When a primary shard uses the read-only engine, it always returns a RefreshResult.NO_REFRESH for refreshes. Since elastic#93600 we added an extra roundtrip to hook unpromotable shard refresh logic. This hook is always executed, even if there are no unpromotable shards, but the UnpromotableShardRefreshRequest would fail if the primary shard returns a RefreshResult.NO_REFRESH result. (elastic#129338)

tlrx · web-flow · commit 16f2d81eb70a · 2025-06-13T00:38:51.000+10:00
Fix to be backported to several versions as it's annoying. Closes elastic#129036 Backport of elastic#129176 for 8.18.3
diff --git a/build-tools-internal/src/main/resources/changelog-schema.json b/build-tools-internal/src/main/resources/changelog-schema.json
@@ -86,6 +86,7 @@
             "Rollup",
             "SQL",
             "Search",
+            "Searchable Snapshots",
             "Security",
             "Snapshot/Restore",
             "Stats",
diff --git a/docs/changelog/129176.yaml b/docs/changelog/129176.yaml
@@ -0,0 +1,6 @@
+pr: 129176
+summary: Adjust unpromotable shard refresh request validation to allow `RefreshResult.NO_REFRESH`
+area: Searchable Snapshots
+type: bug
+issues:
+ - 129036
diff --git a/server/src/main/java/org/elasticsearch/action/admin/indices/refresh/TransportShardRefreshAction.java b/server/src/main/java/org/elasticsearch/action/admin/indices/refresh/TransportShardRefreshAction.java
@@ -118,17 +118,13 @@ public void onPrimaryOperationComplete(
             IndexShardRoutingTable indexShardRoutingTable,
             ActionListener<Void> listener
         ) {
-            assert replicaRequest.primaryRefreshResult.refreshed() : "primary has not refreshed";
-            UnpromotableShardRefreshRequest unpromotableReplicaRequest = new UnpromotableShardRefreshRequest(
-                indexShardRoutingTable,
-                replicaRequest.primaryRefreshResult.primaryTerm(),
-                replicaRequest.primaryRefreshResult.generation(),
-                false
-            );
+            var primaryTerm = replicaRequest.primaryRefreshResult.primaryTerm();
+            var generation = replicaRequest.primaryRefreshResult.generation();
+
             transportService.sendRequest(
                 transportService.getLocalNode(),
                 TransportUnpromotableShardRefreshAction.NAME,
-                unpromotableReplicaRequest,
+                new UnpromotableShardRefreshRequest(indexShardRoutingTable, primaryTerm, generation, false),
                 new ActionListenerResponseHandler<>(listener.safeMap(r -> null), in -> ActionResponse.Empty.INSTANCE, refreshExecutor)
             );
         }
diff --git a/server/src/main/java/org/elasticsearch/action/admin/indices/refresh/TransportUnpromotableShardRefreshAction.java b/server/src/main/java/org/elasticsearch/action/admin/indices/refresh/TransportUnpromotableShardRefreshAction.java
@@ -16,6 +16,7 @@
 import org.elasticsearch.cluster.action.shard.ShardStateAction;
 import org.elasticsearch.cluster.service.ClusterService;
 import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.index.engine.Engine;
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.injection.guice.Inject;
 import org.elasticsearch.tasks.Task;
@@ -73,12 +74,13 @@ protected void unpromotableShardOperation(
             return;
         }
 
+        var primaryTerm = request.getPrimaryTerm();
+        assert Engine.UNKNOWN_PRIMARY_TERM < primaryTerm : primaryTerm;
+        var segmentGeneration = request.getSegmentGeneration();
+        assert Engine.RefreshResult.UNKNOWN_GENERATION < segmentGeneration : segmentGeneration;
+
         ActionListener.run(responseListener, listener -> {
-            shard.waitForPrimaryTermAndGeneration(
-                request.getPrimaryTerm(),
-                request.getSegmentGeneration(),
-                listener.map(l -> ActionResponse.Empty.INSTANCE)
-            );
+            shard.waitForPrimaryTermAndGeneration(primaryTerm, segmentGeneration, listener.map(l -> ActionResponse.Empty.INSTANCE));
         });
     }
 
diff --git a/server/src/main/java/org/elasticsearch/action/admin/indices/refresh/UnpromotableShardRefreshRequest.java b/server/src/main/java/org/elasticsearch/action/admin/indices/refresh/UnpromotableShardRefreshRequest.java
@@ -47,7 +47,9 @@ public UnpromotableShardRefreshRequest(StreamInput in) throws IOException {
     @Override
     public ActionRequestValidationException validate() {
         ActionRequestValidationException validationException = super.validate();
-        if (segmentGeneration == Engine.RefreshResult.UNKNOWN_GENERATION) {
+        if (segmentGeneration == Engine.RefreshResult.UNKNOWN_GENERATION && primaryTerm == Engine.UNKNOWN_PRIMARY_TERM) {
+            // read-only primary shards (like searchable snapshot shard) return Engine.RefreshResult.NO_REFRESH during refresh
+        } else if (segmentGeneration == Engine.RefreshResult.UNKNOWN_GENERATION) {
             validationException = addValidationError("segment generation is unknown", validationException);
         }
         return validationException;
diff --git a/server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java b/server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java
@@ -445,7 +445,7 @@ public RefreshResult refresh(String source) {
 
     @Override
     public void maybeRefresh(String source, ActionListener<RefreshResult> listener) throws EngineException {
-        listener.onResponse(RefreshResult.NO_REFRESH);
+        ActionListener.completeWith(listener, () -> refresh(source));
     }
 
     @Override
diff --git a/x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/FrozenSearchableSnapshotsIntegTests.java b/x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/FrozenSearchableSnapshotsIntegTests.java
@@ -71,6 +71,7 @@
 import static org.elasticsearch.search.aggregations.AggregationBuilders.dateHistogram;
 import static org.elasticsearch.snapshots.SearchableSnapshotsSettings.SEARCHABLE_SNAPSHOT_STORE_TYPE;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
+import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertNoFailures;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertNoFailuresAndResponse;
 import static org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshots.SNAPSHOT_RECOVERY_STATE_FACTORY_KEY;
 import static org.hamcrest.Matchers.arrayWithSize;
@@ -542,6 +543,37 @@ public void testRequestCacheOnFrozen() throws Exception {
         }
     }
 
+    public void testRefreshPartiallyMountedIndex() throws Exception {
+        internalCluster().ensureAtLeastNumDataNodes(2);
+
+        final var index = "index";
+        createIndex(index, 1, 0);
+        populateIndex(index, 1_000);
+
+        final var repository = "repository";
+        createRepository(repository, FsRepository.TYPE, Settings.builder().put("location", randomRepoPath()));
+
+        final var snapshot = "repository";
+        createFullSnapshot(repository, snapshot);
+
+        assertAcked(indicesAdmin().prepareDelete(index));
+
+        final var partialIndex = "partial-" + index;
+        mountSnapshot(
+            repository,
+            snapshot,
+            index,
+            partialIndex,
+            Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, randomInt(1)).build(),
+            MountSearchableSnapshotRequest.Storage.SHARED_CACHE
+        );
+        ensureGreen(partialIndex);
+
+        // before the fix this would have failed
+        var refreshResult = indicesAdmin().prepareRefresh(partialIndex).execute().actionGet();
+        assertNoFailures(refreshResult);
+    }
+
     public void testTierPreferenceCannotBeRemovedForFrozenIndex() throws Exception {
         final String fsRepoName = randomAlphaOfLength(10);
         final String indexName = randomAlphaOfLength(10).toLowerCase(Locale.ROOT);
diff --git a/x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/SearchableSnapshotsIntegTests.java b/x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/SearchableSnapshotsIntegTests.java
@@ -51,6 +51,7 @@
 import org.elasticsearch.index.shard.IndexLongFieldRange;
 import org.elasticsearch.indices.IndicesService;
 import org.elasticsearch.repositories.RepositoryData;
+import org.elasticsearch.repositories.fs.FsRepository;
 import org.elasticsearch.rest.RestStatus;
 import org.elasticsearch.search.SearchResponseUtils;
 import org.elasticsearch.snapshots.SnapshotId;
@@ -89,6 +90,7 @@
 import static org.elasticsearch.snapshots.SearchableSnapshotsSettings.SEARCHABLE_SNAPSHOT_STORE_TYPE;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAcked;
 import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertHitCount;
+import static org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertNoFailures;
 import static org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshots.SNAPSHOT_RECOVERY_STATE_FACTORY_KEY;
 import static org.hamcrest.Matchers.allOf;
 import static org.hamcrest.Matchers.containsString;
@@ -1274,6 +1276,37 @@ public void onFailure(Exception e) {
         }
     }
 
+    public void testRefreshFullyMountedIndex() throws Exception {
+        internalCluster().ensureAtLeastNumDataNodes(2);
+
+        final var index = "index";
+        createIndex(index, 1, 0);
+        populateIndex(index, 1_000);
+
+        final var repository = "repository";
+        createRepository(repository, FsRepository.TYPE, Settings.builder().put("location", randomRepoPath()));
+
+        final var snapshot = "repository";
+        createFullSnapshot(repository, snapshot);
+
+        assertAcked(indicesAdmin().prepareDelete(index));
+
+        final var fullIndex = "full-" + index;
+        mountSnapshot(
+            repository,
+            snapshot,
+            index,
+            fullIndex,
+            Settings.builder().put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, randomInt(1)).build(),
+            MountSearchableSnapshotRequest.Storage.FULL_COPY
+        );
+        ensureGreen(fullIndex);
+
+        // before the fix this would have failed
+        var refreshResult = indicesAdmin().prepareRefresh(fullIndex).execute().actionGet();
+        assertNoFailures(refreshResult);
+    }
+
     private TaskInfo getTaskForActionFromMaster(String action) {
         ListTasksResponse response = client().execute(
             TransportListTasksAction.TYPE,

Original file line number	Diff line number	Diff line change
`@@ -445,7 +445,7 @@ public RefreshResult refresh(String source) {`
`445`	`445`
`446`	`446`	`@Override`
`447`	`447`	`public void maybeRefresh(String source, ActionListener<RefreshResult> listener) throws EngineException {`
`448`		`- listener.onResponse(RefreshResult.NO_REFRESH);`
	`448`	`+ ActionListener.completeWith(listener, () -> refresh(source));`
`449`	`449`	`}`
`450`	`450`
`451`	`451`	`@Override`