Re: should the functional-testing scratch_file() utility guarantee that the path it returns doesn't exist?

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Sep 08, 2025 at 02:13:54PM +0100, Peter Maydell wrote:
> The functional testing framework includes a utility function
> scratch_file() which you can use to get a path for a scratch
> file to use in the test. However, it doesn't do anything to
> ensure that the path it returns doesn't already exist.
> Should it?

No, we need to be able to use scratch_file() to refer to
previously created resources. For example, when we extract
files from an archive, we want to then reference those, or
when we uncompress a file.

> I ran into this with the aarch64/test_rme_sbsaref.py test:
> this does:
>         rme_stack = self.scratch_file('.')
> to get a scratch path, and then uses it both to place
> specific files:
>         pflash0 = join(rme_stack, 'out', 'SBSA_FLASH0.fd')
> and to place subdirectories:
>         efi = join(rme_stack, 'out', 'EFI')
>         os.makedirs(efi, exist_ok=True)
> 
> In the original version of this test we used os.mkdir(efi),
> but this fails sometimes because the directory already exists
> (typically if the test was interrupted during a previous run);
> see this email for the error log:
> https://lore.kernel.org/qemu-devel/CAFEAcA_ZQ13qMRUQsieJiEPV=ULrDbz8=EJaW4_kw=yEysob0w@mail.gmail.co/m

We have a tearDown function which purges everything under the
scratch dir location, so if a test is interrupted in a "normal"
way we should be safe on re-run. Only if python is hard-terminated
would the tearDown method fail to get run.

Perhaps we should proactively purge the scratch location in
the setUp method too, as a safety net against previous unclean
shutdown, as the intent of the 'scratch file' concept is that
the test should have a clean environment when it starts running.


> Secondary question: is it OK to pass '.' to this function
> and then construct filenames based on the return value,
> or would it be better to call scratch_file('out', 'SBSA_FLASH0.fd')
> etc and have the utility function construct the whole path?

My recommendation  / preferred coding style for func tests is to
always pass the set of sub-dir/file components to scratch_file(),
and never try to manually construct file paths using 'join'.

With regards,
Daniel
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