net: fix GetListenPort() to derive the proper port

`GetListenPort()` uses a simple logic: "if `-port=P` is given, then we
must be listening on `P`, otherwise we must be listening on `8333`".
This is however not true if `-bind=` has been provided with `:port` part
or if `-whitebind=` has been provided. Thus, extend `GetListenPort()` to
return the port from `-bind=` or `-whitebind=`, if any.

Fixes https://github.com/bitcoin/bitcoin/issues/20184 (cases 1. 2. 3. 5.)

test/functional/feature_bind_port_externalip.py

@@ -0,0 +1,75 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020-2021 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 the proper port is used for -externalip=
+"""
+
+from test_framework.test_framework import BitcoinTestFramework, SkipTest
+from test_framework.util import assert_equal, p2p_port
+
+# We need to bind to a routable address for this test to exercise the relevant code.
+# To set a routable address on the machine use:
+# Linux:
+# ifconfig lo:0 1.1.1.1/32 up  # to set up
+# ifconfig lo:0 down  # to remove it, after the test
+# FreeBSD:
+# ifconfig lo0 1.1.1.1/32 alias  # to set up
+# ifconfig lo0 1.1.1.1 -alias  # to remove it, after the test
+ADDR = '1.1.1.1'
+
+# array of tuples [arguments, expected port in localaddresses]
+EXPECTED = [
+    [['-externalip=2.2.2.2',       '-port=30001'],                        30001],
+    [['-externalip=2.2.2.2',       '-port=30002', f'-bind={ADDR}'],       30002],
+    [['-externalip=2.2.2.2',                      f'-bind={ADDR}'],       'default_p2p_port'],
+    [['-externalip=2.2.2.2',       '-port=30003', f'-bind={ADDR}:30004'], 30004],
+    [['-externalip=2.2.2.2',                      f'-bind={ADDR}:30005'], 30005],
+    [['-externalip=2.2.2.2:30006', '-port=30007'],                        30006],
+    [['-externalip=2.2.2.2:30008', '-port=30009', f'-bind={ADDR}'],       30008],
+    [['-externalip=2.2.2.2:30010',                f'-bind={ADDR}'],       30010],
+    [['-externalip=2.2.2.2:30011', '-port=30012', f'-bind={ADDR}:30013'], 30011],
+    [['-externalip=2.2.2.2:30014',                f'-bind={ADDR}:30015'], 30014],
+    [['-externalip=2.2.2.2',       '-port=30016', f'-bind={ADDR}:30017',
+                                             f'-whitebind={ADDR}:30018'], 30017],
+    [['-externalip=2.2.2.2',       '-port=30019',
+                                             f'-whitebind={ADDR}:30020'], 30020],
+]
+
+class BindPortExternalIPTest(BitcoinTestFramework):
+    def set_test_params(self):
+        # Avoid any -bind= on the command line. Force the framework to avoid adding -bind=127.0.0.1.
+        self.setup_clean_chain = True
+        self.bind_to_localhost_only = False
+        self.num_nodes = len(EXPECTED)
+        self.extra_args = list(map(lambda e: e[0], EXPECTED))
+
+    def add_options(self, parser):
+        parser.add_argument(
+            "--ihave1111", action='store_true', dest="ihave1111",
+            help=f"Run the test, assuming {ADDR} is configured on the machine",
+            default=False)
+
+    def skip_test_if_missing_module(self):
+        if not self.options.ihave1111:
+            raise SkipTest(
+                f"To run this test make sure that {ADDR} (a routable address) is assigned "
+                "to one of the interfaces on this machine and rerun with --ihave1111")
+
+    def run_test(self):
+        self.log.info("Test the proper port is used for -externalip=")
+        for i in range(len(EXPECTED)):
+            expected_port = EXPECTED[i][1]
+            if expected_port == 'default_p2p_port':
+                expected_port = p2p_port(i)
+            found = False
+            for local in self.nodes[i].getnetworkinfo()['localaddresses']:
+                if local['address'] == '2.2.2.2':
+                    assert_equal(local['port'], expected_port)
+                    found = True
+                    break
+            assert found
+
+if __name__ == '__main__':
+    BindPortExternalIPTest().main()