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.cluster.NotMasterException;
import org.elasticsearch.cluster.RepositoryCleanupInProgress;
import org.elasticsearch.cluster.RestoreInProgress;
import org.elasticsearch.cluster.SnapshotDeletionsInProgress;
import org.elasticsearch.cluster.SnapshotsInProgress;
import org.elasticsearch.cluster.SnapshotsInProgress.ShardSnapshotStatus;
import org.elasticsearch.cluster.SnapshotsInProgress.ShardState;
import org.elasticsearch.cluster.SnapshotsInProgress.State;
import org.elasticsearch.cluster.block.ClusterBlockException;
import org.elasticsearch.cluster.coordination.FailedToCommitClusterStateException;
import org.elasticsearch.cluster.metadata.DataStream;
import org.elasticsearch.cluster.metadata.DataStreamAlias;
import org.elasticsearch.cluster.metadata.IndexMetadata;
import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.metadata.RepositoriesMetadata;
import org.elasticsearch.cluster.metadata.RepositoryMetadata;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.node.DiscoveryNodes;
import org.elasticsearch.cluster.routing.IndexRoutingTable;
import org.elasticsearch.cluster.routing.IndexShardRoutingTable;
import org.elasticsearch.cluster.routing.RoutingTable;
import org.elasticsearch.cluster.routing.ShardRouting;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.cluster.service.MasterService;
import org.elasticsearch.cluster.service.MasterServiceTaskQueue;
import org.elasticsearch.common.Priority;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.UUIDs;
import org.elasticsearch.common.collect.ImmutableOpenMap;