ReadOnlyEngine should update translog recovery state information (elastic#39238)

`ReadOnlyEngine` never recovers operations from translog and never 
updates translog information in the index shard's recovery state, even 
though the recovery state goes through the `TRANSLOG` stage during 
the recovery. It means that recovery information for frozen shards indicates 
an unkown number of recovered translog ops in the Recovery APIs 
(translog_ops: `-1` and translog_ops_percent: `-1.0%`) and this is confusing.

This commit changes the `recoverFromTranslog()` method in `ReadOnlyEngine` 
so that it always recover from an empty translog snapshot, allowing the recovery 
state translog information to be correctly updated.

Related to elastic#33888

Author: tlrx

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

@@ -34,6 +34,7 @@
 import org.elasticsearch.Version;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.index.ElasticsearchDirectoryReader;
+import org.elasticsearch.common.util.concurrent.ReleasableLock;
 import org.elasticsearch.core.internal.io.IOUtils;
 import org.elasticsearch.index.mapper.MapperService;
 import org.elasticsearch.index.seqno.SeqNoStats;
@@ -287,18 +288,7 @@ public Translog.Snapshot newChangesSnapshot(String source, MapperService mapperS
 
     @Override
     public Translog.Snapshot readHistoryOperations(String source, MapperService mapperService, long startingSeqNo) throws IOException {
-        return new Translog.Snapshot() {
-            @Override
-            public void close() { }
-            @Override
-            public int totalOperations() {
-                return 0;
-            }
-            @Override
-            public Translog.Operation next() {
-                return null;
-            }
-        };
+        return newEmptySnapshot();
     }
 
     @Override
@@ -429,7 +419,15 @@ public int fillSeqNoGaps(long primaryTerm) {
     }
 
     @Override
-    public Engine recoverFromTranslog(TranslogRecoveryRunner translogRecoveryRunner, long recoverUpToSeqNo) {
+    public Engine recoverFromTranslog(final TranslogRecoveryRunner translogRecoveryRunner, final long recoverUpToSeqNo) {
+        try (ReleasableLock lock = readLock.acquire()) {
+            ensureOpen();
+            try (Translog.Snapshot snapshot = newEmptySnapshot()) {
+                translogRecoveryRunner.run(this, snapshot);
+            } catch (final Exception e) {
+                throw new EngineException(shardId, "failed to recover from empty translog snapshot", e);
+            }
+        }
         return this;
     }
 
@@ -468,4 +466,22 @@ protected void processReaders(IndexReader reader, IndexReader previousReader) {
     public boolean refreshNeeded() {
         return false;
     }
+
+    private Translog.Snapshot newEmptySnapshot() {
+        return new Translog.Snapshot() {
+            @Override
+            public void close() {
+            }
+
+            @Override
+            public int totalOperations() {
+                return 0;
+            }
+
+            @Override
+            public Translog.Operation next() {
+                return null;
+            }
+        };
+    }
 }

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

@@ -210,4 +210,36 @@ public void testVerifyShardBeforeIndexClosingIsNoOp() throws IOException {
             }
         }
     }
+
+    public void testRecoverFromTranslogAppliesNoOperations() throws IOException {
+        IOUtils.close(engine, store);
+        final AtomicLong globalCheckpoint = new AtomicLong(SequenceNumbers.NO_OPS_PERFORMED);
+        try (Store store = createStore()) {
+            EngineConfig config = config(defaultSettings, store, createTempDir(), newMergePolicy(), null, null, globalCheckpoint::get);
+            int numDocs = scaledRandomIntBetween(10, 1000);
+            try (InternalEngine engine = createEngine(config)) {
+                for (int i = 0; i < numDocs; i++) {
+                    if (rarely()) {
+                        continue; // gap in sequence number
+                    }
+                    ParsedDocument doc = testParsedDocument(Integer.toString(i), null, testDocument(), new BytesArray("{}"), null);
+                    engine.index(new Engine.Index(newUid(doc), doc, i, primaryTerm.get(), 1, null, Engine.Operation.Origin.REPLICA,
+                        System.nanoTime(), -1, false, SequenceNumbers.UNASSIGNED_SEQ_NO, 0));
+                    if (rarely()) {
+                        engine.flush();
+                    }
+                    globalCheckpoint.set(i);
+                }
+                engine.syncTranslog();
+                engine.flushAndClose();
+            }
+            try (ReadOnlyEngine readOnlyEngine = new ReadOnlyEngine(config, null , null, true, Function.identity())) {
+                final TranslogHandler translogHandler = new TranslogHandler(xContentRegistry(), config.getIndexSettings());
+                readOnlyEngine.initializeMaxSeqNoOfUpdatesOrDeletes();
+                readOnlyEngine.recoverFromTranslog(translogHandler, randomNonNegativeLong());
+
+                assertThat(translogHandler.appliedOperations(), equalTo(0L));
+            }
+        }
+    }
 }

x-pack/plugin/core/src/test/java/org/elasticsearch/index/engine/FrozenIndexTests.java

@@ -9,12 +9,14 @@
 import org.elasticsearch.action.admin.cluster.state.ClusterStateResponse;
 import org.elasticsearch.action.admin.indices.stats.IndicesStatsResponse;
 import org.elasticsearch.action.delete.DeleteResponse;
+import org.elasticsearch.action.index.IndexResponse;
 import org.elasticsearch.action.search.SearchRequest;
 import org.elasticsearch.action.search.SearchResponse;
 import org.elasticsearch.action.search.SearchType;
 import org.elasticsearch.action.support.IndicesOptions;
 import org.elasticsearch.cluster.block.ClusterBlockException;
 import org.elasticsearch.cluster.metadata.IndexMetaData;
+import org.elasticsearch.cluster.routing.RecoverySource;
 import org.elasticsearch.common.Strings;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
@@ -27,6 +29,7 @@
 import org.elasticsearch.index.shard.IndexShard;
 import org.elasticsearch.index.shard.IndexShardTestCase;
 import org.elasticsearch.indices.IndicesService;
+import org.elasticsearch.indices.recovery.RecoveryState;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.search.SearchService;
@@ -49,8 +52,10 @@
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertHitCount;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
 import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.notNullValue;
 
 public class FrozenIndexTests extends ESSingleNodeTestCase {
 
@@ -372,4 +377,36 @@ public void testFreezeEmptyIndexWithTranslogOps() throws Exception {
         assertAcked(new XPackClient(client()).freeze(new TransportFreezeIndexAction.FreezeRequest(indexName)));
         assertIndexFrozen(indexName);
     }
+
+    public void testRecoveryState() throws ExecutionException, InterruptedException {
+        final String indexName = "index_recovery_state";
+        createIndex(indexName, Settings.builder()
+            .put("index.number_of_replicas", 0)
+            .build());
+
+        final long nbDocs = randomIntBetween(0, 50);
+        for (long i = 0; i < nbDocs; i++) {
+            final IndexResponse indexResponse = client().prepareIndex(indexName, "_doc", Long.toString(i)).setSource("field", i).get();
+            assertThat(indexResponse.status(), is(RestStatus.CREATED));
+        }
+
+        assertAcked(new XPackClient(client()).freeze(new TransportFreezeIndexAction.FreezeRequest(indexName)));
+        assertIndexFrozen(indexName);
+
+        final IndexMetaData indexMetaData = client().admin().cluster().prepareState().get().getState().metaData().index(indexName);
+        final IndexService indexService = getInstanceFromNode(IndicesService.class).indexService(indexMetaData.getIndex());
+        for (int i = 0; i < indexMetaData.getNumberOfShards(); i++) {
+            final IndexShard indexShard = indexService.getShardOrNull(i);
+            assertThat("Shard [" + i + "] is missing for index " + indexMetaData.getIndex(), indexShard, notNullValue());
+            final RecoveryState recoveryState = indexShard.recoveryState();
+            assertThat(recoveryState.getRecoverySource(), is(RecoverySource.ExistingStoreRecoverySource.INSTANCE));
+            assertThat(recoveryState.getStage(), is(RecoveryState.Stage.DONE));
+            assertThat(recoveryState.getTargetNode(), notNullValue());
+            assertThat(recoveryState.getIndex().totalFileCount(), greaterThan(0));
+            assertThat(recoveryState.getIndex().reusedFileCount(), greaterThan(0));
+            assertThat(recoveryState.getTranslog().recoveredOperations(), equalTo(0));
+            assertThat(recoveryState.getTranslog().totalOperations(), equalTo(0));
+            assertThat(recoveryState.getTranslog().recoveredPercent(), equalTo(100.0f));
+        }
+    }
 }