util/helper: add stderr param to ProcessWrapper.check_output()

Allow separation of stderr from the output. For example, stderr can be
ignored by redirecting it to subprocess.DEVNULL or prevent capturing by
passing None, similar to how subprocess handles it.

Signed-off-by: Hoppy <dev.hoppy@gmail.com>
[bst: use default similar to subprocess: subprocess.STDOUT means merging
stdout and stderr, None means no capturing; raise on unsupported stderr
values]
Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: ImHoppy

labgrid/util/helper.py

@@ -41,8 +41,14 @@ class ProcessWrapper:
     loglevel = logging.INFO
 
     @step(args=['command'], result=True, tag='process')
-    def check_output(self, command, *, print_on_silent_log=False, input=None, stdin=None, timeout=None): # pylint: disable=redefined-builtin
+    def check_output(
+        self, command, *, print_on_silent_log=False, input=None, stdin=None, stderr=subprocess.STDOUT, timeout=None
+    ):  # pylint: disable=redefined-builtin
         """Run a command and supply the output to callback functions"""
+
+        if stderr not in (None, subprocess.STDOUT, subprocess.DEVNULL):
+            raise NotImplementedError(f"{stderr=} not supported")
+
         logger = logging.getLogger("Process")
         res = []
         mfd, sfd = pty.openpty()
@@ -59,8 +65,7 @@ def check_output(self, command, *, print_on_silent_log=False, input=None, stdin=
         elif stdin is not None:
             kwargs['stdin'] = stdin
 
-        process = subprocess.Popen(command, stderr=sfd,
-                                   stdout=sfd, bufsize=0, **kwargs)
+        process = subprocess.Popen(command, stderr=stderr, stdout=sfd, bufsize=0, **kwargs)
 
         logger.log(ProcessWrapper.loglevel, "[%d] command: %s", process.pid, " ".join(command))