Expose field mapper type for timestamp field

Author: albertzaharovits

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

@@ -15,6 +15,7 @@
 import org.elasticsearch.index.IndexSettings;
 import org.elasticsearch.index.analysis.IndexAnalyzers;
 import org.elasticsearch.index.analysis.NamedAnalyzer;
+import org.elasticsearch.index.mapper.DateFieldMapper.DateFieldType;
 import org.elasticsearch.inference.InferenceService;
 import org.elasticsearch.search.lookup.SourceFilter;
 
@@ -530,19 +531,18 @@ public boolean isDataStreamTimestampFieldEnabled() {
     }
 
     /**
-     * Returns if this mapping contains a timestamp field that is of type date, has doc values, and is either indexed or uses a doc values
-     * skipper.
-     * @return {@code true} if contains a timestamp field of type date that has doc values and is either indexed or uses a doc values
-     * skipper, {@code false} otherwise.
+     * If this mapping contains a timestamp field that is of type date, has doc values, and is either indexed or uses a doc values
+     * skipper, this returns the field type for it.
      */
-    public boolean hasTimestampField() {
+    public DateFieldType getTimestampFieldType() {
         final MappedFieldType mappedFieldType = fieldTypesLookup().get(DataStream.TIMESTAMP_FIELD_NAME);
-        if (mappedFieldType instanceof DateFieldMapper.DateFieldType dateMappedFieldType) {
+        if (mappedFieldType instanceof DateFieldType dateMappedFieldType) {
             IndexType indexType = dateMappedFieldType.indexType();
-            return indexType.hasPoints() || indexType.hasDocValuesSkipper();
-        } else {
-            return false;
+            if (indexType.hasPoints() || indexType.hasDocValuesSkipper()) {
+                return dateMappedFieldType;
+            }
         }
+        return null;
     }
 
     /**

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

@@ -3842,7 +3842,7 @@ private EngineConfig newEngineConfig(LongSupplier globalCheckpointSupplier) {
                 this.warmer.warm(reader);
             }
         };
-        final boolean isTimeBasedIndex = mapperService == null ? false : mapperService.mappingLookup().hasTimestampField();
+        final boolean isTimeBasedIndex = mapperService == null ? false : mapperService.mappingLookup().getTimestampFieldType() != null;
         return new EngineConfig(
             shardId,
             threadPool,