Function ProtectEvictionCandidatesByRatio

src/net.cpp:961–1032

}

void ProtectEvictionCandidatesByRatio(std::vector<NodeEvictionCandidate>& eviction_candidates)
{
    // Protect the half of the remaining nodes which have been connected the longest.
    // This replicates the non-eviction implicit behavior, and precludes attacks that start later.
    // To favorise the diversity of our peer connections, reserve up to half of these protected
    // spots for Tor/onion, localhost, I2P, and CJDNS peers, even if they're not longest uptime
    // overall. This helps protect these higher-latency peers that tend to be otherwise
    // disadvantaged under our eviction criteria.
    const size_t initial_size = eviction_candidates.size();
    const size_t total_protect_size{initial_size / 2};

    // Disadvantaged networks to protect. In the case of equal counts, earlier array members
    // have the first opportunity to recover unused slots from the previous iteration.
    struct Net { bool is_local; Network id; size_t count; };
    std::array<Net, 4> networks{
        {{false, NET_CJDNS, 0}, {false, NET_I2P, 0}, {/*localhost=*/true, NET_MAX, 0}, {false, NET_ONION, 0}}};

    // Count and store the number of eviction candidates per network.
    for (Net& n : networks) {
        n.count = std::count_if(eviction_candidates.cbegin(), eviction_candidates.cend(),
                                [&n](const NodeEvictionCandidate& c) {
                                    return n.is_local ? c.m_is_local : c.m_network == n.id;
                                });
    }
    // Sort `networks` by ascending candidate count, to give networks having fewer candidates
    // the first opportunity to recover unused protected slots from the previous iteration.
    std::stable_sort(networks.begin(), networks.end(), [](Net a, Net b) { return a.count < b.count; });

    // Protect up to 25% of the eviction candidates by disadvantaged network.
    const size_t max_protect_by_network{total_protect_size / 2};
    size_t num_protected{0};

    while (num_protected < max_protect_by_network) {
        // Count the number of disadvantaged networks from which we have peers to protect.
        auto num_networks = std::count_if(networks.begin(), networks.end(), [](const Net& n) { return n.count; });
        if (num_networks == 0) {
            break;
        }
        const size_t disadvantaged_to_protect{max_protect_by_network - num_protected};
        const size_t protect_per_network{std::max(disadvantaged_to_protect / num_networks, static_cast<size_t>(1))};
        // Early exit flag if there are no remaining candidates by disadvantaged network.
        bool protected_at_least_one{false};

        for (Net& n : networks) {
            if (n.count == 0) continue;
            const size_t before = eviction_candidates.size();
            EraseLastKElements(eviction_candidates, CompareNodeNetworkTime(n.is_local, n.id),
                               protect_per_network, [&n](const NodeEvictionCandidate& c) {
                                   return n.is_local ? c.m_is_local : c.m_network == n.id;
                               });
            const size_t after = eviction_candidates.size();
            if (before > after) {
                protected_at_least_one = true;
                const size_t delta{before - after};
                num_protected += delta;
                if (num_protected >= max_protect_by_network) {
                    break;
                }