wallet: Add coin control option "change_target"

Add a coin control rpc parameter to optionally force a particular change target for coin selection algorithms that result in a change output.
remyers · Aug 22, 2024 · 86e6207 · 86e6207
1 parent 0cbdc6b
commit 86e6207
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Showing 3 changed files with 15 additions and 1 deletion.
diff --git a/src/wallet/coincontrol.h b/src/wallet/coincontrol.h
@@ -115,6 +115,8 @@ class CCoinControl
     std::optional<uint32_t> m_locktime;
     //! Version
     std::optional<uint32_t> m_version;
+    //! Use this change target instead of default behavior if set
+    std::optional<CAmount> m_change_target;
 
     CCoinControl();
 

diff --git a/src/wallet/rpc/spend.cpp b/src/wallet/rpc/spend.cpp
@@ -535,6 +535,7 @@ CreatedTransactionResult FundTransaction(CWallet& wallet, const CMutableTransact
                 {"minconf", UniValueType(UniValue::VNUM)},
                 {"maxconf", UniValueType(UniValue::VNUM)},
                 {"input_weights", UniValueType(UniValue::VARR)},
+                {"change_target", UniValueType()}, // will be checked by AmountFromValue() below
             },
             true, true);
 
@@ -702,6 +703,10 @@ CreatedTransactionResult FundTransaction(CWallet& wallet, const CMutableTransact
         }
     }
 
+    if (options.exists("change_target")) {
+        coinControl.m_change_target = CAmount(AmountFromValue(options["change_target"]));
+    }
+
     if (recipients.empty())
         throw JSONRPCError(RPC_INVALID_PARAMETER, "TX must have at least one output");
 
@@ -787,6 +792,7 @@ RPCHelpMan fundrawtransaction()
                                     },
                                 },
                              },
+                             {"change_target", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to default wallet behavior"}, "Specify a target change amount in " + CURRENCY_UNIT + "."},
                         },
                         FundTxDoc()),
                         RPCArgOptions{
@@ -1241,6 +1247,7 @@ RPCHelpMan send()
                             {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                         },
                     },
+                    {"change_target", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to default wallet behavior"}, "Specify a target change amount in " + CURRENCY_UNIT + "."},
                 },
                 FundTxDoc()),
                 RPCArgOptions{.oneline_description="options"}},
@@ -1690,6 +1697,7 @@ RPCHelpMan walletcreatefundedpsbt()
                                     {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                                 },
                             },
+                            {"change_target", RPCArg::Type::AMOUNT, RPCArg::DefaultHint{"not set, fall back to default wallet behavior"}, "Specify a target change amount in " + CURRENCY_UNIT + "."},
                         },
                         FundTxDoc()),
                         RPCArgOptions{.oneline_description="options"}},

diff --git a/src/wallet/spend.cpp b/src/wallet/spend.cpp
@@ -1077,7 +1077,11 @@ static util::Result<CreatedTransactionResult> CreateTransactionInternal(
     coin_selection_params.m_change_fee = coin_selection_params.m_effective_feerate.GetFee(coin_selection_params.change_output_size);
     coin_selection_params.m_cost_of_change = coin_selection_params.m_discard_feerate.GetFee(coin_selection_params.change_spend_size) + coin_selection_params.m_change_fee;
 
-    coin_selection_params.m_min_change_target = GenerateChangeTarget(std::floor(recipients_sum / vecSend.size()), coin_selection_params.m_change_fee, rng_fast);
+    if (coin_control.m_change_target) {
+        coin_selection_params.m_min_change_target = coin_control.m_change_target.value();
+    } else {
+        coin_selection_params.m_min_change_target = GenerateChangeTarget(std::floor(recipients_sum / vecSend.size()), coin_selection_params.m_change_fee, rng_fast);
+    }
 
     // The smallest change amount should be:
     // 1. at least equal to dust threshold