Move building fieldNames set to class constructor (#131722)

This patch moves building the set of fieldName prefixes in the
FallbackSyntheticSourceBlockLoader to the constructor. This set does not
change between invocations (since the fieldName is final), so we can
just do the work once when constructing the BlockLoader
instead of per-document.

Author: jordan-powers

server/src/main/java/org/elasticsearch/index/mapper/FallbackSyntheticSourceBlockLoader.java

@@ -70,19 +70,16 @@ public SortedSetDocValues ordinals(LeafReaderContext context) throws IOException
         throw new UnsupportedOperationException();
     }
 
-    private record IgnoredSourceRowStrideReader<T>(String fieldName, Reader<T> reader) implements RowStrideReader {
-        @Override
-        public void read(int docId, StoredFields storedFields, Builder builder) throws IOException {
-            var ignoredSource = storedFields.storedFields().get(IgnoredSourceFieldMapper.NAME);
-            if (ignoredSource == null) {
-                builder.appendNull();
-                return;
-            }
-
-            Map<String, List<IgnoredSourceFieldMapper.NameValue>> valuesForFieldAndParents = new HashMap<>();
-
-            // Contains name of the field and all its parents
-            Set<String> fieldNames = new HashSet<>() {
+    private static class IgnoredSourceRowStrideReader<T> implements RowStrideReader {
+        // Contains name of the field and all its parents
+        private final Set<String> fieldNames;
+        private final String fieldName;
+        private final Reader<T> reader;
+
+        IgnoredSourceRowStrideReader(String fieldName, Reader<T> reader) {
+            this.fieldName = fieldName;
+            this.reader = reader;
+            this.fieldNames = new HashSet<>() {
                 {
                     add("_doc");
                 }
@@ -97,6 +94,18 @@ public void read(int docId, StoredFields storedFields, Builder builder) throws I
                 fieldNames.add(current.toString());
             }
 
+        }
+
+        @Override
+        public void read(int docId, StoredFields storedFields, Builder builder) throws IOException {
+            var ignoredSource = storedFields.storedFields().get(IgnoredSourceFieldMapper.NAME);
+            if (ignoredSource == null) {
+                builder.appendNull();
+                return;
+            }
+
+            Map<String, List<IgnoredSourceFieldMapper.NameValue>> valuesForFieldAndParents = new HashMap<>();
+
             for (Object value : ignoredSource) {
                 IgnoredSourceFieldMapper.NameValue nameValue = IgnoredSourceFieldMapper.decode(value);
                 if (fieldNames.contains(nameValue.name())) {