rpc, wallet: Allow users to specify input weights

Coin selection requires knowing the weight of a transaction so that fees
can be estimated. However for external inputs, the weight may not be
avialble, and solving data may not be enough as the input could be one
that we do not support. By allowing users to specify input weights,
those external inputs can be included in the transaction.

Additionally, if the weight for an input is specified, that value will
always be used, regardless of whether the input is in the wallet or
solving data is available. This allows us to account for scenarios where
the wallet may be more conservative and estimate a larger input than may
actually be created.

For example, we assume the maximum DER signature size, but an external
input may be signed by a wallet which does nonce grinding in order to get
a smaller signature. In that case, the user can specify the smaller
input weight to avoid overpaying transaction fees.

1 files changed, 77 insertions, 2 deletions

diff --git a/src/wallet/rpc/spend.cpp b/src/wallet/rpc/spend.cpp
index cae3542a5e..433b5a1815 100644
--- a/src/wallet/rpc/spend.cpp
+++ b/src/wallet/rpc/spend.cpp
@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <consensus/validation.h>
 #include <core_io.h>
 #include <key_io.h>
 #include <policy/policy.h>
@@ -429,6 +430,7 @@ void FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& fee_out,
                 {"replaceable", UniValueType(UniValue::VBOOL)},
                 {"conf_target", UniValueType(UniValue::VNUM)},
                 {"estimate_mode", UniValueType(UniValue::VSTR)},
+                {"input_weights", UniValueType(UniValue::VARR)},
             },
             true, true);
 
@@ -548,6 +550,37 @@ void FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& fee_out,
         }
     }
 
+    if (options.exists("input_weights")) {
+        for (const UniValue& input : options["input_weights"].get_array().getValues()) {
+            uint256 txid = ParseHashO(input, "txid");
+
+            const UniValue& vout_v = find_value(input, "vout");
+            if (!vout_v.isNum()) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, missing vout key");
+            }
+            int vout = vout_v.get_int();
+            if (vout < 0) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout cannot be negative");
+            }
+
+            const UniValue& weight_v = find_value(input, "weight");
+            if (!weight_v.isNum()) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, missing weight key");
+            }
+            int64_t weight = weight_v.get_int64();
+            const int64_t min_input_weight = GetTransactionInputWeight(CTxIn());
+            CHECK_NONFATAL(min_input_weight == 165);
+            if (weight < min_input_weight) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, weight cannot be less than 165 (41 bytes (size of outpoint + sequence + empty scriptSig) * 4 (witness scaling factor)) + 1 (empty witness)");
+            }
+            if (weight > MAX_STANDARD_TX_WEIGHT) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, weight cannot be greater than the maximum standard tx weight of %d", MAX_STANDARD_TX_WEIGHT));
+            }
+
+            coinControl.SetInputWeight(COutPoint(txid, vout), weight);
+        }
+    }
+
     if (tx.vout.size() == 0)
         throw JSONRPCError(RPC_INVALID_PARAMETER, "TX must have at least one output");
 
@@ -585,6 +618,23 @@ void FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& fee_out,
     }
 }
 
+static void SetOptionsInputWeights(const UniValue& inputs, UniValue& options)
+{
+    if (options.exists("input_weights")) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "Input weights should be specified in inputs rather than in options.");
+    }
+    if (inputs.size() == 0) {
+        return;
+    }
+    UniValue weights(UniValue::VARR);
+    for (const UniValue& input : inputs.getValues()) {
+        if (input.exists("weight")) {
+            weights.push_back(input);
+        }
+    }
+    options.pushKV("input_weights", weights);
+}
+
 RPCHelpMan fundrawtransaction()
 {
     return RPCHelpMan{"fundrawtransaction",
@@ -626,6 +676,17 @@ RPCHelpMan fundrawtransaction()
                                     {"vout_index", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "The zero-based output index, before a change output is added."},
                                 },
                             },
+                            {"input_weights", RPCArg::Type::ARR, RPCArg::Optional::OMITTED_NAMED_ARG, "Inputs and their corresponding weights",
+                                {
+                                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
+                                    {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output index"},
+                                    {"weight", RPCArg::Type::NUM, RPCArg::Optional::NO, "The maximum weight for this input, "
+                                        "including the weight of the outpoint and sequence number. "
+                                        "Note that serialized signature sizes are not guaranteed to be consistent, "
+                                        "so the maximum DER signatures size of 73 bytes should be used when considering ECDSA signatures."
+                                        "Remember to convert serialized sizes to weight units when necessary."},
+                                },
+                             },
                         },
                         FundTxDoc()),
                         "options"},
@@ -1007,6 +1068,11 @@ RPCHelpMan send()
                             {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                             {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                             {"sequence", RPCArg::Type::NUM, RPCArg::Optional::NO, "The sequence number"},
+                            {"weight", RPCArg::Type::NUM, RPCArg::DefaultHint{"Calculated from wallet and solving data"}, "The maximum weight for this input, "
+                                        "including the weight of the outpoint and sequence number. "
+                                        "Note that signature sizes are not guaranteed to be consistent, "
+                                        "so the maximum DER signatures size of 73 bytes should be used when considering ECDSA signatures."
+                                        "Remember to convert serialized sizes to weight units when necessary."},
                         },
                     },
                     {"locktime", RPCArg::Type::NUM, RPCArg::Default{0}, "Raw locktime. Non-0 value also locktime-activates inputs"},
@@ -1110,6 +1176,7 @@ RPCHelpMan send()
             // Automatically select coins, unless at least one is manually selected. Can
             // be overridden by options.add_inputs.
             coin_control.m_add_inputs = rawTx.vin.size() == 0;
+            SetOptionsInputWeights(options["inputs"], options);
             FundTransaction(*pwallet, rawTx, fee, change_position, options, coin_control, /* override_min_fee */ false);
 
             bool add_to_wallet = true;
@@ -1250,6 +1317,11 @@ RPCHelpMan walletcreatefundedpsbt()
                                     {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                                     {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                                     {"sequence", RPCArg::Type::NUM, RPCArg::DefaultHint{"depends on the value of the 'locktime' and 'options.replaceable' arguments"}, "The sequence number"},
+                                    {"weight", RPCArg::Type::NUM, RPCArg::DefaultHint{"Calculated from wallet and solving data"}, "The maximum weight for this input, "
+                                        "including the weight of the outpoint and sequence number. "
+                                        "Note that signature sizes are not guaranteed to be consistent, "
+                                        "so the maximum DER signatures size of 73 bytes should be used when considering ECDSA signatures."
+                                        "Remember to convert serialized sizes to weight units when necessary."},
                                 },
                             },
                         },
@@ -1330,10 +1402,12 @@ RPCHelpMan walletcreatefundedpsbt()
         }, true
     );
 
+    UniValue options = request.params[3];
+
     CAmount fee;
     int change_position;
     bool rbf{wallet.m_signal_rbf};
-    const UniValue &replaceable_arg = request.params[3]["replaceable"];
+    const UniValue &replaceable_arg = options["replaceable"];
     if (!replaceable_arg.isNull()) {
         RPCTypeCheckArgument(replaceable_arg, UniValue::VBOOL);
         rbf = replaceable_arg.isTrue();
@@ -1343,7 +1417,8 @@ RPCHelpMan walletcreatefundedpsbt()
     // Automatically select coins, unless at least one is manually selected. Can
     // be overridden by options.add_inputs.
     coin_control.m_add_inputs = rawTx.vin.size() == 0;
-    FundTransaction(wallet, rawTx, fee, change_position, request.params[3], coin_control, /* override_min_fee */ true);
+    SetOptionsInputWeights(request.params[0], options);
+    FundTransaction(wallet, rawTx, fee, change_position, options, coin_control, /* override_min_fee */ true);
 
     // Make a blank psbt
     PartiallySignedTransaction psbtx(rawTx);