An incomplete shard snapshot will prevent a shard from moving away from a node (see `SnapshotInProgressAllocationDecider`). That means that when a shard snapshot completes it may become possible to perform some shard movements that previously were not possible, so we should trigger a reroute to start those movements. AFAICT we do not trigger such a reroute today.
server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java
import org.elasticsearch.common.component.AbstractLifecycleComponent;
import org.elasticsearch.common.regex.Regex;
import org.elasticsearch.common.settings.Setting;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.unit.ByteSizeValue;
import org.elasticsearch.common.util.CollectionUtils;
import org.elasticsearch.common.util.Maps;
import org.elasticsearch.common.util.concurrent.EsExecutors;
import org.elasticsearch.common.util.concurrent.ListenableFuture;
import org.elasticsearch.core.Nullable;
import org.elasticsearch.core.SuppressForbidden;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.index.Index;
import org.elasticsearch.index.IndexVersion;
import org.elasticsearch.index.IndexVersions;
import org.elasticsearch.index.shard.ShardId;
import org.elasticsearch.indices.SystemDataStreamDescriptor;
import org.elasticsearch.indices.SystemIndices;
import org.elasticsearch.repositories.FinalizeSnapshotContext;
import org.elasticsearch.repositories.IndexId;
import org.elasticsearch.repositories.RepositoriesService;
import org.elasticsearch.repositories.Repository;
import org.elasticsearch.repositories.RepositoryData;
import org.elasticsearch.repositories.RepositoryException;
import org.elasticsearch.repositories.RepositoryMissingException;
import org.elasticsearch.repositories.RepositoryShardId;
import org.elasticsearch.repositories.ShardGeneration;
import org.elasticsearch.repositories.ShardGenerations;
import org.elasticsearch.repositories.ShardSnapshotResult;
import org.elasticsearch.tasks.Task;