Fix Ex. Handling in SnapshotsService#snapshots (#47507) (#47727)

We're needlessly wrapping a `SnapshotMissingException` which
itself is a `SnapshotException` when trying to load a missing
snapshot. This leads to failure #47442 which expects a
`SnapshotMissingException` in this case.

Closes #47442

server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java

@@ -210,6 +210,9 @@ public class SnapshotsService extends AbstractLifecycleComponent implements Clus
                 if (ignoreUnavailable) {
                     logger.warn(() -> new ParameterizedMessage("failed to get snapshot [{}]", snapshotId), ex);
                 } else {
+                    if (ex instanceof SnapshotException) {
+                        throw ex;
+                    }
                     throw new SnapshotException(repositoryName, snapshotId, "Snapshot could not be read", ex);
                 }
             }

server/src/test/java/org/elasticsearch/snapshots/SnapshotStatusApisIT.java

@@ -21,13 +21,18 @@ package org.elasticsearch.snapshots;
 import org.elasticsearch.Version;
 import org.elasticsearch.action.ActionFuture;
 import org.elasticsearch.action.admin.cluster.snapshots.create.CreateSnapshotResponse;
+import org.elasticsearch.action.admin.cluster.snapshots.get.GetSnapshotsRequest;
 import org.elasticsearch.action.admin.cluster.snapshots.status.SnapshotStatus;
 import org.elasticsearch.action.admin.cluster.snapshots.status.SnapshotsStatusRequest;
 import org.elasticsearch.client.Client;
 import org.elasticsearch.cluster.SnapshotsInProgress;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.unit.TimeValue;
+import org.elasticsearch.core.internal.io.IOUtils;
+import org.elasticsearch.repositories.blobstore.BlobStoreRepository;
 
+import java.io.IOException;
+import java.nio.file.Path;
 import java.util.List;
 import java.util.concurrent.TimeUnit;
 
@@ -109,4 +114,23 @@ public class SnapshotStatusApisIT extends AbstractSnapshotIntegTestCase {
         logger.info("--> wait for snapshot to finish");
         createSnapshotResponseActionFuture.actionGet();
     }
+
+    public void testExceptionOnMissingSnapBlob() throws IOException {
+        disableRepoConsistencyCheck("This test intentionally corrupts the repository");
+
+        logger.info("--> creating repository");
+        final Path repoPath = randomRepoPath();
+        assertAcked(client().admin().cluster().preparePutRepository("test-repo").setType("fs").setSettings(
+            Settings.builder().put("location", repoPath).build()));
+
+        logger.info("--> snapshot");
+        final CreateSnapshotResponse response =
+            client().admin().cluster().prepareCreateSnapshot("test-repo", "test-snap").setWaitForCompletion(true).get();
+
+        logger.info("--> delete snap-${uuid}.dat file for this snapshot to simulate concurrent delete");
+        IOUtils.rm(repoPath.resolve(BlobStoreRepository.SNAPSHOT_PREFIX + response.getSnapshotInfo().snapshotId().getUUID() + ".dat"));
+
+        expectThrows(SnapshotMissingException.class, () -> client().admin().cluster()
+            .getSnapshots(new GetSnapshotsRequest("test-repo", new String[] {"test-snap"})).actionGet());
+    }
 }