Add 'subtractFeeFromOutputs' option to 'fundrawtransaction'.

qa/rpc-tests/fundrawtransaction.py

@@ -660,5 +660,75 @@ class RawTransactionsTest(BitcoinTestFramework):
         assert_fee_amount(result2['fee'], count_bytes(result2['hex']), 2 * result_fee_rate)
         assert_fee_amount(result3['fee'], count_bytes(result3['hex']), 10 * result_fee_rate)
 
+        ######################################
+        # Test subtractFeeFromOutputs option #
+        ######################################
+
+        # Make sure there is exactly one input so coin selection can't skew the result
+        assert_equal(len(self.nodes[3].listunspent(1)), 1)
+
+        inputs = []
+        outputs = {self.nodes[2].getnewaddress(): 1}
+        rawtx = self.nodes[3].createrawtransaction(inputs, outputs)
+
+        result = [self.nodes[3].fundrawtransaction(rawtx), # uses min_relay_tx_fee (set by settxfee)
+                  self.nodes[3].fundrawtransaction(rawtx, {"subtractFeeFromOutputs": []}), # empty subtraction list
+                  self.nodes[3].fundrawtransaction(rawtx, {"subtractFeeFromOutputs": [0]}), # uses min_relay_tx_fee (set by settxfee)
+                  self.nodes[3].fundrawtransaction(rawtx, {"feeRate": 2*min_relay_tx_fee}),
+                  self.nodes[3].fundrawtransaction(rawtx, {"feeRate": 2*min_relay_tx_fee, "subtractFeeFromOutputs": [0]})]
+
+        dec_tx = [self.nodes[3].decoderawtransaction(tx['hex']) for tx in result]
+        output = [d['vout'][1 - r['changepos']]['value'] for d, r in zip(dec_tx, result)]
+        change = [d['vout'][r['changepos']]['value'] for d, r in zip(dec_tx, result)]
+
+        assert_equal(result[0]['fee'], result[1]['fee'], result[2]['fee'])
+        assert_equal(result[3]['fee'], result[4]['fee'])
+        assert_equal(change[0], change[1])
+        assert_equal(output[0], output[1])
+        assert_equal(output[0], output[2] + result[2]['fee'])
+        assert_equal(change[0] + result[0]['fee'], change[2])
+        assert_equal(output[3], output[4] + result[4]['fee'])
+        assert_equal(change[3] + result[3]['fee'], change[4])
+
+        inputs = []
+        outputs = {self.nodes[2].getnewaddress(): value for value in (1.0, 1.1, 1.2, 1.3)}
+        keys = list(outputs.keys())
+        rawtx = self.nodes[3].createrawtransaction(inputs, outputs)
+
+        result = [self.nodes[3].fundrawtransaction(rawtx),
+                  # split the fee between outputs 0, 2, and 3, but not output 1
+                  self.nodes[3].fundrawtransaction(rawtx, {"subtractFeeFromOutputs": [0, 2, 3]})]
+
+        dec_tx = [self.nodes[3].decoderawtransaction(result[0]['hex']),
+                  self.nodes[3].decoderawtransaction(result[1]['hex'])]
+
+        # Nested list of non-change output amounts for each transaction
+        output = [[out['value'] for i, out in enumerate(d['vout']) if i != r['changepos']]
+                  for d, r in zip(dec_tx, result)]
+
+        # List of differences in output amounts between normal and subtractFee transactions
+        share = [o0 - o1 for o0, o1 in zip(output[0], output[1])]
+
+        # output 1 is the same in both transactions
+        assert_equal(share[1], 0)
+
+        # the other 3 outputs are smaller as a result of subtractFeeFromOutputs
+        assert_greater_than(share[0], 0)
+        assert_greater_than(share[2], 0)
+        assert_greater_than(share[3], 0)
+
+        # outputs 2 and 3 take the same share of the fee
+        assert_equal(share[2], share[3])
+
+        # output 0 takes at least as much share of the fee, and no more than 2 satoshis more, than outputs 2 and 3
+        assert_greater_than_or_equal(share[0], share[2])
+        assert_greater_than_or_equal(share[2] + Decimal(2e-8), share[0])
+
+        # the fee is the same in both transactions
+        assert_equal(result[0]['fee'], result[1]['fee'])
+
+        # the total subtracted from the outputs is equal to the fee
+        assert_equal(share[0] + share[2] + share[3], result[0]['fee'])
+
 if __name__ == '__main__':
     RawTransactionsTest().main()

qa/rpc-tests/test_framework/util.py

@@ -524,14 +524,18 @@ def assert_fee_amount(fee, tx_size, fee_per_kB):
     if fee > (tx_size + 2) * fee_per_kB / 1000:
         raise AssertionError("Fee of %s BTC too high! (Should be %s BTC)"%(str(fee), str(target_fee)))
 
-def assert_equal(thing1, thing2):
-    if thing1 != thing2:
-        raise AssertionError("%s != %s"%(str(thing1),str(thing2)))
+def assert_equal(thing1, thing2, *args):
+    if thing1 != thing2 or any(thing1 != arg for arg in args):
+        raise AssertionError("not(%s)" % " == ".join(str(arg) for arg in (thing1, thing2) + args))
 
 def assert_greater_than(thing1, thing2):
     if thing1 <= thing2:
         raise AssertionError("%s <= %s"%(str(thing1),str(thing2)))
 
+def assert_greater_than_or_equal(thing1, thing2):
+    if thing1 < thing2:
+        raise AssertionError("%s < %s"%(str(thing1),str(thing2)))
+
 def assert_raises(exc, fun, *args, **kwds):
     assert_raises_message(exc, None, fun, *args, **kwds)
 

src/wallet/rpcwallet.cpp

@@ -2464,7 +2464,8 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
         throw runtime_error(
                             "fundrawtransaction \"hexstring\" ( options )\n"
                             "\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 one change output to the outputs.\n"
+                            "This will not modify existing inputs, and 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 signrawtransaction for that.\n"
                             "Note that all existing inputs must have their previous output transaction be in the wallet.\n"
@@ -2476,11 +2477,17 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
                             "1. \"hexstring\"           (string, required) The hex string of the raw transaction\n"
                             "2. options               (object, optional)\n"
                             "   {\n"
-                            "     \"changeAddress\"     (string, optional, default pool address) The bitcoin address to receive the change\n"
-                            "     \"changePosition\"    (numeric, optional, default random) The index of the change output\n"
-                            "     \"includeWatching\"   (boolean, optional, default false) Also select inputs which are watch only\n"
-                            "     \"lockUnspents\"      (boolean, optional, default false) Lock selected unspent outputs\n"
-                            "     \"feeRate\"           (numeric, optional, default not set: makes wallet determine the fee) Set a specific feerate (" + CURRENCY_UNIT + " per KB)\n"
+                            "     \"changeAddress\"          (string, optional, default pool address) The bitcoin address to receive the change\n"
+                            "     \"changePosition\"         (numeric, optional, default random) The index of the change output\n"
+                            "     \"includeWatching\"        (boolean, optional, default false) Also select inputs which are watch only\n"
+                            "     \"lockUnspents\"           (boolean, optional, default false) Lock selected unspent outputs\n"
+                            "     \"feeRate\"                (numeric, optional, default not set: makes wallet determine the fee) Set a specific feerate (" + CURRENCY_UNIT + " per KB)\n"
+                            "     \"subtractFeeFromOutputs\" (array, optional) A json array of integers.\n"
+                            "                              The fee will be equally deducted from the amount of each specified output.\n"
+                            "                              The outputs are specified by their zero-based index, before any change output is added.\n"
+                            "                              Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
+                            "                              If no outputs are specified here, the sender pays the fee.\n"
+                            "                                  [vout_index,...]\n"
                             "   }\n"
                             "                         for backward compatibility: passing in a true instead of an object will result in {\"includeWatching\":true}\n"
                             "\nResult:\n"
@@ -2509,6 +2516,8 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
     bool lockUnspents = false;
     CFeeRate feeRate = CFeeRate(0);
     bool overrideEstimatedFeerate = false;
+    UniValue subtractFeeFromOutputs;
+    set<int> setSubtractFeeFromOutputs;
 
     if (request.params.size() > 1) {
       if (request.params[1].type() == UniValue::VBOOL) {
@@ -2527,6 +2536,7 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
                 {"includeWatching", UniValueType(UniValue::VBOOL)},
                 {"lockUnspents", UniValueType(UniValue::VBOOL)},
                 {"feeRate", UniValueType()}, // will be checked below
+                {"subtractFeeFromOutputs", UniValueType(UniValue::VARR)},
             },
             true, true);
 
@@ -2553,6 +2563,9 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
             feeRate = CFeeRate(AmountFromValue(options["feeRate"]));
             overrideEstimatedFeerate = true;
         }
+
+        if (options.exists("subtractFeeFromOutputs"))
+            subtractFeeFromOutputs = options["subtractFeeFromOutputs"].get_array();
       }
     }
 
@@ -2567,10 +2580,21 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
     if (changePosition != -1 && (changePosition < 0 || (unsigned int)changePosition > tx.vout.size()))
         throw JSONRPCError(RPC_INVALID_PARAMETER, "changePosition out of bounds");
 
+    for (unsigned int idx = 0; idx < subtractFeeFromOutputs.size(); idx++) {
+        int pos = subtractFeeFromOutputs[idx].get_int();
+        if (setSubtractFeeFromOutputs.count(pos))
+            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, duplicated position: %d", pos));
+        if (pos < 0)
+            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, negative position: %d", pos));
+        if (pos >= int(tx.vout.size()))
+            throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Invalid parameter, position too large: %d", pos));
+        setSubtractFeeFromOutputs.insert(pos);
+    }
+
     CAmount nFeeOut;
     string strFailReason;
 
-    if(!pwalletMain->FundTransaction(tx, nFeeOut, overrideEstimatedFeerate, feeRate, changePosition, strFailReason, includeWatching, lockUnspents, changeAddress))
+    if(!pwalletMain->FundTransaction(tx, nFeeOut, overrideEstimatedFeerate, feeRate, changePosition, strFailReason, includeWatching, lockUnspents, setSubtractFeeFromOutputs, changeAddress))
         throw JSONRPCError(RPC_INTERNAL_ERROR, strFailReason);
 
     UniValue result(UniValue::VOBJ);

src/wallet/wallet.cpp

@@ -2181,14 +2181,15 @@ bool CWallet::SelectCoins(const vector<COutput>& vAvailableCoins, const CAmount&
     return res;
 }
 
-bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, bool overrideEstimatedFeeRate, const CFeeRate& specificFeeRate, int& nChangePosInOut, std::string& strFailReason, bool includeWatching, bool lockUnspents, const CTxDestination& destChange)
+bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, bool overrideEstimatedFeeRate, const CFeeRate& specificFeeRate, int& nChangePosInOut, std::string& strFailReason, bool includeWatching, bool lockUnspents, const std::set<int>& setSubtractFeeFromOutputs, const CTxDestination& destChange)
 {
     vector<CRecipient> vecSend;
 
     // Turn the txout set into a CRecipient vector
-    BOOST_FOREACH(const CTxOut& txOut, tx.vout)
+    for (size_t idx = 0; idx < tx.vout.size(); idx++)
     {
-        CRecipient recipient = {txOut.scriptPubKey, txOut.nValue, false};
+        const CTxOut& txOut = tx.vout[idx];
+        CRecipient recipient = {txOut.scriptPubKey, txOut.nValue, setSubtractFeeFromOutputs.count(idx) == 1};
         vecSend.push_back(recipient);
     }
 
@@ -2210,6 +2211,10 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, bool ov
     if (nChangePosInOut != -1)
         tx.vout.insert(tx.vout.begin() + nChangePosInOut, wtx.tx->vout[nChangePosInOut]);
 
+    // Copy output sizes from new transaction; they may have had the fee subtracted from them
+    for (unsigned int idx = 0; idx < tx.vout.size(); idx++)
+        tx.vout[idx].nValue = wtx.tx->vout[idx].nValue;
+
     // Add new txins (keeping original txin scriptSig/order)
     BOOST_FOREACH(const CTxIn& txin, wtx.tx->vin)
     {

src/wallet/wallet.h

@@ -772,7 +772,7 @@ public:
      * Insert additional inputs into the transaction by
      * calling CreateTransaction();
      */
-    bool FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, bool overrideEstimatedFeeRate, const CFeeRate& specificFeeRate, int& nChangePosInOut, std::string& strFailReason, bool includeWatching, bool lockUnspents, const CTxDestination& destChange = CNoDestination());
+    bool FundTransaction(CMutableTransaction& tx, CAmount& nFeeRet, bool overrideEstimatedFeeRate, const CFeeRate& specificFeeRate, int& nChangePosInOut, std::string& strFailReason, bool includeWatching, bool lockUnspents, const std::set<int>& setSubtractFeeFromOutputs, const CTxDestination& destChange = CNoDestination());
 
     /**
      * Create a new transaction paying the recipients with a set of coins