[main] [UA] Preserve hidden status of original index during reindexing (

elastic#209512) (elastic#209540)

Close elastic#209471

# Backport

This will backport the following commits from `8.18` to `main`:
- [[UA] Preserve hidden status of original index during reindexing
(elastic#209512)](elastic#209512)

<!--- Backport version: 9.6.4 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Jean-Louis
Leysens","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-02-04T13:20:51Z","message":"[UA]
Preserve hidden status of original index during reindexing
(elastic#209512)","sha":"ed5b521cb491ccb4e1491190f221209de1c2a90f","branchLabelMapping":{"^v8.16.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Team:Core","release_note:skip","Feature:Upgrade
Assistant","backport:skip"],"title":"[UA] Preserve hidden status of
original index during
reindexing","number":209512,"url":"https://github.com/elastic/kibana/pull/209512","mergeCommit":{"message":"[UA]
Preserve hidden status of original index during reindexing
(elastic#209512)","sha":"ed5b521cb491ccb4e1491190f221209de1c2a90f"}},"sourceBranch":"8.18","suggestedTargetBranches":[],"targetPullRequestStates":[]}]
BACKPORT-->

x-pack/platform/plugins/private/upgrade_assistant/server/lib/reindexing/reindex_service.test.ts

@@ -799,18 +799,42 @@ describe('reindexService', () => {
         expect(updatedOp.attributes.lastCompletedStep).toEqual(ReindexStep.aliasCreated);
         expect(clusterClient.asCurrentUser.indices.updateAliases).toHaveBeenCalledWith({
           actions: [
-            { add: { index: 'myIndex-reindex-0', alias: 'myIndex' } },
+            { add: { index: 'myIndex-reindex-0', alias: 'myIndex', is_hidden: false } },
             { remove_index: { index: 'myIndex' } },
           ],
         });
       });
 
+      it.each([true, 'true'])(
+        'switches aliases, passing on hidden status of index to alias if defined (hidden value: %p)',
+        async (hidden) => {
+          clusterClient.asCurrentUser.indices.getAlias.mockResponseOnce({
+            myIndex: { aliases: {} },
+          });
+          clusterClient.asCurrentUser.indices.getSettings.mockResponseOnce({
+            myIndex: { settings: { index: { hidden } } },
+          });
+          clusterClient.asCurrentUser.indices.updateAliases.mockResponseOnce({
+            acknowledged: true,
+          });
+          const updatedOp = await service.processNextStep(reindexOp);
+          expect(updatedOp.attributes.lastCompletedStep).toEqual(ReindexStep.aliasCreated);
+          expect(clusterClient.asCurrentUser.indices.updateAliases).toHaveBeenCalledWith({
+            actions: [
+              { add: { index: 'myIndex-reindex-0', alias: 'myIndex', is_hidden: true } },
+              { remove_index: { index: 'myIndex' } },
+            ],
+          });
+        }
+      );
+
       it('moves existing aliases over to new index', async () => {
         clusterClient.asCurrentUser.indices.getAlias.mockResponseOnce({
           myIndex: {
             aliases: {
               myAlias: {},
               myFilteredAlias: { filter: { term: { https: true } } },
+              myHiddenAlias: { is_hidden: true },
             },
           },
         });
@@ -821,7 +845,7 @@ describe('reindexService', () => {
         expect(updatedOp.attributes.lastCompletedStep).toEqual(ReindexStep.aliasCreated);
         expect(clusterClient.asCurrentUser.indices.updateAliases).toHaveBeenCalledWith({
           actions: [
-            { add: { index: 'myIndex-reindex-0', alias: 'myIndex' } },
+            { add: { index: 'myIndex-reindex-0', alias: 'myIndex', is_hidden: false } },
             { remove_index: { index: 'myIndex' } },
             { add: { index: 'myIndex-reindex-0', alias: 'myAlias' } },
             {
@@ -831,6 +855,13 @@ describe('reindexService', () => {
                 filter: { term: { https: true } },
               },
             },
+            {
+              add: {
+                index: 'myIndex-reindex-0',
+                alias: 'myHiddenAlias',
+                is_hidden: true,
+              },
+            },
           ],
         });
       });

x-pack/platform/plugins/private/upgrade_assistant/server/lib/reindexing/reindex_service.ts

@@ -349,6 +349,12 @@ export const reindexServiceFactory = (
     return response[indexName]?.aliases ?? {};
   };
 
+  const isIndexHidden = async (indexName: string) => {
+    const response = await esClient.indices.getSettings({ index: indexName });
+    const isHidden = response[indexName]?.settings?.index?.hidden;
+    return isHidden === true || isHidden === 'true';
+  };
+
   /**
    * Restores the original index settings in the new index that had other defaults for reindexing performance reasons
    * @param reindexOp
@@ -391,9 +397,11 @@ export const reindexServiceFactory = (
       add: { index: newIndexName, alias: aliasName, ...existingAliases[aliasName] },
     }));
 
+    const isHidden = await isIndexHidden(indexName);
+
     const aliasResponse = await esClient.indices.updateAliases({
       actions: [
-        { add: { index: newIndexName, alias: indexName } },
+        { add: { index: newIndexName, alias: indexName, is_hidden: isHidden } },
         { remove_index: { index: indexName } },
         ...extraAliases,
       ],