Function AutomaticCoinSelection

src/wallet/spend.cpp:868–977

}

util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, CoinsResult& available_coins, const CAmount& value_to_select, const CoinSelectionParams& coin_selection_params)
{
    // Try to enforce a mixture of cluster limits and ancestor/descendant limits on transactions we create by limiting
    // the ancestors and the maximum cluster count of any UTXO we use. We use the ancestor/descendant limits, which are
    // lower than the cluster limits, to avoid exceeding any ancestor/descendant limits of legacy nodes. This filter is safe
    // because a transaction's ancestor or descendant count cannot be larger than its cluster count.
    // TODO: these limits can be relaxed in the future, and we can replace the ancestor filter with a cluster equivalent.
    unsigned int limit_ancestor_count = 0;
    unsigned int limit_descendant_count = 0;
    wallet.chain().getPackageLimits(limit_ancestor_count, limit_descendant_count);
    const size_t max_ancestors = (size_t)std::max<int64_t>(1, limit_ancestor_count);
    const size_t max_cluster_count = (size_t)std::max<int64_t>(1, limit_descendant_count);
    const bool fRejectLongChains = gArgs.GetBoolArg("-walletrejectlongchains", DEFAULT_WALLET_REJECT_LONG_CHAINS);

    // Cases where we have 101+ outputs all pointing to the same destination may result in
    // privacy leaks as they will potentially be deterministically sorted. We solve that by
    // explicitly shuffling the outputs before processing
    if (coin_selection_params.m_avoid_partial_spends && available_coins.Size() > OUTPUT_GROUP_MAX_ENTRIES) {
        available_coins.Shuffle(coin_selection_params.rng_fast);
    }

    // Coin Selection attempts to select inputs from a pool of eligible UTXOs to fund the
    // transaction at a target feerate. If an attempt fails, more attempts may be made using a more
    // permissive CoinEligibilityFilter.
    {
        // Place coins eligibility filters on a scope increasing order.
        std::vector<SelectionFilter> ordered_filters{
                // If possible, fund the transaction with confirmed UTXOs only. Prefer at least six
                // confirmations on outputs received from other wallets and only spend confirmed change.
                {CoinEligibilityFilter(1, 6, 0), /*allow_mixed_output_types=*/false},
                {CoinEligibilityFilter(1, 1, 0)},
        };
        // Fall back to using zero confirmation change (but with as few ancestors in the mempool as
        // possible) if we cannot fund the transaction otherwise.
        if (wallet.m_spend_zero_conf_change) {
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, 2)});
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, std::min(size_t{4}, max_ancestors/3), std::min(size_t{4}, max_cluster_count/3))});
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, max_ancestors/2, max_cluster_count/2)});
            // If partial groups are allowed, relax the requirement of spending OutputGroups (groups
            // of UTXOs sent to the same address, which are obviously controlled by a single wallet)
            // in their entirety.
            ordered_filters.push_back({CoinEligibilityFilter(0, 1, max_ancestors-1, max_cluster_count-1, /*include_partial=*/true)});
            // Try with unsafe inputs if they are allowed. This may spend unconfirmed outputs
            // received from other wallets.
            if (coin_selection_params.m_include_unsafe_inputs) {
                ordered_filters.push_back({CoinEligibilityFilter(/*conf_mine=*/0, /*conf_theirs=*/0, max_ancestors-1, max_cluster_count-1, /*include_partial=*/true)});
            }
            // Try with unlimited ancestors/clusters. The transaction will still need to meet
            // local mempool policy (i.e. cluster limits) to be accepted to mempool and broadcasted, and
            // limits of other nodes (e.g. ancestor/descendant limits) to propagate, but OutputGroups
            // use heuristics that may overestimate.
            if (!fRejectLongChains) {
                ordered_filters.push_back({CoinEligibilityFilter(0, 1, std::numeric_limits<uint64_t>::max(),
                                                                   std::numeric_limits<uint64_t>::max(),
                                                                   /*include_partial=*/true)});
            }
        }