net, libs: add poll_tcp_connectivity helper

A NAD reference change triggers a live migration. During and immediately
after the migration, ARP/NDP tables are rebuilding and the interface is
reconfiguring on the new VLAN, so network reachability does not appear
(or disappear) instantaneously. A one-shot TCP check after the patch
would produce false negatives on the new VLAN and false positives on the
old one.

poll_tcp_connectivity() wraps iperf3 in a @Retry loop so the test waits
until the expected reachability state is actually observed. It also
accepts client_bind_dev and server_bind_dev to force iperf3 traffic
through specific interfaces, bypassing ECMP when both secondaries share
the same subnet.

Signed-off-by: Asia Khromov <azhivovk@redhat.com>
Assisted-by: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Author: azhivovk

tests/network/libs/connectivity.py

@@ -1,6 +1,11 @@
 import ipaddress
 from typing import Final
 
+from timeout_sampler import TimeoutExpiredError, retry
+
+from libs.net.traffic_generator import IPERF_SERVER_PORT, TcpServer, VMTcpClient
+from libs.vm.vm import BaseVirtualMachine
+
 ARP_ISOLATION_SYSCTL_CMD: Final[list[str]] = [
     # Only answer ARP for the IP assigned to the receiving interface —
     # prevents eth1 from responding to ARP for eth2's IP when queried from the same VLAN.
@@ -26,3 +31,38 @@ def build_ping_command(dst_ip: str, count: int, timeout: int) -> str:
     ip = ipaddress.ip_address(address=dst_ip)
     ping_ipv6_flag = " -6" if ip.version == 6 else ""
     return f"ping{ping_ipv6_flag} {dst_ip} -c {count} -w {timeout}"
+
+
+@retry(wait_timeout=60, sleep=5, exceptions_dict={})
+def poll_tcp_connectivity(
+    client_vm: BaseVirtualMachine,
+    server_vm: BaseVirtualMachine,
+    server_ip: str,
+    client_bind_dev: str | None = None,
+    server_bind_dev: str | None = None,
+    expect_connectivity: bool = True,
+) -> bool:
+    """Poll TCP connectivity (or its absence) between two VMs, retrying until the expected state is reached.
+
+    Args:
+        client_vm: VM initiating the TCP connection.
+        server_vm: VM running the iperf3 server.
+        server_ip: IP address the server binds to.
+        client_bind_dev: Guest network device name to force the client out (e.g. "eth1").
+            Bypasses ECMP routing when both secondary interfaces share the same subnet.
+        server_bind_dev: Guest network device name to force the server responses out (e.g. "eth1").
+            Bypasses ECMP routing on the server VM when it has multiple secondary interfaces.
+        expect_connectivity: When True polls until connectivity exists; when False polls until it does not.
+
+    Returns:
+        True when the observed reachability matches expect_connectivity.
+    """
+    try:
+        with TcpServer(vm=server_vm, port=IPERF_SERVER_PORT, bind_ip=server_ip, bind_dev=server_bind_dev):
+            with VMTcpClient(
+                vm=client_vm, server_ip=server_ip, server_port=IPERF_SERVER_PORT, bind_dev=client_bind_dev
+            ):
+                reachable = True
+    except TimeoutExpiredError:
+        reachable = False
+    return reachable if expect_connectivity else not reachable