Merge bitcoin/bitcoin#22777: net processing: don't request tx relay on feeler connections

eaf6be0114a6d7763767da9496907fe8a670ff9e [net processing] Do not request transaction relay from feeler connections (John Newbery)
0220b834b175dc8c45a2c60213474a72c0ef8193 [test] Add testing for outbound feeler connections (John Newbery)

Pull request description:

  Feelers are short-lived connections used to test the viability of peers. The bitcoind node will periodically open feeler connections to addresses in its addrman, wait for a `version` message from the peer, and then close the connection.

  Currently, we set `fRelay` to `1` in the `version` message for feeler connections, indicating that we want the peer to relay transactions to us. However, we close the connection immediately on receipt of the `version` message, and so never process any incoming transaction announcements. This PR changes that behaviour to instead set `fRelay` to `0` indicating that we do not wish to receive transaction announcements from the peer.

  This PR also extends the `addconnection` RPC to allow creating outbound feeler connections from the node to the test framework, and a test to verify that the node sets `fRelay` to `0` in the `version` message to feeler connections.

ACKs for top commit:
  naumenkogs:
    ACK eaf6be0114a6d7763767da9496907fe8a670ff9e
  MarcoFalke:
    review ACK eaf6be0114a6d7763767da9496907fe8a670ff9e 🏃

Tree-SHA512: 1c56837dbd0a396fe404a5e39f7459864d15f666664d6b35ad109628b13158e077e417e586bf48946a23bd5cbe63716cb4bf22cdf8781b74dfce6047b87b465a

2 files changed, 27 insertions, 5 deletions

diff --git a/test/functional/p2p_add_connections.py b/test/functional/p2p_add_connections.py
index b86502dc85..2d6c03b675 100755
--- a/test/functional/p2p_add_connections.py
+++ b/test/functional/p2p_add_connections.py
@@ -8,6 +8,13 @@ from test_framework.p2p import P2PInterface
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import check_node_connections
 
+class P2PFeelerReceiver(P2PInterface):
+    def on_version(self, message):
+        # The bitcoind node closes feeler connections as soon as a version
+        # message is received from the test framework. Don't send any responses
+        # to the node's version message since the connection will already be
+        # closed.
+        pass
 
 class P2PAddConnections(BitcoinTestFramework):
     def set_test_params(self):
@@ -85,6 +92,16 @@ class P2PAddConnections(BitcoinTestFramework):
 
         check_node_connections(node=self.nodes[1], num_in=5, num_out=10)
 
+        self.log.info("Add 1 feeler connection to node 0")
+        feeler_conn = self.nodes[0].add_outbound_p2p_connection(P2PFeelerReceiver(), p2p_idx=6, connection_type="feeler")
+
+        # Feeler connection is closed
+        assert not feeler_conn.is_connected
+
+        # Verify version message received
+        assert_equal(feeler_conn.message_count["version"], 1)
+        # Feeler connections do not request tx relay
+        assert_equal(feeler_conn.last_message["version"].relay, 0)
 
 if __name__ == '__main__':
     P2PAddConnections().main()
diff --git a/test/functional/test_framework/test_node.py b/test/functional/test_framework/test_node.py
index b3279666b2..ca6e341be8 100755
--- a/test/functional/test_framework/test_node.py
+++ b/test/functional/test_framework/test_node.py
@@ -578,7 +578,7 @@ class TestNode():
 
     def add_outbound_p2p_connection(self, p2p_conn, *, p2p_idx, connection_type="outbound-full-relay", **kwargs):
         """Add an outbound p2p connection from node. Must be an
-        "outbound-full-relay", "block-relay-only" or "addr-fetch" connection.
+        "outbound-full-relay", "block-relay-only", "addr-fetch" or "feeler" connection.
 
         This method adds the p2p connection to the self.p2ps list and returns
         the connection to the caller.
@@ -590,11 +590,16 @@ class TestNode():
 
         p2p_conn.peer_accept_connection(connect_cb=addconnection_callback, connect_id=p2p_idx + 1, net=self.chain, timeout_factor=self.timeout_factor, **kwargs)()
 
-        p2p_conn.wait_for_connect()
-        self.p2ps.append(p2p_conn)
+        if connection_type == "feeler":
+            # feeler connections are closed as soon as the node receives a `version` message
+            p2p_conn.wait_until(lambda: p2p_conn.message_count["version"] == 1, check_connected=False)
+            p2p_conn.wait_until(lambda: not p2p_conn.is_connected, check_connected=False)
+        else:
+            p2p_conn.wait_for_connect()
+            self.p2ps.append(p2p_conn)
 
-        p2p_conn.wait_for_verack()
-        p2p_conn.sync_with_ping()
+            p2p_conn.wait_for_verack()
+            p2p_conn.sync_with_ping()
 
         return p2p_conn