ShardRegistry::reload() is called in many places and internally it performs a call to CatalogManager::getAllShards() and then rebuilds all internal maps.

To encapsulate the reload functionality:
1) Add ShardRegistryData class (ShardRegistry eventually will get stripped of its other responsibilities and so ShardRegistryData can become what is ShardRegistry now)

2) Provide a helper method (pseudocode)

void ShardRegistryData::swap(ShardRegistryData* other) {
        if (other->newerThan(this) {
        // swap
        }
}

shared_ptr<Shard> getShard(txn) {
    auto shard = grid.shardResistry()->getShard();
    if (!shard) {
           ShardRegistryData newShardRegistry(txn);
            grid.shardRegistry().data().swap(&newShardRegistry);
        }
        shard = grid.shardResistry()->getShard();   
   }
   return shard;
}

3) replace calls to ShardRegistry::getShard() with the new method

4) Add a class ShardRegistryUpdater
owned by ShardRegistry that will be used to update ShardRegistryData via swap:

    ShardRegistryData newShardRegistry(txn);
    grid.shardRegistry().data().swap(&newShardRegistry);

Protect the calls with a lock to serialize the reload order.

5) In place of reload() use

grid.shardRegistry()->shardRegistryUpdater()->reload();