coin selection: knapsack, select closest UTXO above target if result exceeds max tx size

The simplest scenario where this is useful is on the 'check_max_weight' unit test
already:

We create 1515 UTXOs with 0.033 BTC each, and 1 UTXO with 50 BTC. Then perform
Coin Selection.

As the selection of the 1515 small UTXOs exceeds the max allowed tx size, the
expectation here is to receive a selection result that only contain the big
UTXO (which is not happening for the reasons stated below).

As knapsack returns a result that exceeds the max allowed transaction size, we
fallback to SRD, which selects coins randomly up until the target is met. So
we end up with a selection result with lot more coins than what is needed.

2 files changed, 19 insertions, 4 deletions

diff --git a/src/wallet/test/coinselector_tests.cpp b/src/wallet/test/coinselector_tests.cpp
index 8f626addde..db8322e3cb 100644
--- a/src/wallet/test/coinselector_tests.cpp
+++ b/src/wallet/test/coinselector_tests.cpp
@@ -87,6 +87,14 @@ static void add_coin(CoinsResult& available_coins, CWallet& wallet, const CAmoun
     available_coins.Add(OutputType::BECH32, {COutPoint(wtx.GetHash(), nInput), txout, nAge, CalculateMaximumSignedInputSize(txout, &wallet, /*coin_control=*/nullptr), /*spendable=*/ true, /*solvable=*/ true, /*safe=*/ true, wtx.GetTxTime(), fIsFromMe, feerate});
 }
 
+// Helper
+std::optional<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, const CAmount& nTargetValue,
+                                              CAmount change_target, FastRandomContext& rng)
+{
+    auto res{KnapsackSolver(groups, nTargetValue, change_target, rng, MAX_STANDARD_TX_WEIGHT)};
+    return res ? std::optional<SelectionResult>(*res) : std::nullopt;
+}
+
 /** Check if SelectionResult a is equivalent to SelectionResult b.
  * Equivalent means same input values, but maybe different inputs (i.e. same value, different prevout) */
 static bool EquivalentResult(const SelectionResult& a, const SelectionResult& b)
@@ -987,7 +995,10 @@ BOOST_AUTO_TEST_CASE(check_max_weight)
             chain);
 
         BOOST_CHECK(result);
-        BOOST_CHECK(has_coin(result->GetInputSet(), CAmount(50 * COIN)));
+        // Verify that only the 50 BTC UTXO was selected
+        const auto& selection_res = result->GetInputSet();
+        BOOST_CHECK(selection_res.size() == 1);
+        BOOST_CHECK((*selection_res.begin())->GetEffectiveValue() == 50 * COIN);
     }
 
     {
diff --git a/src/wallet/test/fuzz/coinselection.cpp b/src/wallet/test/fuzz/coinselection.cpp
index 304190eec1..4878b24c30 100644
--- a/src/wallet/test/fuzz/coinselection.cpp
+++ b/src/wallet/test/fuzz/coinselection.cpp
@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <policy/feerate.h>
+#include <policy/policy.h>
 #include <primitives/transaction.h>
 #include <test/fuzz/FuzzedDataProvider.h>
 #include <test/fuzz/fuzz.h>
@@ -44,6 +45,9 @@ static void GroupCoins(FuzzedDataProvider& fuzzed_data_provider, const std::vect
     if (valid_outputgroup) output_groups.push_back(output_group);
 }
 
+// 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(); }
+
 FUZZ_TARGET(coinselection)
 {
     FuzzedDataProvider fuzzed_data_provider{buffer.data(), buffer.size()};
@@ -90,12 +94,12 @@ FUZZ_TARGET(coinselection)
     if (result_srd) result_srd->ComputeAndSetWaste(cost_of_change, cost_of_change, 0);
 
     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);
+    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 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) {
+    // 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);
     }
 }