wallet: allow mintxfee=0

This check for 0 feerate was inherited from upstream, and subsequently
removed in this commit:

bitcoin/bitcoin@f11eb1f

src/confidential_validation.cpp

@@ -30,8 +30,9 @@ bool HasValidFee(const CTransaction& tx) {
         CAmount fee = 0;
         if (tx.vout[i].IsFee()) {
             fee = tx.vout[i].nValue.GetAmount();
-            if (fee == 0 || !MoneyRange(fee))
+            if (!MoneyRange(fee)) {
                 return false;
+            }
             totalFee[tx.vout[i].nAsset.GetAsset()] += fee;
             if (!MoneyRange(totalFee)) {
                 return false;

src/wallet/wallet.cpp

@@ -3012,7 +3012,8 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
 
     if (args.IsArgSet("-mintxfee")) {
         std::optional<CAmount> min_tx_fee = ParseMoney(args.GetArg("-mintxfee", ""));
-        if (!min_tx_fee || min_tx_fee.value() == 0) {
+        // ELEMENTS: allow mintxfee=0
+        if (!min_tx_fee) {
             error = AmountErrMsg("mintxfee", args.GetArg("-mintxfee", ""));
             return nullptr;
         } else if (min_tx_fee.value() > HIGH_TX_FEE_PER_KB) {

test/functional/test_runner.py

@@ -352,6 +352,7 @@
     'feature_coinstatsindex.py --legacy-wallet',
     'feature_coinstatsindex.py --descriptors',
     'wallet_orphanedreward.py',
+    'wallet_send_zero_fee.py',
     'wallet_timelock.py',
     'p2p_node_network_limited.py',
     'p2p_permissions.py',

test/functional/wallet_send_zero_fee.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+# Copyright (c) 2017-2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+from decimal import Decimal
+
+from test_framework.blocktools import COINBASE_MATURITY
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+)
+
+class WalletTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = True
+        self.num_nodes = 3
+        self.extra_args = [[
+            "-blindedaddresses=1",
+            "-initialfreecoins=2100000000000000",
+            "-con_blocksubsidy=0",
+            "-con_connect_genesis_outputs=1",
+            "-txindex=1",
+            "-minrelaytxfee=0",
+            "-blockmintxfee=0",
+            "-mintxfee=0",
+        ]] * self.num_nodes
+        self.extra_args[0].append("-anyonecanspendaremine=1")
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def run_test(self):
+        self.generate(self.nodes[0], COINBASE_MATURITY + 1)
+        self.sync_all()
+
+        self.nodes[0].sendtoaddress(self.nodes[1].getnewaddress(), 10)
+        self.nodes[0].sendtoaddress(self.nodes[2].getnewaddress(), 20)
+        self.generate(self.nodes[0], 1)
+        assert_equal(self.nodes[0].getblockchaininfo()["blocks"], 102)
+        assert_equal(self.nodes[0].getbalance(), {'bitcoin': Decimal('20999969.99900900')})
+        assert_equal(self.nodes[1].getbalance(), {'bitcoin': Decimal('10.00000000')})
+        assert_equal(self.nodes[2].getbalance(), {'bitcoin': Decimal('20.00000000')})
+
+        addr = self.nodes[1].getnewaddress()
+        txid = self.nodes[0].sendtoaddress(addr, 1, None, None, None, None, None, None, None, None, None, 0, False)
+        tx = self.nodes[0].gettransaction(txid)
+        assert_equal(tx["fee"]["bitcoin"], 0)
+        hex = self.nodes[0].getrawtransaction(txid)
+        self.generate(self.nodes[0], 1)
+        assert_equal(self.nodes[0].getblockchaininfo()["blocks"], 103)
+
+        decoded = self.nodes[0].decoderawtransaction(hex)
+        bitcoin = "b2e15d0d7a0c94e4e2ce0fe6e8691b9e451377f6e46e8045a86f7c4b5d4f0f23"
+        assert_equal(decoded["fee"][bitcoin], 0)
+        assert_equal(self.nodes[0].getbalance(), {'bitcoin': Decimal('20999968.99900900')})
+        assert_equal(self.nodes[1].getbalance(), {'bitcoin': Decimal('11.00000000')})
+        assert_equal(self.nodes[2].getbalance(), {'bitcoin': Decimal('20.00000000')})
+
+if __name__ == '__main__':
+    WalletTest().main()