bugfix: should add snapshot gc label to config

[fuweid]

Ⅰ. Describe what this PR did

# containerd content/snapshot gc reference overview
+----------+        +-------------+     +-----------------+
| manifest +------> | config blob +---> | rootfs snapshot |
+-----+----+        +-------------+     +-----------------+
      |             +-------------+
      +-----------> | layer blob  |
                    +-------------+

For containerd.io/gc.root label means the content/snapshot cannot be
removed during gc. If we use it for snapshot, the snapshot cannot be
removed during removing image.

By default, we should add
containerd.io/gc.ref.snapshot.{snapshot-name} gc label to the image
config so that gc can remove the snapshot if user tries to remove the
image.

Signed-off-by: Wei Fu fuweid89@gmail.com

Ⅱ. Does this pull request fix one issue?

close #2556

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

no need

Ⅳ. Describe how to verify it

# create container
$ sudo pouch run -d --name test1 busybox sh -c "echo 1 > /tmp/1 && sleep 1000"
be0d930081334dfc409e96aa1ec7184c89de3357be82ec28febba09921ee64a8

# commit it
$ sudo pouch commit test1 hello:1
b57ae9798515

# run it make sure that it is correct.
$ sudo pouch run --rm hello:1 sh -c "cat /tmp/1"
1

# check content gc label use by image id(= config digest)
$ sudo ctr -a /var/run/containerd.sock content ls | grep b57ae9798515 | grep snapshot
sha256:b57ae9798515d642a7b9c9ec5beea9dac5870d1cf770b76e70b76d134afb06eb 822 B           2 minutes       containerd.io/gc.ref.snapshot.overlayfs=sha256:964c0a9b08d55c0b51b044c0affe021fdd9fd366deffd1df3abe82baaf95d6ac

# check the snapshot (id is in the gc label)
$ sudo ctr -a /var/run/containerd.sock snapshot info sha256:964c0a9b08d55c0b51b044c0affe021fdd9fd366deffd1df3abe82baaf95d6ac
{
    "Kind": "Committed",
    "Name": "sha256:964c0a9b08d55c0b51b044c0affe021fdd9fd366deffd1df3abe82baaf95d6ac",
    "Parent": "sha256:23bc2b70b2014dec0ac22f27bb93e9babd08cdd6f1115d0c955b9ff22b382f5a",
    "Created": "2018-12-29T05:24:14.337401258Z",
    "Updated": "2018-12-29T05:24:14.337401258Z"
}

# time to remove the image
$ sudo pouch rmi hello:1

# check the snapshot
$ sudo ctr -a /var/run/containerd.sock snapshot info sha256:964c0a9b08d55c0b51b044c0affe021fdd9fd366deffd1df3abe82baaf95d6ac
ctr: snapshot sha256:964c0a9b08d55c0b51b044c0affe021fdd9fd366deffd1df3abe82baaf95d6ac does not exist: not found

Ⅴ. Special notes for reviews

[codecov]

Codecov Report

Merging #2635 into master will increase coverage by 0.03%.
The diff coverage is 65.11%.

@@            Coverage Diff            @@
##           master   #2635      +/-   ##
=========================================
+ Coverage   69.57%   69.6%   +0.03%     
=========================================
  Files         280     280              
  Lines       18844   18844              
=========================================
+ Hits        13111   13117       +6     
+ Misses       4268    4261       -7     
- Partials     1465    1466       +1

Flag	Coverage Δ
#criv1alpha1test	31.45% <0%> (+0.04%)	⬆️
#criv1alpha2test	35.69% <0%> (-0.02%)	⬇️
#integrationtest	41.69% <65.11%> (+0.01%)	⬆️
#nodee2etest	32.74% <0%> (ø)	⬆️
#unittest	26.91% <0%> (ø)	⬆️

Impacted Files	Coverage Δ
ctrd/image_commit.go	68.87% <65.11%> (ø)	⬆️
daemon/logger/jsonfile/utils.go	71.54% <0%> (-1.63%)	⬇️
cri/v1alpha2/cri_wrapper.go	65.59% <0%> (-1.21%)	⬇️
cri/v1alpha2/cri.go	67.82% <0%> (-0.25%)	⬇️
daemon/mgr/container.go	59.21% <0%> (+0.21%)	⬆️
ctrd/container.go	59.28% <0%> (+1.18%)	⬆️
ctrd/image.go	78.18% <0%> (+2.27%)	⬆️

[wangforthinker]

LGTM