Only load FST off heap if we are actually using mmaps for the term dictionary

Given the significant performance impact that NIOFS has when term dicts are
loaded off-heap this change enforces FstLoadMode#AUTO that loads term dicts
off heap only if the underlying index input indicates a memory map.

Relates to elastic#43150

Author: s1monw

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

@@ -67,10 +67,10 @@ public class ReadOnlyEngine extends Engine {
 
     /**
      * Reader attributes used for read only engines. These attributes prevent loading term dictionaries on-heap even if the field is an
-     * ID field.
+     * ID field if we are reading form memory maps.
      */
     public static final Map<String, String> OFF_HEAP_READER_ATTRIBUTES = Collections.singletonMap(BlockTreeTermsReader.FST_MODE_KEY,
-        BlockTreeTermsReader.FSTLoadMode.OFF_HEAP.name());
+        BlockTreeTermsReader.FSTLoadMode.AUTO.name());
     private final SegmentInfos lastCommittedSegmentInfos;
     private final SeqNoStats seqNoStats;
     private final TranslogStats translogStats;

x-pack/plugin/core/src/main/java/org/elasticsearch/snapshots/SourceOnlySnapshot.java

@@ -6,6 +6,7 @@
 package org.elasticsearch.snapshots;
 
 import org.apache.lucene.codecs.Codec;
+import org.apache.lucene.codecs.blocktree.BlockTreeTermsReader;
 import org.apache.lucene.index.CheckIndex;
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.DocValuesType;
@@ -38,7 +39,6 @@
 import org.apache.lucene.util.FixedBitSet;
 import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.core.internal.io.IOUtils;
-import org.elasticsearch.index.engine.ReadOnlyEngine;
 
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
@@ -82,7 +82,7 @@ public synchronized List<String> syncSnapshot(IndexCommit commit) throws IOExcep
         String segmentFileName;
         try (Lock writeLock = targetDirectory.obtainLock(IndexWriter.WRITE_LOCK_NAME);
              StandardDirectoryReader reader = (StandardDirectoryReader) DirectoryReader.open(commit,
-                 ReadOnlyEngine.OFF_HEAP_READER_ATTRIBUTES)) {
+                 Collections.singletonMap(BlockTreeTermsReader.FST_MODE_KEY, BlockTreeTermsReader.FSTLoadMode.OFF_HEAP.name()))) {
             SegmentInfos segmentInfos = reader.getSegmentInfos().clone();
             DirectoryReader wrappedReader = wrapReader(reader);
             List<SegmentCommitInfo> newInfos = new ArrayList<>();

x-pack/plugin/core/src/main/java/org/elasticsearch/snapshots/SourceOnlySnapshotRepository.java

@@ -6,6 +6,7 @@
 package org.elasticsearch.snapshots;
 
 import com.carrotsearch.hppc.cursors.ObjectObjectCursor;
+import org.apache.lucene.codecs.blocktree.BlockTreeTermsReader;
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.IndexCommit;
 import org.apache.lucene.index.SegmentInfos;
@@ -37,6 +38,7 @@
 import java.io.IOException;
 import java.io.UncheckedIOException;
 import java.nio.file.Path;
+import java.util.Collections;
 import java.util.Iterator;
 import java.util.List;
 import java.util.function.Function;
@@ -135,7 +137,8 @@ protected void closeInternal() {
             final long maxDoc = segmentInfos.totalMaxDoc();
             tempStore.bootstrapNewHistory(maxDoc, maxDoc);
             store.incRef();
-            try (DirectoryReader reader = DirectoryReader.open(tempStore.directory(), ReadOnlyEngine.OFF_HEAP_READER_ATTRIBUTES)) {
+            try (DirectoryReader reader = DirectoryReader.open(tempStore.directory(),
+                Collections.singletonMap(BlockTreeTermsReader.FST_MODE_KEY, BlockTreeTermsReader.FSTLoadMode.OFF_HEAP.name()))) {
                 IndexCommit indexCommit = reader.getIndexCommit();
                 super.snapshotShard(tempStore, mapperService, snapshotId, indexId, indexCommit, snapshotStatus);
             } finally {