Use cached SourceLookup for FieldScript

Signed-off-by: Jay Deng <[email protected]>

Author: jed326

server/src/main/java/org/opensearch/search/lookup/SearchLookup.java

@@ -42,6 +42,7 @@
 import java.util.LinkedHashSet;
 import java.util.Objects;
 import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.BiFunction;
 import java.util.function.Supplier;
 
@@ -77,10 +78,10 @@ public class SearchLookup {
      */
     private final Set<String> fieldChain;
     private final DocLookup docMap;
-    private final SourceLookup sourceLookup;
     private final FieldsLookup fieldsLookup;
     private final BiFunction<MappedFieldType, Supplier<SearchLookup>, IndexFieldData<?>> fieldDataLookup;
     private final int shardId;
+    private final ConcurrentHashMap<Long, SourceLookup> sourceLookupMap = new ConcurrentHashMap<>();
 
     /**
      * Constructor for backwards compatibility. Use the one with explicit shardId argument.
@@ -107,7 +108,6 @@ public SearchLookup(
             mapperService,
             fieldType -> fieldDataLookup.apply(fieldType, () -> forkAndTrackFieldReferences(fieldType.name()))
         );
-        sourceLookup = new SourceLookup();
         fieldsLookup = new FieldsLookup(mapperService);
         this.fieldDataLookup = fieldDataLookup;
         this.shardId = shardId;
@@ -126,7 +126,6 @@ private SearchLookup(SearchLookup searchLookup, Set<String> fieldChain) {
             searchLookup.docMap.mapperService(),
             fieldType -> searchLookup.fieldDataLookup.apply(fieldType, () -> forkAndTrackFieldReferences(fieldType.name()))
         );
-        this.sourceLookup = searchLookup.sourceLookup;
         this.fieldsLookup = searchLookup.fieldsLookup;
         this.fieldDataLookup = searchLookup.fieldDataLookup;
         this.shardId = searchLookup.shardId;
@@ -160,7 +159,9 @@ public LeafSearchLookup getLeafSearchLookup(LeafReaderContext context) {
         return new LeafSearchLookup(
             context,
             docMap.getLeafDocLookup(context),
-            new SourceLookup(),
+            sourceLookupMap.computeIfAbsent(
+                Thread.currentThread().threadId(), K -> new SourceLookup()
+            ),
             fieldsLookup.getLeafFieldsLookup(context)
         );
     }
@@ -173,7 +174,9 @@ public DocLookup doc() {
      * Returned SourceLookup will be unrelated to any created LeafSearchLookups. Instead, use {@link LeafSearchLookup#source()} to access the related {@link SearchLookup}.
      */
     public SourceLookup source() {
-        return sourceLookup;
+        return sourceLookupMap.computeIfAbsent(
+            Thread.currentThread().threadId(), K -> new SourceLookup()
+        );
     }
 
     public int shardId() {