support nydus image topmost layer view

[changweige]

Without inline layer fs meta blob, we can still support the topmost View for nydus image. It is useful for Buildkit to use nydus image as the base image.

Next PR will implement View on any layer of the nydus image.

➜  ~ ctr snapshot view --mounts cccddd sha256:62b1c52c224c7907ca34194f2c7e819b8057251b6d69623ff3bf8a7c9393894a
[
    {
        "Type": "overlay",
        "Source": "overlay",
        "Target": "",
        "Options": [
            "lowerdir=/var/lib/containerd-nydus/snapshots/510/mnt:/var/lib/containerd-nydus/snapshots/536/fs"
        ]
    }
]

[sctb512]

Fantastic! This PR is useful for mounting a nydus image into a directory with ctr i mount.
I also recommend improving the Prepare API. Because the command with --rw (ctr i mount --rw IMAGE TARGET ) does not work now.
The ctr source code about it:

if context.Bool("rw") {
	mounts, err = s.Prepare(ctx, target, chainID)
} else {
	mounts, err = s.View(ctx, target, chainID)
}

Here is a comparison with stargz:

mkdir mkdir test.bootstrap.stargz
nerdctl --snapshotter stargz pull ghcr.io/stargz-containers/alpine:3.15.3-esgz
ctr --debug i mount --rw --snapshotter stargz ghcr.io/stargz-containers/alpine:3.15.3-esgz test.bootstrap.stargz
ls test.bootstrap.stargz

Output:

bin dev etc home lib media mnt opt proc root run sbin srv sys tmp usr var

Unfortunately this does not work for nydus:

mkdir test.bootstrap.nydus
nerdctl --snapshotter nydus pull sctb512/golang:nydusv6
ctr --debug i mount --rw --snapshotter nydus docker.io/sctb512/golang:nydusv6 test.bootstrap.nydus
ls test.bootstrap.nydus

Output: None

[imeoer]

if err := o.fs.WaitUntilReady(pID); err != nil {

[changweige]

fixed

[changweige]

o.fs.WaitUntilReady(pID)

Prepare is now implemented

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ab8c6fe) 37.15% compared to head (1db810a) 37.15%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #447   +/-   ##
=======================================
  Coverage   37.15%   37.15%           
=======================================
  Files          60       60           
  Lines        6909     6909           
=======================================
  Hits         2567     2567           
  Misses       4045     4045           
  Partials      297      297           

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