[Upgrade Assistant] Avoid creating the new index if it already exists when reindexing

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

@@ -191,15 +191,26 @@ export const reindexServiceFactory = (
 
     const { settings, mappings } = transformFlatSettings(flatSettings);
 
-    const { body: createIndex } = await esClient.indices.create({
-      index: newIndexName,
-      body: {
-        settings,
-        mappings,
-      },
-    });
+    let createIndex;
+    try {
+      createIndex = await esClient.indices.create({
+        index: newIndexName,
+        body: {
+          settings,
+          mappings,
+        },
+      });
+    } catch (err) {
+      // If for any reason the new index name generated by the `generateNewIndexName` already
+      // exists (this could happen if kibana is restarted during reindexing), we can just go
+      // ahead with the process without needing to create the index again.
+      // See: https://github.com/elastic/kibana/issues/123816
+      if (err?.body?.error?.type !== 'resource_already_exists_exception') {
+        throw err;
+      }
+    }
 
-    if (!createIndex.acknowledged) {
+    if (createIndex && !createIndex?.body?.acknowledged) {
       throw error.cannotCreateIndex(`Index could not be created: ${newIndexName}`);
     }
 

x-pack/test/upgrade_assistant_integration/upgrade_assistant/reindexing.js

@@ -83,6 +83,32 @@ export default function ({ getService }) {
       });
     });
 
+    it('can resume after reindexing was stopped right after creating the new index', async () => {
+      await esArchiver.load('x-pack/test/functional/es_archives/upgrade_assistant/reindex');
+
+      // This new index is the new soon to be created reindexed index. We create it
+      // upfront to simulate a situation in which the user restarted kibana half
+      // way through the reindex process and ended up with an extra index.
+      await es.indices.create({ index: 'reindexed-v7-dummydata' });
+
+      const { body } = await supertest
+        .post(`/api/upgrade_assistant/reindex/dummydata`)
+        .set('kbn-xsrf', 'xxx')
+        .expect(200);
+
+      expect(body.indexName).to.equal('dummydata');
+      expect(body.status).to.equal(ReindexStatus.inProgress);
+
+      const lastState = await waitForReindexToComplete('dummydata');
+      expect(lastState.errorMessage).to.equal(null);
+      expect(lastState.status).to.equal(ReindexStatus.completed);
+
+      // Cleanup newly created index
+      await es.indices.delete({
+        index: lastState.newIndexName,
+      });
+    });
+
     it('should update any aliases', async () => {
       await esArchiver.load('x-pack/test/functional/es_archives/upgrade_assistant/reindex');