qa: add -paytxfee and -mintxfee interaction test

Test that the interaction between the wallet parameters
-paytxfee and -mintxfee function as intended. This has to
be done using rpc tests rather than unit tests because it
tests the actual parameters passed to the executables.

qa/pull-tester/rpc-tests.py

@@ -155,6 +155,7 @@ testScripts = [
     # 'nulldummy.py',
     'import-rescan.py',
     'harddustlimit.py',
+    'paytxfee.py',
     # While fee bumping should work in Doge, these tests depend on free transactions, which we don't support.
     # Disable until we can do a full rewrite of the tests (possibly upstream), or revise fee schedule, or something
     'bumpfee.py',

qa/rpc-tests/paytxfee.py

@@ -0,0 +1,82 @@
+#!/usr/bin/env python3
+# Copyright (c) 2021 The Dogecoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""PayTxFee QA test.
+
+# Tests wallet behavior of -paytxfee in relation to -mintxfee
+"""
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import *
+from decimal import Decimal
+
+class PayTxFeeTest(BitcoinTestFramework):
+
+    def __init__(self):
+        super().__init__()
+        self.setup_clean_chain = True
+        self.num_nodes = 4
+
+    def setup_nodes(self, split=False):
+        nodes = []
+
+        # node 0 has txindex to track txs
+        nodes.append(start_node(0, self.options.tmpdir,
+            ["-debug", '-txindex']))
+
+        # node 1 pays 0.1 DOGE on all txs due to implicit mintxfee = paytxfee
+        nodes.append(start_node(1, self.options.tmpdir,
+            ["-paytxfee=0.1", "-debug"]))
+
+        # node 2 will always pay 1 DOGE on all txs because of explicit mintxfee
+        nodes.append(start_node(2, self.options.tmpdir,
+            ["-mintxfee=1", "-paytxfee=0.1", "-debug"]))
+
+        # node 3 will always pay 0.1 DOGE on all txs despite explicit mintxfee of 0.01
+        nodes.append(start_node(3, self.options.tmpdir,
+            ["-mintxfee=0.01", "-paytxfee=0.1", "-debug"]))
+
+        return nodes
+
+    def run_test(self):
+
+        seed = 1000 # the amount to seed wallets with
+        amount = 995 # the amount to send back
+        targetAddress = self.nodes[0].getnewaddress()
+
+        # mine some blocks and prepare some coins
+        self.nodes[0].generate(102)
+        self.nodes[0].sendtoaddress(self.nodes[1].getnewaddress(), seed)
+        self.nodes[0].sendtoaddress(self.nodes[2].getnewaddress(), seed)
+        self.nodes[0].sendtoaddress(self.nodes[3].getnewaddress(), seed)
+        self.nodes[0].generate(1)
+        self.sync_all()
+
+        # create transactions
+        txid1 = self.nodes[1].sendtoaddress(targetAddress, amount)
+        txid2 = self.nodes[2].sendtoaddress(targetAddress, amount)
+        txid3 = self.nodes[3].sendtoaddress(targetAddress, amount)
+        self.sync_all()
+
+        # make sure correct fees were paid
+        tx1 = self.nodes[0].getrawtransaction(txid1, True)
+        tx2 = self.nodes[0].getrawtransaction(txid2, True)
+        tx3 = self.nodes[0].getrawtransaction(txid3, True)
+
+        assert_equal(tx1['vout'][0]['value'] + tx1['vout'][1]['value'], Decimal("999.9"))
+        assert_equal(tx2['vout'][0]['value'] + tx2['vout'][1]['value'], Decimal("999"))
+        assert_equal(tx3['vout'][0]['value'] + tx3['vout'][1]['value'], Decimal("999.9"))
+
+        # mine a block
+        self.nodes[0].generate(1);
+        self.sync_all()
+
+        # make sure all fees were mined
+        block = self.nodes[0].getblock(self.nodes[0].getbestblockhash())
+        coinbaseTx = self.nodes[0].getrawtransaction(block['tx'][0], True)
+
+        assert_equal(coinbaseTx['vout'][0]['value'], Decimal("500001.2"))
+
+if __name__ == '__main__':
+    PayTxFeeTest().main()