Merge bitcoin/bitcoin#27585: fuzz: improve `coinselection`

bf26f978ffbe7e2fc681825de631600e24e5c93e fuzz: coinselection, fix `m_cost_of_change` (brunoerg)
6d9b26d56ab5295dfcfe0f80a3069046a263fb2f fuzz: coinselection, BnB should never produce change (brunoerg)
b2eb55840778515d61465acc8106b27e16af1c88 fuzz: coinselection, compare `GetSelectedValue` with target (brunoerg)
0df0438c60e27df1aced6d31a192d6f334cef2d1 fuzz: coinselection, improve `ComputeAndSetWaste` (brunoerg)
1e351e5db1ced6a32681ceea8a111148bd83e323 fuzz: coinselection, add coverage for `Merge` (brunoerg)
f0244a8614ee35caef03bc326519823972ec61b4 fuzz: coinselection, add coverage for `GetShuffledInputVector`/`GetInputSet` (brunoerg)
808618b8a25b1d9cfc4e4f1a5b4c6fff02972396 fuzz: coinselection, add coverage for `AddInputs` (brunoerg)
90c4e6a241eee605809ab1b4331e620b92f05933 fuzz: coinselection, add coverage for `EligibleForSpending` (brunoerg)
2a031cb2c218e288a9784d677705a7d2bc1c2d2b fuzz: coinselection, add `CreateCoins` (brunoerg)

Pull request description:

  This PR:

  - Moves coin creation to its own function called `CreateCoins`.
  - Add coverage for `EligibleForSpending`
  - Add coverage for `AddInputs`: get result of each algorithm (srd, knapsack and bnb), call `CreateCoins` and add into them.
  - Add coverage for `GetShuffledInputVector` and `GetInputSet` using the result of each algorithm (srd, knapsack and bnb).
  - Add coverage for `Merge`: Call SRD with the new utxos and, if successful, try to merge with the previous SRD result.

ACKs for top commit:
  murchandamus:
    reACK with some minimal fuzzing bf26f978ffbe7e2fc681825de631600e24e5c93e
  achow101:
    ACK bf26f978ffbe7e2fc681825de631600e24e5c93e
  furszy:
    re-ACK bf26f97

Tree-SHA512: bdd2b0a39de37be0a9b21a7c51260b6b8abe538cc0ea74312eb658b90a121a1ae07306c09fb0e75e93b531ce9ea2402feb041b0d852902d07739257f792e64ab

1 files changed, 90 insertions, 17 deletions

diff --git a/src/wallet/test/fuzz/coinselection.cpp b/src/wallet/test/fuzz/coinselection.cpp
index bc935157b1..4caf96b18d 100644
--- a/src/wallet/test/fuzz/coinselection.cpp
+++ b/src/wallet/test/fuzz/coinselection.cpp
@@ -45,6 +45,35 @@ static void GroupCoins(FuzzedDataProvider& fuzzed_data_provider, const std::vect
     if (valid_outputgroup) output_groups.push_back(output_group);
 }
 
+static CAmount CreateCoins(FuzzedDataProvider& fuzzed_data_provider, std::vector<COutput>& utxo_pool, CoinSelectionParams& coin_params, int& next_locktime)
+{
+    CAmount total_balance{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>(41, 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, coin_params.m_effective_feerate);
+        total_balance += amount;
+    }
+
+    return total_balance;
+}
+
+static SelectionResult ManualSelection(std::vector<COutput>& utxos, const CAmount& total_amount, const bool& subtract_fee_outputs)
+{
+    SelectionResult result(total_amount, SelectionAlgorithm::MANUAL);
+    std::set<std::shared_ptr<COutput>> utxo_pool;
+    for (const auto& utxo : utxos) {
+        utxo_pool.insert(std::make_shared<COutput>(utxo));
+    }
+    result.AddInputs(utxo_pool, subtract_fee_outputs);
+    return result;
+}
+
 // Returns true if the result contains an error and the message is not empty
 static bool HasErrorMsg(const util::Result<SelectionResult>& res) { return !util::ErrorString(res).empty(); }
 
@@ -55,7 +84,9 @@ FUZZ_TARGET(coinselection)
 
     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)};
+    // Discard feerate must be at least dust relay feerate
+    const CFeeRate discard_fee_rate{fuzzed_data_provider.ConsumeIntegralInRange<CAmount>(DUST_RELAY_TX_FEE, COIN)};
+    const CAmount min_viable_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()};
 
@@ -64,47 +95,89 @@ FUZZ_TARGET(coinselection)
     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;
+    coin_params.min_viable_change = min_viable_change;
     coin_params.change_output_size = fuzzed_data_provider.ConsumeIntegralInRange<int>(10, 1000);
     coin_params.m_change_fee = effective_fee_rate.GetFee(coin_params.change_output_size);
+    coin_params.m_discard_feerate = discard_fee_rate;
+    coin_params.change_spend_size = fuzzed_data_provider.ConsumeIntegralInRange<int>(41, 1000);
+    coin_params.m_cost_of_change = coin_params.m_change_fee + coin_params.m_discard_feerate.GetFee(coin_params.change_spend_size);
 
-    // 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, coin_params.m_effective_feerate);
-        total_balance += amount;
-    }
+    CAmount total_balance{CreateCoins(fuzzed_data_provider, utxo_pool, coin_params, next_locktime)};
 
     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);
 
+    for (const OutputGroup& group : group_all) {
+        const CoinEligibilityFilter filter(fuzzed_data_provider.ConsumeIntegral<int>(), fuzzed_data_provider.ConsumeIntegral<int>(), fuzzed_data_provider.ConsumeIntegral<uint64_t>());
+        (void)group.EligibleForSpending(filter);
+    }
+
     // Run coinselection algorithms
-    const auto result_bnb = SelectCoinsBnB(group_pos, target, cost_of_change, MAX_STANDARD_TX_WEIGHT);
+    auto result_bnb = SelectCoinsBnB(group_pos, target, coin_params.m_cost_of_change, MAX_STANDARD_TX_WEIGHT);
+    if (result_bnb) {
+        assert(result_bnb->GetChange(coin_params.m_cost_of_change, CAmount{0}) == 0);
+        assert(result_bnb->GetSelectedValue() >= target);
+        (void)result_bnb->GetShuffledInputVector();
+        (void)result_bnb->GetInputSet();
+    }
 
     auto result_srd = SelectCoinsSRD(group_pos, target, coin_params.m_change_fee, fast_random_context, MAX_STANDARD_TX_WEIGHT);
     if (result_srd) {
+        assert(result_srd->GetSelectedValue() >= target);
         assert(result_srd->GetChange(CHANGE_LOWER, coin_params.m_change_fee) > 0); // Demonstrate that SRD creates change of at least CHANGE_LOWER
-        result_srd->ComputeAndSetWaste(cost_of_change, cost_of_change, 0);
+        result_srd->ComputeAndSetWaste(coin_params.min_viable_change, coin_params.m_cost_of_change, coin_params.m_change_fee);
+        (void)result_srd->GetShuffledInputVector();
+        (void)result_srd->GetInputSet();
     }
 
     CAmount change_target{GenerateChangeTarget(target, coin_params.m_change_fee, fast_random_context)};
     auto result_knapsack = KnapsackSolver(group_all, target, change_target, fast_random_context, MAX_STANDARD_TX_WEIGHT);
-    if (result_knapsack) result_knapsack->ComputeAndSetWaste(cost_of_change, cost_of_change, 0);
+    if (result_knapsack) {
+        assert(result_knapsack->GetSelectedValue() >= target);
+        result_knapsack->ComputeAndSetWaste(coin_params.min_viable_change, coin_params.m_cost_of_change, coin_params.m_change_fee);
+        (void)result_knapsack->GetShuffledInputVector();
+        (void)result_knapsack->GetInputSet();
+    }
 
     // If the total balance is sufficient for the target and we are not using
     // effective values, Knapsack should always find a solution (unless the selection exceeded the max tx weight).
     if (total_balance >= target && subtract_fee_outputs && !HasErrorMsg(result_knapsack)) {
         assert(result_knapsack);
     }
+
+    std::vector<COutput> utxos;
+    std::vector<util::Result<SelectionResult>> results{result_srd, result_knapsack, result_bnb};
+    CAmount new_total_balance{CreateCoins(fuzzed_data_provider, utxos, coin_params, next_locktime)};
+    if (new_total_balance > 0) {
+        std::set<std::shared_ptr<COutput>> new_utxo_pool;
+        for (const auto& utxo : utxos) {
+            new_utxo_pool.insert(std::make_shared<COutput>(utxo));
+        }
+        for (auto& result : results) {
+            if (!result) continue;
+            const auto weight{result->GetWeight()};
+            result->AddInputs(new_utxo_pool, subtract_fee_outputs);
+            assert(result->GetWeight() > weight);
+        }
+    }
+
+    std::vector<COutput> manual_inputs;
+    CAmount manual_balance{CreateCoins(fuzzed_data_provider, manual_inputs, coin_params, next_locktime)};
+    if (manual_balance == 0) return;
+    auto manual_selection{ManualSelection(manual_inputs, manual_balance, coin_params.m_subtract_fee_outputs)};
+    for (auto& result : results) {
+        if (!result) continue;
+        const CAmount old_target{result->GetTarget()};
+        const std::set<std::shared_ptr<COutput>> input_set{result->GetInputSet()};
+        const int old_weight{result->GetWeight()};
+        result->Merge(manual_selection);
+        assert(result->GetInputSet().size() == input_set.size() + manual_inputs.size());
+        assert(result->GetTarget() == old_target + manual_selection.GetTarget());
+        assert(result->GetWeight() == old_weight + manual_selection.GetWeight());
+    }
 }
 
 } // namespace wallet