Remove auto_expand_replicas setting during index clone in searchable_snapshot (#137111)

In #133954, we modified the `searchable_snapshot` ILM action to clone
the index with 0 replicas before performing the force-merge. We didn't
take the `index.auto_expand_replicas` setting into account, which could
result in the clone having indices after all. That's harmless, as it
merely nullifies the optimization of that PR, but we should remove the
setting to ensure we achieve the intended optimizations.

Author: nielsbauman

docs/changelog/137111.yaml

@@ -0,0 +1,5 @@
+pr: 137111
+summary: Remove `auto_expand_replicas` setting during index clone in `searchable_snapshot`
+area: ILM+SLM
+type: bug
+issues: []

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/SearchableSnapshotAction.java

@@ -78,7 +78,11 @@ public class SearchableSnapshotAction implements LifecycleAction {
         indexName,
         state) -> state.forceMergeCloneIndexName() != null ? state.forceMergeCloneIndexName() : indexName;
 
-    private static final Settings CLONE_SETTINGS = Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0).build();
+    /** The cloned index should have 0 replicas, so we also need to remove the auto_expand_replicas setting if present. */
+    private static final Settings CLONE_SETTINGS = Settings.builder()
+        .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0)
+        .put(IndexMetadata.SETTING_AUTO_EXPAND_REPLICAS, (String) null)
+        .build();
     private static final Function<IndexMetadata, Settings> CLONE_SETTINGS_SUPPLIER = indexMetadata -> CLONE_SETTINGS;
 
     private static final ConstructingObjectParser<SearchableSnapshotAction, Void> PARSER = new ConstructingObjectParser<>(

x-pack/plugin/ilm/qa/multi-node/src/javaRestTest/java/org/elasticsearch/xpack/ilm/actions/SearchableSnapshotActionIT.java

@@ -1033,11 +1033,17 @@ private String prepareDataStreamWithDocs(String phase, int numberOfPrimaries, in
         createSnapshotRepo(client(), snapshotRepo, randomBoolean());
         createNewSingletonPolicy(client(), policy, phase, new SearchableSnapshotAction(snapshotRepo, true));
 
+        final var indexSettings = indexSettings(numberOfPrimaries, numberOfReplicas);
+        // Randomly enable auto-expand replicas to test that we remove the setting for the clone with 0 replicas.
+        if (numberOfReplicas > 0 && randomBoolean()) {
+            logger.info("--> enabling auto-expand replicas on backing index");
+            indexSettings.put(IndexMetadata.SETTING_AUTO_EXPAND_REPLICAS, "1-all");
+        }
         createComposableTemplate(
             client(),
             randomAlphaOfLengthBetween(5, 10).toLowerCase(Locale.ROOT),
             dataStream,
-            new Template(indexSettings(numberOfPrimaries, numberOfReplicas).build(), null, null)
+            new Template(indexSettings.build(), null, null)
         );
         for (int i = 0; i < randomIntBetween(5, 10); i++) {
             indexDocument(client(), dataStream, true);