Document that writing to stdout/stderr does not work on workers

[nicoddemus]

No description provided.

[pelme]

Why is this the case? I understand the motivation for remote runs over ssh where stdout/stderr would have to be serialized and communicated back. But what about local runs (i.e. -n1), wouldn't stdout/stderr just be printed from the subprocess?

[RonnyPfannschmidt]

no, execnet uses stdin/out for communication
so we either have to add explicit support, or remove it

[pelme]

I see. Would adding explicit support be hard (in theory)? If you think it wouldn't require any insane refactors I would like to give it a try.

print() and log handlers which just dumps stuff on stdout/stderr is such an obvious and easy way to get some debug output. I wrestle with some hard-to-debug intermittent failures on our CI server where I cannot just disable xdist because of slowness.

(I tried to redirect my log messages to some temporary files, but I found it not really straightforward to get that working either, with different nodes etc.)

[RonnyPfannschmidt]

a python only solution is thinkable (it just needs a handler for stdin/out)
if this gets documented as limited python-only support it seems doable

[pelme]

(I have not gotten around to do anything about this yet, if anyone is interested, feel free to tackle this!)

[leorochael]

If execnet uses stdin/stdout for communications but doesn't need stderr, it would be nice if sys.stderr was available when not being captured by pytest.

I know, for example, that stderr is available during pytest_sessionstart() in workers, but in pytest_sessionend() its's not. It'd be nice if it was.

[leorochael]

Though I guess one could always resort to printing to sys.__stderr__:

def pytest_sessionfinish(session, exitstatus):
    workerinput = getattr(session.config, 'workerinput', None)
    if workerinput is None:
        print("\nExiting the Master Process, or running serially", file=sys.__stderr__)
    else:
        print(f"\nExiting Worker: {workerinput['workerid']}\n", file=sys.__stderr__)

[jiasli]

In my scenario, stderr is indeed shown when --capture=no is specified, according to @RonnyPfannschmidt answered in pytest-dev/pytest#1693 (comment).