Fix get field mapping API returns 404 error in mixed cluster with mul…

…tiple versions (#13624) (#13625)

* Fix get field mapping API returns 404 error in mixed cluster with multiple versions



* modify changelog



* Add comment and remove unit test



---------


(cherry picked from commit f28e673)

Signed-off-by: Gao Binlong <gbinlong@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

CHANGELOG.md

@@ -23,6 +23,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 
 ### Fixed
 - Fix negative RequestStats metric issue ([#13553](https://github.com/opensearch-project/OpenSearch/pull/13553))
+- Fix get field mapping API returns 404 error in mixed cluster with multiple versions ([#13624](https://github.com/opensearch-project/OpenSearch/pull/13624))
 
 ### Security
 

server/src/main/java/org/opensearch/action/admin/indices/mapping/get/GetFieldMappingsResponse.java

@@ -52,6 +52,7 @@
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
@@ -116,6 +117,11 @@ public class GetFieldMappingsResponse extends ActionResponse implements ToXConte
             String index = in.readString();
             if (in.getVersion().before(Version.V_2_0_0)) {
                 int typesSize = in.readVInt();
+                // if the requested field doesn't exist, type size in the received response from 1.x node is 0
+                if (typesSize == 0) {
+                    indexMapBuilder.put(index, Collections.emptyMap());
+                    continue;
+                }
                 if (typesSize != 1) {
                     throw new IllegalStateException("Expected single type but received [" + typesSize + "]");
                 }