Reject remounting snapshot of a searchable snapshot (elastic#68816)

Today you can mount a snapshot of a searchable snapshot index, but the
shard fails to allocate since the underlying snapshot is devoid of
content. Doing this is a mistake, you probably meant to restore the
index instead, so this commit rejects it earlier with a more helpful
message.

Closes elastic#68792

server/src/main/java/org/elasticsearch/index/store/Store.java

@@ -1376,7 +1376,7 @@ public void markStoreCorrupted(IOException exception) throws IOException {
                 BytesRef ref = bytes.toBytesRef();
                 output.writeBytes(ref.bytes, ref.offset, ref.length);
                 CodecUtil.writeFooter(output);
-            } catch (IOException ex) {
+            } catch (IOException | ImmutableDirectoryException ex) {
                 logger.warn("Can't mark store as corrupted", ex);
             }
             directory().sync(Collections.singleton(corruptionMarkerName));

x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/SearchableSnapshotsIntegTests.java

@@ -9,6 +9,7 @@
 import com.carrotsearch.hppc.cursors.ObjectCursor;
 import org.apache.lucene.index.IndexFileNames;
 import org.apache.lucene.search.TotalHits;
+import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.ResourceNotFoundException;
 import org.elasticsearch.action.admin.cluster.snapshots.restore.RestoreSnapshotResponse;
 import org.elasticsearch.action.admin.cluster.snapshots.status.SnapshotIndexShardStatus;
@@ -98,7 +99,9 @@
 import static org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshots.getDataTiersPreference;
 import static org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshotsConstants.SNAPSHOT_DIRECTORY_FACTORY_KEY;
 import static org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshotsConstants.SNAPSHOT_RECOVERY_STATE_FACTORY_KEY;
+import static org.hamcrest.Matchers.allOf;
 import static org.hamcrest.Matchers.anyOf;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
 import static org.hamcrest.Matchers.greaterThanOrEqualTo;
@@ -1292,6 +1295,34 @@ public void testSnapshotOfSearchableSnapshotIncludesNoDataButCanBeRestored() thr
         logger.info("--> finished restoring snapshot-2");
 
         assertTotalHits(restoredIndexName, originalAllHits, originalBarHits);
+
+        final IllegalArgumentException remountException = expectThrows(IllegalArgumentException.class, () -> {
+            try {
+                mountSnapshot(
+                    restoreRepositoryName,
+                    snapshotTwo.getName(),
+                    restoredIndexName,
+                    randomAlphaOfLength(10).toLowerCase(Locale.ROOT),
+                    Settings.EMPTY
+                );
+            } catch (Exception e) {
+                final Throwable cause = ExceptionsHelper.unwrap(e, IllegalArgumentException.class);
+                throw cause == null ? e : cause;
+            }
+        });
+        assertThat(
+            remountException.getMessage(),
+            allOf(
+                containsString("is a snapshot of a searchable snapshot index backed by index"),
+                containsString(repositoryName),
+                containsString(snapshotOne.getName()),
+                containsString(indexName),
+                containsString(restoreRepositoryName),
+                containsString(snapshotTwo.getName()),
+                containsString(restoredIndexName),
+                containsString("cannot be mounted; did you mean to restore it instead?")
+            )
+        );
     }
 
     private void assertTotalHits(String indexName, TotalHits originalAllHits, TotalHits originalBarHits) throws Exception {

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/action/TransportMountSearchableSnapshotAction.java

@@ -42,13 +42,15 @@
 import org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshotsConstants;
 
 import java.util.Arrays;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Objects;
 import java.util.Optional;
 
 import static org.elasticsearch.index.IndexModule.INDEX_RECOVERY_TYPE_SETTING;
 import static org.elasticsearch.index.IndexModule.INDEX_STORE_TYPE_SETTING;
 import static org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshots.getDataTiersPreference;
+import static org.elasticsearch.xpack.searchablesnapshots.SearchableSnapshotsConstants.isSearchableSnapshotStore;
 
 /**
  * Action that mounts a snapshot as a searchable snapshot, by converting the mount request into a restore request with specific settings
@@ -177,6 +179,25 @@ protected void masterOperation(
             final String[] ignoreIndexSettings = Arrays.copyOf(request.ignoreIndexSettings(), request.ignoreIndexSettings().length + 1);
             ignoreIndexSettings[ignoreIndexSettings.length - 1] = IndexMetadata.SETTING_DATA_PATH;
 
+            final IndexMetadata indexMetadata = repository.getSnapshotIndexMetaData(repoData, snapshotId, indexId);
+            if (isSearchableSnapshotStore(indexMetadata.getSettings())) {
+                throw new IllegalArgumentException(
+                    String.format(
+                        Locale.ROOT,
+                        "index [%s] in snapshot [%s/%s:%s] is a snapshot of a searchable snapshot index "
+                            + "backed by index [%s] in snapshot [%s/%s:%s] and cannot be mounted; did you mean to restore it instead?",
+                        indexName,
+                        repoName,
+                        repository.getMetadata().uuid(),
+                        snapName,
+                        SearchableSnapshots.SNAPSHOT_INDEX_NAME_SETTING.get(indexMetadata.getSettings()),
+                        SearchableSnapshots.SNAPSHOT_REPOSITORY_NAME_SETTING.get(indexMetadata.getSettings()),
+                        SearchableSnapshots.SNAPSHOT_REPOSITORY_UUID_SETTING.get(indexMetadata.getSettings()),
+                        SearchableSnapshots.SNAPSHOT_SNAPSHOT_NAME_SETTING.get(indexMetadata.getSettings())
+                    )
+                );
+            }
+
             client.admin()
                 .cluster()
                 .restoreSnapshot(