[9.0] Preventing ConcurrentModificationException when updating settings for more than one index (#126077)

docs/changelog/126077.yaml

@@ -0,0 +1,6 @@
+pr: 126077
+summary: Preventing `ConcurrentModificationException` when updating settings for more
+  than one index
+area: Indices APIs
+type: bug
+issues: []

server/src/internalClusterTest/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsServiceIT.java

@@ -39,7 +39,8 @@ public void testThatNonDynamicSettingChangesTakeEffect() throws Exception {
          * This test makes sure that when non-dynamic settings are updated that they actually take effect (as opposed to just being set
          * in the cluster state).
          */
-        createIndex("test", Settings.EMPTY);
+        createIndex("test-1", Settings.EMPTY);
+        createIndex("test-2", Settings.EMPTY);
         MetadataUpdateSettingsService metadataUpdateSettingsService = internalCluster().getCurrentMasterNodeInstance(
             MetadataUpdateSettingsService.class
         );

server/src/main/java/org/elasticsearch/cluster/metadata/MetadataUpdateSettingsService.java

@@ -219,7 +219,7 @@ ClusterState execute(ClusterState currentState) {
                         allocationService.getShardRoutingRoleStrategy(),
                         currentState.routingTable()
                     );
-                    for (Index index : openIndices) {
+                    for (Index index : new HashSet<>(openIndices)) {
                         // We only want to take on the expense of reopening all shards for an index if the setting is really changing
                         Settings existingSettings = currentState.getMetadata().index(index).getSettings();
                         boolean needToReopenIndex = false;