Verify image components are "ready"/available for use

[estesp]

Per conversation at MobySummit with @stevvooe @Random-Liu :

Need an "ensure" or "verify" function in the client library to properly walk an image's components to verify all pulled/unpacked and in a "ready" state to be used, e.g., for a container start referenced against that image ref. Without this, it is possible today to rely on an image whose reference is in the content store but whose components may or may not be ready/fully unpacked.

[crosbymichael]

@estesp did #1567 resolve this issue?

[estesp]

Let's just verify with @Random-Liu, but it was definitely meant to. I just didn't follow whether the commentary between @stevvooe and Lantau was incorporated into that PR or whether there was anything else outside of cri-containerd to add.

[crosbymichael]

@Random-Liu can you verify if this issue is resolved or if there are still aspects that need to be address?

[Random-Liu]

@estesp @crosbymichael We've added this.

Besides checking image readiness across cri-containerd restart, we also use IsUnpacked to check whether an image is unpacked, and unpack it again if not before creating each container:

• https://github.com/kubernetes-incubator/cri-containerd/blob/master/pkg/server/container_create.go#L155
• https://github.com/kubernetes-incubator/cri-containerd/blob/master/pkg/containerd/opts/container.go#L31