, randomBoolean(), IntStream.range(0, numDocs)
+            .mapToObj(n -> client().prepareIndex(indexName, "_doc").setSource("num", n)).collect(toList()));
+        assertAcked(client().admin().indices().prepareUpdateSettings(indexName)
+            .setSettings(Settings.builder().put("index.number_of_replicas", 1)));
+        ensureGreen(indexName);
+        ShardId shardId = null;
+        for (ShardStats shardStats : client().admin().indices().prepareStats(indexName).get().getIndex(indexName).getShards()) {
+            shardId = shardStats.getShardRouting().shardId();
+            if (shardStats.getShardRouting().primary() == false) {
+                assertThat(shardStats.getCommitStats().getNumDocs(), equalTo(numDocs));
+                SequenceNumbers.CommitInfo commitInfo = SequenceNumbers.loadSeqNoInfoFromLuceneCommit(
+                    shardStats.getCommitStats().getUserData().entrySet());
+                assertThat(commitInfo.localCheckpoint, equalTo(shardStats.getSeqNoStats().getLocalCheckpoint()));
+                assertThat(commitInfo.maxSeqNo, equalTo(shardStats.getSeqNoStats().getMaxSeqNo()));
+            }
+        }
+        SyncedFlushUtil.attemptSyncedFlush(logger, internalCluster(), shardId);
+        assertBusy(() -> assertThat(client().admin().indices().prepareSyncedFlush(indexName).get().failedShards(), equalTo(0)));
+        assertAcked(client().admin().indices().prepareUpdateSettings(indexName)
+            .setSettings(Settings.builder().put("index.number_of_replicas", 2)));
+        ensureGreen(indexName);
+        // Recovery should keep syncId if no indexing activity on the primary after synced-flush.
+        Set<String> syncIds = Stream.of(client().admin().indices().prepareStats(indexName).get().getIndex(indexName).getShards())
+            .map(shardStats -> shardStats.getCommitStats().syncId())
+            .collect(Collectors.toSet());
+        assertThat(syncIds, hasSize(1));
+    }
+
     public static final class TestAnalysisPlugin extends Plugin implements AnalysisPlugin {
         final AtomicBoolean throwParsingError = new AtomicBoolean();
         @Override