Add index commit id to searcher

server/src/internalClusterTest/java/org/elasticsearch/indices/state/CloseIndexIT.java

@@ -34,9 +34,11 @@
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.ByteSizeUnit;
 import org.elasticsearch.common.unit.ByteSizeValue;
+import org.elasticsearch.common.util.iterable.Iterables;
 import org.elasticsearch.common.util.set.Sets;
 import org.elasticsearch.index.IndexNotFoundException;
 import org.elasticsearch.index.IndexSettings;
+import org.elasticsearch.index.engine.Engine;
 import org.elasticsearch.index.shard.IndexShard;
 import org.elasticsearch.indices.IndexClosedException;
 import org.elasticsearch.indices.IndicesService;
@@ -472,6 +474,34 @@ public void testResyncPropagatePrimaryTerm() throws Exception {
         }
     }
 
+    public void testCommitIdInSearcher() throws Exception {
+        final String indexName = "test_commit_id";
+        createIndex(indexName, Settings.builder()
+            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, 1)
+            .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
+            .build());
+        indexRandom(randomBoolean(), randomBoolean(), randomBoolean(), IntStream.range(0, randomIntBetween(0, 50))
+            .mapToObj(n -> client().prepareIndex(indexName).setSource("num", n)).collect(toList()));
+        ensureGreen(indexName);
+        assertAcked(client().admin().indices().prepareClose(indexName));
+        assertIndexIsClosed(indexName);
+        ensureGreen(indexName);
+        final String nodeWithPrimary = Iterables.get(internalCluster().nodesInclude(indexName), 0);
+        IndexShard shard = internalCluster().getInstance(IndicesService.class, nodeWithPrimary)
+            .indexService(resolveIndex(indexName)).getShard(0);
+        final String commitId;
+        try (Engine.SearcherSupplier searcherSupplier = shard.acquireSearcherSupplier(randomFrom(Engine.SearcherScope.values()))) {
+            assertNotNull(searcherSupplier.getCommitId());
+            commitId = searcherSupplier.getCommitId();
+        }
+        internalCluster().restartNode(nodeWithPrimary);
+        ensureGreen(indexName);
+        shard = internalCluster().getInstance(IndicesService.class, nodeWithPrimary).indexService(resolveIndex(indexName)).getShard(0);
+        try (Engine.SearcherSupplier searcherSupplier = shard.acquireSearcherSupplier(randomFrom(Engine.SearcherScope.values()))) {
+            assertThat(searcherSupplier.getCommitId(), equalTo(commitId));
+        }
+    }
+
     private static void closeIndices(final String... indices) {
         closeIndices(client().admin().indices().prepareClose(indices));
     }

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

@@ -1205,6 +1205,15 @@ public final void close() {
         protected abstract void doClose();
 
         protected abstract Searcher acquireSearcherInternal(String source);
+
+        /**
+         * Returns a commit id associated with this searcher if it's opened from an index commit; otherwise, return null.
+         * Two searcher with the same commit id must have identical content at the Lucene document level.
+         */
+        @Nullable
+        public String getCommitId() {
+            return null;
+        }
     }
 
     public static final class Searcher extends IndexSearcher implements Releasable {

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

@@ -26,6 +26,7 @@
 import org.apache.lucene.search.ReferenceManager;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.Lock;
+import org.apache.lucene.util.StringHelper;
 import org.elasticsearch.Version;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.index.ElasticsearchDirectoryReader;
@@ -76,6 +77,7 @@ public class ReadOnlyEngine extends Engine {
     private final boolean requireCompleteHistory;
 
     protected volatile TranslogStats translogStats;
+    protected final String commitId;
 
     /**
      * Creates a new ReadOnlyEngine. This ctor can also be used to open a read-only engine on top of an already opened
@@ -107,6 +109,7 @@ public ReadOnlyEngine(EngineConfig config, SeqNoStats seqNoStats, TranslogStats
                 // yet this makes sure nobody else does. including some testing tools that try to be messy
                 indexWriterLock = obtainLock ? directory.obtainLock(IndexWriter.WRITE_LOCK_NAME) : null;
                 this.lastCommittedSegmentInfos = Lucene.readSegmentInfos(directory);
+                this.commitId = StringHelper.idToString(this.lastCommittedSegmentInfos.getId());
                 if (seqNoStats == null) {
                     seqNoStats = buildSeqNoStats(config, lastCommittedSegmentInfos);
                     ensureMaxSeqNoEqualsToGlobalCheckpoint(seqNoStats);
@@ -497,4 +500,25 @@ protected static DirectoryReader openDirectory(Directory directory) throws IOExc
     public CompletionStats completionStats(String... fieldNamePatterns) {
         return completionStatsCache.get(fieldNamePatterns);
     }
+
+    @Override
+    public SearcherSupplier acquireSearcherSupplier(Function<Searcher, Searcher> wrapper, SearcherScope scope) throws EngineException {
+        final SearcherSupplier delegate = super.acquireSearcherSupplier(wrapper, scope);
+        return new SearcherSupplier(Function.identity()) {
+            @Override
+            protected void doClose() {
+                delegate.close();
+            }
+
+            @Override
+            protected Searcher acquireSearcherInternal(String source) {
+                return delegate.acquireSearcherInternal(source);
+            }
+
+            @Override
+            public String getCommitId() {
+                return commitId;
+            }
+        };
+    }
 }

x-pack/plugin/frozen-indices/src/main/java/org/elasticsearch/index/engine/FrozenEngine.java

@@ -193,6 +193,11 @@ public Searcher acquireSearcherInternal(String source) {
             protected void doClose() {
                 store.decRef();
             }
+
+            @Override
+            public String getCommitId() {
+                return commitId;
+            }
         };
     }