// LRU iterator for the key and block.
    std::shared_ptr<Block> block = Lookup(key);
    if (!block) {
      std::cerr << "No block for key " << key.first << "@" << key.second;
      abort();
    }
    MaybeFetch(key, block, status);
    if (TF_GetCode(status) != TF_OK) return -1;
    UpdateLRU(key, block, status);
    if (TF_GetCode(status) != TF_OK) return -1;

                                                   const void* proto,
                                                   size_t proto_len,
                                                   TF_Status* status);

// Aborts all ongoing collectives with the specified status. After abortion,
// subsequent collectives will error with this status immediately. To reset the
// collectives, create a new EagerContext.
//