Reject shrinking disk during YAML validation

[songponssw]

I wish we would detect the attempt to downsize the disk size, and then reject the change as invalid, but that can be a separate PR.

As @jandubois suggested in #3533.

Currently, rejecting shrinking disk size happens during instance startup.
This PR adds validation earlier during YAML validation.
The function compares values between the new config and the latest config YAML files to reject disk size shrinking during validation.

[Copilot]

Pull Request Overview

This PR implements early YAML validation to reject configurations that attempt to shrink the disk size rather than waiting until instance startup.

• Added a ValidateYAMLAgainstLatest function that compares disk sizes from the new and latest YAML configurations.
• Integrated the new validation into the limactl edit flow to save and report rejected configurations.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pkg/limayaml/validate.go	Added a YAML validation function to prevent disk size shrinking.
cmd/limactl/edit.go	Integrated validation into the edit command and added a function to handle rejected YAML.

[Copilot]

The error message wraps two errors using '%w: %w', which might be confusing. Consider simplifying the error wrapping to clearly associate the cause of failure with a single wrapped error.

Suggested change

	return fmt.Errorf("the YAML is invalid, attempted to save the buffer as %q but failed: %w: %w", rejectedYAML, writeErr, origErr)
	return fmt.Errorf("the YAML is invalid, attempted to save the buffer as %q but failed: %w", rejectedYAML, writeErr)

[songponssw]

The disk value can be nil when editing some YAML file.
For example, the editing kernel and initrd location in hack/inject-cmdline-to-template.sh.

[jandubois]

Looks mostly good, but there is some nitpicking I think should be addressed.

[jandubois]

As Copilot suggested above, you cannot use 2 %w formatting codes in an error; only the last one will be wrapped; the other will be treated as %v.

So either explicitly choose which one should be wrapped, and use %v for the other one explicitly. Or combine both errors with errors.Join():

Suggested change

	return fmt.Errorf("the YAML is invalid, attempted to save the buffer as %q but failed: %w: %w", rejectedYAML, writeErr, origErr)
	return fmt.Errorf("the YAML is invalid, attempted to save the buffer as %q but failed: %w", rejectedYAML, errors.Join(writeErr, origErr))

[jandubois]

Suggested change

	if err = Unmarshal(yNew, &n, "Unmarshal new YAML byte"); err != nil {
	if err = Unmarshal(yNew, &n, "Unmarshal new YAML bytes); err != nil {

[jandubois]

You are dereferencing both, so you need to bail if either one is nil.

Suggested change

	if n.Disk == nil && l.Disk == nil {
	if n.Disk == nil || l.Disk == nil {

[alexandear]

We can avoid err declaration:

Suggested change

	var err error
	if err = Unmarshal(yLatest, &l, "Unmarshal latest YAML bytes"); err != nil {
	return err
	}
	if err = Unmarshal(yNew, &n, "Unmarshal new YAML byte"); err != nil {
	if err := Unmarshal(yLatest, &l, "Unmarshal latest YAML bytes"); err != nil {
	return err
	}
	if err := Unmarshal(yNew, &n, "Unmarshal new YAML byte"); err != nil {

[alexandear]

Let's cover this function with unit tests.