// FIXME: use Store::topoSortPaths().
StorePaths topoSortPaths(const std::map<StorePath, ValidPathInfo> & paths)
{
    StorePaths sorted;
    StorePathSet visited;
    std::function<void(const StorePath & path)> dfsVisit;
    dfsVisit = [&](const StorePath & path) {
        if (!visited.insert(path).second) return;
        auto info = paths.find(path);
        auto references = info == paths.end() ? StorePathSet() : info->second.references;
        for (auto & i : references)
            /* Don't traverse into paths that don't exist.  That can
               happen due to substitutes for non-existent paths. */
            if (i != path && paths.count(i))
                dfsVisit(i);
        sorted.push_back(path);
    };
    for (auto & i : paths)
        dfsVisit(i.first);
    std::reverse(sorted.begin(), sorted.end());
    return sorted;
}

        // FIXME: rewrite to use Store.

            to << SERVE_MAGIC_1 << 0x203;

            to << SERVE_MAGIC_1 << 0x204;

            /* Query the size of the output paths. */

            /* Get info about each output path. */
            std::map<StorePath, ValidPathInfo> infos;

                if (readString(from) == "") break;

                auto storePathS = readString(from);
                if (storePathS == "") break;
                ValidPathInfo info(localStore->parseStorePath(storePathS));
                assert(outputs.count(info.path));

                readStrings<PathSet>(from); // references

                info.references = readStorePaths<StorePathSet>(*localStore, from);

                totalNarSize += readLongLong(from);

                info.narSize = readLongLong(from);
                totalNarSize += info.narSize;
                info.narHash = Hash(readString(from), htSHA256);
                info.ca = parseContentAddressOpt(readString(from));
                readStrings<StringSet>(from); // sigs
                infos.insert_or_assign(info.path, info);

            /* Copy each path. */

            to << cmdExportPaths << 0;
            writeStorePaths(*localStore, to, outputs);
            to.flush();
            destStore->importPaths(from, /* result.accessor, */ NoCheckSigs);

            auto pathsSorted = topoSortPaths(infos);
            for (auto & path : pathsSorted) {
                auto & info = infos.find(path)->second;
                to << cmdDumpStorePath << localStore->printStorePath(path);
                to.flush();
                destStore->addToStore(info, from);
            }