[test] Test the add_outbound_p2p_connection functionality

Open max number of full-relay and block-relay-only connections from a
functional test with different sorts of behaviors to ensure it behaves as
expected.

test/functional/p2p_add_connections.py

@@ -0,0 +1,97 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 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 add_outbound_p2p_connection test framework functionality"""
+
+from test_framework.p2p import P2PInterface
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import assert_equal
+
+
+def check_node_connections(*, node, num_in, num_out):
+    info = node.getnetworkinfo()
+    assert_equal(info["connections_in"], num_in)
+    assert_equal(info["connections_out"], num_out)
+
+
+class P2PAddConnections(BitcoinTestFramework):
+    def set_test_params(self):
+        self.setup_clean_chain = False
+        self.num_nodes = 2
+
+    def setup_network(self):
+        self.setup_nodes()
+        # Don't connect the nodes
+
+    def run_test(self):
+        self.log.info("Add 8 outbounds to node 0")
+        for i in range(8):
+            self.log.info(f"outbound: {i}")
+            self.nodes[0].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i, connection_type="outbound-full-relay")
+
+        self.log.info("Add 2 block-relay-only connections to node 0")
+        for i in range(2):
+            self.log.info(f"block-relay-only: {i}")
+            # set p2p_idx based on the outbound connections already open to the
+            # node, so add 8 to account for the previous full-relay connections
+            self.nodes[0].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i + 8, connection_type="block-relay-only")
+
+        self.log.info("Add 2 block-relay-only connections to node 1")
+        for i in range(2):
+            self.log.info(f"block-relay-only: {i}")
+            self.nodes[1].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i, connection_type="block-relay-only")
+
+        self.log.info("Add 5 inbound connections to node 1")
+        for i in range(5):
+            self.log.info(f"inbound: {i}")
+            self.nodes[1].add_p2p_connection(P2PInterface())
+
+        self.log.info("Add 8 outbounds to node 1")
+        for i in range(8):
+            self.log.info(f"outbound: {i}")
+            # bump p2p_idx to account for the 2 existing outbounds on node 1
+            self.nodes[1].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i + 2)
+
+        self.log.info("Check the connections opened as expected")
+        check_node_connections(node=self.nodes[0], num_in=0, num_out=10)
+        check_node_connections(node=self.nodes[1], num_in=5, num_out=10)
+
+        self.log.info("Disconnect p2p connections & try to re-open")
+        self.nodes[0].disconnect_p2ps()
+        check_node_connections(node=self.nodes[0], num_in=0, num_out=0)
+
+        self.log.info("Add 8 outbounds to node 0")
+        for i in range(8):
+            self.log.info(f"outbound: {i}")
+            self.nodes[0].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i)
+        check_node_connections(node=self.nodes[0], num_in=0, num_out=8)
+
+        self.log.info("Add 2 block-relay-only connections to node 0")
+        for i in range(2):
+            self.log.info(f"block-relay-only: {i}")
+            # bump p2p_idx to account for the 8 existing outbounds on node 0
+            self.nodes[0].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i + 8, connection_type="block-relay-only")
+        check_node_connections(node=self.nodes[0], num_in=0, num_out=10)
+
+        self.log.info("Restart node 0 and try to reconnect to p2ps")
+        self.restart_node(0)
+
+        self.log.info("Add 4 outbounds to node 0")
+        for i in range(4):
+            self.log.info(f"outbound: {i}")
+            self.nodes[0].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i)
+        check_node_connections(node=self.nodes[0], num_in=0, num_out=4)
+
+        self.log.info("Add 2 block-relay-only connections to node 0")
+        for i in range(2):
+            self.log.info(f"block-relay-only: {i}")
+            # bump p2p_idx to account for the 4 existing outbounds on node 0
+            self.nodes[0].add_outbound_p2p_connection(P2PInterface(), p2p_idx=i + 4, connection_type="block-relay-only")
+        check_node_connections(node=self.nodes[0], num_in=0, num_out=6)
+
+        check_node_connections(node=self.nodes[1], num_in=5, num_out=10)
+
+
+if __name__ == '__main__':
+    P2PAddConnections().main()

test/functional/test_runner.py

@@ -261,6 +261,7 @@ BASE_SCRIPTS = [
     'feature_filelock.py',
     'feature_loadblock.py',
     'p2p_dos_header_tree.py',
+    'p2p_add_connections.py',
     'p2p_unrequested_blocks.py',
     'p2p_blockfilters.py',
     'feature_includeconf.py',