[test] Add testing for outbound feeler connections

Extend the addconnection RPC method to allow creating outbound
feeler connections. Extend the test framework to accept those
feeler connections.

src/net.cpp

@@ -1217,7 +1217,6 @@ bool CConnman::AddConnection(const std::string& address, ConnectionType conn_typ
     switch (conn_type) {
     case ConnectionType::INBOUND:
     case ConnectionType::MANUAL:
-    case ConnectionType::FEELER:
         return false;
     case ConnectionType::OUTBOUND_FULL_RELAY:
         max_connections = m_max_outbound_full_relay;
@@ -1228,6 +1227,9 @@ bool CConnman::AddConnection(const std::string& address, ConnectionType conn_typ
     // no limit for ADDR_FETCH because -seednode has no limit either
     case ConnectionType::ADDR_FETCH:
         break;
+    // no limit for FEELER connections since they're short-lived
+    case ConnectionType::FEELER:
+        break;
     } // no default case, so the compiler can warn about missing cases
 
     // Count existing connections

src/net.h

@@ -892,8 +892,8 @@ public:
      * Attempts to open a connection. Currently only used from tests.
      *
      * @param[in]   address     Address of node to try connecting to
-     * @param[in]   conn_type   ConnectionType::OUTBOUND or ConnectionType::BLOCK_RELAY
-     *                          or ConnectionType::ADDR_FETCH
+     * @param[in]   conn_type   ConnectionType::OUTBOUND, ConnectionType::BLOCK_RELAY,
+     *                          ConnectionType::ADDR_FETCH or ConnectionType::FEELER
      * @return      bool        Returns false if there are no available
      *                          slots for this connection:
      *                          - conn_type not a supported ConnectionType

src/rpc/net.cpp

@@ -343,7 +343,7 @@ static RPCHelpMan addconnection()
         "\nOpen an outbound connection to a specified node. This RPC is for testing only.\n",
         {
             {"address", RPCArg::Type::STR, RPCArg::Optional::NO, "The IP address and port to attempt connecting to."},
-            {"connection_type", RPCArg::Type::STR, RPCArg::Optional::NO, "Type of connection to open (\"outbound-full-relay\", \"block-relay-only\" or \"addr-fetch\")."},
+            {"connection_type", RPCArg::Type::STR, RPCArg::Optional::NO, "Type of connection to open (\"outbound-full-relay\", \"block-relay-only\", \"addr-fetch\" or \"feeler\")."},
         },
         RPCResult{
             RPCResult::Type::OBJ, "", "",
@@ -371,6 +371,8 @@ static RPCHelpMan addconnection()
         conn_type = ConnectionType::BLOCK_RELAY;
     } else if (conn_type_in == "addr-fetch") {
         conn_type = ConnectionType::ADDR_FETCH;
+    } else if (conn_type_in == "feeler") {
+        conn_type = ConnectionType::FEELER;
     } else {
         throw JSONRPCError(RPC_INVALID_PARAMETER, self.ToString());
     }

test/functional/p2p_add_connections.py

@@ -14,6 +14,13 @@ def check_node_connections(*, node, num_in, num_out):
     assert_equal(info["connections_in"], num_in)
     assert_equal(info["connections_out"], num_out)
 
+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):
@@ -91,6 +98,14 @@ 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)
 
 if __name__ == '__main__':
     P2PAddConnections().main()

test/functional/test_framework/test_node.py

@@ -558,7 +558,7 @@ 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.
@@ -570,11 +570,16 @@ 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