tests: wait for test client to connect before running test

Vtysh has been improved to startup very quickly this exposed a race in this
test, where the `clear ip rip...` command ran before the test client that
handles it had finished connecting to mgmtd. Add a retried check for the test
client being connected before issuing the `clear ip rip ...` test command.

Signed-off-by: Christian Hopps <chopps@labn.net>

tests/topotests/mgmt_rpc/test_rpc.py

@@ -14,6 +14,7 @@ import os
 import threading
 
 import pytest
+from lib.common_config import retry
 from lib.topogen import Topogen
 from lib.topotest import json_cmp
 
@@ -40,12 +41,20 @@ def tgen(request):
     tgen.stop_topology()
 
 
+# Verify the backend test client has connected
+@retry(retry_timeout=10)
+def check_client_connect(r1):
+    out = r1.vtysh_cmd("show mgmt backend-adapter all")
+    assert "mgmtd-testc" in out
+
+
 def test_backend_rpc(tgen):
     if tgen.routers_have_failure():
         pytest.skip(tgen.errors)
 
     r1 = tgen.gears["r1"]
 
+    # Run the backend test client which registers to handle the `clear ip rip` command.
     be_client_path = "/usr/lib/frr/mgmtd_testc"
     rc, _, _ = r1.net.cmd_status(be_client_path + " --help")
 
@@ -63,6 +72,10 @@ def test_backend_rpc(tgen):
     t = threading.Thread(target=run_testc)
     t.start()
 
+    # We need to wait for mgmtd_testc to connect before issuing the command.
+    res = check_client_connect(r1)
+    assert res is None
+
     r1.vtysh_cmd("clear ip rip vrf testname")
 
     t.join()