[tests] Test that nodes respond to getdata with notfound

If a node has not announced a tx at all, then it should respond to
getdata messages for that tx with notfound, to avoid leaking tx
origination privacy.

test/functional/p2p_leak_tx.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+# Copyright (c) 2017-2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test that we don't leak txs to inbound peers that we haven't yet announced to"""
+
+from test_framework.messages import msg_getdata, CInv
+from test_framework.mininode import P2PDataStore
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import (
+    assert_equal,
+)
+
+
+class P2PNode(P2PDataStore):
+    def on_inv(self, msg):
+        pass
+
+
+class P2PLeakTxTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_wallet()
+
+    def run_test(self):
+        gen_node = self.nodes[0]  # The block and tx generating node
+        gen_node.generate(1)
+
+        inbound_peer = self.nodes[0].add_p2p_connection(P2PNode())  # An "attacking" inbound peer
+
+        MAX_REPEATS = 100
+        self.log.info("Running test up to {} times.".format(MAX_REPEATS))
+        for i in range(MAX_REPEATS):
+            self.log.info('Run repeat {}'.format(i + 1))
+            txid = gen_node.sendtoaddress(gen_node.getnewaddress(), 0.01)
+
+            want_tx = msg_getdata()
+            want_tx.inv.append(CInv(t=1, h=int(txid, 16)))
+            inbound_peer.last_message.pop('notfound', None)
+            inbound_peer.send_message(want_tx)
+            inbound_peer.sync_with_ping()
+
+            if inbound_peer.last_message.get('notfound'):
+                self.log.debug('tx {} was not yet announced to us.'.format(txid))
+                self.log.debug("node has responded with a notfound message. End test.")
+                assert_equal(inbound_peer.last_message['notfound'].vec[0].hash, int(txid, 16))
+                inbound_peer.last_message.pop('notfound')
+                break
+            else:
+                self.log.debug('tx {} was already announced to us. Try test again.'.format(txid))
+                assert int(txid, 16) in [inv.hash for inv in inbound_peer.last_message['inv'].inv]
+
+
+if __name__ == '__main__':
+    P2PLeakTxTest().main()