Allow Overriding the Default Mount Point of the Stage Directory in Containers

[mredenti]

Description

Currently, ReFrame mounts the test’s stage directory inside the container to /rfm_workdir by default. This behavior is hard-coded and does not provide flexibility for users to specify an alternative mount point. - see https://reframe-hpc.readthedocs.io/en/stable/regression_test_api.html#reframe.core.containers.ContainerPlatform.workdir

Proposes feature

Introduce a configuration option that allows users to override the stage directory as the default mount point that points to /rfm_workdir directory inside the container.

Motivation

In environments with non-shared filesystems, input files are not located on the login node where the stage directory resides. As a result, the default mounting behavior restricts the accessibility of input files within the container. In this case one has to specify and additional mount point and and also set the self.container_platform.workdir. It would be more efficient to allow the possibility of changing the default mount point in such situations

[vkarak]

I am not sure that this is actually needed, since you can still achieve the desired behaviour by setting two variables instead of one. Am I missing something?

[mredenti]

So you can definitely mount any other directory anywhere else and also change the default working directory inside the container.
I guess it is just a matter of redundancy, why mount something in the container if you do not need it.

Mounting two host directories to /rfm_workdir makes we wonder whether this is defined behaviour. I need to check whether container engines allow this or the second one specified takes precedence. In this case you would not be able to mount into /rfm_workdir but it would be nice to keep a certain consistency for all tests such that the destination directory inside the container can always be
/rfm_workdir but need not be the stage directory being mounted

[vkarak]

You would still need the stagedir mounted inside the container if you wanted to copy out some files for inspection. I guess in your example, everything is in stdout so you don't need this. Maybe a variable defined as:

mount_stagedir = variable(str, type(None), value='/rfm_workdir')

would make sense. If None, the stage directory would not be mounted at all. Afaiu this would work nicely for your case, wouldn't it?

[mredenti]

Yes, that would work nicely