feat: enable Artifact Streaming on Flatcar

[chewi]

What type of PR is this?

/kind cleanup
/kind feature

What this PR does / why we need it:

Artifact Streaming is an important feature for Flatcar.

This uses Flatcar's own systemd system extension for overlaybd and Azure Linux's RPM for acr and the configuration scripts. It's not perfect, but we cannot improve on this until acr changes its build process or goes open source.

Which issue(s) this PR fixes:

Fixes Flatcar for AKS work item #34435545

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version
• commits are GPG signed and Github marks them as verified

Release note:

* Enabled Artifact Streaming on Flatcar.

[chewi]

It took a long time, but I was finally able to test this under RP. I couldn't see any details, but after removing Flatcar's exclusion of the ArtifactStreaming test, I saw that it passed along with the other E2Ev3 tests.

[chewi]

To be extra sure, I also ran E2Ev3 without this change and saw that Scenario_Flatcar_Artifact_Streaming failed while the other tests passed.

[Copilot]

Pull request overview

This pull request enables Artifact Streaming on Flatcar by refactoring the isARM64 function and implementing Flatcar-specific installation logic. Artifact Streaming is an important performance feature for container image management.

Changes:

• Refactored isARM64() function to return an exit code directly instead of echoing 0/1, simplifying all usage patterns
• Added Flatcar-specific Artifact Streaming installation using Flatcar's overlaybd system extension and Azure Linux's RPM package
• Updated Flatcar custom data to enable the overlaybd system extension and mask related services initially

Reviewed changes

Copilot reviewed 15 out of 75 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
parts/linux/cloud-init/artifacts/cse_helpers.sh	Refactored isARM64() to return exit code directly
parts/linux/cloud-init/artifacts/cse_install.sh	Updated 3 call sites to use new isARM64 pattern
parts/linux/cloud-init/artifacts/cse_config.sh	Updated 2 call sites and added unmask logic for Flatcar overlaybd services
parts/linux/cloud-init/artifacts/ubuntu/cse_install_ubuntu.sh	Updated 1 call site to use new isARM64 pattern
vhdbuilder/packer/install-dependencies.sh	Added Flatcar-specific artifact streaming installation logic and updated 4 call sites
vhdbuilder/scripts/linux/tool_installs.sh	Updated 1 call site to use new isARM64 pattern
vhdbuilder/scripts/linux/ubuntu/tool_installs_ubuntu.sh	Updated 1 call site to use new isARM64 pattern
vhdbuilder/packer/test/linux-vhd-content-test.sh	Updated 2 call sites to use new isARM64 pattern
vhdbuilder/packer/flatcar-customdata.yaml	Added overlaybd system extension configuration and service masking
vhdbuilder/packer/flatcar-customdata.json	Compiled JSON version of the YAML custom data
pkg/agent/testdata/MarinerV2+Kata/CustomData	Regenerated test data with updated code

[Copilot]

Pull request overview

Copilot reviewed 15 out of 75 changed files in this pull request and generated 1 comment.

[Copilot]

systemctl unmask is executed unconditionally, but the inline comment indicates this is Flatcar-specific. If this script runs with set -e (common in CSE), systemctl unmask can fail on non-Flatcar (or before the units exist) and abort provisioning. Make this conditional on Flatcar (e.g., if isFlatcar "$OS"; then ... fi) and/or make the unmask tolerant to missing units (e.g., append || true).

Suggested change

	systemctl unmask overlaybd-{tcmu,snapshotter}.service # Flatcar masks these initially.
	systemctl unmask overlaybd-{tcmu,snapshotter}.service || true # Flatcar masks these initially.