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
public void executeSnapshot(final CreateSnapshotRequest request, final ActionListener<SnapshotInfo> listener) {
        createSnapshot(request, listener.delegateFailureAndWrap((l, snapshot) -> addListener(snapshot, l)));
    }
    /**
     * Initializes the snapshotting process.
     * <p>
     * This method is used by clients to start snapshot. It makes sure that there is no snapshots are currently running and
     * creates a snapshot record in cluster state metadata.
     *
     * @param request  snapshot request
     * @param listener snapshot creation listener
     */
    public void createSnapshot(final CreateSnapshotRequest request, final ActionListener<Snapshot> listener) {
        final String repositoryName = request.repository();
        final String snapshotName = IndexNameExpressionResolver.resolveDateMathExpression(request.snapshot());
        validate(repositoryName, snapshotName);
        
        
        final SnapshotId snapshotId = new SnapshotId(snapshotName, UUIDs.randomBase64UUID()); 
        Repository repository = repositoriesService.repository(request.repository());
        if (repository.isReadOnly()) {
            listener.onFailure(new RepositoryException(repository.getMetadata().name(), "cannot create snapshot in a readonly repository"));
            return;
        }
        submitCreateSnapshotRequest(request, listener, repository, new Snapshot(repositoryName, snapshotId), repository.getMetadata());
    }
    private void submitCreateSnapshotRequest(
        CreateSnapshotRequest request,
        ActionListener<Snapshot> listener,
        Repository repository,