Today if we detect concurrent modification of a snapshot repository then we mark it with `CORRUPTED_REPO_GEN` which blocks further writes to try and prevent additional damage to its contents. You can remove the marker by calling the register-repository API again, but this API will discard no-op re-registrations without taking any action. You can work around this by unregistering the repository first (which may be impossible if it's still in use for searchable snapshots) or by re-registering it with some trivial setting change (which seems hacky).

We should not consider it a no-op re-registration if the repository is currently marked with `CORRUPTED_REPO_GEN`, because doing this re-registration will re-read the repo contents and clear the corruption marker and therefore is not a no-op.
server/src/internalClusterTest/java/org/elasticsearch/snapshots/ConcurrentSnapshotsIT.java
*/
package org.elasticsearch.snapshots;
import org.elasticsearch.ElasticsearchException;
import org.elasticsearch.ExceptionsHelper;
import org.elasticsearch.action.ActionFuture;
import org.elasticsearch.action.ActionListener;
import org.elasticsearch.action.StepListener;
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.restore.RestoreSnapshotResponse;
import org.elasticsearch.action.admin.cluster.snapshots.status.SnapshotStatus;
import org.elasticsearch.action.admin.cluster.snapshots.status.SnapshotsStatusResponse;
import org.elasticsearch.action.support.GroupedActionListener;
import org.elasticsearch.action.support.PlainActionFuture;
import org.elasticsearch.action.support.master.AcknowledgedResponse;
import org.elasticsearch.cluster.SnapshotDeletionsInProgress;
import org.elasticsearch.cluster.SnapshotsInProgress;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.UncategorizedExecutionException;
import org.elasticsearch.discovery.AbstractDisruptionTestCase;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.repositories.IndexId;
import org.elasticsearch.repositories.RepositoryData;
import org.elasticsearch.repositories.RepositoryException;
import org.elasticsearch.repositories.ShardGenerations;
import org.elasticsearch.repositories.blobstore.BlobStoreRepository;
import org.elasticsearch.snapshots.mockstore.MockRepository;
import org.elasticsearch.test.ESIntegTestCase;
import org.elasticsearch.test.disruption.NetworkDisruption;