Merge branch '6.x' into ccr-6.x

* 6.x:
  [DOCS] Fixes kibana security file location
  Fix synced flush docs
  REST high-level client: add synced flush API (2) (#30650)
  stable filemode for zip distributions (#30854)
  Fix index_prefixes cross-type compatibility check (#30956)
  Add deprecation notice for missing option
  Add missing_bucket option in the composite agg (#29465)
  Amend skip version for snapshot REST test Relates #18543
  Reintroduces SSL tests (#30947)
  Fsync state file before exposing it (#30929)
  Rename index_prefix to index_prefixes (#30932)

client/rest-high-level/src/main/java/org/elasticsearch/client/IndicesClient.java

@@ -34,6 +34,7 @@
 import org.elasticsearch.action.admin.indices.delete.DeleteIndexResponse;
 import org.elasticsearch.action.admin.indices.flush.FlushRequest;
 import org.elasticsearch.action.admin.indices.flush.FlushResponse;
+import org.elasticsearch.action.admin.indices.flush.SyncedFlushRequest;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeRequest;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeResponse;
 import org.elasticsearch.action.admin.indices.get.GetIndexRequest;
@@ -269,6 +270,28 @@ public void flushAsync(FlushRequest flushRequest, ActionListener<FlushResponse>
                 listener, emptySet(), headers);
     }
 
+    /** Initiate a synced flush manually using the synced flush API
+      * <p>
+      * See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-synced-flush.html">
+      *     Synced flush API on elastic.co</a>
+      */
+    public SyncedFlushResponse flushSynced(SyncedFlushRequest syncedFlushRequest, Header... headers) throws IOException {
+        return restHighLevelClient.performRequestAndParseEntity(syncedFlushRequest, RequestConverters::flushSynced,
+                SyncedFlushResponse::fromXContent, emptySet(), headers);
+    }
+
+    /**
+     * Asynchronously initiate a synced flush manually using the synced flush API
+     * <p>
+     * See <a href="https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-synced-flush.html">
+     *     Synced flush API on elastic.co</a>
+     */
+    public void flushSyncedAsync(SyncedFlushRequest syncedFlushRequest, ActionListener<SyncedFlushResponse> listener, Header... headers) {
+        restHighLevelClient.performRequestAsyncAndParseEntity(syncedFlushRequest, RequestConverters::flushSynced,
+                SyncedFlushResponse::fromXContent, listener, emptySet(), headers);
+    }
+
+
     /**
      * Retrieve the settings of one or more indices
      * <p>

client/rest-high-level/src/main/java/org/elasticsearch/client/RequestConverters.java

@@ -41,6 +41,7 @@
 import org.elasticsearch.action.admin.indices.create.CreateIndexRequest;
 import org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest;
 import org.elasticsearch.action.admin.indices.flush.FlushRequest;
+import org.elasticsearch.action.admin.indices.flush.SyncedFlushRequest;
 import org.elasticsearch.action.admin.indices.forcemerge.ForceMergeRequest;
 import org.elasticsearch.action.admin.indices.get.GetIndexRequest;
 import org.elasticsearch.action.admin.indices.mapping.put.PutMappingRequest;
@@ -214,6 +215,14 @@ static Request flush(FlushRequest flushRequest) {
         return request;
     }
 
+    static Request flushSynced(SyncedFlushRequest syncedFlushRequest) {
+        String[] indices = syncedFlushRequest.indices() == null ? Strings.EMPTY_ARRAY : syncedFlushRequest.indices();
+        Request request = new Request(HttpPost.METHOD_NAME, endpoint(indices, "_flush/synced"));
+        Params parameters = new Params(request);
+        parameters.withIndicesOptions(syncedFlushRequest.indicesOptions());
+        return request;
+    }
+
     static Request forceMerge(ForceMergeRequest forceMergeRequest) {
         String[] indices = forceMergeRequest.indices() == null ? Strings.EMPTY_ARRAY : forceMergeRequest.indices();
         Request request = new Request(HttpPost.METHOD_NAME, endpoint(indices, "_forcemerge"));