From 640e4ae422260d6053a0a41beebee0ba51fbec7e Mon Sep 17 00:00:00 2001
From: Byron Hambly <bhambly@blockstream.com>
Date: Wed, 22 May 2024 10:46:06 +0200
Subject: [PATCH] wallet: allow mintxfee=0

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

https://github.com/bitcoin/bitcoin/commit/f11eb1fe279c4a92e1bfc2c139e8838c73459d12
---
 src/confidential_validation.cpp         |  3 +-
 src/wallet/wallet.cpp                   |  3 +-
 test/functional/test_runner.py          |  1 +
 test/functional/wallet_send_zero_fee.py | 60 +++++++++++++++++++++++++
 4 files changed, 65 insertions(+), 2 deletions(-)
 create mode 100755 test/functional/wallet_send_zero_fee.py

diff --git a/src/confidential_validation.cpp b/src/confidential_validation.cpp
index efc7b753c4..c101852fbc 100644
--- a/src/confidential_validation.cpp
+++ b/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;
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index e7444556ea..39e83f5a6b 100644
--- a/src/wallet/wallet.cpp
+++ b/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) {
diff --git a/test/functional/test_runner.py b/test/functional/test_runner.py
index 198a3eaeb1..6806b33fed 100755
--- a/test/functional/test_runner.py
+++ b/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',
diff --git a/test/functional/wallet_send_zero_fee.py b/test/functional/wallet_send_zero_fee.py
new file mode 100755
index 0000000000..49deea6585
--- /dev/null
+++ b/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()