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/test/java/org/elasticsearch/snapshots/SnapshotResiliencyTests.java
transportService,
                        clusterService,
                        threadPool,
                        snapshotsService,
                        actionFilters,
                        indexNameExpressionResolver
                    )
                );
                actions.put(
                    ClusterRerouteAction.INSTANCE,
                    new TransportClusterRerouteAction(
                        transportService,
                        clusterService,
                        threadPool,
                        allocationService,
                        actionFilters,
                        indexNameExpressionResolver
                    )
                );
                actions.put(
                    ClusterStateAction.INSTANCE,
                    new TransportClusterStateAction(
                        transportService,
                        clusterService,
                        threadPool,
                        actionFilters,
                        indexNameExpressionResolver
                    )
                );
                actions.put(