Merge bitcoin/bitcoin#24602: fuzz: add target for coinselection algorithms

21520b95515676d45145df624f430cdd39db7515 fuzz: add target for coinselection (Martin Zumsande)

Pull request description:

  This adds a fuzz target for the coinselection algorithms by creating random `OutputGroup`s and running all three coin selection algorithms for them.
  It does not fuzz higher-level wallet logic for selecting eligible coins (as in `SelectCoins()`), thought it probably would make sense to have a fuzz target for that too.

ACKs for top commit:
  achow101:
    ACK 21520b95515676d45145df624f430cdd39db7515
  vasild:
    ACK 21520b95515676d45145df624f430cdd39db7515

Tree-SHA512: c763003cf5ff5317f929d3d0b2f06fa739ae41dd642042d9a5c5c96e6cb9b349a6c7aeabc77bc2b846d12c8bcb60e07ee20a9f38539429c65723ab76aeee6b2e

1 files changed, 99 insertions, 0 deletions

diff --git a/src/wallet/test/fuzz/coinselection.cpp b/src/wallet/test/fuzz/coinselection.cpp
new file mode 100644
index 0000000000..2693b68cca
--- /dev/null
+++ b/src/wallet/test/fuzz/coinselection.cpp
@@ -0,0 +1,99 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <policy/feerate.h>
+#include <primitives/transaction.h>
+#include <test/fuzz/FuzzedDataProvider.h>
+#include <test/fuzz/fuzz.h>
+#include <test/fuzz/util.h>
+#include <test/util/setup_common.h>
+#include <wallet/coinselection.h>
+
+#include <vector>
+
+namespace wallet {
+
+static void AddCoin(const CAmount& value, int n_input, int n_input_bytes, int locktime, std::vector<COutput>& coins)
+{
+    CMutableTransaction tx;
+    tx.vout.resize(n_input + 1);
+    tx.vout[n_input].nValue = value;
+    tx.nLockTime = locktime; // all transactions get different hashes
+    coins.emplace_back(COutPoint(tx.GetHash(), n_input), tx.vout.at(n_input), /*depth=*/0, n_input_bytes, /*spendable=*/true, /*solvable=*/true, /*safe=*/true, /*time=*/0, /*from_me=*/true);
+}
+
+// Randomly distribute coins to instances of OutputGroup
+static void GroupCoins(FuzzedDataProvider& fuzzed_data_provider, const std::vector<COutput>& coins, const CoinSelectionParams& coin_params, bool positive_only, std::vector<OutputGroup>& output_groups)
+{
+    auto output_group = OutputGroup(coin_params);
+    bool valid_outputgroup{false};
+    for (auto& coin : coins) {
+        output_group.Insert(coin, /*ancestors=*/0, /*descendants=*/0, positive_only);
+        // If positive_only was specified, nothing may have been inserted, leading to an empty outpout group
+        // that would be invalid for the BnB algorithm
+        valid_outputgroup = !positive_only || output_group.GetSelectionAmount() > 0;
+        if (valid_outputgroup && fuzzed_data_provider.ConsumeBool()) {
+            output_groups.push_back(output_group);
+            output_group = OutputGroup(coin_params);
+            valid_outputgroup = false;
+        }
+    }
+    if (valid_outputgroup) output_groups.push_back(output_group);
+}
+
+FUZZ_TARGET(coinselection)
+{
+    FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
+    std::vector<COutput> utxo_pool;
+
+    const CFeeRate long_term_fee_rate{ConsumeMoney(fuzzed_data_provider, /*max=*/COIN)};
+    const CFeeRate effective_fee_rate{ConsumeMoney(fuzzed_data_provider, /*max=*/COIN)};
+    const CAmount cost_of_change{ConsumeMoney(fuzzed_data_provider, /*max=*/COIN)};
+    const CAmount target{fuzzed_data_provider.ConsumeIntegralInRange<CAmount>(1, MAX_MONEY)};
+    const bool subtract_fee_outputs{fuzzed_data_provider.ConsumeBool()};
+
+    FastRandomContext fast_random_context{ConsumeUInt256(fuzzed_data_provider)};
+    CoinSelectionParams coin_params{fast_random_context};
+    coin_params.m_subtract_fee_outputs = subtract_fee_outputs;
+    coin_params.m_long_term_feerate = long_term_fee_rate;
+    coin_params.m_effective_feerate = effective_fee_rate;
+
+    // Create some coins
+    CAmount total_balance{0};
+    int next_locktime{0};
+    LIMITED_WHILE(fuzzed_data_provider.ConsumeBool(), 10000)
+    {
+        const int n_input{fuzzed_data_provider.ConsumeIntegralInRange<int>(0, 10)};
+        const int n_input_bytes{fuzzed_data_provider.ConsumeIntegralInRange<int>(100, 10000)};
+        const CAmount amount{fuzzed_data_provider.ConsumeIntegralInRange<CAmount>(1, MAX_MONEY)};
+        if (total_balance + amount >= MAX_MONEY) {
+            break;
+        }
+        AddCoin(amount, n_input, n_input_bytes, ++next_locktime, utxo_pool);
+        total_balance += amount;
+    }
+
+    std::vector<OutputGroup> group_pos;
+    GroupCoins(fuzzed_data_provider, utxo_pool, coin_params, /*positive_only=*/true, group_pos);
+    std::vector<OutputGroup> group_all;
+    GroupCoins(fuzzed_data_provider, utxo_pool, coin_params, /*positive_only=*/false, group_all);
+
+    // Run coinselection algorithms
+    const auto result_bnb = SelectCoinsBnB(group_pos, target, cost_of_change);
+
+    auto result_srd = SelectCoinsSRD(group_pos, target, fast_random_context);
+    if (result_srd) result_srd->ComputeAndSetWaste(cost_of_change);
+
+    CAmount change_target{GenerateChangeTarget(target, fast_random_context)};
+    auto result_knapsack = KnapsackSolver(group_all, target, change_target, fast_random_context);
+    if (result_knapsack) result_knapsack->ComputeAndSetWaste(cost_of_change);
+
+    // If the total balance is sufficient for the target and we are not using
+    // effective values, Knapsack should always find a solution.
+    if (total_balance >= target && subtract_fee_outputs) {
+        assert(result_knapsack);
+    }
+}
+
+} // namespace wallet