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
continue;
                                            }
                                            if (clones == null) {
                                                clones = ImmutableOpenMap.builder(snapshot.shardsByRepoShardId());
                                            }
                                            
                                            
                                            
                                            
                                            
                                            
                                            clones.put(
                                                repositoryShardId,
                                                new ShardSnapshotStatus(nodes.getLocalNodeId(), failureEntry.getValue().generation())
                                            );
                                        }
                                    }
                                    if (clones != null) {
                                        changed = true;
                                        updatedEntriesForRepo.add(snapshot.withClones(clones.build()));
                                    } else {
                                        updatedEntriesForRepo.add(snapshot);
                                    }
                                }
                            } else {
                                ImmutableOpenMap<ShardId, ShardSnapshotStatus> shards = processWaitingShardsAndRemovedNodes(
                                    snapshot,
                                    routingTable,
                                    nodes,