Reduce synchronization on field data cache

The field data cache can come under heavy contention in cases when lots
of search threads are hitting it for doc values. This commit reduces the
amount of contention here by using a double-checked locking strategy to
only lock when the cache needs to be initialized.

Relates #27365

core/src/main/java/org/elasticsearch/index/fielddata/IndexFieldDataService.java

@@ -110,19 +110,24 @@ public <IFD extends IndexFieldData<?>> IFD getForField(MappedFieldType fieldType
         final String fieldName = fieldType.name();
         IndexFieldData.Builder builder = fieldType.fielddataBuilder();
 
-        IndexFieldDataCache cache;
-        synchronized (this) {
-            cache = fieldDataCaches.get(fieldName);
-            if (cache == null) {
-                String cacheType = indexSettings.getValue(INDEX_FIELDDATA_CACHE_KEY);
-                if (FIELDDATA_CACHE_VALUE_NODE.equals(cacheType)) {
-                    cache = indicesFieldDataCache.buildIndexFieldDataCache(listener, index(), fieldName);
-                } else if ("none".equals(cacheType)){
-                    cache = new IndexFieldDataCache.None();
-                } else {
-                    throw new IllegalArgumentException("cache type not supported [" + cacheType + "] for field [" + fieldName + "]");
+        IndexFieldDataCache cache = fieldDataCaches.get(fieldName);
+        if (cache == null) {
+            //for perf reason, only synchronize when cache is null
+            synchronized (this) {
+                cache = fieldDataCaches.get(fieldName);
+                //double checked locking to make sure it is thread safe
+                //especially when other threads calling clear() or clearField()
+                if (cache == null) {
+                    String cacheType = indexSettings.getValue(INDEX_FIELDDATA_CACHE_KEY);
+                    if (FIELDDATA_CACHE_VALUE_NODE.equals(cacheType)) {
+                        cache = indicesFieldDataCache.buildIndexFieldDataCache(listener, index(), fieldName);
+                    } else if ("none".equals(cacheType)){
+                        cache = new IndexFieldDataCache.None();
+                    } else {
+                        throw new IllegalArgumentException("cache type not supported [" + cacheType + "] for field [" + fieldName + "]");
+                    }
+                    fieldDataCaches.put(fieldName, cache);
                 }
-                fieldDataCaches.put(fieldName, cache);
             }
         }