nix-super/src/libstore/length-prefixed-protocol-helper.hh

#pragma once
/**
 * @file Reusable serialisers for serialization container types in a
 * length-prefixed manner.
 *
 * Used by both the Worker and Serve protocols.
 */

#include "types.hh"

namespace nix {

class Store;

/**
 * Reusable serialisers for serialization container types in a
 * length-prefixed manner.
 *
 * @param T The type of the collection being serialised
 *
 * @param Inner This the most important parameter; this is the "inner"
 * protocol. The user of this will substitute `MyProtocol` or similar
 * when making a `MyProtocol::Serialiser<Collection<T>>`. Note that the
 * inside is allowed to call to call `Inner::Serialiser` on different
 * types. This is especially important for `std::map` which doesn't have
 * a single `T` but one `K` and one `V`.
 */
template<class Inner, typename T>
struct LengthPrefixedProtoHelper;

/*!
 * \typedef LengthPrefixedProtoHelper::S
 *
 * Read this as simply `using S = Inner::Serialise;`.
 *
 * It would be nice to use that directly, but C++ doesn't seem to allow
 * it. The `typename` keyword needed to refer to `Inner` seems to greedy
 * (low precedence), and then C++ complains that `Serialise` is not a
 * type parameter but a real type.
 *
 * Making this `S` alias seems to be the only way to avoid these issues.
 */

#define LENGTH_PREFIXED_PROTO_HELPER(Inner, T) \
    struct LengthPrefixedProtoHelper< Inner, T > \
    { \
        static T read(const Store & store, typename Inner::ReadConn conn); \
        static void write(const Store & store, typename Inner::WriteConn conn, const T & str); \
    private: \
        template<typename U> using S = typename Inner::template Serialise<U>; \
    }

template<class Inner, typename T>
LENGTH_PREFIXED_PROTO_HELPER(Inner, std::vector<T>);

template<class Inner, typename T>
LENGTH_PREFIXED_PROTO_HELPER(Inner, std::set<T>);

template<class Inner, typename... Ts>
LENGTH_PREFIXED_PROTO_HELPER(Inner, std::tuple<Ts...>);

template<class Inner, typename K, typename V>
#define _X std::map<K, V>
LENGTH_PREFIXED_PROTO_HELPER(Inner, _X);
#undef _X

template<class Inner, typename T>
std::vector<T>
LengthPrefixedProtoHelper<Inner, std::vector<T>>::read(
    const Store & store, typename Inner::ReadConn conn)
{
    std::vector<T> resSet;
    auto size = readNum<size_t>(conn.from);
    while (size--) {
        resSet.push_back(S<T>::read(store, conn));
    }
    return resSet;
}

template<class Inner, typename T>
void
LengthPrefixedProtoHelper<Inner, std::vector<T>>::write(
    const Store & store, typename Inner::WriteConn conn, const std::vector<T> & resSet)
{
    conn.to << resSet.size();
    for (auto & key : resSet) {
        S<T>::write(store, conn, key);
    }
}

template<class Inner, typename T>
std::set<T>
LengthPrefixedProtoHelper<Inner, std::set<T>>::read(
    const Store & store, typename Inner::ReadConn conn)
{
    std::set<T> resSet;
    auto size = readNum<size_t>(conn.from);
    while (size--) {
        resSet.insert(S<T>::read(store, conn));
    }
    return resSet;
}

template<class Inner, typename T>
void
LengthPrefixedProtoHelper<Inner, std::set<T>>::write(
    const Store & store, typename Inner::WriteConn conn, const std::set<T> & resSet)
{
    conn.to << resSet.size();
    for (auto & key : resSet) {
        S<T>::write(store, conn, key);
    }
}

template<class Inner, typename K, typename V>
std::map<K, V>
LengthPrefixedProtoHelper<Inner, std::map<K, V>>::read(
    const Store & store, typename Inner::ReadConn conn)
{
    std::map<K, V> resMap;
    auto size = readNum<size_t>(conn.from);
    while (size--) {
        auto k = S<K>::read(store, conn);
        auto v = S<V>::read(store, conn);
        resMap.insert_or_assign(std::move(k), std::move(v));
    }
    return resMap;
}

template<class Inner, typename K, typename V>
void
LengthPrefixedProtoHelper<Inner, std::map<K, V>>::write(
    const Store & store, typename Inner::WriteConn conn, const std::map<K, V> & resMap)
{
    conn.to << resMap.size();
    for (auto & i : resMap) {
        S<K>::write(store, conn, i.first);
        S<V>::write(store, conn, i.second);
    }
}

template<class Inner, typename... Ts>
std::tuple<Ts...>
LengthPrefixedProtoHelper<Inner, std::tuple<Ts...>>::read(
    const Store & store, typename Inner::ReadConn conn)
{
    return std::tuple<Ts...> {
        S<Ts>::read(store, conn)...,
    };
}

template<class Inner, typename... Ts>
void
LengthPrefixedProtoHelper<Inner, std::tuple<Ts...>>::write(
    const Store & store, typename Inner::WriteConn conn, const std::tuple<Ts...> & res)
{
    std::apply([&]<typename... Us>(const Us &... args) {
        (S<Us>::write(store, conn, args), ...);
    }, res);
}

}