[PATCH net-next 1/3] selftests: net: py: avoid masking exceptions in bkg() failures

[Jakub Kicinski <kuba@kernel.org>]

bkg() failures are currently quite hard to debug and spot.
Often we have code along the lines of:

  with bkg("./cmd_rx_something -p PORT"):
       wait_port_listen(PORT)
       cmd("./cmd_tx_something", host=remote)

When wait_port_listen() fails we don't get to see the exit status
of bkg(). Even tho very often it's a failure in the bkg() command
that's actually to blame. Try not to interfere with the bkg()
command error checking.

With:

   with bkg("false", exit_wait=True):
        time.sleep(0.01)  # let the 'false' cmd run
        raise Exception("bla")

Before:

  .. stack trace ..
  # Exception| Exception: bla

After:

  .. stack trace ..
  # Exception| Exception: bla
  # Exception|
  # Exception| During handling of the above exception, another exception occurred:
  .. stack trace ..
  # Exception| lib.py.utils.CmdExitFailure: Command failed: false
  # Exception| STDOUT: b''
  # Exception| STDERR: b''

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
 tools/testing/selftests/net/lib/py/utils.py | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/lib/py/utils.py b/tools/testing/selftests/net/lib/py/utils.py
index 85884f3e827b..8fa1c2fabfc2 100644
--- a/tools/testing/selftests/net/lib/py/utils.py
+++ b/tools/testing/selftests/net/lib/py/utils.py
@@ -159,8 +159,11 @@ import time
         return self
 
     def __exit__(self, ex_type, ex_value, ex_tb):
-        # Force termination on exception
-        terminate = self.terminate or (self._exit_wait and ex_type is not None)
+        terminate = self.terminate
+        # Force termination on exception, but only if bkg() didn't already exit
+        # since forcing termination silences failures with fail=None
+        if self.proc.poll() is None:
+            terminate = terminate or (self._exit_wait and ex_type is not None)
         return self.process(terminate=terminate, fail=self.check_fail)
 
 
-- 
2.53.0