Accommodate hollow engine changes (elastic#129535)

* Field infos calculation method inside Engine
* buildSeqNoStats as static public method

So it can be overriden in stateless if/as needed.

Relates ES-11457

Author: kingherc

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

@@ -262,6 +262,15 @@ public ShardFieldStats shardFieldStats() {
         }
     }
 
+    /**
+     * @throws AlreadyClosedException if the shard is closed
+     */
+    public FieldInfos shardFieldInfos() {
+        try (var searcher = acquireSearcher("field_has_value")) {
+            return FieldInfos.getMergedFieldInfos(searcher.getIndexReader());
+        }
+    }
+
     protected static ShardFieldStats shardFieldStats(List<LeafReaderContext> leaves) {
         int numSegments = 0;
         int totalFields = 0;
@@ -2407,6 +2416,13 @@ public final EngineConfig getEngineConfig() {
         return engineConfig;
     }
 
+    public static SeqNoStats buildSeqNoStats(EngineConfig config, SegmentInfos infos) {
+        final SequenceNumbers.CommitInfo seqNoStats = SequenceNumbers.loadSeqNoInfoFromLuceneCommit(infos.userData.entrySet());
+        long maxSeqNo = seqNoStats.maxSeqNo();
+        long localCheckpoint = seqNoStats.localCheckpoint();
+        return new SeqNoStats(maxSeqNo, localCheckpoint, config.getGlobalCheckpointSupplier().getAsLong());
+    }
+
     /**
      * Allows registering a listener for when the index shard is on a segment generation >= minGeneration.
      *

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

@@ -243,13 +243,6 @@ protected void closeNoLock(String reason, CountDownLatch closedLatch) {
         }
     }
 
-    private static SeqNoStats buildSeqNoStats(EngineConfig config, SegmentInfos infos) {
-        final SequenceNumbers.CommitInfo seqNoStats = SequenceNumbers.loadSeqNoInfoFromLuceneCommit(infos.userData.entrySet());
-        long maxSeqNo = seqNoStats.maxSeqNo();
-        long localCheckpoint = seqNoStats.localCheckpoint();
-        return new SeqNoStats(maxSeqNo, localCheckpoint, config.getGlobalCheckpointSupplier().getAsLong());
-    }
-
     private static TranslogStats translogStats(final EngineConfig config, final SegmentInfos infos) throws IOException {
         assert config.getTranslogConfig().hasTranslog();
         final String translogUuid = infos.getUserData().get(Translog.TRANSLOG_UUID_KEY);

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

@@ -4308,9 +4308,9 @@ public void afterRefresh(boolean didRefresh) {
     }
 
     private FieldInfos loadFieldInfos() {
-        try (Engine.Searcher hasValueSearcher = getEngine().acquireSearcher("field_has_value")) {
-            return FieldInfos.getMergedFieldInfos(hasValueSearcher.getIndexReader());
-        } catch (AlreadyClosedException ignore) {
+        try {
+            return getEngine().shardFieldInfos();
+        } catch (AlreadyClosedException ignored) {
             // engine is closed - no update to FieldInfos is fine
         }
         return FieldInfos.EMPTY;