Preempt Swift slots to enforce mutual exclusion updating Swift container

[zhijie-yang]

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Ping the @canonical/rocks team.

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Description

This PR adds a Prepare upload job to the Image.yaml workflow, which preempts the Swift slot right after retrieving the revision number from the Swift container to prevent a race condition. For the workflow before this PR, such race conditions can happen when a workflow is triggered, while another workflow with the same image name runs for a job between Build and Upload (both included). Since the job Build can take up 20 minutes, it's easy to trigger such a workflow that races with other workflows. The eventual consequence of such a race condition can result in duplicated revisions of images of different tracks, thus letting the workflow release wrong artefacts to some tracks (tags).

Changelog

• Using {name}_{commit}_{directory} as identifiers for OCI image artefacts;
• Validation of the uniqueness of such identifiers;
• Forming a critical section in the job Prepare upload that makes a write-after-read operation to the Swift container;
• Preempting slots in Swift container for concurrent uploads in the job Upload;
• Protecting such critical sections by wrapping them with "lock" and "unlock" operations using a lock file (on a per-image basis);
• Busy waiting when the lock file exists;
• Unlocking when the workflow is cancelled or the preemption operation fails;
  • Won't unlock if the lock operation fails (usually means waiting for other Prepare upload jobs for the same image to finish)
• Adding a timestamp to the revision data cache key to enable the re-run of failed Prepare upload jobs that can give different revision numbers.

Related issues

• https://bugs.launchpad.net/ubuntu-docker-images/+bug/2069652
• Docker images on ubuntu/python chiselled-python#21

Before merge:

• Revert the changes 92e27a6 and e364915.

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[cjdcordeiro]

ty for the changes, can we merge?

[zhijie-yang]

Yes, I reverted the changes disabling ppc64el for mock-rock. We can merge now (even if the lpci build in the workflow can fail).

[cjdcordeiro]

there's a conflict that needs resolution

[zhijie-yang]

Done.

[cjdcordeiro]

why is the prometheus rock being built in the CI?

[zhijie-yang]

Because of the merge commit 2acd80e.