Fix SquashFS extraction when it contains directory symlinks

[Flamefire]

Description of the Pull Request (PR):

A SquashFS image might contain directory symlinks such as /var/tmp -> /tmp. Before extraction a base environment is created (makeBaseEnv) by pre-creating some directories. Among them /var/tmp. Although unsquashfs is called with -force it fails to replace that by the symlink in the image as it removes only directories.

As per suggestion from @dtrudg the base environment causing the trouble is created after the extraction as done for e.g. OCI images.

This fixes or addresses the following GitHub issues:

• Fixes Error while building from image: "failed to create symlink /image/rootfs/var/tmp" #3084

[dtrudg]

Hi @Flamefire - thanks for sharing this. However, it's not the approach we want to take.

• Relying on scanning the output of unsquashfs is brittle to any changes in that tool, and adds significant complexity to that code. (edit... we are now using squashfs-tools-ng tools elsewhere, and may want to here at some point, so this is why parsing unsquashfs output is additionally an issue)
• In other code paths, where we build e.g. from an OCI source, rather than a local image file, the directories are created after image extraction, which avoids this issue. We believe we can swap the order of operations for the localimage conveyorpacker code to fix it in a much simpler manner.

I've been discussing this with @cyanezstange and he will be tackling the issue in the next week.

[Flamefire]

Relying on scanning the output of unsquashfs is brittle to any changes in that tool, and adds significant complexity to that code. > (edit... we are now using squashfs-tools-ng tools elsewhere, and may want to here at some point, so this is why parsing unsquashfs output is additionally an issue)

I made it as simple as possible with as few assumptions as possible. E.g. using -ls instead of -ll to just get a plain list of names. One issue were warnings output by the singularity sandbox run:

WARNING: Skipping mount /etc/hosts [binds]: /etc/hosts doesn't exist in container
WARNING: Skipping mount /etc/localtime [binds]: /etc/localtime doesn't exist in container
WARNING: Skipping mount /installed/var/singularity/mnt/session/tmp [tmp]: /tmp doesn't exist in container
WARNING: Skipping mount /installed/var/singularity/mnt/session/var/tmp [tmp]: /var/tmp doesn't exist in container
WARNING: Skipping mount /installed/var/singularity/mnt/session/etc/resolv.conf [files]: /etc/resolv.conf doesn't exist in container

So the only assumption I did was that the output of unsquashfs -ls is a list of paths, one per file, in top-down order or at least with the root path first. In case that changes (which I find rather unlikely) then directories might not get removed leading to the same behavior as before. Not too bad IMO.

In other code paths, where we build e.g. from an OCI source, rather than a local image file, the directories are created after image extraction, which avoids this issue. We believe we can swap the order of operations for the localimage conveyorpacker code to fix it in a much simpler manner.

You are right, that is likely better

[cstangeyanez]

Hi @Flamefire , thank you for this PR. I have tidy some thing in #3151 which keep you as an contributor. Please let us know if you don't want to be listed in the CONTRIBUTORS.md file or have any others concerns.

[Flamefire]

That's fine for me, thanks!