[wallet, rpc]: add `max_tx_weight` to tx funding options

This allows a transaction's weight to be bound under a certain
weight if possible and desired. This can be beneficial for future
RBF attempts, or whenever a more restricted spend topology is
desired.

Co-authored-by: Greg Sanders <gsanders87@gmail.com>

7 files changed, 73 insertions, 17 deletions

diff --git a/src/rpc/client.cpp b/src/rpc/client.cpp
index 7dfe69a6c5..b866fa484b 100644
--- a/src/rpc/client.cpp
+++ b/src/rpc/client.cpp
@@ -146,6 +146,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "fundrawtransaction", 1, "conf_target"},
     { "fundrawtransaction", 1, "replaceable"},
     { "fundrawtransaction", 1, "solving_data"},
+    { "fundrawtransaction", 1, "max_tx_weight"},
     { "fundrawtransaction", 2, "iswitness" },
     { "walletcreatefundedpsbt", 0, "inputs" },
     { "walletcreatefundedpsbt", 1, "outputs" },
@@ -164,6 +165,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "walletcreatefundedpsbt", 3, "conf_target"},
     { "walletcreatefundedpsbt", 3, "replaceable"},
     { "walletcreatefundedpsbt", 3, "solving_data"},
+    { "walletcreatefundedpsbt", 3, "max_tx_weight"},
     { "walletcreatefundedpsbt", 4, "bip32derivs" },
     { "walletprocesspsbt", 1, "sign" },
     { "walletprocesspsbt", 3, "bip32derivs" },
@@ -208,6 +210,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "send", 4, "conf_target"},
     { "send", 4, "replaceable"},
     { "send", 4, "solving_data"},
+    { "send", 4, "max_tx_weight"},
     { "sendall", 0, "recipients" },
     { "sendall", 1, "conf_target" },
     { "sendall", 3, "fee_rate"},
diff --git a/src/wallet/coincontrol.h b/src/wallet/coincontrol.h
index b2f813383d..d36314312a 100644
--- a/src/wallet/coincontrol.h
+++ b/src/wallet/coincontrol.h
@@ -115,6 +115,8 @@ public:
     std::optional<uint32_t> m_locktime;
     //! Version
     std::optional<uint32_t> m_version;
+    //! Caps weight of resulting tx
+    std::optional<int> m_max_tx_weight{std::nullopt};
 
     CCoinControl();
 
diff --git a/src/wallet/coinselection.cpp b/src/wallet/coinselection.cpp
index c37d42d22a..a54d1a4668 100644
--- a/src/wallet/coinselection.cpp
+++ b/src/wallet/coinselection.cpp
@@ -597,11 +597,12 @@ util::Result<SelectionResult> SelectCoinsSRD(const std::vector<OutputGroup>& utx
  *                              nTargetValue, with indices corresponding to groups. If the ith
  *                              entry is true, that means the ith group in groups was selected.
  * param@[out]  nBest           Total amount of subset chosen that is closest to nTargetValue.
+ * paramp[in]   max_selection_weight  The maximum allowed weight for a selection result to be valid.
  * param@[in]   iterations      Maximum number of tries.
  */
 static void ApproximateBestSubset(FastRandomContext& insecure_rand, const std::vector<OutputGroup>& groups,
                                   const CAmount& nTotalLower, const CAmount& nTargetValue,
-                                  std::vector<char>& vfBest, CAmount& nBest, int iterations = 1000)
+                                  std::vector<char>& vfBest, CAmount& nBest, int max_selection_weight, int iterations = 1000)
 {
     std::vector<char> vfIncluded;
 
@@ -613,6 +614,7 @@ static void ApproximateBestSubset(FastRandomContext& insecure_rand, const std::v
     {
         vfIncluded.assign(groups.size(), false);
         CAmount nTotal = 0;
+        int selected_coins_weight{0};
         bool fReachedTarget = false;
         for (int nPass = 0; nPass < 2 && !fReachedTarget; nPass++)
         {
@@ -627,9 +629,9 @@ static void ApproximateBestSubset(FastRandomContext& insecure_rand, const std::v
                 if (nPass == 0 ? insecure_rand.randbool() : !vfIncluded[i])
                 {
                     nTotal += groups[i].GetSelectionAmount();
+                    selected_coins_weight += groups[i].m_weight;
                     vfIncluded[i] = true;
-                    if (nTotal >= nTargetValue)
-                    {
+                    if (nTotal >= nTargetValue && selected_coins_weight <= max_selection_weight) {
                         fReachedTarget = true;
                         // If the total is between nTargetValue and nBest, it's our new best
                         // approximation.
@@ -639,6 +641,7 @@ static void ApproximateBestSubset(FastRandomContext& insecure_rand, const std::v
                             vfBest = vfIncluded;
                         }
                         nTotal -= groups[i].GetSelectionAmount();
+                        selected_coins_weight -= groups[i].m_weight;
                         vfIncluded[i] = false;
                     }
                 }
@@ -652,6 +655,7 @@ util::Result<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, c
 {
     SelectionResult result(nTargetValue, SelectionAlgorithm::KNAPSACK);
 
+    bool max_weight_exceeded{false};
     // List of values less than target
     std::optional<OutputGroup> lowest_larger;
     // Groups with selection amount smaller than the target and any change we might produce.
@@ -662,6 +666,10 @@ util::Result<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, c
     Shuffle(groups.begin(), groups.end(), rng);
 
     for (const OutputGroup& group : groups) {
+        if (group.m_weight > max_selection_weight) {
+            max_weight_exceeded = true;
+            continue;
+        }
         if (group.GetSelectionAmount() == nTargetValue) {
             result.AddInput(group);
             return result;
@@ -677,11 +685,18 @@ util::Result<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, c
         for (const auto& group : applicable_groups) {
             result.AddInput(group);
         }
-        return result;
+        if (result.GetWeight() <= max_selection_weight) return result;
+        else max_weight_exceeded = true;
+
+        // Try something else
+        result.Clear();
     }
 
     if (nTotalLower < nTargetValue) {
-        if (!lowest_larger) return util::Error();
+        if (!lowest_larger) {
+            if (max_weight_exceeded) return ErrorMaxWeightExceeded();
+            return util::Error();
+        }
         result.AddInput(*lowest_larger);
         return result;
     }
@@ -691,9 +706,9 @@ util::Result<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, c
     std::vector<char> vfBest;
     CAmount nBest;
 
-    ApproximateBestSubset(rng, applicable_groups, nTotalLower, nTargetValue, vfBest, nBest);
+    ApproximateBestSubset(rng, applicable_groups, nTotalLower, nTargetValue, vfBest, nBest, max_selection_weight);
     if (nBest != nTargetValue && nTotalLower >= nTargetValue + change_target) {
-        ApproximateBestSubset(rng, applicable_groups, nTotalLower, nTargetValue + change_target, vfBest, nBest);
+        ApproximateBestSubset(rng, applicable_groups, nTotalLower, nTargetValue + change_target, vfBest, nBest, max_selection_weight);
     }
 
     // If we have a bigger coin and (either the stochastic approximation didn't find a good solution,
@@ -728,7 +743,7 @@ util::Result<SelectionResult> KnapsackSolver(std::vector<OutputGroup>& groups, c
             LogPrint(BCLog::SELECTCOINS, "%stotal %s\n", log_message, FormatMoney(nBest));
         }
     }
-
+    Assume(result.GetWeight() <= max_selection_weight);
     return result;
 }
 
diff --git a/src/wallet/coinselection.h b/src/wallet/coinselection.h
index 1f234427d2..8a81cfc268 100644
--- a/src/wallet/coinselection.h
+++ b/src/wallet/coinselection.h
@@ -174,10 +174,13 @@ struct CoinSelectionParams {
      * 1) Received from other wallets, 2) replacing other txs, 3) that have been replaced.
      */
     bool m_include_unsafe_inputs = false;
+    /** The maximum weight for this transaction. */
+    std::optional<int> m_max_tx_weight{std::nullopt};
 
     CoinSelectionParams(FastRandomContext& rng_fast, int change_output_size, int change_spend_size,
                         CAmount min_change_target, CFeeRate effective_feerate,
-                        CFeeRate long_term_feerate, CFeeRate discard_feerate, int tx_noinputs_size, bool avoid_partial)
+                        CFeeRate long_term_feerate, CFeeRate discard_feerate, int tx_noinputs_size, bool avoid_partial,
+                        std::optional<int> max_tx_weight = std::nullopt)
         : rng_fast{rng_fast},
           change_output_size(change_output_size),
           change_spend_size(change_spend_size),
@@ -186,7 +189,8 @@ struct CoinSelectionParams {
           m_long_term_feerate(long_term_feerate),
           m_discard_feerate(discard_feerate),
           tx_noinputs_size(tx_noinputs_size),
-          m_avoid_partial_spends(avoid_partial)
+          m_avoid_partial_spends(avoid_partial),
+          m_max_tx_weight(max_tx_weight)
     {
     }
     CoinSelectionParams(FastRandomContext& rng_fast)
diff --git a/src/wallet/rpc/spend.cpp b/src/wallet/rpc/spend.cpp
index ac2a4826f0..8ea51e65c7 100644
--- a/src/wallet/rpc/spend.cpp
+++ b/src/wallet/rpc/spend.cpp
@@ -542,6 +542,7 @@ CreatedTransactionResult FundTransaction(CWallet& wallet, const CMutableTransact
                 {"minconf", UniValueType(UniValue::VNUM)},
                 {"maxconf", UniValueType(UniValue::VNUM)},
                 {"input_weights", UniValueType(UniValue::VARR)},
+                {"max_tx_weight", UniValueType(UniValue::VNUM)},
             },
             true, true);
 
@@ -701,6 +702,10 @@ CreatedTransactionResult FundTransaction(CWallet& wallet, const CMutableTransact
         }
     }
 
+    if (options.exists("max_tx_weight")) {
+        coinControl.m_max_tx_weight = options["max_tx_weight"].getInt<int>();
+    }
+
     if (recipients.empty())
         throw JSONRPCError(RPC_INVALID_PARAMETER, "TX must have at least one output");
 
@@ -786,6 +791,8 @@ RPCHelpMan fundrawtransaction()
                                     },
                                 },
                              },
+                            {"max_tx_weight", RPCArg::Type::NUM, RPCArg::Default{MAX_STANDARD_TX_WEIGHT}, "The maximum acceptable transaction weight.\n"
+                                                          "Transaction building will fail if this can not be satisfied."},
                         },
                         FundTxDoc()),
                         RPCArgOptions{
@@ -1240,6 +1247,8 @@ RPCHelpMan send()
                             {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                         },
                     },
+                    {"max_tx_weight", RPCArg::Type::NUM, RPCArg::Default{MAX_STANDARD_TX_WEIGHT}, "The maximum acceptable transaction weight.\n"
+                                                  "Transaction building will fail if this can not be satisfied."},
                 },
                 FundTxDoc()),
                 RPCArgOptions{.oneline_description="options"}},
@@ -1287,6 +1296,9 @@ RPCHelpMan send()
             // Automatically select coins, unless at least one is manually selected. Can
             // be overridden by options.add_inputs.
             coin_control.m_allow_other_inputs = rawTx.vin.size() == 0;
+            if (options.exists("max_tx_weight")) {
+                coin_control.m_max_tx_weight = options["max_tx_weight"].getInt<int>();
+            }
             SetOptionsInputWeights(options["inputs"], options);
             // Clear tx.vout since it is not meant to be used now that we are passing outputs directly.
             // This sets us up for a future PR to completely remove tx from the function signature in favor of passing inputs directly
@@ -1697,6 +1709,8 @@ RPCHelpMan walletcreatefundedpsbt()
                                     {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                                 },
                             },
+                            {"max_tx_weight", RPCArg::Type::NUM, RPCArg::Default{MAX_STANDARD_TX_WEIGHT}, "The maximum acceptable transaction weight.\n"
+                                                          "Transaction building will fail if this can not be satisfied."},
                         },
                         FundTxDoc()),
                         RPCArgOptions{.oneline_description="options"}},
diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
index e0546c6210..1ad570442e 100644
--- a/src/wallet/spend.cpp
+++ b/src/wallet/spend.cpp
@@ -696,7 +696,12 @@ util::Result<SelectionResult> ChooseSelectionResult(interfaces::Chain& chain, co
     };
 
     // Maximum allowed weight for selected coins.
-    int max_selection_weight = MAX_STANDARD_TX_WEIGHT - (coin_selection_params.tx_noinputs_size * WITNESS_SCALE_FACTOR);
+    int max_transaction_weight = coin_selection_params.m_max_tx_weight.value_or(MAX_STANDARD_TX_WEIGHT);
+    int tx_weight_no_input = coin_selection_params.tx_noinputs_size * WITNESS_SCALE_FACTOR;
+    int max_selection_weight = max_transaction_weight - tx_weight_no_input;
+    if (max_selection_weight <= 0) {
+        return util::Error{_("Maximum transaction weight is less than transaction weight without inputs")};
+    }
 
     // SFFO frequently causes issues in the context of changeless input sets: skip BnB when SFFO is active
     if (!coin_selection_params.m_subtract_fee_outputs) {
@@ -706,7 +711,11 @@ util::Result<SelectionResult> ChooseSelectionResult(interfaces::Chain& chain, co
     }
 
     // Deduct change weight because remaining Coin Selection algorithms can create change output
-    max_selection_weight -= (coin_selection_params.change_output_size * WITNESS_SCALE_FACTOR);
+    int change_outputs_weight = coin_selection_params.change_output_size * WITNESS_SCALE_FACTOR;
+    max_selection_weight -= change_outputs_weight;
+    if (max_selection_weight < 0 && results.empty()) {
+        return util::Error{_("Maximum transaction weight is too low, can not accommodate change output")};
+    }
 
     // The knapsack solver has some legacy behavior where it will spend dust outputs. We retain this behavior, so don't filter for positive only here.
     if (auto knapsack_result{KnapsackSolver(groups.mixed_group, nTargetValue, coin_selection_params.m_min_change_target, coin_selection_params.rng_fast, max_selection_weight)}) {
@@ -801,7 +810,7 @@ util::Result<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& av
                                                 coin_selection_params.m_change_fee);
 
         // Verify we haven't exceeded the maximum allowed weight
-        int max_inputs_weight = MAX_STANDARD_TX_WEIGHT - (coin_selection_params.tx_noinputs_size * WITNESS_SCALE_FACTOR);
+        int max_inputs_weight = coin_selection_params.m_max_tx_weight.value_or(MAX_STANDARD_TX_WEIGHT) - (coin_selection_params.tx_noinputs_size * WITNESS_SCALE_FACTOR);
         if (op_selection_result->GetWeight() > max_inputs_weight) {
             return util::Error{_("The combination of the pre-selected inputs and the wallet automatic inputs selection exceeds the transaction maximum weight. "
                                  "Please try sending a smaller amount or manually consolidating your wallet's UTXOs")};
@@ -1002,7 +1011,11 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
     CoinSelectionParams coin_selection_params{rng_fast}; // Parameters for coin selection, init with dummy
     coin_selection_params.m_avoid_partial_spends = coin_control.m_avoid_partial_spends;
     coin_selection_params.m_include_unsafe_inputs = coin_control.m_include_unsafe_inputs;
-
+    coin_selection_params.m_max_tx_weight = coin_control.m_max_tx_weight.value_or(MAX_STANDARD_TX_WEIGHT);
+    int minimum_tx_weight = MIN_STANDARD_TX_NONWITNESS_SIZE * WITNESS_SCALE_FACTOR;
+    if (coin_selection_params.m_max_tx_weight.value() < minimum_tx_weight || coin_selection_params.m_max_tx_weight.value() > MAX_STANDARD_TX_WEIGHT) {
+        return util::Error{strprintf(_("Maximum transaction weight must be between %d and %d"), minimum_tx_weight, MAX_STANDARD_TX_WEIGHT)};
+    }
     // Set the long term feerate estimate to the wallet's consolidate feerate
     coin_selection_params.m_long_term_feerate = wallet.m_consolidate_feerate;
 
diff --git a/src/wallet/test/fuzz/coinselection.cpp b/src/wallet/test/fuzz/coinselection.cpp
index 72975c3d68..209c87fd42 100644
--- a/src/wallet/test/fuzz/coinselection.cpp
+++ b/src/wallet/test/fuzz/coinselection.cpp
@@ -256,29 +256,34 @@ FUZZ_TARGET(coinselection)
         (void)group.EligibleForSpending(filter);
     }
 
+    int max_selection_weight = fuzzed_data_provider.ConsumeIntegralInRange<int>(0, std::numeric_limits<int>::max());
+
     // Run coinselection algorithms
     auto result_bnb = coin_params.m_subtract_fee_outputs ? util::Error{Untranslated("BnB disabled when SFFO is enabled")} :
-                      SelectCoinsBnB(group_pos, target, coin_params.m_cost_of_change, MAX_STANDARD_TX_WEIGHT);
+                      SelectCoinsBnB(group_pos, target, coin_params.m_cost_of_change, max_selection_weight);
     if (result_bnb) {
         assert(result_bnb->GetChange(coin_params.min_viable_change, coin_params.m_change_fee) == 0);
         assert(result_bnb->GetSelectedValue() >= target);
+        assert(result_bnb->GetWeight() <= max_selection_weight);
         (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);
+    auto result_srd = SelectCoinsSRD(group_pos, target, coin_params.m_change_fee, fast_random_context, max_selection_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
+        assert(result_srd->GetWeight() <= max_selection_weight);
         result_srd->RecalculateWaste(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);
+    auto result_knapsack = KnapsackSolver(group_all, target, change_target, fast_random_context, max_selection_weight);
     if (result_knapsack) {
         assert(result_knapsack->GetSelectedValue() >= target);
+        assert(result_knapsack->GetWeight() <= max_selection_weight);
         result_knapsack->RecalculateWaste(coin_params.min_viable_change, coin_params.m_cost_of_change, coin_params.m_change_fee);
         (void)result_knapsack->GetShuffledInputVector();
         (void)result_knapsack->GetInputSet();