Switch away from ctr to nerdctl (orthogonal to the provider discussion)

[AkihiroSuda]

Note

This topic is orthogonal to the current on-going discussion about nerdctl provider:

• Initial provider implementation for nerdctl (+finch) #3429

What should be cleaned up or changed:

Switch away from ctr to nerdctl:

kind/pkg/build/nodeimage/imageimporter.go

Lines 49 to 64 in 5549e91

	func (c *containerdImporter) Pull(image, platform string) error {
	return c.containerCmder.Command(
	"ctr", "--namespace=k8s.io", "content", "fetch", "--platform="+platform, image,
	).SetStdout(io.Discard).SetStderr(io.Discard).Run()
	}
	func (c *containerdImporter) LoadCommand() exec.Cmd {
	return c.containerCmder.Command(
	// TODO: ideally we do not need this in the future. we have fixed at least one image
	"ctr", "--namespace=k8s.io", "images", "import", "--all-platforms", "--no-unpack", "--digests", "-",
	)
	}
	func (c *containerdImporter) ListImported() ([]string, error) {
	return exec.OutputLines(c.containerCmder.Command("ctr", "--namespace=k8s.io", "images", "list", "-q"))
	}

kind/pkg/cluster/nodeutils/util.go

Lines 79 to 90 in 5549e91

	// LoadImageArchive loads image onto the node, where image is a Reader over an image archive
	func LoadImageArchive(n nodes.Node, image io.Reader) error {
	snapshotter, err := getSnapshotter(n)
	if err != nil {
	return err
	}
	cmd := n.Command("ctr", "--namespace=k8s.io", "images", "import", "--all-platforms", "--digests", "--snapshotter="+snapshotter, "-").SetStdin(image)
	if err := cmd.Run(); err != nil {
	return errors.Wrap(err, "failed to load image")
	}
	return nil
	}

kind/pkg/cluster/nodeutils/util.go

Lines 152 to 156 in 5549e91

	// ReTagImage is used to tag an ImageID with a custom tag specified by imageName parameter
	func ReTagImage(n nodes.Node, imageID, imageName string) error {
	var out bytes.Buffer
	return n.Command("ctr", "--namespace=k8s.io", "images", "tag", "--force", imageID, imageName).SetStdout(&out).Run()
	}

Why is this needed:
Because ctr does not guarantee to keep compatibility:
https://github.com/containerd/containerd/blob/v1.7.10/RELEASES.md#ctr-tool

The ctr tool provides the ability to introspect and understand the containerd
API. It is not considered a primary offering of the project and is unsupported in
that sense. While we understand its value as a debug tool, it may be completely
refactored or have breaking changes in minor releases.

Targeting ctr for feature additions reflects a misunderstanding of the containerd
architecture. Feature addition should focus on the client Go API and additions to
ctr may or may not be accepted at the discretion of the maintainers.

We will do our best to not break compatibility in the tool in patch releases.

[aojea]

Is nerctl already available in the kind node images?

[BenTheElder]

Because ctr does not guarantee to keep compatibility:

This is a good point, however we control which ctr version we're using and kind tries not to break compatibility so we'd need a while before we can switch, otherwise we introduce another breaking change to

We will do our best to not break compatibility in the tool in patch releases.

This also makes it much less risky in the short term?

Is nerctl already available in the kind node images?

No, it's a separate project and we're not building it yet. We'd probably have to start including nerdctl for N releases since ctr has not broken us yet.

Also, I know that other users are using ctr via docker exec $node so there's that to consider ... As much as we try to tell people otherwise they do depend on what is in the node, and we'll wind up increasing the image size again or breaking them.