[9.3] Fix system index mapping update for reindexed indices after migration (#144782)

docs/changelog/144782.yaml

@@ -0,0 +1,6 @@
+area: Infra/Core
+issues:
+ - 144764
+pr: 144782
+summary: Fix system index mapping update for reindexed indices after migration
+type: bug

server/src/internalClusterTest/java/org/elasticsearch/indices/SystemIndexMappingUpdateServiceIT.java

@@ -9,6 +9,8 @@
 
 package org.elasticsearch.indices;
 
+import org.elasticsearch.action.admin.indices.alias.IndicesAliasesRequest;
+import org.elasticsearch.action.admin.indices.create.CreateIndexRequest;
 import org.elasticsearch.action.admin.indices.mapping.get.GetMappingsRequest;
 import org.elasticsearch.action.admin.indices.mapping.get.GetMappingsResponse;
 import org.elasticsearch.action.admin.indices.settings.get.GetSettingsRequest;
@@ -93,6 +95,40 @@ public void testSystemIndexManagerLeavesNewerMappingsAlone() throws Exception {
         assertBusy(() -> assertMappingsAndSettings(TestSystemIndexDescriptor.getNewMappings()));
     }
 
+    /**
+     * Reproduces a bug where system index mappings are not updated after a major-version reindex migration.
+     * <a href="https://github.com/elastic/elasticsearch/issues/144764"></a>
+     */
+    public void testSystemIndexManagerUpgradesMappingsOfReindexedIndex() throws Exception {
+        internalCluster().startNodes(1);
+
+        // Directly set up the post-migration state: a reindexed concrete index with old mappings,
+        // and the original primary index name as an alias pointing to it.
+        // Use MIGRATE_SYSTEM_INDEX_CAUSE so that TransportCreateIndexAction allows creating a system
+        // index whose name is not the descriptor's primary index, mirroring SystemIndexMigrator.
+        final String reindexedIndexName = PRIMARY_INDEX_NAME + SystemIndices.UPGRADED_INDEX_SUFFIX;
+        assertAcked(
+            indicesAdmin().create(
+                new CreateIndexRequest(reindexedIndexName).cause(SystemIndices.MIGRATE_SYSTEM_INDEX_CAUSE)
+                    .settings(TestSystemIndexDescriptor.SETTINGS)
+                    .mapping(TestSystemIndexDescriptor.getOldMappings())
+            ).actionGet()
+        );
+        ensureGreen(reindexedIndexName);
+        assertAcked(
+            indicesAdmin().aliases(
+                new IndicesAliasesRequest(TEST_REQUEST_TIMEOUT, TEST_REQUEST_TIMEOUT).addAliasAction(
+                    IndicesAliasesRequest.AliasActions.add().index(reindexedIndexName).alias(PRIMARY_INDEX_NAME)
+                ).addAliasAction(IndicesAliasesRequest.AliasActions.add().index(reindexedIndexName).alias(INDEX_NAME))
+            ).actionGet()
+        );
+
+        TestSystemIndexDescriptor.useNewMappings.set(true);
+        triggerClusterStateUpdates();
+
+        assertBusy(() -> assertMappingsAndSettings(TestSystemIndexDescriptor.getNewMappings(), reindexedIndexName));
+    }
+
     /**
      * Ensures that we can clear any blocks that get set on managed system indices.
      *
@@ -119,29 +155,33 @@ private void triggerClusterStateUpdates() {
         indicesAdmin().putTemplate(new PutIndexTemplateRequest(name).patterns(List.of(name))).actionGet();
     }
 
+    private void assertMappingsAndSettings(String expectedMappings) {
+        assertMappingsAndSettings(expectedMappings, PRIMARY_INDEX_NAME);
+    }
+
     /**
      * Fetch the mappings and settings for {@link TestSystemIndexDescriptor#INDEX_NAME} and verify that they match the expected values.
      */
-    private void assertMappingsAndSettings(String expectedMappings) {
+    private void assertMappingsAndSettings(String expectedMappings, String primaryIndexName) {
         final GetMappingsResponse getMappingsResponse = indicesAdmin().getMappings(
             new GetMappingsRequest(TEST_REQUEST_TIMEOUT).indices(INDEX_NAME)
         ).actionGet();
 
         final Map<String, MappingMetadata> mappings = getMappingsResponse.getMappings();
         assertThat(
-            "Expected mappings to contain a key for [" + PRIMARY_INDEX_NAME + "], but found: " + mappings.toString(),
-            mappings.containsKey(PRIMARY_INDEX_NAME),
+            "Expected mappings to contain a key for [" + primaryIndexName + "], but found: " + mappings.toString(),
+            mappings.containsKey(primaryIndexName),
             equalTo(true)
         );
-        final Map<String, Object> sourceAsMap = mappings.get(PRIMARY_INDEX_NAME).getSourceAsMap();
+        final Map<String, Object> sourceAsMap = mappings.get(primaryIndexName).getSourceAsMap();
 
         assertThat(sourceAsMap, equalTo(XContentHelper.convertToMap(XContentType.JSON.xContent(), expectedMappings, false)));
 
         final GetSettingsResponse getSettingsResponse = indicesAdmin().getSettings(
             new GetSettingsRequest(TEST_REQUEST_TIMEOUT).indices(INDEX_NAME)
         ).actionGet();
 
-        final Settings actual = getSettingsResponse.getIndexToSettings().get(PRIMARY_INDEX_NAME);
+        final Settings actual = getSettingsResponse.getIndexToSettings().get(primaryIndexName);
 
         for (String settingName : TestSystemIndexDescriptor.SETTINGS.keySet()) {
             assertThat(actual.get(settingName), equalTo(TestSystemIndexDescriptor.SETTINGS.get(settingName)));

server/src/main/java/org/elasticsearch/indices/SystemIndexMappingUpdateService.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.cluster.ProjectState;
 import org.elasticsearch.cluster.health.ClusterHealthStatus;
 import org.elasticsearch.cluster.health.ClusterIndexHealth;
+import org.elasticsearch.cluster.metadata.IndexAbstraction;
 import org.elasticsearch.cluster.metadata.IndexMetadata;
 import org.elasticsearch.cluster.metadata.MappingMetadata;
 import org.elasticsearch.cluster.metadata.ProjectId;
@@ -249,7 +250,7 @@ public void onFailure(Exception e) {
      * @return a summary of the index state, or <code>null</code> if the index doesn't exist
      */
     static State calculateIndexState(ProjectState state, SystemIndexDescriptor descriptor) {
-        final IndexMetadata indexMetadata = state.metadata().index(descriptor.getPrimaryIndex());
+        IndexMetadata indexMetadata = getSystemIndexMetadata(state, descriptor);
 
         if (indexMetadata == null) {
             return null;
@@ -278,6 +279,21 @@ static State calculateIndexState(ProjectState state, SystemIndexDescriptor descr
         return new State(indexState, indexHealth, isIndexUpToDate, isMappingIsUpToDate);
     }
 
+    private static IndexMetadata getSystemIndexMetadata(ProjectState state, SystemIndexDescriptor descriptor) {
+        String primaryIndexName = descriptor.getPrimaryIndex();
+        ProjectMetadata projectMetadata = state.metadata();
+        IndexMetadata indexMetadata = projectMetadata.index(primaryIndexName);
+        if (indexMetadata == null) {
+            // The primary index name might be an alias pointing to the concrete index
+            // (e.g. ".fleet-agents-7" → ".fleet-agents-7-reindexed-for-9").
+            IndexAbstraction indexAbstraction = projectMetadata.getIndicesLookup().get(primaryIndexName);
+            if (indexAbstraction != null && indexAbstraction.getWriteIndex() != null) {
+                indexMetadata = projectMetadata.getIndexSafe(indexAbstraction.getWriteIndex());
+            }
+        }
+        return indexMetadata;
+    }
+
     /**
      * Checks whether an index's mappings are up-to-date. If an index is encountered that has
      * a version higher than Version.CURRENT, it is still considered up-to-date.

server/src/test/java/org/elasticsearch/indices/SystemIndexMappingUpdateServiceTests.java

@@ -286,6 +286,42 @@ public void testManagerSubmitsPutRequest() {
         verify(client, times(1)).execute(same(TransportPutMappingAction.TYPE), any(PutMappingRequest.class), any());
     }
 
+    /**
+     * Reproduces a bug where SystemIndexMappingUpdateService incorrectly reports UP_TO_DATE for an index whose mappings are
+     * outdated after a major-version migration (reindex).
+     */
+    public void testManagerDetectsOutdatedMappingsInReindexedIndex() {
+        final String reindexedIndexName = SYSTEM_INDEX_NAME + SystemIndices.UPGRADED_INDEX_SUFFIX;
+        final var projectId = randomProjectIdOrDefault();
+
+        // Simulate the post-migration state: the concrete index has the reindexed suffix and an
+        // outdated managed_index_mappings_version, while the original primary index name and the
+        // alias are both aliases pointing to the new concrete index.
+        IndexMetadata.Builder reindexedIndexMeta = IndexMetadata.builder(reindexedIndexName)
+            .settings(Settings.builder().put(getSettings()).put(IndexMetadata.INDEX_FORMAT_SETTING.getKey(), 6))
+            .putMapping(Strings.toString(getMappings("1.0.0", 4)))
+            .putAlias(AliasMetadata.builder(SYSTEM_INDEX_NAME).build())
+            .putAlias(AliasMetadata.builder(DESCRIPTOR.getAliasName()).build());
+
+        final Metadata metadata = Metadata.builder()
+            .generateClusterUuidIfNeeded()
+            .put(ProjectMetadata.builder(projectId).put(reindexedIndexMeta))
+            .build();
+        final DiscoveryNode node = DiscoveryNodeUtils.builder("1").roles(new HashSet<>(DiscoveryNodeRole.roles())).build();
+        final DiscoveryNodes nodes = DiscoveryNodes.builder().add(node).masterNodeId(node.getId()).localNodeId(node.getId()).build();
+        final ClusterState clusterState = ClusterState.builder(CLUSTER_NAME)
+            .nodes(nodes)
+            .metadata(metadata)
+            .routingTable(GlobalRoutingTableTestHelper.buildRoutingTable(metadata, RoutingTable.Builder::addAsNew))
+            .build();
+        ProjectState projectState = clusterState.projectState(projectId);
+
+        assertThat(
+            SystemIndexMappingUpdateService.getUpgradeStatus(projectState, DESCRIPTOR),
+            equalTo(UpgradeStatus.NEEDS_MAPPINGS_UPDATE)
+        );
+    }
+
     /**
      * Check that this
      */