selftests: drv-net: try to check if port is in use

We use random ports for communication. As Willem predicted
this leads to occasional failures. Try to check if port is
already in use by opening a socket and binding to that port.
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240626012456.2326192-2-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/lib/py/utils.py

 # SPDX-License-Identifier: GPL-2.0
 
+import errno
 import json as _json
 import random
 import re
+import socket
 import subprocess
 import time
 
@@ -79,9 +81,18 @@ def ip(args, json=None, ns=None, host=None):
 
 def rand_port():
     """
-    Get unprivileged port, for now just random, one day we may decide to check if used.
+    Get a random unprivileged port, try to make sure it's not already used.
     """
-    return random.randint(10000, 65535)
+    for _ in range(1000):
+        port = random.randint(10000, 65535)
+        try:
+            with socket.socket(socket.AF_INET6, socket.SOCK_STREAM) as s:
+                s.bind(("", port))
+            return port
+        except OSError as e:
+            if e.errno != errno.EADDRINUSE:
+                raise
+    raise Exception("Can't find any free unprivileged port")
 
 
 def wait_port_listen(port, proto="tcp", ns=None, host=None, sleep=0.005, deadline=5):