bootc ukify fails when O_TMPFILE is not supported #2340

I have opened this issue, which is very similar to: #1977 as unfortunately it does not seem to be fully resolved in certain situations.

Using bootc 1.16.4 the following error occurs when trying to use bootc ukify:

DEBUG argv0="bootc"
error: Building UKI: Computing composefs digest: Reading container root: Reading container root: Ensuring object from file descriptor: Ensuring object from reader: Creating object tmpfile: Opening temp file in objects directory: Operation not supported (os error 95)
subprocess exited with status 1
subprocess exited with status 1

This is triggered by: https://github.com/composefs/composefs-rs/blob/67ea2f8c56725bb342bf712127baec5a7b6efbe5/crates/composefs/src/repository.rs#L1677

It might be its not possible to do this without O_TMPFILE support, but since it was resolved in the other issue, I was hoping this might be resolvable in a similar way.

Temporary (or not if this isn't resolvable) Workaround for Gitlab CI:

Since Gitlab CI has some of the CI environment on overlayfs and some (including your build folder) on normal ext4, it is possible to use --volume for podman build to put /var/tmp directly on ext4 instead of overlay.

Example:

• Create tmp folder in your build context folder (and a README.md or something so the folder is created on git checkout)
• Add something like this to .gitignore (only really needed if you commit back during CI):

tmp/**/*
!tmp/README.md

• Add a .containerignore file with tmp (prevents inflating build context)
• Add --volume=${CI_PROJECT_DIR}/tmp:/var/tmp to your podman build