Support schema 1 image.

[Random-Liu]

Now containerd only supports schema 2 manifest.

However, a lot of images are still using schema 1 manifest.

We should at least support pulling schema 1 image.

[stevvooe]

@Random-Liu I just took a look at the code here and it looks more than doable. The only missing aspect is that schema1 has BlobSums instead of descriptors, but we can generate them.

[Random-Liu]

@stevvooe Cool~ It would be very helpful if we could support schema 1 before 1.0, if it's not that complex. :)

If we don't have bandwidth, it's also fine for us to leave it to next release. But we need to do some work around on our side for the schema 1 system container images.

[Random-Liu]

We've get most pieces for alpha integration ready (at least in-queue for reviewing).

@dmcgowan @stevvooe This becomes one of biggest blockers for our integration test.

Do we have plan to prioritize this? Does this need much effort? If not, maybe I could try it out. :)

[dmcgowan]

@Random-Liu I can probably get to this next week, first we want to get this client model completed and integrated into the dist tool. The plan is to translate schema1 to schema2 client side on pull and not support it on push. This is not much effort if you want to take it on, the most difficult part will be updating the image config with the rootfs. The diff ids may be missing from the schema 1 history section, requiring computation of the digest of the uncompressed blobs.

[Random-Liu]

@dmcgowan Thanks! Given that I've got no experience on the image format, it would be super great if you could get to this. I'll not run our integration test which needs schema 1 support for now.

I'll work on the other 2 items on our side listed in containerd/cri#35, so that we could have a better test result for the alpha integration.

[Random-Liu]

@crosbymichael Thanks for including this in alpha release 1.