test: Test max_selection_weight edge cases

Slays Mutant 37 from Bruno’s report:
https://gist.github.com/brunoerg/834063398d5002f738506d741513e310

diff --git a/src/wallet/coinselection.cpp b/muts/coinselection.mutant.37.cpp
index cee5580..9747cd26c9 100644
--- a/src/wallet/coinselection.cpp
+++ b/muts/coinselection.mutant.37.cpp
@@ -128,7 +128,7 @@ util::Result<SelectionResult> SelectCoinsBnB(std::vector<OutputGroup>& utxo_pool
             curr_value > selection_target + cost_of_change || // Selected value is out of range, go back and try other branch
             (curr_waste > best_waste && is_feerate_high)) { // Don't select things which we know will be more wasteful if the waste is increasing
             backtrack = true;
-        } else if (curr_selection_weight > max_selection_weight) { // Selected UTXOs weight exceeds the maximum weight allowed, cannot find more solutions by adding more inputs
+        } else if (curr_selection_weight >= max_selection_weight) { // Selected UTXOs weight exceeds the maximum weight allowed, cannot find more solutions by adding more inputs
             max_tx_weight_exceeded = true; // at least one selection attempt exceeded the max weight
             backtrack = true;
         } else if (curr_value >= selection_target) {       // Selected value is within range

Author: murchandamus

src/wallet/test/coinselection_tests.cpp

@@ -15,20 +15,23 @@ BOOST_FIXTURE_TEST_SUITE(coinselection_tests, TestingSetup)
 static int next_lock_time = 0;
 static FastRandomContext default_rand;
 
+static const int P2WPKH_INPUT_VSIZE = 68;
+static const int P2WPKH_OUTPUT_VSIZE = 31;
+
 /** Default coin selection parameters (dcsp) allow us to only explicitly set
  * parameters when a diverging value is relevant in the context of a test.
  * We use P2WPKH input and output weights for the change weights. */
 static CoinSelectionParams init_default_params()
 {
     CoinSelectionParams dcsp{
         /*rng_fast*/default_rand,
-        /*change_output_size=*/31,
-        /*change_spend_size=*/68,
+        /*change_output_size=*/P2WPKH_OUTPUT_VSIZE,
+        /*change_spend_size=*/P2WPKH_INPUT_VSIZE,
         /*min_change_target=*/50'000,
         /*effective_feerate=*/CFeeRate(5000),
         /*long_term_feerate=*/CFeeRate(10'000),
         /*discard_feerate=*/CFeeRate(3000),
-        /*tx_noinputs_size=*/11 + 31, //static header size + output size
+        /*tx_noinputs_size=*/11 + P2WPKH_OUTPUT_VSIZE, //static header size + output size
         /*avoid_partial=*/false,
     };
     dcsp.m_change_fee = /*155 sats=*/dcsp.m_effective_feerate.GetFee(dcsp.change_output_size);
@@ -41,7 +44,7 @@ static CoinSelectionParams init_default_params()
 static const CoinSelectionParams default_cs_params = init_default_params();
 
 /** Make one OutputGroup with a single UTXO that either has a given effective value (default) or a given amount (`is_eff_value = false`). */
-static OutputGroup MakeCoin(const CAmount& amount, bool is_eff_value = true, CoinSelectionParams cs_params = default_cs_params, int custom_spending_vsize = 68)
+static OutputGroup MakeCoin(const CAmount& amount, bool is_eff_value = true, CoinSelectionParams cs_params = default_cs_params, int custom_spending_vsize = P2WPKH_INPUT_VSIZE)
 {
     // Always assume that we only have one input
     CMutableTransaction tx;
@@ -93,7 +96,7 @@ static std::string InputAmountsToString(const SelectionResult& selection)
     return "[" + util::Join(selection.GetInputSet(), " ", [](const auto& input){ return util::ToString(input->txout.nValue);}) + "]";
 }
 
-static void TestBnBSuccess(std::string test_title, std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const std::vector<CAmount>& expected_input_amounts, const CoinSelectionParams& cs_params = default_cs_params, int custom_spending_vsize = 68)
+static void TestBnBSuccess(std::string test_title, std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const std::vector<CAmount>& expected_input_amounts, const CoinSelectionParams& cs_params = default_cs_params, const int custom_spending_vsize = P2WPKH_INPUT_VSIZE, const int max_selection_weight = MAX_STANDARD_TX_WEIGHT)
 {
     SelectionResult expected_result(CAmount(0), SelectionAlgorithm::BNB);
     CAmount expected_amount = 0;
@@ -103,15 +106,15 @@ static void TestBnBSuccess(std::string test_title, std::vector<OutputGroup>& utx
         expected_result.AddInput(group);
     }
 
-    const auto result = SelectCoinsBnB(utxo_pool, selection_target, /*cost_of_change=*/default_cs_params.m_cost_of_change, /*max_selection_weight=*/MAX_STANDARD_TX_WEIGHT);
+    const auto result = SelectCoinsBnB(utxo_pool, selection_target, /*cost_of_change=*/default_cs_params.m_cost_of_change, max_selection_weight);
     BOOST_CHECK_MESSAGE(result, "Falsy result in BnB-Success: " + test_title);
     BOOST_CHECK_MESSAGE(HaveEquivalentValues(expected_result, *result), strprintf("Result mismatch in BnB-Success: %s. Expected %s, but got %s", test_title, InputAmountsToString(expected_result), InputAmountsToString(*result)));
     BOOST_CHECK_MESSAGE(result->GetSelectedValue() == expected_amount, strprintf("Selected amount mismatch in BnB-Success: %s. Expected %d, but got %d", test_title, expected_amount, result->GetSelectedValue()));
 }
 
-static void TestBnBFail(std::string test_title, std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, const bool expect_max_weight_exceeded = false)
+static void TestBnBFail(std::string test_title, std::vector<OutputGroup>& utxo_pool, const CAmount& selection_target, int max_selection_weight = MAX_STANDARD_TX_WEIGHT, const bool expect_max_weight_exceeded = false)
 {
-    const auto result = SelectCoinsBnB(utxo_pool, selection_target, /*cost_of_change=*/default_cs_params.m_cost_of_change, /*max_selection_weight=*/MAX_STANDARD_TX_WEIGHT);
+    const auto result = SelectCoinsBnB(utxo_pool, selection_target, /*cost_of_change=*/default_cs_params.m_cost_of_change, max_selection_weight);
     BOOST_CHECK_MESSAGE(!result, "BnB-Fail: " + test_title);
     bool max_weight_exceeded = util::ErrorString(result).original.find("The inputs size exceeds the maximum weight") != std::string::npos;
     BOOST_CHECK(expect_max_weight_exceeded == max_weight_exceeded);
@@ -145,6 +148,10 @@ BOOST_AUTO_TEST_CASE(bnb_test)
         // BnB fails to find changeless solution when overshooting by cost_of_change + 1 sat
         TestBnBFail("Overshoot upper bound", utxo_pool, /*selection_target=*/4 * CENT - default_cs_params.m_cost_of_change - 1);
 
+        TestBnBSuccess("Select max weight", utxo_pool, /*selection_target=*/4 * CENT, /*expected_input_amounts=*/{1 * CENT, 3 * CENT}, cs_params, /*custom_spending_vsize=*/P2WPKH_INPUT_VSIZE, /*max_selection_weight=*/4 * 2 * P2WPKH_INPUT_VSIZE);
+
+        TestBnBFail("Exceed max weight", utxo_pool, /*selection_target=*/4 * CENT, /*max_selection_weight=*/4 * 2 * P2WPKH_INPUT_VSIZE - 1, /*expect_max_weight_exceeded=*/true);
+
         // Simple cases without BnB solution
         TestBnBFail("Smallest combination too big", utxo_pool, /*selection_target=*/0.5 * CENT);
         TestBnBFail("No UTXO combination in target window", utxo_pool, /*selection_target=*/7 * CENT);