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.
test/framework/src/main/java/org/elasticsearch/test/ESTestCase.java
* Create a copy of an original {@link NamedWriteable} object by running it through a {@link BytesStreamOutput} and
     * reading it in again using a provided {@link Writeable.Reader}.
     */
    public static <C extends NamedWriteable, T extends C> C copyNamedWriteable(
        T original,
        NamedWriteableRegistry namedWriteableRegistry,
        Class<C> categoryClass
    ) throws IOException {
        return copyNamedWriteable(original, namedWriteableRegistry, categoryClass, TransportVersion.current());
    }
    /**
     * Same as {@link #copyNamedWriteable(NamedWriteable, NamedWriteableRegistry, Class)} but also allows to provide
     * a {@link TransportVersion} argument which will be used to write and read back the object.
     * @return
     */
    @SuppressWarnings("unchecked")
    public static <C extends NamedWriteable, T extends C> C copyNamedWriteable(
        T original,
        NamedWriteableRegistry namedWriteableRegistry,
        Class<C> categoryClass,
        TransportVersion version
    ) throws IOException {
        return copyInstance(
            original,
            namedWriteableRegistry,
            StreamOutput::writeNamedWriteable,
            in -> in.readNamedWriteable(categoryClass),
            version
        );
    }