Add a generic method for reading shard field stats from an Engine (elastic#125055)

Support reading shard field stats outside of `IndexShard` in read-only engines.

See ES-11062

Author: arteam

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

@@ -60,6 +60,7 @@
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.index.VersionType;
+import org.elasticsearch.index.codec.FieldInfosWithUsages;
 import org.elasticsearch.index.mapper.DocumentParser;
 import org.elasticsearch.index.mapper.FieldNamesFieldMapper;
 import org.elasticsearch.index.mapper.LuceneDocument;
@@ -76,6 +77,7 @@
 import org.elasticsearch.index.shard.DenseVectorStats;
 import org.elasticsearch.index.shard.DocsStats;
 import org.elasticsearch.index.shard.IndexShard;
+import org.elasticsearch.index.shard.ShardFieldStats;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.index.shard.ShardLongFieldRange;
 import org.elasticsearch.index.shard.SparseVectorStats;
@@ -243,6 +245,34 @@ protected final DocsStats docsStats(IndexReader indexReader) {
         return new DocsStats(numDocs, numDeletedDocs, sizeInBytes);
     }
 
+    /**
+     * @throws AlreadyClosedException if the shard is closed
+     */
+    public ShardFieldStats shardFieldStats() {
+        try (var searcher = acquireSearcher("shard_field_stats", Engine.SearcherScope.INTERNAL)) {
+            return shardFieldStats(searcher.getLeafContexts());
+        }
+    }
+
+    protected static ShardFieldStats shardFieldStats(List<LeafReaderContext> leaves) {
+        int numSegments = 0;
+        int totalFields = 0;
+        long usages = 0;
+        for (LeafReaderContext leaf : leaves) {
+            numSegments++;
+            var fieldInfos = leaf.reader().getFieldInfos();
+            totalFields += fieldInfos.size();
+            if (fieldInfos instanceof FieldInfosWithUsages ft) {
+                if (usages != -1) {
+                    usages += ft.getTotalUsages();
+                }
+            } else {
+                usages = -1;
+            }
+        }
+        return new ShardFieldStats(numSegments, totalFields, usages);
+    }
+
     /**
      * Returns the {@link DenseVectorStats} for this engine
      */

server/src/main/java/org/elasticsearch/index/shard/IndexShard.java

@@ -18,7 +18,6 @@
 import org.apache.lucene.index.FilterDirectoryReader;
 import org.apache.lucene.index.IndexCommit;
 import org.apache.lucene.index.LeafReader;
-import org.apache.lucene.index.LeafReaderContext;
 import org.apache.lucene.index.SegmentInfos;
 import org.apache.lucene.search.QueryCachingPolicy;
 import org.apache.lucene.search.ReferenceManager;
@@ -84,7 +83,6 @@
 import org.elasticsearch.index.cache.query.TrivialQueryCachingPolicy;
 import org.elasticsearch.index.cache.request.ShardRequestCache;
 import org.elasticsearch.index.codec.CodecService;
-import org.elasticsearch.index.codec.FieldInfosWithUsages;
 import org.elasticsearch.index.engine.CommitStats;
 import org.elasticsearch.index.engine.Engine;
 import org.elasticsearch.index.engine.Engine.GetResult;
@@ -4160,26 +4158,9 @@ public void beforeRefresh() {
         @Override
         public void afterRefresh(boolean didRefresh) {
             if (shardFieldStats == null || didRefresh) {
-                try (var searcher = getEngine().acquireSearcher("shard_field_stats", Engine.SearcherScope.INTERNAL)) {
-                    int numSegments = 0;
-                    int totalFields = 0;
-                    long usages = 0;
-                    for (LeafReaderContext leaf : searcher.getLeafContexts()) {
-                        numSegments++;
-                        var fieldInfos = leaf.reader().getFieldInfos();
-                        totalFields += fieldInfos.size();
-                        if (fieldInfos instanceof FieldInfosWithUsages ft) {
-                            if (usages != -1) {
-                                usages += ft.getTotalUsages();
-                            }
-                        } else {
-                            usages = -1;
-                        }
-                    }
-                    shardFieldStats = new ShardFieldStats(numSegments, totalFields, usages);
-                } catch (AlreadyClosedException ignored) {
-
-                }
+                try {
+                    shardFieldStats = getEngine().shardFieldStats();
+                } catch (AlreadyClosedException ignored) {}
             }
         }
     }