test: improve robustness of connect_nodes()

The 'connect_nodes' function in the test framework relies
on a stable number of peer connections to verify the new
connection between the nodes is successfully established.
This approach is fragile, as any of the peers involved in
the process can drop, lose, or create a connection at any
step, causing subsequent 'wait_until' checks to stall
indefinitely even when the peers in question are connected
successfully.

This commit improves the situation by using the nodes' subversion
and the connection direction (inbound/outbound) to identify the
exact peer connection and perform the checks exclusively on it.

test/functional/test_framework/test_framework.py

@@ -610,8 +610,6 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         """
         from_connection = self.nodes[a]
         to_connection = self.nodes[b]
-        from_num_peers = 1 + len(from_connection.getpeerinfo())
-        to_num_peers = 1 + len(to_connection.getpeerinfo())
         ip_port = "127.0.0.1:" + str(p2p_port(b))
 
         if peer_advertises_v2 is None:
@@ -627,19 +625,32 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         if not wait_for_connect:
             return
 
+        # Use subversion as peer id. Test nodes have their node number appended to the user agent string
+        from_connection_subver = from_connection.getnetworkinfo()['subversion']
+        to_connection_subver = to_connection.getnetworkinfo()['subversion']
+
+        def find_conn(node, peer_subversion, inbound):
+            return next(filter(lambda peer: peer['subver'] == peer_subversion and peer['inbound'] == inbound, node.getpeerinfo()), None)
+
         # poll until version handshake complete to avoid race conditions
         # with transaction relaying
         # See comments in net_processing:
         # * Must have a version message before anything else
         # * Must have a verack message before anything else
-        self.wait_until(lambda: sum(peer['version'] != 0 for peer in from_connection.getpeerinfo()) == from_num_peers)
-        self.wait_until(lambda: sum(peer['version'] != 0 for peer in to_connection.getpeerinfo()) == to_num_peers)
-        self.wait_until(lambda: sum(peer['bytesrecv_per_msg'].pop('verack', 0) >= 21 for peer in from_connection.getpeerinfo()) == from_num_peers)
-        self.wait_until(lambda: sum(peer['bytesrecv_per_msg'].pop('verack', 0) >= 21 for peer in to_connection.getpeerinfo()) == to_num_peers)
+        self.wait_until(lambda: find_conn(from_connection, to_connection_subver, inbound=False) is not None)
+        self.wait_until(lambda: find_conn(to_connection, from_connection_subver, inbound=True) is not None)
+
+        def check_bytesrecv(peer, msg_type, min_bytes_recv):
+            assert peer is not None, "Error: peer disconnected"
+            return peer['bytesrecv_per_msg'].pop(msg_type, 0) >= min_bytes_recv
+
+        self.wait_until(lambda: check_bytesrecv(find_conn(from_connection, to_connection_subver, inbound=False), 'verack', 21))
+        self.wait_until(lambda: check_bytesrecv(find_conn(to_connection, from_connection_subver, inbound=True), 'verack', 21))
+
         # The message bytes are counted before processing the message, so make
         # sure it was fully processed by waiting for a ping.
-        self.wait_until(lambda: sum(peer["bytesrecv_per_msg"].pop("pong", 0) >= 29 for peer in from_connection.getpeerinfo()) == from_num_peers)
-        self.wait_until(lambda: sum(peer["bytesrecv_per_msg"].pop("pong", 0) >= 29 for peer in to_connection.getpeerinfo()) == to_num_peers)
+        self.wait_until(lambda: check_bytesrecv(find_conn(from_connection, to_connection_subver, inbound=False), 'pong', 29))
+        self.wait_until(lambda: check_bytesrecv(find_conn(to_connection, from_connection_subver, inbound=True), 'pong', 29))
 
     def disconnect_nodes(self, a, b):
         def disconnect_nodes_helper(node_a, node_b):

test/functional/test_framework/test_node.py

@@ -106,7 +106,7 @@ class TestNode():
             "-debugexclude=libevent",
             "-debugexclude=leveldb",
             "-debugexclude=rand",
-            "-uacomment=testnode%d" % i,
+            "-uacomment=testnode%d" % i,  # required for subversion uniqueness across peers
         ]
         if self.descriptors is None:
             self.args.append("-disablewallet")