independently unpacking layer functions

[xiekeyang]

cc @wking @runcom @stevvooe

At early discussion, @wking and I doubt that manifest.unpack() is not very close to manifest structure.
Actually, manifest.unpack() is only responsible for unpacking layers, which list can be gotten from manifest. It can be done without a real manifest. It is odd to use it as a method of manifest structure.
So, I place 2 layer functions to layer.go file. and place their unit test to layer_test.go.
From my side it looks more reasonable and go easily forward to reconstructing. And I'd like to get your proposal.

Signed-off-by: xiekeyang xiekeyang@huawei.com

[wking]

On Sun, Sep 18, 2016 at 08:04:04PM -0700, xiekeyang wrote:

Actually, manifest.unpack() is only responsible for unpacking
layers, which list can be gotten from manifest. It can be done
without a real manifest. It is odd to use it as a method of manifest
structure. So, I place 2 layer functions to layer.go file. and
place their unit test to layer_test.go.

This makes sense to me. However, if the in-flight #8 lands pulling in
an external library for manipulating layers it may be a moot point.
We should probably idle this PR until the maintainers and other
interested parties have had time to weigh in on #8.

[xiekeyang]

PTAL.

[glestaris]

Also #3 is refactoring the unpacking area slightly. One of us may have to rebase :)

[xiekeyang]

@glestaris Yes. According to previous discussion, the manifest_test.go is inproper because it is just for layer unpacking. So this PR should stand firstly, and it need more refactor following to #4 .
I read #3 and think that rebase is necessary but not very difficult, which I can help to do.
Thanks.

[vbatts]

please rebase

[xiekeyang]

rebased. PTAL
cc @wking @vbatts

[xiekeyang]

@vbatts It has been rebased.

[xiekeyang]

@opencontainers/image-tools-maintainers
I think this PR is ready to be merged. PTAL.
The main purpose is to set up layer.go and unit test file, to collect layer unpacking functions, which mix with manifest. We are and will handle layer. It had better to involve them in individual files.

[wking]

On Thu, Nov 17, 2016 at 07:41:48PM -0800, xiekeyang wrote:

I think this PR is ready to be merged. PTAL.

I still 1 think this depends on how we intend to handle #8. As
@glestaris pointed out 2, using a vendored/forked library only for
creation but not for unpacking is strangely asymmetric. Do we have a
consensus on where #8 is headed? Is it likely to land? Is it's new
utils/ package (forked from github.com/containers/storage, which is
forked from docker/docker) expected to grow/fork unpacking logic?

[xiekeyang]

As evolution on image-spec, this PR should be closed, and waiting for a solid design framework of CAS.
@coolljt0725 please review #8
@wking please review #5
Consider how to handle them.