[PATCH net-next v2 1/4] selftests: drv-net: try to check if port is in use

[Jakub Kicinski <kuba@kernel.org>]

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.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
v2:
 - remove v4 check (Willem)
 - update comment (David, Przemek)
 - cap the iterations (Przemek)
---
 tools/testing/selftests/net/lib/py/utils.py | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/lib/py/utils.py b/tools/testing/selftests/net/lib/py/utils.py
index 0540ea24921d..16907b51e034 100644
--- a/tools/testing/selftests/net/lib/py/utils.py
+++ b/tools/testing/selftests/net/lib/py/utils.py
@@ -3,6 +3,7 @@
 import json as _json
 import random
 import re
+import socket
 import subprocess
 import time
 
@@ -79,9 +80,18 @@ import time
 
 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 != 98:  # already in use
+                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):
-- 
2.45.2