[RCI] Add NoOpEngine for closed indices (#33903)

This commit adds a new NoOpEngine implementation based on the current 
ReadOnlyEngine. This new implementation uses an empty DirectoryReader 
with no segments readers and will always returns 0 docs. The NoOpEngine 
is the default Engine created for IndexShards of closed indices. It expects 
an empty translog when it is instantiated.

Relates to #33888

Author: tlrx

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

@@ -0,0 +1,149 @@
+/*
+ * 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.index.engine;
+
+import org.apache.lucene.index.DirectoryReader;
+import org.apache.lucene.index.IndexCommit;
+import org.apache.lucene.index.IndexWriter;
+import org.apache.lucene.index.LeafReader;
+import org.apache.lucene.store.Directory;
+import org.elasticsearch.common.Nullable;
+import org.elasticsearch.core.internal.io.IOUtils;
+import org.elasticsearch.index.translog.Translog;
+import org.elasticsearch.index.translog.TranslogConfig;
+import org.elasticsearch.index.translog.TranslogCorruptedException;
+import org.elasticsearch.index.translog.TranslogDeletionPolicy;
+
+import java.io.IOException;
+import java.util.List;
+import java.util.Map;
+import java.util.function.LongSupplier;
+import java.util.stream.Stream;
+
+/**
+ * NoOpEngine is an engine implementation that does nothing but the bare minimum
+ * required in order to have an engine. All attempts to do something (search,
+ * index, get), throw {@link UnsupportedOperationException}. This does maintain
+ * a translog with a deletion policy so that when flushing, no translog is
+ * retained on disk (setting a retention size and age of 0).
+ *
+ * It's also important to notice that this does list the commits of the Store's
+ * Directory so that the last commit's user data can be read for the historyUUID
+ * and last committed segment info.
+ */
+public final class NoOpEngine extends ReadOnlyEngine {
+
+    public NoOpEngine(EngineConfig engineConfig) {
+        super(engineConfig, null, null, true, directoryReader -> directoryReader);
+        boolean success = false;
+        try {
+            // The deletion policy for the translog should not keep any translogs around, so the min age/size is set to -1
+            final TranslogDeletionPolicy translogDeletionPolicy = new TranslogDeletionPolicy(-1, -1);
+
+            // The translog is opened and closed to validate that the translog UUID from lucene is the same as the one in the translog
+            try (Translog translog = openTranslog(engineConfig, translogDeletionPolicy, engineConfig.getGlobalCheckpointSupplier())) {
+                final int nbOperations = translog.totalOperations();
+                if (nbOperations != 0) {
+                    throw new IllegalArgumentException("Expected 0 translog operations but there were " + nbOperations);
+                }
+            }
+            success = true;
+        } catch (IOException | TranslogCorruptedException e) {
+            throw new EngineCreationFailureException(shardId, "failed to create engine", e);
+        } finally {
+            if (success == false) {
+                IOUtils.closeWhileHandlingException(this);
+            }
+        }
+    }
+
+    @Override
+    protected DirectoryReader open(final Directory directory) throws IOException {
+        final List<IndexCommit> indexCommits = DirectoryReader.listCommits(directory);
+        assert indexCommits.size() == 1 : "expected only one commit point";
+        IndexCommit indexCommit = indexCommits.get(indexCommits.size() - 1);
+        return new DirectoryReader(directory, new LeafReader[0]) {
+            @Override
+            protected DirectoryReader doOpenIfChanged() throws IOException {
+                return null;
+            }
+
+            @Override
+            protected DirectoryReader doOpenIfChanged(IndexCommit commit) throws IOException {
+                return null;
+            }
+
+            @Override
+            protected DirectoryReader doOpenIfChanged(IndexWriter writer, boolean applyAllDeletes) throws IOException {
+                return null;
+            }
+
+            @Override
+            public long getVersion() {
+                return 0;
+            }
+
+            @Override
+            public boolean isCurrent() throws IOException {
+                return true;
+            }
+
+            @Override
+            public IndexCommit getIndexCommit() throws IOException {
+                return indexCommit;
+            }
+
+            @Override
+            protected void doClose() throws IOException {
+            }
+
+            @Override
+            public CacheHelper getReaderCacheHelper() {
+                return null;
+            }
+        };
+    }
+
+    private Translog openTranslog(EngineConfig engineConfig, TranslogDeletionPolicy translogDeletionPolicy,
+                                  LongSupplier globalCheckpointSupplier) throws IOException {
+        final TranslogConfig translogConfig = engineConfig.getTranslogConfig();
+        final String translogUUID = loadTranslogUUIDFromLastCommit();
+        // We expect that this shard already exists, so it must already have an existing translog else something is badly wrong!
+        return new Translog(translogConfig, translogUUID, translogDeletionPolicy, globalCheckpointSupplier,
+                engineConfig.getPrimaryTermSupplier());
+    }
+
+    /**
+     * Reads the current stored translog ID from the last commit data.
+     */
+    @Nullable
+    private String loadTranslogUUIDFromLastCommit() {
+        final Map<String, String> commitUserData = getLastCommittedSegmentInfos().getUserData();
+        if (commitUserData.containsKey(Translog.TRANSLOG_GENERATION_KEY) == false) {
+            throw new IllegalStateException("Commit doesn't contain translog generation id");
+        }
+        return commitUserData.get(Translog.TRANSLOG_UUID_KEY);
+    }
+
+    @Override
+    public boolean ensureTranslogSynced(Stream<Translog.Location> locations) {
+        throw new UnsupportedOperationException("Translog synchronization should never be needed");
+    }
+}

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

@@ -57,7 +57,7 @@
  *
  * @see #ReadOnlyEngine(EngineConfig, SeqNoStats, TranslogStats, boolean, Function)
  */
-public final class ReadOnlyEngine extends Engine {
+public class ReadOnlyEngine extends Engine {
 
     private final SegmentInfos lastCommittedSegmentInfos;
     private final SeqNoStats seqNoStats;

server/src/main/java/org/elasticsearch/indices/IndicesService.java

@@ -84,6 +84,7 @@
 import org.elasticsearch.index.engine.CommitStats;
 import org.elasticsearch.index.engine.EngineFactory;
 import org.elasticsearch.index.engine.InternalEngineFactory;
+import org.elasticsearch.index.engine.NoOpEngine;
 import org.elasticsearch.index.fielddata.IndexFieldDataCache;
 import org.elasticsearch.index.flush.FlushStats;
 import org.elasticsearch.index.get.GetStats;
@@ -517,6 +518,12 @@ private synchronized IndexService createIndexService(final String reason,
     }
 
     private EngineFactory getEngineFactory(final IndexSettings idxSettings) {
+        final IndexMetaData indexMetaData = idxSettings.getIndexMetaData();
+        if (indexMetaData != null && indexMetaData.getState() == IndexMetaData.State.CLOSE) {
+            // NoOpEngine takes precedence as long as the index is closed
+            return NoOpEngine::new;
+        }
+
         final List<Optional<EngineFactory>> engineFactories =
                 engineFactoryProviders
                         .stream()