`tee-fd`/`capteefd` - file descriptor level capture with pass-through

[i-ky]

What's the problem this feature will solve?

It does not seem to be possible to capture all output including subprocesses in JUnit report while being able to judge the progress of test execution by the output passed through to the terminal.

--capture=fd does the former, but not the latter. --capture=no does the latter, but not the former. --capture=tee-sys does the pass-through, but fails to capture subprocess output, which will only be available in the terminal without the structure of JUnit report.

Describe the solution you'd like

It would be nice to have tee-fd capture mode and, perhaps, an accompanying capteefd fixture. They would capture at file descriptor level like fd/capfd, but would also pass-through test output like tee-fd/capteefd.

This would be best of both worlds - all output of tests including subprocesses would be captured and stored in JUnit report while pass-through output would allow to monitor progress.

Alternative Solutions

Not sure this can be worked around using a plugin since capturing is a core feature.

Additional context

We are using pytest to run high level end-to-end tests of non-Python code. This involves launching subprocesses and the output of subprocesses is crucial for failure analysis. Tests are long-running and if there is no output it is difficult to judge whether everything progresses as usual or the test simply hanged. Tests produce a lot of output, so we use JUnit reports to add some structure.

[i-ky]

Related discussion - #12782.

[RonnyPfannschmidt]

A key problem with that is that it needs a process/ thread that does the cloning

So far im unaware of any erklärt towards a reliable implementation

Personally be unwilling to take on a naive implementation due to the likelihood of requiring maintenance/ support work

[i-ky]

A solution, I guess, could be to set up a thread in FDCaptureBase to read from self.tmpfile and write to self.targetfd_save.

[i-ky]

I was also thinking about a "lightweight" alternative of file descriptor level capturing plus sys level pass-through that can be achieved by making TeeCaptureIO work with tempfile passed to SysCaptureBase. This would not be ideal, but would likely address 80% of my use case (where capture is way more important than pass-through).