[rpc] fundrawtransaction: add_inputs option to control automatic input adding

Author: Sjors

doc/release-notes-16377.md

@@ -4,3 +4,6 @@ RPC changes
   `Insufficient funds` when inputs are manually selected but are not enough to cover
   the outputs and fee. Additional inputs can automatically be added through the
   new `add_inputs` option.
+
+- The `fundrawtransaction` RPC now supports `add_inputs` option that when `false`
+  prevents adding more inputs if necessary and consequently the RPC fails.

src/wallet/rpcwallet.cpp

@@ -3148,8 +3148,8 @@ static UniValue fundrawtransaction(const JSONRPCRequest& request)
     }
 
     RPCHelpMan{"fundrawtransaction",
-                "\nAdd inputs to a transaction until it has enough in value to meet its out value.\n"
-                "This will not modify existing inputs, and will add at most one change output to the outputs.\n"
+                "\nIf the transaction has no inputs, they will be automatically selected to meet its out value.\n"
+                "It will add at most one change output to the outputs.\n"
                 "No existing outputs will be modified unless \"subtractFeeFromOutputs\" is specified.\n"
                 "Note that inputs which were signed may need to be resigned after completion since in/outputs have been added.\n"
                 "The inputs added will not be signed, use signrawtransactionwithkey\n"
@@ -3163,6 +3163,7 @@ static UniValue fundrawtransaction(const JSONRPCRequest& request)
                     {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The hex string of the raw transaction"},
                     {"options", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED_NAMED_ARG, "for backward compatibility: passing in a true instead of an object will result in {\"includeWatching\":true}",
                         {
+                            {"add_inputs", RPCArg::Type::BOOL, /* default */ "true", "For a transaction with existing inputs, automatically include more if they are not enough."},
                             {"changeAddress", RPCArg::Type::STR, /* default */ "pool address", "The bitcoin address to receive the change"},
                             {"changePosition", RPCArg::Type::NUM, /* default */ "random", "The index of the change output"},
                             {"change_type", RPCArg::Type::STR, /* default */ "set by -changetype", "The output type to use. Only valid if changeAddress is not specified. Options are \"legacy\", \"p2sh-segwit\", and \"bech32\"."},
@@ -3229,6 +3230,8 @@ static UniValue fundrawtransaction(const JSONRPCRequest& request)
     CAmount fee;
     int change_position;
     CCoinControl coin_control;
+    // Automatically select (additional) coins. Can be overriden by options.add_inputs.
+    coin_control.m_add_inputs = true;
     FundTransaction(pwallet, tx, fee, change_position, request.params[1], coin_control);
 
     UniValue result(UniValue::VOBJ);

test/functional/rpc_fundrawtransaction.py

@@ -271,7 +271,11 @@ def test_coin_selection(self):
         assert_equal(utx['txid'], dec_tx['vin'][0]['txid'])
         assert_equal("00", dec_tx['vin'][0]['scriptSig']['hex'])
 
+        # Should fail without add_inputs:
+        assert_raises_rpc_error(-4, "Insufficient funds", self.nodes[2].fundrawtransaction, rawtx, {"add_inputs": False})
+        # add_inputs is enabled by default
         rawtxfund = self.nodes[2].fundrawtransaction(rawtx)
+
         dec_tx  = self.nodes[2].decoderawtransaction(rawtxfund['hex'])
         totalOut = 0
         matchingOuts = 0
@@ -299,7 +303,10 @@ def test_two_vin(self):
         dec_tx  = self.nodes[2].decoderawtransaction(rawtx)
         assert_equal(utx['txid'], dec_tx['vin'][0]['txid'])
 
-        rawtxfund = self.nodes[2].fundrawtransaction(rawtx)
+        # Should fail without add_inputs:
+        assert_raises_rpc_error(-4, "Insufficient funds", self.nodes[2].fundrawtransaction, rawtx, {"add_inputs": False})
+        rawtxfund = self.nodes[2].fundrawtransaction(rawtx, {"add_inputs": True})
+
         dec_tx  = self.nodes[2].decoderawtransaction(rawtxfund['hex'])
         totalOut = 0
         matchingOuts = 0
@@ -330,7 +337,10 @@ def test_two_vin_two_vout(self):
         dec_tx  = self.nodes[2].decoderawtransaction(rawtx)
         assert_equal(utx['txid'], dec_tx['vin'][0]['txid'])
 
-        rawtxfund = self.nodes[2].fundrawtransaction(rawtx)
+        # Should fail without add_inputs:
+        assert_raises_rpc_error(-4, "Insufficient funds", self.nodes[2].fundrawtransaction, rawtx, {"add_inputs": False})
+        rawtxfund = self.nodes[2].fundrawtransaction(rawtx, {"add_inputs": True})
+
         dec_tx  = self.nodes[2].decoderawtransaction(rawtxfund['hex'])
         totalOut = 0
         matchingOuts = 0