fix(stargz-snapshotter): set default root path

[daper]

Description

This PR adds the default root path parameter to stargz-snapshotter extension as an argument: --root. It sets the path to /var/lib/containerd/io.containerd.snapshotter.v1.stargz as it is a more common place to find it. The snapshotter default path is defined in stargz-snapshotter/cmd/containerd-stargz-grpc/main.go#L66 and points to /var/lib/containerd-stargz-grpc.

Motivation

When trying the stargz-snapshotter extension I found some errors and seemed that the snapshotter wasn't working properly. I didn't copy the errors but they were something in the lines of:

• missing directory /var/lib/containerd/io.containerd.snapshotter.v1.stargz.
• stargz-snapshotter plugin is not exporting its root directory.

Investigation

Searching around I found some issues:

• stargz-snapshotter doesn't work #245
• stargz-snapshotter uses up all available disk space containerd/stargz-snapshotter#1349
• Kubelet log has error when use default root dir containerd/nydus-snapshotter#288

The last of the issues listed above pointed me to the right track. There is a linked PR (containerd/containerd#10127) where it is added to containerd the functionality to inform the remote snapshotter's root dir. Although it is advised to point it (if possible) to /var/lib/containerd/ for better compatibility.

containerd/nydus-snapshotter#288 (comment)

But we'd better still set snapshotter's default root path to /var/lib/containerd/io.containerd.snapshotter.v1.nydus for the better compatibility.

Testing

I've built the extension with the change, then built an installer image and upgraded a testing cluster with it. After it all the logs seemed normal. Also these logs from the CRI stream, leads me to think that cleanup of stale snapshots is working as expected:

{"level":"debug","msg":"schedule snapshotter cleanup","snapshotter":"stargz","time":"2024-08-15T15:17:49.143802230Z"}
{"key":"k8s.io/138/96b15cc0376d1abec916fb9fa9af3e6e83c7d0862ab82abd026fd309cee54723","level":"debug","msg":"removed snapshot","snapshotter":"stargz","time":"2024-08-15T15:17:49.147120976Z"}

[rsmitty]

Oh shoot, this is awesome. I poked at this same bit quite a while back without success. Glad to see you cracked it!

We'll sign this and get it committed.

[smira]

/m