wallet: Avoid leaking locktime fingerprint when anti-fee-sniping

test/functional/wallet_create_tx.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+# Copyright (c) 2018 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 test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+)
+
+
+class CreateTxWalletTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = False
+        self.num_nodes = 1
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def run_test(self):
+        self.log.info('Check that we have some (old) blocks and that anti-fee-sniping is disabled')
+        assert_equal(self.nodes[0].getblockchaininfo()['blocks'], 200)
+        txid = self.nodes[0].sendtoaddress(self.nodes[0].getnewaddress(), 1)
+        tx = self.nodes[0].decoderawtransaction(self.nodes[0].gettransaction(txid)['hex'])
+        assert_equal(tx['locktime'], 0)
+
+        self.log.info('Check that anti-fee-sniping is enabled when we mine a recent block')
+        self.nodes[0].generate(1)
+        txid = self.nodes[0].sendtoaddress(self.nodes[0].getnewaddress(), 1)
+        tx = self.nodes[0].decoderawtransaction(self.nodes[0].gettransaction(txid)['hex'])
+        assert 0 < tx['locktime'] <= 201
+
+
+if __name__ == '__main__':
+    CreateTxWalletTest().main()