[launch_pytest]: Output buffered in fixture.

Bug report

Required Info:

• Operating System:
  • Ubuntu 22.04
• Installation type:
  • Docker
• Version or commit hash:
  • ros:rolling-ros-core-jammy
• Client library (if applicable):
  • launch_pytest

Steps to reproduce issue

I run pytest with the -s flag, but output is buffered until the test actually starts to execute.

@pytest.fixture(scope="function")
def client():
    _client = FastapiClient()
    _client.assert_wait_for_services(timeout_sec=120)
    return _client

@pytest.mark.launch(fixture=generate_test_description)
def test_fastapi_client(client):
    assert True

Expected behavior

I expect launch log output to print to screen while my all my processes launch.

Actual behavior

Nothing prints in the terminal until test_fastapi_client actually started. This could be 120 seconds given the _client.assert_wait_for_services line. I simplified this example but its not as easy to move construction into my actual test.

Happy to help contribute if someone with more experience can point me in the right direction.

Start by reproducing the launch_pytest fixture behavior with pytest -s and the provided FastapiClient example. Trace how launch output is handled during fixture setup, and consider the issue done when process and launch logs appear while the fixture is waiting rather than only after test_fastapi_client starts.