[pytest-dev] How to add delay before processing captured output

[Nikolaus Rath]

Hello,

I have a test that starts a background process, and then checks its
response to various queries. Now, I would like to test for particular
bug where some query causes the server to crash. However, there's an
interesting problem with this:


I when running with capturing disabled, I can see the final words of the
server on the screen. When running with capturing (fd capturing, to be
exact), these last messages are lost.

I eventually figured out that the problem is that the test fails before
the server crashes (because the server returns an invalid response
before crashing), and capfd then shows the captured output before the
server had a chance to print its last words.

If I explicitly ignore the test failure, I am able to see the full
captured output. In other words, if

def my_test():
    check_server_response()

becomes

def my_test():
    try:
       check_server_response()
    except:
       time.sleep(1)
       wait

then the server gets enough time to die before the test fails.


Now, I realize that adding a delay is rather hacky, but I do not see a
better way to implement this (after all, I have to take into account
that the server could also hang). However, I really do not want to
insert sleep calls into every single test.

Therefore, I tried to define a custom "wait a little" autouse fixture
that just does this:

@pytest.fixture(autouse=True)
def wait_for_server(request, capfd):
    def wait_a_moment():
        time.sleep(1)
    request.addfinalizer(wait_a_moment)


...but this did not actually have the desired effect. It seems the
fixture is only run *after* capfd has read the the output.


Is there a way to add a delay that is (1) only incurred when a test
fails and (2) executed right after the test fails (ie., before the capfd
finalizer runs)?


Thanks,
-Nikolaus


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