selftests: net: py: check process exit code in bkg() and background cmd()

We're a bit too loose with error checking for background
processes. cmd() completely ignores the fail argument
passed to the constructor if background is True.
Default to checking for errors if process is not terminated
explicitly. Caller can override with True / False.

For bkg() the processing step is called magically by __exit__
so record the value passed in the constructor.
Reported-by: Willem de Bruijn <willemb@google.com>
Tested-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240502025325.1924923-1-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -26,6 +26,9 @@ class cmd:
             self.process(terminate=False, fail=fail)
 
     def process(self, terminate=True, fail=None):
+        if fail is None:
+            fail = not terminate
+
         if terminate:
             self.proc.terminate()
         stdout, stderr = self.proc.communicate(timeout=5)
@@ -43,17 +46,18 @@ class cmd:
 
 
 class bkg(cmd):
-    def __init__(self, comm, shell=True, fail=True, ns=None, host=None,
+    def __init__(self, comm, shell=True, fail=None, ns=None, host=None,
                  exit_wait=False):
         super().__init__(comm, background=True,
                          shell=shell, fail=fail, ns=ns, host=host)
         self.terminate = not exit_wait
+        self.check_fail = fail
 
     def __enter__(self):
         return self
 
     def __exit__(self, ex_type, ex_value, ex_tb):
-        return self.process(terminate=self.terminate)
+        return self.process(terminate=self.terminate, fail=self.check_fail)
 
 
 def tool(name, args, json=None, ns=None, host=None):